Search

In this research work, enhancement and induction of magnetization and other related properties of ferro-electromagnet is reported. The ascertained enhancement and induction is supported by the Hamiltonian composed of itinerant electrons, localized electrons, and their mutual interactions. Using double time temperature dependent Green’s function formalism and equation of motion, the study exhibited the effect of external electric field that can induce and enhance the magnetization and other related parameters of the material. The expressions for spin wave energy, magnetization, magnetic susceptibility, and heat capacity of the system have been obtained and influenced by the action of an external electric field. These enhanced and induced functional responses of this material will present an opportunity to create and implement thin film devices that actively couple the magnetic and ferroelectric order parameters. The results obtained in this work are in a good agreement with the previous theoretical and experimental findings.

В работе показано, что в результате спекания образцов композита титанат бария (80 объемных %) - феррит бария (20 объемных %) в фарфоровом тигле при температуре 1300 °С возникает эвтектика. В результате сравнения свойств образцов, полученных при различных температурах спекания, установлено, что оптимальные свойства имеют образцы, спеченные при 1250 °С. Проведено сравнение структуры и диэлектрических свойств образцов керамики титаната бария и композита титанат бария (80 объемных %) - феррит бария (20 объемных %), спеченных при температуре 1250°С. Показано, что добавление в состав титаната бария 20% феррита бария повышает значение диэлектрической проницаемости, пироэлектрического коэффициента и пьезоэлектрического модуля d композита в 1,5 - 2 раза по сравнению с керамикой титаната бария, тогда как значение пьезоэлектрического модуля d остается без изменения. Введение в состав керамики титаната бария 20 % феррита бария достаточно для того, чтобы полученный композит имел магнитные характеристики, соответствующий чистому ферриту бария. It is shown in the work that as a result of sintering of composite samples of barium titanate (80 vol. %) - barium ferrite (20 vol. %) in a porcelain crucible at a temperature of 1300 °C, a eutectic appears. As a result of comparing the properties of the samples obtained at different sintering temperatures, it was found that the samples sintered at 1250 °C have optimal properties. The structure and dielectric properties of barium titanate ceramic samples and barium titanate (80 vol. %) - barium ferrite (20 vol. %) composite sintered at a temperature of 1250 °C are compared. It has been shown that the addition of 20% barium ferrite to the composition of barium titanate increases the dielectric constant, pyroelectric coefficient, and piezoelectric modulus d of the composite of 1,5 - 2 times compared to barium titanate ceramics, while the value of the piezoelectric modulus d remains unchanged. The introduction of 20 % barium ferrite into the barium titanate ceramics is sufficient for the resulting composite to have magnetic characteristics corresponding to pure barium ferrite.

The dielectric performances as well as the effects of Dy3+ ions content at A sites of YMn2O5 (YMO) [x ​= ​0 (YDM0) and x ​= ​0.4 (YDM0.4)] polycrystalline samples were explored. These compounds were synthesized via sol–gel method. X-Ray diffraction and Raman measurements proved the high quality of the compounds that crystallized in an orthorhombic structure with the Pbam space group. Besides, impedance spectroscopy and electrical modulus studies revealed that both samples exhibited a non-Debye's type of relaxation. The decrease of impedance for YDM0.4 compared to the pure YDM0, may be attributed to the decrease in charge transfer resistance. Concerning the fit of impedance spectra, they confirmed that both compounds were simultaneously capacitive and resistive. Furthermore, the activation energies obtained from Modulus and complex impedance were proven to be neighboring, suggesting that the relaxation process refers to electron hopping. For each compound, the evolution of e' and tan ​δ with temperature indicated a clearer relaxor behavior, namely the existence of two relaxations. The first one detected at low temperature is related to the charge carriers hopping between Mn3+ and Mn4+ and the second at high temperature is associated with oxygen vacancies. The reduced hopping rate with Dy3+ substitution for Y3+ ions at A sites is responsible for the decrease in dielectric constant and dielectric loss. These significant findings demonstrate that these materials can be invested in a fruitful use in UV photo-detector and power applications in high frequency as microwave, millimeter wave signal processing, for civilian, military and space applications.

Magnetization M , specific heat Cp, polarization P and dielectric constant of DyFeWO6 are investigated for the first time using a microscopic model and the Green’s function technique. The magnetic field dependence of M for temperatures below and above TN is observed. Cp shows an anomaly at the Neel temperature TN which disappears by applying an external magnetic field h. P decreases with increasing h. The antiferromagnetic transition in DyFeWO6 is accompanied by a peak in dielectric constant. With increasing magnetic field the peak decreases, shifts to smaller TN values, and for strong magnetic fields vanishes. The magnetic field behaviour of P and dielectric constant is an evidence for the multiferroicity of DyFeWO6. The observed results are in good qualitative agreement with the existing experimental data

Nanocrystalline La0.9A0.1Mn0.9Cr0.1O3 (A: Li, K, Na) powders have been synthesized by combustion method. The powders were used to prepare ceramics by high-pressure low-temperature sintering technique. For all samples the structure, elemental composition and morphology were studied using X-ray diffraction (XRD), Raman spectroscopy, Energy-Dispersive X-ray Spectroscopy (EDS) and Scanning electron microscopy (SEM). Magnetic properties were studied using magnetometry methods and the valency changes of the cations after alkali ions doping were studied using X-ray photoelectron spectroscopy (XPS). The influence of the sintering pressure on the structural and magnetic properties of the manganites doped with different alkali ions and chromium was also investigated. Magnetization properties were studied as a function of sintering pressure and type of the dopant. Chemical doping with alkali ions as well as external pressure significantly changed the magnetic properties of the compounds. It was found that the magnetic properties of the manganites could be predictably modified through the use of a suitable dopant element.

Multilayered magnetoelectric materials are of great interest for investigations due to their unique tuneable properties and giant values of magnetoelectric effect. The flexible layered structures consisting of soft components can reveal lower values of the resonant frequency for the dynamic magnetoelectric effect appearing in bending deformation mode. The double-layered structure based on the piezoelectric polymer polyvinylidene fluoride and a magnetoactive elastomer (MAE) with carbonyl iron particles in a cantilever configuration was investigated in this work. The gradient AC magnetic field was applied to the structure, causing the bending of the sample due to the attraction acting on the magnetic component. The resonant enhancement of the magnetoelectric effect was observed. The main resonant frequency for the samples depended on the MAE properties, namely, their thickness and concentration of iron particles, and was 156–163 Hz for a 0.3 mm MAE layer and 50–72 Hz for a 3 mm MAE layer; the resonant frequency depended on bias DC magnetic field as well. The results obtained can extend the application area of these devices for energy harvesting.

Radial vortex-based spin torque nano-oscillators (RV-STNOs) have attracted extensive attention as potential nano microwave signal generators due to their advantages over other topological states, such as their higher oscillation, higher microwave power, and lower power consumption. However, the current driving the oscillation frequency of the STNOs must be limited in a small range of adjustment, which means less data transmission channels. In this paper, a new RV-STNO system is proposed with a multiferroic nanostructure, which consists of an ultrathin magnetic multilayer and a piezoelectric layer. Phase diagrams of oscillation frequency and amplitude with respect to piezostrain and current are obtained through micromagnetic simulation. The results show that the threshold current density of −4000-ppm compressive strain-assisted RV-STNOs is reduced from 2 × 109 A/m2 to 2 × 108 A/m2, showing one order of magnitude lower than that of conventional current-driven nano-oscillators. Meanwhile, the range of oscillation frequency adjustment is significantly enhanced, and there is an increased amplitude at the low oscillation point. Moreover, a promising digital binary frequency-shift key (BFSK) and binary amplitude-shift key (BASK) modulation technique is proposed under the combined action of current pulse and piezostrain pulse. They can transmit bit signals and show good modulation characteristics with a minimal transient state. These results provide a reference for developing the next generation of spintronic nano-oscillators with a wide frequency range and low power consumption, showing potential for future wireless communication applications.

Over recent decades, the scientific community has managed to make great progress in the theoretical investigation and practical characterization of bismuth ferrite thin films. However, there is still much work to be completed in the field of magnetic property analysis. Under a normal operational temperature, the ferroelectric properties of bismuth ferrite could overcome the magnetic properties due to the robustness of ferroelectric alignment. Therefore, investigation of the ferroelectric domain structure is crucial for functionality of any potential devices. This paper reports deposition and analyzation of bismuth ferrite thin films by Piezoresponse Force Microscopy (PFM) and XPS methods, aiming to provide a characterization of deposited thin films. In this paper, thin films of 100 nm thick bismuth ferrite material were prepared by pulsed laser deposition on multilayer substrates Pt/Ti(TiO2)/Si. Our main purpose for the PFM investigation in this paper is to determine which magnetic pattern will be observed on Pt/Ti/Si and Pt/TiO2/Si multilayer substrates under certain deposition parameters by utilizing the PLD method and using samples of a deposited thickness of 100 nm. It was also important to determine how strong the measured piezoelectric response will be, considering parameters mentioned previously. By establishing a clear understanding of how prepared thin films react on various biases, we have provided a foundation for future research involving the formation of piezoelectric grains, thickness-dependent domain wall formations, and the effect of the substrate topology on the magnetic properties of bismuth ferrite films.

In this work, we have prepared intergrowth of multiferroic compounds namely Bi4RTi3Fe0.7Co0.3O15-Bi3RTi2Fe0.7Co0.3O12−δ (BRTFCO15-BRTFCO12) (rare earth (R) = Dy, Sm, La) by solid-state reaction method. From the X-ray diffraction Rietveld refinement, the structure of the intergrowths was found to be orthorhombic in which satisfactory fittings establish the existence of three-layered (space group: b 2 c b) and four-layered compounds (space group: A21am). Analysis of magnetic measurements confirmed a larger magnetization for theSm-modified intergrowth compound (BSTFCO15-BSTFCO12) compared to Dy- and La-doped ones. The emergence of higher magnetic properties can be due to distortion in the unit cell when some Bi3+ ions are replaced with the Sm3+, bonding of Fe3+-O-Co3+ as well as a possible mixture of FexCoy-type nanoparticles that are formed generally in the synthesis of intergrowths. The changes in the magnetic state of the Aurivillius intergrowths have been reflected in the magnetoelectric (ME) coupling: higher ME coefficient (~30 mV/Cm-Oe) at lower magnetic fields and is constant up to 3 kOe. The results were corroborated by Raman spectroscopy and variation of temperature with magnetization data. The results revealed that the RE-modified intergrowth route is an effective preparative method for higher-layer Aurivillius multiferroic ceramics.

This paper reports the synthesis and characterization of the Bi0:94Gd0:06FeO3 sample obtained by solid state reaction method. The structural and morphological analysis was performed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Rietveld refinement analysis, confirming the obtaining of a rhombohedral crystalline phase and R3c space group (a = b = 5.577 Å and c = 13.847 Å), with an interconnected structure. Studies by X-ray photoelectron spectroscopy (XPS) revealed trivalent oxidation states of Bi, Fe and Gd ions. The synthesized sample exhibited a non-linear M–H loop indicating a weak ferromagnetic behavior with a remnant magnetization of 6.34 emu˙mol−1 and a coercive field of 295 Oe. The obtained structural and magnetic characteristics make these materials of great interest as multiferroic components.

Effect of starting Fe precursor (Fe2O3 and Fe3O4) on the multiferroic properties of BST (Ba0.96Sr0.04Ti0.91Fe0.09O3) ceramics has been investigated. Considerable changes in the ferroelectric and magnetic properties of these ceramics were observed. High crystallinity, tetragonal structure and phase purity of both the samples were confirmed using X-ray diffraction analysis. The field emission scanning electron microscopy analysis confirmed the uniform grain growth in both the samples with an average grain size of ≈0.40 μm. X-ray photoelectron spectroscopy analysis confirmed the presence of Fe2+ only and both Fe2+ and Fe3+ in Fe2O3- and Fe3O4-doped samples. The Fe3O4-doped BST samples displayed higher Pr and Mr values (2.32 μC/cm2and 0.012 emu/g) as compared to Fe2O3-doped BST samples. The M-E coupling response (≈10.56%) too was observed to be more pronounced in Fe3O4-doped BST samples. Our research work suggests that Fe3O4 doping in BST is better suited for multiferroic applications due to the presence of multivalent Fe ions.

Bismuth ferrite (BiFeO3) is one of multiferroic material group, but it is difficult to produce BiFeO3 in single phase as multiferroic material because it occurs leakage of current arising from non stoichiometric. So, to minimize it, it has already been engineering processed to synthesis BiFeO3 doped by Mg to produce Bi0.9Mg0.1FeO3 and Bi0.93Mg0.07FeO3. It used sol-gel method to produce the ceramics. The result of TGA/DTA(Thermo Gravimetric Analysis/Differential Thermal Analysis) test shows that the temperature of calcination is about of 150 and 175oC and temperature of sintering is about of 650oC. Characterization of the powder has already been done by using X-Ray Diffraction (XRD) test and electrical properties test. The results of XRD test show that the powder of Bi0.9Mg0.1FeO3has minimum impurities with total oxide of 6.9% (bismite 3.5% and silenite 3.4%) at calcination temperature of 175oC for 4 hours and sintering at 650oC for 6 hours. Meanwhile at same parameter, Bi0.93Mg0.07FeO3 has more oxide phases with total oxide of 14.5% which consists of silenite (2.5%) and Bi2O4 (12%). Presence of oxide phases could cause leakage of current decreasing electrical properties. The values of electrical saturation polarization for ceramic having minimum total oxide (Bi0.9Mg0.1FeO3) is higher than ceramic having more oxide (Bi0.93Mg0.07FeO3). The value of electric saturation polarization for Bi0.9Mg0.1FeO3 is of 0.26 kv/cm and for Bi0.93Mg0.07FeO3 is of 0.11 kV/cm.

A conventional solid-state method was used to prepare Pb(Fe1/3Ti1/3W1/3)O3 single perovskite material. The sample sintered at 900?C was studied by XRD and after Rietveld refinement the orthorhombic structure with Pbam space group was confirmed. The FTIR frequency bands corresponding to Pb-O, Ti-O, O-lattice, W- O-W and Fe-O stretching vibrations together with the XRD data also confirmed that Fe2+ and W6+ ions are incorporated in the structure and most probably substituted Ti4+ ions at the B site of the perovskite ABO3 structure. The average grain size of the perovskite ceramics, estimated from SEM micrograph, was 1 to 2 ?m. Variation of dielectric properties and AC conductivity with frequency and temperature revealed that the material had semiconducting nature and negative temperature coefficient of resistance (NTCR). The Nyquist plots were modelled with an equivalent circuit consisted of parallel capacitance, resistance and constant phase element corresponding to the grains (bulk) and grain boundaries. It was observed that grain resistance (Rb) of the sample decreases with the rise of temperature, which supports the NTCR behaviour. The semi-circular arcs of the Nyquist and Cole-Cole plots confirm the semiconducting nature of the sample.

Domain walls in Cu–Cl boracite develop as a consequence of an improper ferroelastic, improper ferroelectric transition, and have attracted close interest because some are conductive and all can be mechanically written and repositioned by application of an electric field. The phase transition and its associated dynamical properties have been analysed here from the perspective of strain and elasticity. Determination of spontaneous strains from published lattice parameter data has allowed the equilibrium long-range order parameter for F 4 ̄ 3c → Pca21 to be modelled simply as being close to the order–disorder limit. High acoustic loss in the cubic phase, revealed by resonant ultrasound spectroscopy, is consistent with the presence of dynamical microdomains of the orthorhombic structure with relaxation times in the vicinity of ∼10−5–10−6 s. Low acoustic loss in the stability field of the orthorhombic structure signifies, on the other hand, that ferroelastic twin walls which develop as a consequence of the order–disorder process are immobile on this time scale. A Debye loss peak accompanied by ∼1% elastic stiffening at ∼40 K is indicative of some freezing of defects which couple with strain or of some more intrinsic freezing process. The activation energy of ⩾∼0.01–0.02 eV implies a mechanism which could involve strain relaxation clouds around local ferroelectric dipoles or freezing of polarons that determine the conductivity of twin walls.

This thesis represents an effort to understand the structure of anisotropically strained Bismuth Ferrite (BiFeO3 BFO). This is executed by using anisotropic epitaxy and exploring the structure, magnetism, and electromechanical response in anisotropically strained BFO at various levels of average in-plane strain. This includes in the vicinity of the strain-induced morphotropic phase boundary where large enhancements to the electromechanical performance are identified. Bismuth ferrite (BFO) is a room-temperature magnetoelectric material that is able to easily adapt its crystal structure to accomodate any strain that is applied to it. By utilising high-index crystallographic substrates the effect anisotropic epitaxial strain has been explored using three different substrate materials (SrTiO3, (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT), and LaAlO3) each with four orientations. The unit cell parameters of the BFO films behave linearly when weakly compressively strained on SrTiO3, and become more non-linear on LSAT. The strain-driven morphotropic phase boundary in BFO films grown on tilted LaAlO3(310) surfaces is able to stabilise a low-symmetry bridging phase between the well known M_A and M_C symmetries of BFO when deposited on SrTiO3 and LaAlO3 respectively. The anisotropic strain conditions of the substrate miscut force the BFO film to maintain strain along a high-symmetry in-plane direction whilst partially relaxing in the orthogonal low-symmetry in-plane direction. Interferometric displacement sensor (IDS) measurements indicate that the intrinsic piezoresponse of this new phase of BFO is double that of the R'-like version. Moreover we see spectroscopic indications through IDS and band-excitation frequency response measurements that there is a field-induced phase transition occurring under electric field wherein the low-symmetry phase is reversibly interconverted into the tetragonal-like phase creating a giant effective electromechanical response. These observations are fully supported by density functional theory and effective Hamiltonian calculations. When growing thicker films of this soft low-symmetry phase, a rich and detailed phase coexistence between the R', T', and bridging phase arise that is reminiscent of a highly tilted mixed-phase BFO. The topography of these samples also exhibit domain-like periodic stripes that evolve with the crystallography and are intimately linked together. \\ At the end of this thesis a number of neutron scattering experiments are presented on BFO films on YAlO3, LaAlO3, LSAT, and SrTiO3 substrates. Despite calculations and some experimental hints of a C-type antiferromagnetic phase in T'-BFO, there appears to be no evidence of this magnetic phase in BFO//YAO and BFO//LAO. Additionally, a cycloid model has been developed and implemented in order to fit ambiguous cycloidal peaks with a constrained model. This model is applied to two different systems of BFO with the results and interpretations discussed.

The strength and dynamics of magnetoelastic coupling through the paramagnetic (PM) – antiferromagnetic (AFM) – ferrimagnetic (FIM) transitions in multiferroic hexagonal ErMnO3 have been investigated by Resonant Ultrasound Spectroscopy. Elastic stiffening by up to 2% below the PM – AFM transition at 80 K arises from biquadratic coupling between strain and the magnetic order parameter with relaxation times longer than ~10-6 s for the response of spins to changes in strain. In contrast with YMnO3, the PM – AFM transition in ErMnO3 is accompanied by a peak in acoustic loss immediately below the Néel point which is interpreted in terms of strain relaxation accompanying ordering of spins of Er3+ at 4b sites. Changes in the magnetic ordering scheme at the AFM – FIM transition near 3 K are accompanied by elastic softening of ~0.03 %. Poling of the low temperature ferrimagnetic structure round magnetic hysteresis loops is detected as small changes in elastic stiffness which arise due to the contribution of piezomagnetic and/or piezoelectric moduli. Contributions of piezoelectric moduli to acoustic resonance frequencies also permit changes in the configuration of ferroelectric domains to be detected in response both to cycling through this transition and to application of a magnetic field. A peak in acoustic loss in the vicinity of 250 K is attributed to strain-mediated pinning/freezing of some aspect of the domain microstructure with an activation energy of ~0.25-0.3 eV. A return to the original elastic properties on heating to temperatures above ~250 K is interpreted in terms of backswitching of domains to the configuration they had at the start. These observations confirm the existence of subtle variations in magnetoelastic coupling behaviour relating to both the magnetic order parameters and magnetic domain structures.

Agradecimentos: We acknowledge support from Facultad de Ciencias and Vicerrectoria de Investigaciones of Universidad de los Andes, Bogotá, Colombia. A.C.R acknowledges financial supportfrom proyectos semilla-Facultad de Ciencias at Universidad de los Andes (2019-II). E.R acknowledges Colciencias-Colfuturo Universidad Nacional de Colombia for its support under "convocatoria 617- Doctorados Nacionales". MK, A.S.E.C and FB acknowledge the support from Brazilian agencies Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (projects #2017/10581-1 and #2020/07397-7) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) –Finance code 001 and projects 436573/2018-0 and #312762/2021-6 Abstract: BiFeO3 (BFO) nanoparticles (NPs) were studied using First-Order Reversal Curve (FORC) and temperaturedependent magnetometry measurements. The BFO NPs were fabricated by a sol-gel method, while the crystal structure and the average particle radius were obtained by powder X-ray diffraction analysis and Small-Angle XRay Scattering (SAXS) measurements, respectively. The NP size varies below and above the typical bulk BFO spin cycloid length (lambda = 62 nm). Below lambda, the NPs show ferromagnetic-like hysteresis loops where the saturation magnetization decreases while nanoparticle size rises. This magnetic behavior changes for NP size over lambda, which only exhibits a paramagnetic contribution. The FORC distributions indicate the presence of two competing sizedependent contributions to the observed magnetic signal Also, the FORC distributions show that in the ferromagnetic regime there are two competing size-dependent contributions to the observed magnetic signal. Our results suggest the existence of a magnetic core-shell structure in NPs below lambda, possibly driven by the strong spin-lattice coupling FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ Aberto

After the discovery of bulk photovoltaic effect more than half a century ago, ferro-electrical and magneto-optical experiments have provided insights into various related topics, revealing above bandgap open voltages and non-central symmetrical current mechanisms. However, the nature of the photon-generated carriers responses and their microscopic mechanisms remain unclear. Here, all-inorganic perovskite Bi0.85Gd0.15Fe1−xMnxO3 thin films were prepared by a sol-gel process and the effects of Gd and Mn co-doped bismuth ferrites on their microtopography, grain boundries, multiferroic, and optical properties were studied. We discovered a simple “proof of principle” type new method that by one-step measuring the leakage current, one can demonstrate the value of photo generated current being the sum of ballistic current and shift current, which are combined to form the so-called bulk photovoltaic current, and can be related to the prototype intrinsic properties such as magneto-optical coupling and ferroelectric polarization. This result has significant potential influence on design principles for engineering multiferroic optoelectronic devices and future photovoltaic industry development.

Strain-mediated magnetoelastic composite materials are being considered for communication in lossy environments. Their consideration is attributable to predictions stating order of magnitude improvements over current antenna technology. The magnetic antenna design considered herein consists of three layers: 1) a piezoelectric layer, 2) a linear elastic layer, and 3) a magnetoelastic layer. The antenna operates by mediating strain through the device in a resonant bending mode. The magnetoelastic layer is stressed which results in a changing magnetization ultimately leading to a changing magnetostatic field in free space which acts as a signal for information transfer. In order to prove the efficacy of this approach finite element models have been developed to aid in the design and optimization process. Where these models fall short is in their overall run-time to fully resolve the coupled dynamics. It is for this reason that the work presented in this thesis focuses on the development of a figure of merit capable of predicting optimal bias conditions and geometries needing only the data from a static bias study from FEA. The material level magnetomechanical coupling factor is chosen as the foundation for the figure of merit. The figure of merit is then augmented to include structure level information regarding the demagnetizing field and the non-uniform stress distribution. The main results presented are the effects of including demagnetization and stress distributions, and most importantly the ability of the metric to predict the change in magnetization of the device. It is shown that for aspect ratios greater than roughly 2.5 the metric trends the same as the change in magnetization predicted by finite element simulations. The region of disagreement between the metric and the fully resolved finite element simulation is explained by tying back to underlying assumptions made during the formulation of the magnetometric demagnetization factor used in the analysis. The case is made for the figure of merit to be included in the analysis of strain-mediated antennas for its ability to find optimum designs while reducing the overall simulation run-time by an order of magnitude. Master of Science Many communication devices are readily available however there are a few key gaps in communication technology that are yet to be filled. Notably, communication in lossy environments using small scale, low frequency, devices has proven difficult due to the fundamental limits of antennas (a cell phone cannot communicate into a mine shaft for search and rescue operations, nor can they communicate underwater to submarines or divers for instance). A promising new approach of communication using smart magnetic materials is under consideration in this thesis. Specifically, the goal herein is to develop an analysis tool capable of predicting device performance without having to run computationally expensive/time consuming finite element simulations. In this thesis it is shown that the analysis tool is capable of predicting device performance while reducing the necessary simulation run-time by an order of magnitude. Using this tool, researches will be able to design better prototypes; moving one step closer to portable communication in lossy environments.

The paper considers how a film of bismuth ferrite BiFeO3 (BFO) is formed on a polymeric flexible polyimide substrate at low temperature ALD (250 °C). Two samples of BFO/Polyimide with different thicknesses (42 nm, 77 nm) were studied. As the thickness increases, a crystalline BFO phase with magnetic and electrical properties inherent to a multiferroic is observed. An increase in the film thickness promotes clustering. The competition between the magnetic and electrical subsystems creates an anomalous behavior of the magnetization at a temperature of 200 K. This property is probably related to the multiferroic/polymer interface. This paper explores the prerequisites for the low-temperature growth of BFO films on organic materials as promising structural components for flexible and quantum electronics.

Eu1−xBaxTi1−yMyO3 (M = Co or Ni) was sintered at 1400 °C under a reduction atmosphere. X-ray photoelectron spectroscopy revealed the mixed valences of Eu2+/Eu3+ and Ti4+/Ti3+ in EuTiO3 and Eu0.7Ba0.3TiO3, as well as some oxygen vacancies required to keep the charge neutrality. The co-doping of Co2+/Ni2+ in Eu0.7Ba0.3TiO3 resulted in the disappearance of oxygen vacancies, as a result of a reduction in Ti3+ numbers and an increase in Eu3+ numbers. On the other hand, Ba2+ doping led to an increased lattice parameter due to its larger ionic size than Eu2+, whereas the Co2+/Ni2+ co-doping resulted in smaller lattice parameters because of the combined effects of ionic size and variation in the oxygen-vacancy numbers. Eu0.7Ba0.3TiO3 exhibited a clear ferroelectricity, which persisted in the Co2+/Ni2+ co-doped samples until the doping levels of y = 0.05 and 0.10, respectively. Eu0.7Ba0.3TiO3 remained to be antiferromagnetic with a reduced transition temperature of 3.1 K, but co-doping of Co2+/Ni2+ turned the samples from antiferromagnetic to ferromagnetic with transition temperatures of 2.98 K and 2.72 K, respectively. The cause for such a transition could not be explained by the larger lattice volume, oxygen vacancies and mixed valences of Eu2+/Eu3+, which were proposed in previous works. Instead, it was more likely to arise from a large asymmetric distortion of the Eu–O polyhedron introduced by the aliovalent doping, which promotes the admixture of Eu 5d and 4f states.

Complex oxides with tunable structures have many fascinating properties, though high-quality complex oxide epitaxy with precisely controlled composition is still out of reach. Here we have successfully developed solution-based single-crystalline epitaxy for multiferroic (1-x)BiTi(1-y)/2FeyMg(1-y)/2O3–(x)CaTiO3 (BTFM–CTO) solid solution in large area, confirming its ferroelectricity at the atomic scale with strong spontaneous polarization. Careful compositional tuning leads to a bulk magnetization of 0.07 ± 0.035 μB/Fe at room temperature, enabling magnetically induced polarization switching exhibiting a large magnetoelectric coefficient of 2.7–3.0 × 10−7 s/m. This work demonstrates the great potential of solution processing in large-scale complex oxide epitaxy and establishes novel room-temperature magnetoelectric coupling in epitaxial BTFM–CTO film, making it possible to explore a much wider space of composition, phase, and structure that can be easily scaled up for industrial applications.

Achieving large magnetoelectric coupling is of interest for memory and communication applications. In multiferroic hybrid structures (combining ferroelectric and magnetic materials) in the presence of a magnetoelectric coupling, the ferroelectric properties can be modulated by a magnetic field. This is called the direct magnetoelectric effect. Measuring the ferroelectric properties in multiferroic materials most commonly requires using metallic electrodes that sandwich the ferroelectric material. In the present work, we use the series resistance introduced by the metallic electrode (La2/3Sr1/3MnO3) to manipulate one relevant ferroelectric parameter, i.e., the coercive voltage, of an adjacent ferroelectric layer (BaTiO3) by a magnetic field. We will show that the variations are fully reversible and more apparent at high frequencies; thus, of particular interest for applications, where high commutation rates are required., Financial support from the Spanish Ministry of Science, Innovation and Universities, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496) and the MAT2017-85232-R (AEI/FEDER, EU), MAT2015-73839-JIN (AEI/FEDER, EU) projects, and from Generalitat de Catalunya (2017 SGR 1377) is acknowledged. I.F. acknowledges RyC contract RYC-2017-22531. SG work has been done as a part of his Master of Nanoscience and Nanotechnology thesis program of Universitat de Barcelona in Applied Physics Department at Universitat de Barcelona.

The direct and converse magnetoelectric (ME) effects in a flexible structure containing a mechanically coupled layers of amorphous ferromagnet FeBSiC and a piezo-copolymer poly(viniledene fluoride trifluoroethylen) (P(VDF-TrFE)) are investigated. The mutual transformation of magnetic and electric fields in the structure arises due to a combination of magnetostriction and piezoelectric effects in the ferromagnetic and piezoelectric layer, respectively. The ME effects were induced by exciting the structure with alternating magnetic fields of 0–100 kHz frequency and 1–5 Oe amplitude, or alternating electric fields of amplitudes up to 500 V/cm in the presence of a constant H field. For the direct ME effect the conversion coefficient reached 7.2 V/(cm·Oe) at a bending resonance frequency of 412 Hz and 44 V/(cm·Oe) at a planar resonance frequency of 25.15 kHz. Increasing the excitation magnetic field at the bending resonance frequency, the nonlinear second harmonic generation with an efficiency of 0.24 V/(cm·Oe2) was observed. For the converse ME effect, the conversion coefficient at the planar resonance frequency was 0.09 G·cm/V. The dependences of the efficiencies for the direct and converse ME transformations on the constant field and the amplitudes of the excitation fields are well explained by theory. These results could be used to develop magnetic and electric field sensors, as well as autonomous energy harvesting sources.

Organic cocrystals based on noncovalent intermolecular interactions (weak interactions) have aroused interest owing to their unpredicted and versatile chemicophysical properties and their applications. In this Minireview, we highlight recent research on organic cocrystals on reducing the aggregation-caused quenching (ACQ) effect, tuning light emission, ferroelectricity and multiferroics, optical waveguides, and stimuli-responsiveness. We also summarize the progress made in this field including revealing the structure-property relationships and developing unusual properties. Moreover, we provide a discussion on current achievements, limitations and perspectives as well as some directions and inspiration for further investigation on organic cocrystals. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Accepted version Q.Z. acknowledges financial support from AcRF Tier 1 (RG111/17, RG 2/17, RG 114/16, RG 8/16) and Tier 2 (MOE2017-T2-1-021 and MOE2018-T2-1-070), Singapore.Q.Z. and Z.C.acknowledge financial support from A*STAR funding (SERC 1528000048), Singapore.

International audience; The magnetoelectric coupling mechanism in Ni/BiFe0.95Mn0.05O3 heterostructures is investigated through a detailed microstructural study of the BiFe0.95Mn0.05O3 film and a chemical analysis of the Ni/BiFe0.95Mn0.05O3 interface. The four structural variants expected in BiFe0.95Mn0.05O3 are detected by high resolution X-ray diffraction reciprocal space mapping method. The ferroelectric domain structure is imaged with transmission electron microscopy. Oxidized Ni is evidenced at the Ni/BiFe0.95Mn0.05O3 interface by electron energy loss spectroscopy and X-ray photoemission spectroscopy, although the degree of Ni oxidation is independent of the magnetoelectric state of the heterostructure.

BiFeO3 (BFO) is a multiferroic material with excellent ferroelectric properties but with poor magnetic behavior. Therefore, we focused principally on the enhancement of the magnetic order of BFO. These multiferroic properties make BFO an excellent candidate for magnetoelectric devices at room temperature. Pure and Co-BiFeO3 powders were successfully synthesized via the sol–gel method at 700 °C. The effect of Co substitution on the corresponding structural and magnetic properties of BFO was studied. X-ray diffraction and Fourier Transform Infrared Spectroscopy measurements confirmed the rhombohedral perovskite structure in all samples. A secondary phase of CoFe2O4 (CFO) was detected for 9, 10, and 15% of Co doping. The scanning electron microscopy images of the Co-BFO particles showed a reduction in the particle size compared to the pure BFO powders. Vibrating sample magnetometry measurements evidenced the ferromagnetic hysteresis loop for the Co-BFO powders with values of saturation magnetization of 4.1 emu/g and a coercivity of 1083 Oe for 15% of Co doping. In this work, we report impurity free samples with notable magnetic properties at the same time, which is a difficult challenge in bismuth ferrite synthesis. This is the first step for later applications in future technology.

Using a microscopic model and the Green’s function technique we have studied the influence of different surface, size and substrate effects on the magnetic properties of BaCoF 4 thin films. The magnetization and the magnetic phase transition temperature increase for a compressible substrate Al 2 O 3 , whereas they decrease for a tensile substrate such as MgO. The film thickness influences also the magnetic properties. We have discussed a competition, a concurrence between the surface and substrate effects. Doping of bulk BaCoF 4 with Al ions leads to weak room temperature ferromagnetism. Due to the magnetoelectric coupling is obtained that the polarization increases with increasing external magnetic field.

With flexibility in tuning their electric and magnetic properties, multiferroics can be used in information exchange and storage in ways that are very different from the present electronic materials. Here we use resonant soft X-ray scattering spectroscopy to study the F-type (0,τ,0) and C-type (0, 1–2τ, 0) diffraction peaks from sinusoidal antiferromagnetic spin order in multiferroic DyMnO3. By comparing the temperature dependence of ordering wave vectors τ, peak intensities I, and correlation lengths λ measured at Mn L2-, O K-, and Dy M5-edges, we show that the nearly perfect locking between the ordering wave vectors from Dy 4f states and Mn 3d orbitals manifesting the second harmonic diffraction peak implies the notable orbital involvement in the coupling between Mn and Dy spins. Our DFT calculations further suggest that the lattice response to different antiferromagnetic ground states (A-type versus E-type) is much weaker in TbMnO3, in agreement with previous claim that the symmetric exchange interaction can be an important factor for understanding the ferroelectricity in DyMnO3 than in TbMnO3.

Domain walls in Cu–Cl boracite develop as a consequence of an improper ferroelastic, improper ferroelectric transition, and have attracted close interest because some are conductive and all can be mechanically written and repositioned by application of an electric field. The phase transition and its associated dynamical properties have been analysed here from the perspective of strain and elasticity. Determination of spontaneous strains from published lattice parameter data has allowed the equilibrium long-range order parameter for F 4̄ 3c → Pca21 to be modelled simply as being close to the order–disorder limit. High acoustic loss in the cubic phase, revealed by resonant ultrasound spectroscopy, is consistent with the presence of dynamical microdomains of the orthorhombic structure with relaxation times in the vicinity of ∼10−5–10−6 s. Low acoustic loss in the stability field of the orthorhombic structure signifies, on the other hand, that ferroelastic twin walls which develop as a consequence of the order–disorder process are immobile on this time scale. A Debye loss peak accompanied by ∼1% elastic stiffening at ∼40 K is indicative of some freezing of defects which couple with strain or of some more intrinsic freezing process. The activation energy of ⩾∼0.01–0.02 eV implies a mechanism which could involve strain relaxation clouds around local ferroelectric dipoles or freezing of polarons that determine the conductivity of twin walls.

The composite material filled with nano-sized BaTiO3 and Fe3O4 was designed and studied. The aluminium phosphate ceramics was used as a matrix. The XRD analysis demonstrates only the crystalline structure of the fillers used. The thermogravimetric analysis proves the thermal stability of the composites up to 950 K. The Maxwell–Wagner relaxation was observed in the dielectric spectra of the investigated composites. The dielectric spectroscopy proves the close contact between the nanoparticles with the different ferroic ordering. The phosphate-based composites have been proved to be a prospective candidate for the multiphase multiferroic materials design and development.

International audience; Multiferroics are materials combining several ferroic orders such as ferroelectricity, ferro-(or anti-ferro-) magnetism, ferroelasticity and ferrotoroidicity [1]. They are of interest both from a fundamental perspective , as they have multiple (coupled) non-linear functional responses-providing a veritable myriad of correlated phenomena, and because of the opportunity to apply these functionalities for new device applications. One application is, for instance, in no-volatile memory, which has led to special attention being devoted to ferroelectric and magnetic multiferroics. For this application, the vision is to combine the low writing power of ferroelectric information with the easy, non-volatile, reading of magnetic information to give a 'best of both worlds' computer memory: however, for this to be realised the two ferroic orders need to be intimately linked via the magnetoelec-tric effect. The magnetoelectric coupling-the way polarization and magnetization reverse-is manifested by the formation and interactions of domains and domain walls, and so to understand how to engineer future devices one must first understand the interactions of domains and domain walls. In this article, we provide a short introduction to the domain formation in ferroelectrics and ferromagnets, as well as different microscopy techniques that enable the visualization of such domains. We then review the recent research on multiferroic domains and domain walls, including their manipulation and intriguing properties, such as enhanced conductivity and anomalous magnetic order. Finally, we discuss future perspectives concerning the field of multiferroic domain walls and emergent topological structures such as ferroelectric vortices and skyrmions. 1 Domain structures in (multi-)ferroics 1.1 Introduction to ferroic domains and domain walls Ferroic materials are defined by the appearance of an order parameter (e.g., elastic, electric or magnetic) at a non-disruptive phase transition. This order parameter can point in at least two symmetrically equivalent directions (polarities) between which it can be switched by the application of an external field. When cooling through the phase transition in zero-field, the polarities have the same energy and, as a consequence, both polarities appear inside the ferroic material. These regions are called domains and the interfaces that separate them are call domain walls. While this shows, trivially, that multiple domains form naturally in ferroics, the details of how many domains, their size, and where they form, depend on several energy terms, as well as the local defect structure. In the following, we will give a brief outline of how ferroic domains and domain walls form, using a ferroelectric as an illustrative example; although, as we will see, analogous arguments can be made for the formation of domains and domains walls in ferromagnets. For a more complete and in-depth descriptions of the physics of ferroelectric and ferromagnetic domains we refer to, for instance, the textbooks by Tagantsev et al. [2] and by Hubert & Schäfer [3]. A proper ferroelectric is a material for which the spontaneous electric polarization plays the role of the primary symmetry breaking order parameter, which can completely describe the phase transition into the ferroic state [4],[5]. Practically, this means energy contributions related to this order parameter are very important in the system, particularly for the domain formation. For instance, in ferroelectrics, surfaces perpendicular to the ferroelectric polarization comprise bound charges that create a strong depolarizing field (Fig. 1a): this is a major driving force for domain formation. The depolar-izing field can be minimized by either (i) screening the surface charges of the ferroelectric with surface adsorbates or metallic electrodes (Fig. 1b), or (ii) the formation of ferroelectric domains, for example, 180° domains, so that the net polarization at the surface averages to zero (Fig. 1c). The number and size of the ferroelectric domains that form will

The magnetoelectric properties of exchange-coupled Ni/CoFeB-based composite multiferroic microstructures are investigated. The strength and sign of the magnetoelastic effect are found to be strongly correlated with the ratio between the thicknesses of two magnetostrictive materials. In cases where the thickness ratio deviates significantly from one, the magnetoelastic behavior of the multiferroic microstructures is dominated by the thicker layer, which contributes more strongly to the observed magnetoelastic effect. More symmetric structures with a thickness ratio equal to one show an emergent interfacial behavior which cannot be accounted for simply by summing up the magnetoelastic effects occurring in the two constituent layers. This aspect is clearly visible in the case of ultrathin bilayers, where the exchange coupling drastically affects the magnetic behavior of the Ni layer, making the Ni/CoFeB bilayer a promising next-generation synthetic magnetic system entirely. This study demonstrates the richness and high tunability of composite multiferroic systems based on coupled magnetic bilayers compared to their single magnetic layer counterparts. Furthermore, because of the compatibility of CoFeB with present magnetic tunnel junction-based spintronic technologies, the reported findings are expected to be of great interest for the development of ultralow-power magnetoelectric memory devices.

Pulsed laser deposition (PLD) has established itself as one of the pillar techniques for the growth of thin films and nanostructures. The fundamental aspects and applications of PLD described in this chapter do not aim to offer an exhaustive view about this field of science, but rather to describe a brief evolution of its conceptual background in order to settle on the most prominent state-of-the-art achievements. The fundamental discussion around the technique emphasizes how initial efforts to achieve a comprehensive picture that relates target laser ablation to plasma kinetics and to thin film growth were critical in understanding the overwhelming complexity of the relationship between these processes and experimental parameters, such as laser pulse properties, type of vacuum or ambient atmosphere, substrate characteristics, and others. With these in mind, state-of-the art thin films grown by PLD came to rely on an ab initio design of interfacial characteristics that was achieved by correlating their underlying physical interactions to experimental conditions. This approach has yielded remarkable results, particularly for multicomponent thin films, in view of next-generation applications in ferroelectrics and multiferroics, superlattices, photocatalysis, photovoltaics, etc. The perspectives of PLD in relation to the latest developments in the field are also discussed.

Agradecimentos: This work was initiated at the Nanomagnetism Laboratory at Universidad de los Andes. We acknowledge support from the FAPA program by means of Facultad de Ciencias and Vicerrectoria de Investigaciones of Universidad de los Andes, Bogotá, Colombia. Funds were also provided by the bilateral grant 'Tec de Monterrey- Uniandes Program' and Interdisciplinary grant. The authors acknowledge the Brazilian Nanotechnology National Laboratory (LNNano) for the use of the electron microscopy facility under the projects # 22424. DM acknowledges the Brazilian agencies FAPESP (2017/1058-1) and CNPq (303236/2017-5). ACR acknowledges support from the Facultad de Ciencias, Universidad de los Andes (2019-II). ER wants acknowledge support from Colciencias-Colfuturo for supporting a doctoral scholarship through the 'Convocatoria Doctorados Nacionales No. 617, 2014-2 Abstract: Multiferroic materials are at the core of voltage-controlled spintronic devices. Therefore, an understanding of the underlying electronic correlations and their effects associated with their complex energy landscape is an important and ongoing task. One key parameter in oxide-based perovskite multiferroics is their sensitivity to strain effects under confinement. Here, we report on the ferromagnetism induced by strain-engineering at the nanoscale on BiFeO3 (BFO) nanoparticles. By controlling synthesis parameters, we were able to modify the BFO lattice parameters up to 0.15% and as a consequence, induce ferromagnetism in otherwise antiferromagnetic bulk BFO. In order to understand the driving mechanisms behind such an effect, we performed density functional theory calculations (DFT) using the BFO parameters obtained from the experiment. We found that small distortions of the structural lattice parameters of the order of 0.01% are sufficient to induce a significant spin imbalance close to the Fermi energy at the Fe sites. This may explain the appearance of weak ferromagnetism in strained BFO thin films reported earlier and offers a new route to novel voltage-controlled spintronic devices based on multiferroic materials FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ Fechado

The study of formation of the skyrmion lattice with the non-collinear magnetoelectric interaction at the interface in a ferromagnetic/antiferroelectric heterostructure is carried out. The ground state spin configuration is calculated by using the steepest descent method. We found the formation perfect skyrmions structure at acceptable values of the magnetoelectric interaction between the antiferroelectric and magnetic layers, both with the triangilar lattice. Monte Carlo simulation has been used to study the phase transition occurring in the ferromagnetic/antiferroelectric heterostructure with and without an applied field. Skyrmions have been shown to be stable at finite temperatures. The ferromagnetic films undergo two transitions, one is due to the destruction of the skyrmion structure and one magnetic transition, separately. The first skyrmion transition occurs at a lower temperature than magnetic transition. Between these two critical temperatures the ferromagnetic/antiferroelectric heterostructure is partially disordered.

Combining various (multi-)ferroic materials into heterostructures is a promising route to enhance their inherent properties, such as the magnetoelectric coupling in BiFeO3 thin films. We have previously reported on the up-to-tenfold increase of the magnetoelectric voltage coefficient α ME in BaTiO3-BiFeO3 multilayers relative to BiFeO3 single layers. Unraveling the origin and mechanism of this enhanced effect is a prerequisite to designing new materials for the application of magnetoelectric devices. By careful variations in the multilayer design we now present an evaluation of the influences of the BaTiO3-BiFeO3 thickness ratio, oxygen pressure during deposition, and double layer thickness. Our findings suggest an interface driven effect at the core of the magnetoelectric coupling effect in our multilayers superimposed on the inherent magnetoelectric coupling of BiFeO3 thin films, which leads to a giant α ME coefficient of 480 V c m -1 Oe-1 for a 16 × (BaTiO3-BiFeO3) superlattice with a 4 . 8 nm double layer periodicity.

Multiferroic thin-film heterostructures have attracted a great deal of attention due to the increasing demand for novel energy-efficient micro/nano-electronic devices. Both single phase multiferroic materials like BiFeO3 (BFO) thin films, and strain-mediated magnetoelectric (ME) nanocomposites, have the potential to fulfill a number of functional requirements in actual applications—principally, direct control of magnetization by the application of an electric field (E) and vice-versa. From the perspective of material science, however, it is essential to develop a fuller understanding of the complex fabrication-structure-property triangle relationship for these multiferroic thin films. Pulsed laser deposition (PLD) was used in this study to fabricate diverse epitaxial thin film heterostructures on top of single crystal substrates. The crystal structure, phase transition processes (amongst nanodomain distributions, dielectric phases, magnetic spin states, etc.), and various ME-related properties were characterized under different E or temperature environments. Resulting data enabled us to determine the structure-property relationships for a range of multiferroic systems. First, BFO-based heterostructures were studied. Epitaxial BFO thin films were deposited on top of (001)-oriented Pb(Mg1/3Nb2/3)O3-30PbTiO3 (PMN-30PT) single crystal substrates. The strain states of BFO and crystal structural phases were tunable by E applied on the PMN-30PT via both the in-plane and out-of-plane modes. The strain-mediated antiferromagnetic state changes of BFO were also studied using neutron diffraction spectroscopy under E. Then, CoFe2O4(CFO)/tetragonal BFO nanocomposites were successfully fabricated on top of (001)-oriented LaAlO3 single crystal substrates. The surface morphology, crystal structure, magnetic properties, and ME effects were evaluated and compared with CFO/rhombohedral BFO nanocomposites. To enhance the performance of ME heterostructures with PMN-PT substrates, PMN-30PT single crystals with nanograted electrodes were also studied, which evidenced an enhancement in piezoelectric properties and dielectric constant by 36.7% and 38.3%, respectively. X-ray diffraction reciprocal space mapping (RSM) was used to monitor E-induced changes in the apparent symmetry and domain distribution of near-surface regions for the nanograted PMN-30PT crystals. Finally, in order to add antiferroelectric thin films to the family of strain-mediated multiferroic nanocomposites, epitaxial antiferroelectric thin films were prepared. Epitaxial (Pb0.98La0.02)(Zr0.95Ti0.05)O3 (PLZT) thin films were deposited on differently oriented SrTiO3 single crystal substrates. A thickness dependent incommensurate/commensurate antiferroelectric-to-ferroelectric phase transition was identified. The crystal structure, phase transition characteristics and pathways, and energy storage behaviors from room temperature to 250 ℃ were studied, enabling a more systematic understanding of PLZT-based AFE epitaxial thin films. To summarize, a range of epitaxial thin films were prepared using PLD, whose crystal structures and multiferroic properties were related through the strain. Accordingly, properties such as dielectricity, antiferroelectricity, and antiferromagnetism could be adjusted by E. This study sheds further light on the potential for designing desirable strain-mediated multiferroic nano-/micro-devices in the future. Doctor of Philosophy As a general definition, the class of materials known as multiferroics possess more than one ferroic order parameter. Multiferroic thin-film heterostructures have attracted a great deal of attention due to the increasing demand for novel energy-efficient micro/nano-electronic devices. Both single phase multiferroic materials like BiFeO3 (BFO) thin films and strain-mediated magnetoelectric (ME) nanocomposites show significant potential for use in next-generation devices due to the fact that one can control magnetic properties via the application of an electric field (E) and vice-versa. From the perspective of material science, however, it is essential to develop a fuller understanding of the complex fabrication-structure-property triangle relationship for these multiferroic thin films. In this study, diverse epitaxial thin film heterostructures were fabricated on top of single crystal substrates. The crystal structure, phase transition processes (amongst nanodomain distributions, dielectric phases, magnetic spin states, etc.), and various ME-related properties were characterized under different E or temperature environments. Resulting data enabled us to determine the structure-property relationships for a range of multiferroic systems. First, BFO-based heterostructures were studied. Epitaxial BFO thin films were deposited on top of (001)-oriented Pb(Mg1/3Nb2/3)O3-30PbTiO3 (PMN-30PT) single crystal substrates. The strain states of BFO and crystal structural phases were tunable by E applied on the PMN-30PT via both the in-plane and out-of-plane modes. The strain-mediated antiferromagnetic state changes of BFO were studied using powerful neutron diffraction spectroscopy under E. Then, CoFe2O4(CFO)/tetragonal BFO nanocomposites were successfully fabricated on top of (001)-oriented LaAlO3 single crystal substrates. The surface morphology, crystal structure, magnetic properties, and ME effects were discussed and compared with CFO/rhombohedral BFO nanocomposites. To enhance the performance of ME heterostructures with PMN-PT substrates, PMN-PT single crystals with nanograted electrodes were also studied, which evidenced an enhancement in piezoelectric properties and dielectric constant by 36.7% and 38.3%, respectively. X-ray diffraction reciprocal space mapping (RSM) technique was used to monitor E-induced changes in the apparent symmetry and domain distribution of near-surface regions for nanograted PMN-PT crystals. Finally, in order to add antiferroelectric thin films to the family of strain-mediated multiferroic nanocomposites, epitaxial antiferroelectric thin films were prepared. Epitaxial (Pb0.98La0.02)(Zr0.95Ti0.05)O3 (PLZT) thin films were deposited on differently oriented SrTiO3 substrates. A thickness dependent incommensurate antiferroelectric-to-ferroelectric phase transition was identified. The crystal structure, phase transition characteristics and pathways, and energy storage behaviors from room temperature to 250 ℃ were studied, enabling a more systematic understanding of PLZT-based AFE epitaxial thin films. To summarize, a range of epitaxial perovskite thin films were prepared, whose crystal structures and multiferroic properties were related through the strain. Accordingly, the properties such as dielectricity, antiferroelectricity, and antiferromagnetism could be adjusted by E. This study sheds further light on the potential for designing desirable strain-mediated multiferroic nano-/micro-devices in the future.

In improper ferroelectrics, the spontaneous ordering is typically driven by a structural distortion or a magnetic spin alignment. The induced electric polarization is only a secondary effect. This dependence is a rich source for unusual phenomena and ferroelectric domain configurations for proper, polarization-driven ferroelectrics. This study focuses on the polar domain structure and the hysteretic behavior at the ferroelectric phase transition in Ca3Mn1.9Ti0.1O7 as a representative of the recently discovered hybrid improper ferroelectric class of multiferroics. Combining optical second harmonic generation and Raman spectroscopy gives access to the spontaneous structural distortion and the resulting improper electric polarization. This study shows that hybrid improper ferroelectrics contrast proper and improper ferroelectrics in several ways. Most intriguingly, adjacent ferroelectric domains favor head-to-head and tail-to-tail domain walls over charge-neutral configurations. Furthermore, the phase transition occurs in an asymmetric fashion. The regime of phase coexistence of the nonpolar and polar phases shows a clear and abrupt upper temperature limit. In contrast, the coexistence toward low temperatures is best described as a fade-out process, where 100-nm-sized islands of the nonpolar phase expand deep into the polar phase., Advanced Electronic Materials, 8 (6), ISSN:2199-160X

This short review article provides the reader with a summary of the history of organic conductors. To retain a neutral and objective point of view regarding the history, background, novelty, and details of each research subject within this field, a thousand references have been cited with full titles and arranged in chronological order. Among the research conducted over ~70 years, topics from the last two decades are discussed in more detail than the rest. Unlike other papers in this issue, this review will help readers to understand the origin of each topic within the field of organic conductors and how they have evolved. Due to the advancements achieved over these 70 years, the field is nearing new horizons. As history is often a reflection of the future, this review is expected to show the future directions of this research field.

Multiferroic composites with enhanced magneto-electric coefficient are suitable candidates for various multifunctional devices. Here, we chose a particulate composite, which is the combination of multiferroic (PbFe0.5Nb0.5O3, PFN) as matrix and magnetostrictive (Co0.6Zn0.4Fe1.7Mn0.3O4, CZFMO) material as the dispersive phase. The X-ray diffraction analysis confirmed the formation of the composite having both perovskite PFN and magnetostrictive CZFMO phases. The scanning electron micrograph (SEM) showed dispersion of the CZFMO phase in the matrix of the PFN phase. The temperature-dependent magnetization curves suggested the transition arising due to PFN and CZFMO phase. The temperature-dependent dielectric study revealed a second-order ferroelectric to the paraelectric phase transition of the PFN phase in the composite with a small change in the transition temperature as compared to pure PFN. The magnetocapacitance (MC%) and magnetoimpedance (MI%) values (obtained from the magneto-dielectric study at room temperature (RT)) at 10 kHz were found to be 0.18% and 0.17% respectively. The intrinsic magneto-electric coupling value for this composite was calculated to be 0.14 mVcm−1Oe−1, which is comparable to other typical multiferroic composites in bulk form. The composite PFN-CZFMO exhibited a converse magneto-electric effect with a change in remanent magnetization value of −58.34% after electrical poling of the material. The obtained outcomes from the present study may be utilized in the understanding and development of new technologies of this composite for spintronics applications.

In this work thedependence of electrical properties on operation temperature (27 degrees C to 200 degrees C) of Au-Cr/Bi0.7Dy0.3FeO3 (BDFO)/ZnO/p-Si (MIS device) are discussed. The thin film of BDFO was deposited by pulsed laser deposition (PLD) on p-Si. From the electrical characterization, the devices properties like ideality factor (eta), barrier height) (BH) of Au-Cr/BDFO/ZnO/p-Si MIS were determined. The values of eta and BH were found to be 134.and 0.86 eV at 200 degrees C and 2.16 and 0.30 eV; respectively, at room temperature (27 degrees C). The leakage current conduction mechanism of the device was investigated and found to be Schottky emission (SE) in the low electric field (

Exclusive and unprecedented interest was accorded in this paper to the synthesis of BiFeO3 nanopowders by the polyol process. The synthesis protocol was explored and adjusted to control the purity and the grain size of the final product. The optimum parameters were carefully established and an average crystallite size of about 40 nm was obtained. XRD and Mössbauer measurements proved the high purity of the synthesized nanostructurated powders and confirmed the persistence of the rhombohedral R3c symmetry. The first studies on the magnetic properties show a noticeable widening of the hysteresis loop despite the remaining cycloidal magnetic structure, promoting the enhancement of the ferromagnetic order and consequently the magnetoelectric coupling compared to micrometric size powders.

BiFeO3 is a multiferroic material with the perovskite structure which is promising for use in sensors and transducers. Single phase production of BiFeO3 remains a challenge, however. In this study, the optimal calcination temperature to obtain close to single phase powder was determined to be 750°C. The sintering temperature of 775°C was also found to obtain high density ceramics (≈ 95 % of theoretical density). It is shown that off-stoichiometry of bismuth oxide in precursors effects the content of secondary phase. Impedance spectroscopy indicates that the content of secondary phases has a large effect on the electrical conductivity BiFeO3.

Here, we report the catalytic degradation of organic compounds by exploiting the magnetoelectric (ME) nature of cobalt ferrite-bismuth ferrite (CFO-BFO) core-shell nanoparticles. The combination of magnetostrictive CFO with the multiferroic BFO gives rise to a magnetoelectric engine that purifies water under wireless magnetic fields via advanced oxidation processes, without involvement of any sacrificial molecules or co-catalysts. Magnetostrictive CoFe2O4 nanoparticles are fabricated using hydrothermal synthesis, followed by sol-gel synthesis to create the multiferroic BiFeO3 shell. We perform theoretical modeling to study the magnetic field induced polarization on the surface of magnetoelectric nanoparticles. The results obtained from these simulations are consistent with the experimental findings of the piezo-force microscopy analysis, where we observe changes in the piezoresponse of the nanoparticles under magnetic fields. Next, we investigate the magnetoelectric effect induced catalytic degradation of organic pollutants under AC magnetic fields and obtained 97% removal efficiency for synthetic dyes and over 85% removal efficiency for routinely used pharmaceuticals. Additionally, we perform trapping experiments to elucidate the mechanism behind the magnetic field induced catalytic degradation of organic pollutants by using scavengers for each of the reactive species. Our results indicate that hydroxyl and superoxide radicals are the main reactive species in the magnetoelectrically induced catalytic degradation of organic compounds.

Dual-particulate composites of cobalt ferrite dispersed in a fully dense PZT matrix are produced by quite-fast sintering of mechanically activated powders. By high-energy milling of the powders a bi-modal grain size distribution, with octahedral nano-grains and larger grains grown by multiple parallel twinning, are achieved in the final microstructure. The material display a “wasp-waisted” magnetic response as a consequence of the two main CoFeO grain populations and can be exploited for broadband energy harvesting or field sensors. After poling under 5 kV/mm, a maximum d of 30 pC/N was achieved. This value is an order of magnitude lower than that of bulk PZT. Nevertheless, a magnetoelectric coefficient of 1.74 mV cm Oe is obtained, which suggests the high potentiality of these materials, since this value is higher than that shown by magnetoelectric composites with similar composition and connectivity reported in literature. This is so for a partially poled material and thus, magnetoelectric coefficients should be significantly increased by improving the poling process., PG acknowledges the contribution of the Nanoscience Foundries and Fine Analysis (NFFA) European Research Infrastructure. In fact,this project has received funding from the EU-H2020 research andinnovation programme under grant agreement No 654360 having benefitted from the access provided by Laboratoire d'electroniquedes technologies de l'information (CEA-LETI) in Grenoble and Foundation for Research and Technology - Hellas (FORTH) in Her-aklion within the framework of the NFFA-Europe Transnationa lAccess Activity. Guillaume Audoit (CEA-LETI) and Audrey Jannaud(SERMA Technologies) are gratefully acknowledged for the prepa-ration of the sample for TEM analysis. NB and NG acknowledge the“Recherches Technologiques de Base”Program of the French Min-istry of Research which has supported the NanoCharacterisationPlatForm (PFNC). MA also acknowledges funding by SpanishMINECO (projects MAT2014-58816-R and MAT2017-88788-R)

Orientadores: Julio Criginski Cezar, Marcos César de Oliveira Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin Resumo: O objetivo principal deste trabalho é obter heteroestruturas de BaTiO3 (BTO) e Fe com um acoplamento magnetoelétrico na interface. Podemos dividir os resultados em duas linhas principais: a optimização da camada de BTO e o crescimento da estrutura de multicamadas usada para caracterizar o acoplamento. No primeiro grupo, pudemos crescer os filmes finos por deposição por laser pulsado (PLD), optimizando os parâmetros de crescimento a fim de obter os melhores resultados. O uso de diferentes substratos também foi objeto de análise. Nós tentamos crescer amostras policristalinas em substratos de silício. Os resultados mostram que elas puderam ser cristalizadas após um tratamento térmico in-situ até 873 K. A morfologia do crescimento sobre substratos de SrTiO3 e Nb:SrTiO3 também foi estudada. A camada de BTO foi crescida bem espessa (~ 500 nm) para garantir que os filmes fossem menos epitaxiais. No segundo grupo, crescemos eletrodos inferiores de SrRuO3 por PLD, enquanto eletrodos superiores de ouro titânio e uma camada fina de Fe foram crescidos por evaporação por feixe de elétrons. As amostras foram caracterizadas por microscopia de força de piezorresposta (PFM), indicando que elas eram ferroelétricas. A espectroscopia de absorção de raios X (XAS) apontou que as amostras eram metálicas no momento em que entraram no microscópio. Enquanto a caracterização preliminar por microscopia eletrônica de fotoemissão (PEEM) pode mostrar indícios que há um acoplamento entre as camadas ferroelétricas e ferromagnéticas Abstract: The main goal of this work is to obtain heterostructures of BaTiO3 (BTO) and Fe with a magnetoelectric coupling at the interface. We can divide the results into two main lines: the optimization of the BTO layer and the growth of the bilayered structure used to characterize the coupling. In the first group, we were able to grow thin films by pulsed laser deposition (PLD), optimizing the growth parameters to obtain the best results. The use of different substrates is also a subject of analysis. We tried growing polycrystalline samples on silicon substrates. The results show that they could be crystallized after annealing in-situ up to 873 K. The morphology of the growth on SrTiO3 and Nb:SrTiO3 substrates was also studied. The BTO layer was grown very thick (~ 200 nm) to ensure that the films would be less epitaxial. In the second group, we have grown the bottom electrodes of SrRuO3 by PLD while the titanium gold top electrode and the thin Fe layer were grown by electron beam evaporation. The samples were characterized by piezoresponse force microscopy (PFM), showing that they were ferroelectric. The x-ray absorption spectroscopy (XAS) showed that the sample was metallic by the time it reached the microscope. While preliminary characterization by photoemission electron microscopy (PEEM) could indicate that there was a coupling between the ferromagnetic and ferroelectric layers Mestrado Física Mestre em Física CAPES 88887.145777/2017-00 FAPESP CNPQ CNPEM

In the present work the acoustic band structure of a two-dimensional phononic crystal containing an organic ferroelectric (PVDF- polyvinylidene fluoride) and muliferroic material (LiVCuO4) were investigated by the plane-wave-expansion method. A two-dimensional PC with square lattices composed of LiVCuO4 cylindrical rods embedded in the PVDF matrix are studied to find the existence of stop bands for the waves of certain energy. This phononic bandgap - forbidden frequency range - allows sound to be controlled in many useful ways in structures that can act as sonic filters, waveguides or resonant cavities. Phononic band diagram omega = omega(k) for a 2D PC, in which non-dimensional frequencies omega a/2 pi c (c-velocity of wave) were plotted versus the wavevector k along the Gamma-X-M-Gamma path in the square Brillouin zone show four stop bands in the frequency range 0.01-8.0 kHz. The ferroelectric properties of PVDF and unusual properties of multiferroic LiVCuO4 give us the ability to control the wave propagation through the PC in over a wide frequency range.

Multiferroic oxide heterostructures exhibit superior functional properties as a result of the interplay between the lattice, charge, orbital and electronic degrees of freedom. In particular these interactions can lead to emergent functionalities such as ferromagnetism and ferroelectricity at the interfaces. This offers excellent prospects for advanced devices based on the manipulation of the spin and charge degrees of freedom. However, the understanding of the underlying physics at the interface of these complex structures is still incomplete. This thesis investigates, (i) the effect of structural, electronic and compositional dependencies on the ferroelectric domain switching properties of bismuth ferrite (BiFeO3) multiferroic thin-films and, (ii) the emergent magnetism at the interface of perovskite manganite superlattices comprised of lanthanum strontium manganese oxide (LaMnO3) and calcium manganese oxide (CaMnO3) layers. Pulsed laser deposition (PLD) equipped with in-situ reflection high energy electron diffraction (RHEED) technique is employed for the deposition of the samples to ensure atomically precise layer by layer growth. The deposited structures are characterized using an array of characterization techniques such as atomic force microscopy (AFM), X-ray diffraction techniques, piezoresponse force microscopy (PFM) and physical property measurement systems (PPMS) to investigate the effect of internal and external influences upon the strong electron correlations in multiferroics with exciting potential for future spintronic applications. The thesis is made of three key studies; the first part focuses on the fabrication of BiFeO3 (BFO) thin-films of varying thickness on a SrTiO3 (STO) substrate of two different orientations, namely (100) and (110) with and without a bottom electrode. Through unique epitaxial constraints of a (110)-oriented STO substrate and an intermediate layer of SrRuO3 (SRO), single domain BFO thin-films are realized. Further, the samples revealed that the crystallographic orientation and electronic boundary conditions play a vital role in determining the structural and electronic properties in nanoscale BFO thin-films. Presence of the bottom electrode and the thickness of the film played a vital role in imposing considerable strain in BFO thin-film. It is shown that in the absence of the SRO layer when the thickness is increased from 50 to 100 and 150 nm, BFO films relaxes into a two-domain state. A significant reduction in the out-of-plane lattice parameter has also been observed with the increase in the thickness of the film. The results demonstrate that oxide heteroepitaxy of BFO can be carefully tuned to achieve various ferroic ground states. As the second stage, thin BFO films (~12 nm) have been successfully realised on LaAlO3 (100) substrates with LaSrMnO3 (LSMO) buffer layer in between. AFM topography scans show the absence of the characteristic stripe-like morphology. It is demonstrated that the surface roughness increases with the increase in bottom electrode layer thickness. XRD and RSM reveal the evolution of the phase with the electrode thickness. BFO films with 5nm and 10nm LSMO Buffer layers revealed a typical rhombohedral like R’ phase whereas with the increase LSMO layer thickness to 20nm; the phase evolved to tilted S’ like a phase. The PFM studies revealed that the domain size increases with the increase in thickness of the bottom electrode and sample exhibited both in-plane and out of plane domain signals. As the final stage of this study and the groundwork for future possibilities of coupling perovskite manganites and multiferroic BFO, LMO/CMO superlattices are fabricated on STO (100) substrate. The structures are then characterized using AFM, XRD and PPMS techniques. It is observed that varying the periodicity of the SL layers; we can vary the presence of manganese (Mn) ions with different valence states (Mn2+ and Mn3+) and thus introduce an antiferromagnetic state that co-exists with a robust ferromagnetic state. In summary, the study shows that it is possible to engineer novel functional performances via the interfacial engineering of mechanical and electrical boundary conditions in complex metal oxide heterostructures.

Agradecimentos: This work was supported in part by the European H2020-MSCARISE-2017-ENGIMA Action, in part by the Slovenian Research Agency Program under Grant P1-0125, in part by the CNRST Priority Program under Grant PPR 15/2015, in part by the National Council for Scientific and Technological Development (CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO), and in part by São Paulo Research Foundation (FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO) Abstract: High-performance lead-free multiferroic composites are desired to replace the lead-based ceramics in multifunctional device applications. Laminated compounds were prepared from ferroelectric and ferromagnetic materials. In this work, we present the laminated ceramics compound by considering the ferromagnetic La0.5Ca0.5MnO3 (LCMO) and the ferroelectric BaTi0.8Sn0.2O3 (BTSO) in two different proportions. Compounds (1 - x) LCMO-(x) BTSO with x = 1 and 0 (pure materials) were synthesized by the sol-gel method, and x = 0.7 and 0.5 (laminated) compounds were elaborated by welding appropriate mass ratios of each pure material by using the silver paste technique. Structural, dielectric, ferroelectric, microstructure, and magnetic characterizations were conducted on these samples. X-ray scattering results showed pure perovskite phases confirming the successful formation of both LCMO and BTSO. Scanning electron microscope (SEM) images evidenced the laminated structure and good quality of the interfaces. The laminated composite materials have demonstrated a multiferroic behavior characterized by the ferroelectric and the ferromagnetic hysteresis loops. Furthermore, the enhancement of the dielectric constant in the laminated composite samples is mainly attributed to the Maxwell-Wagner polarization CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP Aberto