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Multiferroic materials simultaneously host at least two primary ferroic orders (ferroelectricity, ferromagnetism, or ferroelasticity) in the same phase [1]. The possibility to control ferroelectric properties via magnetic field, or the control of magnetic properties by applied electric field, makes these systems particularly interesting for high-tech application. Multiferroics can be used to manufacture multifunctional microelectronic or microwave devices, ferroelectric memory components, magnetic data storage, etc. YMnO3 is a multiferroic compound which appears in two crystal modifications, the hexagonal thermically stable form and the rhombic metastabile crystal structure. The goal of our research is to obtain multiferroic YMnO3 which would simultaneously show ferroelectric and antiferromagnetic properties. With this in mind, we compared two methods of synthesis: mechanochemical as well as chemical synthesis from citrate polymer precursors. Our results show that chemical synthesis yields powders which, apart from h-YMnO3 and o-YmnO3, also contain YMn2O5 as a secondary phase. However, conventional sintering of these powders at 1400 °C during 2 h produces h-ordered YMnO3 which is simultaneously ferroelectric as well as antiferromagnetically.

Producción Científica, Noncollinear spin systems with magnetically induced ferroelectricity from changes in spiral magnetic ordering have attracted significant interest in recent research due to their remarkable magnetoelectric effects with promising applications. Single phase multiferroics are of great interest for these new multifunctional devices, being Y-type hexaferrites good candidates, and among them the ZnY compounds due to their ordered magnetic behaviour over room temperature. Polycrystalline Y type hexaferrites with composition Ba0.5Sr1.5Zn2Fe2O22 (BSZFO) were sintered in 1050 °C–1250 °C temperature range. Transverse susceptibility measurements carried out on these BSZFO samples in the temperature range 80–350 K with DC fields up to ± 5000 Oe reveal different behaviour depending on the sintering temperature. Sample sintered at 1250 °C is qualitatively different, suggesting a mixed Y and Z phase like CoY hexaferrites. Sintering at lower temperatures produce single phase Y-type, but the transverse susceptibility behaviour of the sample sintered at 1150 °C is shifted at temperatures 15 K higher. Regarding the DC field sweeps the observed behaviour is a peak that shifts to lower values with increasing temperature, and the samples corresponding to single Y phase exhibit several maxima and minima in the 250 K–330 K range at low DC applied field as a result of the magnetic field induced spin transitions in this compound., Ministerio de Ciencia, Innovación y Universidades; Agencia Estatal de Investigación with FEDER (MAT2016-80784-P)

Room-temperature, long-range (300 nm), chirality-induced spin-selective electron conduction is found in chiral metal-organic Cu(II) phenylalanine crystals, using magnetic conductive-probe atomic force microscopy. These crystals are found to be also weakly ferromagnetic and ferroelectric. Notably, the observed ferromagnetism is thermally activated, so that the crystals are antiferromagnetic at low temperatures and become ferromagnetic above similar to 50 K. Electron paramagnetic resonance measurements and density functional theory calculations suggest that these unusual magnetic properties result from indirect exchange interaction of the Cu(II) ions through the chiral lattice.

Room-temperature, long-range (300 nm), chirality-induced spin-selective electron conduction is found in chiral metal-organic Cu(II) phenylalanine crystals, using magnetic conductive-probe atomic force microscopy. These crystals are found to be also weakly ferromagnetic and ferroelectric. Notably, the observed ferromagnetism is thermally activated, so that the crystals are antiferromagnetic at low temperatures and become ferromagnetic above similar to 50 K. Electron paramagnetic resonance measurements and density functional theory calculations suggest that these unusual magnetic properties result from indirect exchange interaction of the Cu(II) ions through the chiral lattice.

Room-temperature, long-range (300 nm), chirality-induced spin-selective electron conduction is found in chiral metal-organic Cu(II) phenylalanine crystals, using magnetic conductive-probe atomic force microscopy. These crystals are found to be also weakly ferromagnetic and ferroelectric. Notably, the observed ferromagnetism is thermally activated, so that the crystals are antiferromagnetic at low temperatures and become ferromagnetic above similar to 50 K. Electron paramagnetic resonance measurements and density functional theory calculations suggest that these unusual magnetic properties result from indirect exchange interaction of the Cu(II) ions through the chiral lattice.

Producción Científica, Noncollinear spin systems with magnetically induced ferroelectricity from changes in spiral magnetic ordering have attracted significant interest in recent research due to their remarkable magnetoelectric effects with promising applications. Single phase multiferroics are of great interest for these new multifunctional devices, being Y-type hexaferrites good candidates, and among them the ZnY compounds due to their ordered magnetic behaviour over room temperature. Polycrystalline Y type hexaferrites with composition Ba0.5Sr1.5Zn2Fe2O22 (BSZFO) were sintered in 1050 °C–1250 °C temperature range. Transverse susceptibility measurements carried out on these BSZFO samples in the temperature range 80–350 K with DC fields up to ± 5000 Oe reveal different behaviour depending on the sintering temperature. Sample sintered at 1250 °C is qualitatively different, suggesting a mixed Y and Z phase like CoY hexaferrites. Sintering at lower temperatures produce single phase Y-type, but the transverse susceptibility behaviour of the sample sintered at 1150 °C is shifted at temperatures 15 K higher. Regarding the DC field sweeps the observed behaviour is a peak that shifts to lower values with increasing temperature, and the samples corresponding to single Y phase exhibit several maxima and minima in the 250 K–330 K range at low DC applied field as a result of the magnetic field induced spin transitions in this compound., Ministerio de Ciencia, Innovación y Universidades; Agencia Estatal de Investigación with FEDER (MAT2016-80784-P)

Producción Científica, Noncollinear spin systems with magnetically induced ferroelectricity from changes in spiral magnetic ordering have attracted significant interest in recent research due to their remarkable magnetoelectric effects with promising applications. Single phase multiferroics are of great interest for these new multifunctional devices, being Y-type hexaferrites good candidates, and among them the ZnY compounds due to their ordered magnetic behaviour over room temperature. Polycrystalline Y type hexaferrites with composition Ba0.5Sr1.5Zn2Fe2O22 (BSZFO) were sintered in 1050 °C–1250 °C temperature range. Transverse susceptibility measurements carried out on these BSZFO samples in the temperature range 80–350 K with DC fields up to ± 5000 Oe reveal different behaviour depending on the sintering temperature. Sample sintered at 1250 °C is qualitatively different, suggesting a mixed Y and Z phase like CoY hexaferrites. Sintering at lower temperatures produce single phase Y-type, but the transverse susceptibility behaviour of the sample sintered at 1150 °C is shifted at temperatures 15 K higher. Regarding the DC field sweeps the observed behaviour is a peak that shifts to lower values with increasing temperature, and the samples corresponding to single Y phase exhibit several maxima and minima in the 250 K–330 K range at low DC applied field as a result of the magnetic field induced spin transitions in this compound., Ministerio de Ciencia, Innovación y Universidades; Agencia Estatal de Investigación with FEDER (MAT2016-80784-P)

Room-temperature, long-range (300 nm), chirality-induced spin-selective electron conduction is found in chiral metal-organic Cu(II) phenylalanine crystals, using magnetic conductive-probe atomic force microscopy. These crystals are found to be also weakly ferromagnetic and ferroelectric. Notably, the observed ferromagnetism is thermally activated, so that the crystals are antiferromagnetic at low temperatures and become ferromagnetic above similar to 50 K. Electron paramagnetic resonance measurements and density functional theory calculations suggest that these unusual magnetic properties result from indirect exchange interaction of the Cu(II) ions through the chiral lattice.

Producción Científica, Noncollinear spin systems with magnetically induced ferroelectricity from changes in spiral magnetic ordering have attracted significant interest in recent research due to their remarkable magnetoelectric effects with promising applications. Single phase multiferroics are of great interest for these new multifunctional devices, being Y-type hexaferrites good candidates, and among them the ZnY compounds due to their ordered magnetic behaviour over room temperature. Polycrystalline Y type hexaferrites with composition Ba0.5Sr1.5Zn2Fe2O22 (BSZFO) were sintered in 1050 °C–1250 °C temperature range. Transverse susceptibility measurements carried out on these BSZFO samples in the temperature range 80–350 K with DC fields up to ± 5000 Oe reveal different behaviour depending on the sintering temperature. Sample sintered at 1250 °C is qualitatively different, suggesting a mixed Y and Z phase like CoY hexaferrites. Sintering at lower temperatures produce single phase Y-type, but the transverse susceptibility behaviour of the sample sintered at 1150 °C is shifted at temperatures 15 K higher. Regarding the DC field sweeps the observed behaviour is a peak that shifts to lower values with increasing temperature, and the samples corresponding to single Y phase exhibit several maxima and minima in the 250 K–330 K range at low DC applied field as a result of the magnetic field induced spin transitions in this compound., Ministerio de Ciencia, Innovación y Universidades; Agencia Estatal de Investigación with FEDER (MAT2016-80784-P)

Room-temperature, long-range (300 nm), chirality-induced spin-selective electron conduction is found in chiral metal-organic Cu(II) phenylalanine crystals, using magnetic conductive-probe atomic force microscopy. These crystals are found to be also weakly ferromagnetic and ferroelectric. Notably, the observed ferromagnetism is thermally activated, so that the crystals are antiferromagnetic at low temperatures and become ferromagnetic above similar to 50 K. Electron paramagnetic resonance measurements and density functional theory calculations suggest that these unusual magnetic properties result from indirect exchange interaction of the Cu(II) ions through the chiral lattice.

Producción Científica, Single phase multiferroics in which ordered magnetism and ferroelectricity coexist, are of great interest for new multifunctional devices, and among them Y-type hexaferrites are good candidates. Transverse susceptibility measurements, which have been proved to be a versatile tool to study singular properties of bulk and nanoparticle magnetic systems, have been carried out with a broadband system on polycrystalline Y type hexaferrites with composition Ba0.5Sr1.5Co2Fe2O22, optimal to exhibit multiferroic properties. In the temperature range 80–350 K transverse susceptibility measurements with DC fields up to ±5000 Oe reveal different behaviour depending on the sintering temperature. The thermal evolution of the anisotropy field peak exhibits four regions with different slopes: positive in 80–130 K, negative in 130–200 K, constant in 200–280 K and negative in 280–350 K, which can be considered a signature of spin transitions in this compound., Ministerio de Ciencia, Innovación y Universidades; Agencia Estatal de Investigación with FEDER ( project MAT2016-80784-P)

Producción Científica, Single phase multiferroics in which ordered magnetism and ferroelectricity coexist, are of great interest for new multifunctional devices, and among them Y-type hexaferrites are good candidates. Transverse susceptibility measurements, which have been proved to be a versatile tool to study singular properties of bulk and nanoparticle magnetic systems, have been carried out with a broadband system on polycrystalline Y type hexaferrites with composition Ba0.5Sr1.5Co2Fe2O22, optimal to exhibit multiferroic properties. In the temperature range 80–350 K transverse susceptibility measurements with DC fields up to ±5000 Oe reveal different behaviour depending on the sintering temperature. The thermal evolution of the anisotropy field peak exhibits four regions with different slopes: positive in 80–130 K, negative in 130–200 K, constant in 200–280 K and negative in 280–350 K, which can be considered a signature of spin transitions in this compound., Ministerio de Ciencia, Innovación y Universidades; Agencia Estatal de Investigación with FEDER ( project MAT2016-80784-P)

The lacunar spinel GaV4S8 was recently suggested to be a prototype multiferroic material hosting skyrmion lattice states with a sizable polarization P coupled to magnetic order. We explain this phenomenon on the microscopic level. On the basis of density functional theory, we construct an effective model describing the behavior of magnetically active electrons in a weakly coupled lattice formed by molecular orbitals of the (V4S4)5+ clusters. By applying superexchange theory combined with the Berry-phase theory for P, we derive a compass model relating the energy and polarization change with the directions of spins ei in magnetic bonds. We argue that, although each skyrmion layer is mainly formed by superexchange interactions in the same plane, the spin dependence of P arises from the stacking misalignment of such planes in the perpendicular direction, which is inherent to the lacunar spinel structure. We predict a strong competition of isotropic, ∼eiej, and antisymmetric, ∼ei×ej, contributions to P that explains the experimentally observed effect. © 2019 American Physical Society.

Producción Científica, Single phase multiferroics in which ordered magnetism and ferroelectricity coexist, are of great interest for new multifunctional devices, and among them Y-type hexaferrites are good candidates. Transverse susceptibility measurements, which have been proved to be a versatile tool to study singular properties of bulk and nanoparticle magnetic systems, have been carried out with a broadband system on polycrystalline Y type hexaferrites with composition Ba0.5Sr1.5Co2Fe2O22, optimal to exhibit multiferroic properties. In the temperature range 80–350 K transverse susceptibility measurements with DC fields up to ±5000 Oe reveal different behaviour depending on the sintering temperature. The thermal evolution of the anisotropy field peak exhibits four regions with different slopes: positive in 80–130 K, negative in 130–200 K, constant in 200–280 K and negative in 280–350 K, which can be considered a signature of spin transitions in this compound., Ministerio de Ciencia, Innovación y Universidades; Agencia Estatal de Investigación with FEDER ( project MAT2016-80784-P)

Producción Científica, Single phase multiferroics in which ordered magnetism and ferroelectricity coexist, are of great interest for new multifunctional devices, and among them Y-type hexaferrites are good candidates. Transverse susceptibility measurements, which have been proved to be a versatile tool to study singular properties of bulk and nanoparticle magnetic systems, have been carried out with a broadband system on polycrystalline Y type hexaferrites with composition Ba0.5Sr1.5Co2Fe2O22, optimal to exhibit multiferroic properties. In the temperature range 80–350 K transverse susceptibility measurements with DC fields up to ±5000 Oe reveal different behaviour depending on the sintering temperature. The thermal evolution of the anisotropy field peak exhibits four regions with different slopes: positive in 80–130 K, negative in 130–200 K, constant in 200–280 K and negative in 280–350 K, which can be considered a signature of spin transitions in this compound., Ministerio de Ciencia, Innovación y Universidades; Agencia Estatal de Investigación with FEDER ( project MAT2016-80784-P)

Multiferroic materials exhibit at least two of the so-called ferroic properties (ferroelectric, (anti)ferromagnetic and ferroelastic) in the same time. They are very interesting from the theoretical point of view because of a different nature of those properties, but coupling between the properties opens up huge possibilities for application as magnetoresistors, memory devices, sensors and many other devices [1]. Being ferroelectric up to 830 °C and antiferromagnetic (weakly ferromagnetic) up to 370 °C, bismuth ferrite (BiFeO3) is one of the very few room-temperature singlephase multiferroic materials and one of the most studied ceramic materials in the last two decades. BiFeO3 has also good potential to be used as a pigment, catalyst, photocatalyst or solar cell material [1,2]. However, because of specific obstacles in obtaining pure, dense and highly resistive ceramics, harnessing of those properties is still far from being achieved and the possibility of its application as a multiferroic material is arguable. High volatility of bismuth above 800 °C and thermodynamic instability of BiFeO3 between 447 °C and 767 °C make the densification of BiFeO3 ceramics very difficult, especially by conventional methods. High leakage currents in BiFeO3 (originating mostly from oxygen and bismuth vacancies) disable ceramic samples to be polarized and to exhibit ferroelectric properties. Spiral structure of magnetic moments lowers the coupling between ferroelectric and magnetic orders [3,4]. BiFeO3 ceramic materials presented in this study were synthesized by autocombustion method with idea to lower the temperature needed for effective sintering in order to prevent volatilisation and improve the density and phase composition. Auto-combustion is a type of sol-gel route which enables high homogeneity in solutions stabilized by organic compounds, which oxidize vigorously producing the ash powders as a wanted product. Because of such fast reaction, the defects are incorporated into st

The characterization of the crystal structure, phase transitions, magnetic structure and dielectric properties has been carried out on [CH3 NH3 ][Co(COOH)3 ] (1) perovskite-like metal-organic compound through variable-temperature single-crystal and powder neutron and X-ray diffraction and relative permittivity measurements. The paraelectric to antiferroelectric-like phase transition observed at around 90 K is triggered by a structural phase transition; the structural studies show a change from Pnma space group at RT (1A) to P21 /n space group at low temperature (1B). This phase transition involves the occurrence of small distortions in the framework and counterions. Neutron diffraction studies have shown a magnetic order showing spontaneous magnetization below 15 K, due to the occurrence of a non-collinear antiferromagnetic structure with a weak ferromagnetic component, mainly due to the single-ion anisotropy of the CoII ions.

Strain-mediated magneto-electric materials, also known as magneto-electro-elastic materials, have received tremendous attention due to their potential in realizing electric-field control of magnetism. More importantly, novel wave propagating behavior can be found in such materials, which involves coupling between elastodynamics and electrodynamics. Yet little work has been done on numerical simulations that allow us to look into the wave phenomena in these materials. This dissertation is devoted to the development and validation of finite element methods that are capable of handling this coupling, with an emphasis on the acoustically-driven electromagnetic radiation from piezoelectrics and piezomagnetic media.

Strain-mediated magneto-electric materials, also known as magneto-electro-elastic materials, have received tremendous attention due to their potential in realizing electric-field control of magnetism. More importantly, novel wave propagating behavior can be found in such materials, which involves coupling between elastodynamics and electrodynamics. Yet little work has been done on numerical simulations that allow us to look into the wave phenomena in these materials. This dissertation is devoted to the development and validation of finite element methods that are capable of handling this coupling, with an emphasis on the acoustically-driven electromagnetic radiation from piezoelectrics and piezomagnetic media.

BiFeO3 (BFO) is a classical multiferroic material with both ferroelectric and magnetic ordering at room temperature. Doping of this material with rare-earth oxides was found to be an efficient way to enhance the otherwise low piezoelectric response of unmodified BFO ceramics. In this work, we studied two types of bulk Sm-modified BFO ceramics with compositions close to the morphotropic phase boundary (MPB) prepared by different solid-state processing methods. In both samples, coexistence of polar R3c and antipolar Pbam phases was detected by conventional X-ray diffraction (XRD); the non-polar Pnma or Pbnm phase also has potential to be present due to the compositional proximity to the polar-to-non-polar phase boundary. Two approaches to separate the phases based on the piezoresponse force microscopy measurements have been proposed. The obtained fractions of the polar and non-polar/anti-polar phases were close to those determined by quantitative XRD analysis. The results thus reveal a useful method for quantitative determination of the phase composition in multi-phase ceramic systems, including the technologically most important MPB systems. © 2015 AIP Publishing LLC.

This is the final report for the ARO/MURI program Electrical Control of Magnetic Dynamics in Hybrid Metal-Semiconductor Systems, W911NF-08-2-0032. It describes the main accomplishments of the MURI., The original document contains color images.

The objectives of this program are to design, synthesize, and characterize multiferroic materials and heterostructures and to demonstrate novel solid-state cooling down to 70K based on these ferroic materials. This program combines studies of fundamental materials chemistry, the nature of order parameter coupling and interactions, and the fabrication of rudimentary devices based on multiferroic materials. At the heart of this program is the ability to synthesize high quality thin film samples using state-of-the-art growth techniques that combine both classic molecular beam epitaxy (MBE) and pulsed-laser deposition (PLD). The program will develop new high-performance multiferroics, will study the coupling across interfaces in multiferroic-based heterostructures with the goal of developing a better understanding of coupling in multiferroic-based device structures, and will generate the fundamental understanding of these materials needed for novel magneto-, electro-, and magneto-electro-caloric cooling applications. Outcomes will include the identification and study of new multiferroic phases and heterostructures, better understanding of coupling across interfaces in multiferroic-based structures, and unprecedented pathways to low temperature cooling. This work will develop novel materials and material performance relevant to future Army capabilities including the development of highly efficient, low vibration, solid-state cooling capabilities for a wide array of applications including night vision systems., Pub. in ACS NANO, v8 n1, p894-903, 2014.

Multiferroic materials showing magnetoelectric coupling are required in various technological applications. Many synthetical approaches can be used to improve the magnetic and/or electrical properties, in particular when the materials exhibit cationic valence fluctuations, as in the Co 2MnO4 cubic spinel. In this compound, Co and Mn ions are in competition at the tetrahedral and octahedral positions, depending on their various oxidation states. The Co2MnO4 was prepared following two techniques: by a soft chemical route based on a modified polymer precursor method, and by a mechanoactivation route. Both approaches yield polycrystalline powders, but their crystallites sizes and particles morphologies differ as a function of the calcination conditions. The magnetic characterization (ZFC/FC cycles, ordering temperatures, ferromagnetic coercive fields and saturation magnetizations) showed that the synthesis procedure influenced the physical properties of Co2MnO4 mainly through the size of the magnetic domains, which play an important role on the magnetic interactions between the Co/Mn cations.

The present project focuses on the theoretical study of suspensions of nano-particles of different nature (ferroelectric, ferromagnetic, multiferroic) with size of 10-100nm in isotropic and anisotropic host. We shall investigate the effect of the dispersed nano-particles on the dielectric, magnetic and optical (both linear and non-linear) properties of the suspensions depending on the nano-particles nature, size, structure and shape. To describe the electric response of suspension of ferroelectric nanoparticles in insulating non-polar (isotropic) fluid we use a theory which is based on the Fokker-Planck (Smoluchowski) equation for a rotator with electric dipole under the external electric field. We also use the dynamic equation for reorientation of ferroelectric nano-particles in the isotropic suspension under the external electric field. We developed the generalized Maxwell-Garnett type theory to study dielectric properties of a dilute suspension of ferroelectric particles in a nematic liquid crystal host. It is supposed that submicron particles do not disturb the LC alignment and the suspension macroscopically appears similar to a pure LC. We propose theoretical model for effective dielectric function of ferroelectric LC suspension. It is found that particles permanent polarisation may significantly increase the effective value of suspension dielectric function in comparison with pure LC. For more elongated particles depolarisation factor is smaller and respectively the contribution of induced polarisation of particles to the effective dielectric function is higher. We show that there are two sets of the effective permittivity tensors. One of these can be used to derive the dielectric properties like capacitances. A second set must be used in free-energy calculations to predict director profile in the cells where, for example, director field is inhomogeneous., The original document contains color images.

Yttrium manganite (YMnO3) is a multiferroic material, which means that it exhibits both ferromagnetic and ferroelectric properties, so making it interesting for a variety of technological applications. In this work, single-phase YMnO3 was prepared for the first time by mechanochemical synthesis in a planetary ball mill. The YMnO3 was formed directly from the highly activated constituent oxides, Y2O3 and Mn2O3, after 60 min of milling time. During prolonged milling, the growth of the particles occurred. The cumulative energy introduced into the system during milling for 60 min was 86 kJ/g. The X-ray powder-diffraction analysis indicates that the as-prepared samples crystallize with an orthorhombic (Pnma) YMnO3 structure. The morphology and chemical composition of the powder were investigated by SEM and FESEM. The magnetic properties of the obtained YMnO3 powders were found to change as a function of the milling time in a manner consistent with the variation in the nanocomposite microstructure.

Multiferroic materials simultaneously possess two or more ferroic orders, and enable a coupling interaction between them. Multiferroic bismuth manganite is known as a material that exhibits both ferromagnetic and ferroelectric properties making it interesting for various technological applications. Unfortunately, preparation of BiMnO3 is not possible by conventional solid state reaction and BiMnO3 has been synthesized from the mixture of oxides only at high pressures (>40 kbar). The aim of this work was to synthesize BiMnO3 (BMO) without additional heating or application of high pressures. Nanocrystalline single-phased BMnO3 was prepared for the first time by mechanochemical synthesis directly from the highly activated constituent oxides, Bi2O3 and Mn2O3, in a planetary ball mill. The obtained materials were characterized by X-ray diffraction, SEM with EDS analysis, HRTEM and magnetization measurements. All the samples were found to be tetragonal perovskite with P4mm crystallographic group. The broad maxima reflections of BMO samples can be ascribed to an amorphous/disordered phase. HRTEM micrographs give clear evidence of core-shell structure with amorphous shell around the nanocrystalline BMO particles. The magnetic hysteresis behavior is similar to that of a soft ferromagnet. The magnetic properties of the obtained BMO powders were found to change as a function of milling time in a manner consistent with the variation in the nanocomposite microstructure.

Multiferroic materials simultaneously possess two or more ferroic orders, and enable a coupling interaction between them. Multiferroic bismuth manganite is known as a material that exhibits both ferromagnetic and ferroelectric properties making it interesting for various technological applications. Unfortunately, preparation of BiMnO3 is not possible by conventional solid state reaction and BiMnO3 has been synthesized from the mixture of oxides only at high pressures (>40 kbar). The aim of this work was to synthesize BiMnO3 (BMO) without additional heating or application of high pressures. Nanocrystalline single-phased BMnO3 was prepared for the first time by mechanochemical synthesis directly from the highly activated constituent oxides, Bi2O3 and Mn2O3, in a planetary ball mill. The obtained materials were characterized by X-ray diffraction, SEM with EDS analysis, HRTEM and magnetization measurements. All the samples were found to be tetragonal perovskite with P4mm crystallographic group. The broad maxima reflections of BMO samples can be ascribed to an amorphous/disordered phase. HRTEM micrographs give clear evidence of core-shell structure with amorphous shell around the nanocrystalline BMO particles. The magnetic hysteresis behavior is similar to that of a soft ferromagnet. The magnetic properties of the obtained BMO powders were found to change as a function of milling time in a manner consistent with the variation in the nanocomposite microstructure.

A mutual ferromagnetic and ferroelectric coupling (multiferroic behavior) in Cu‐doped ZnO is demonstrated via deterministic control of Cu doping and defect engineering. The coexistence of multivalence Cu ions and oxygen vacancies is important to multiferroic behaviors in ZnO:Cu. The samples show clear ferroelectric and ferromagnetic domain patterns. These domain structures may be written reversibly via electric and magnetic bias.

Multiferroic materials have recently begun to attract significant scientific interest due to their potential applications in the design of modern electronic devices. Currently, the magnetic properties of materials form the basis of our electronic data storage and have the potential to enhance the logic operations performed in electronic devices (such as computers and sensors). Non-volatile magnetic memory is used in data storage devices, such as the hard drives found in personal computers, where data is encoded via the magnetisation state of magnetic domains in the device with one of two states: either up or down (M ↑ or M ↓); the state is determined or changed by interacting with the magnetic flux about the domain. Furthermore, in current computing and sensor technology, logic operations are performed with arrays of transistors; however, in spintronics ("spin transport electronics") the electric current itself is spin polarised and there is data encoded in the current itself. Circuit elements in such a system are magnetic devices that interact with the electron spin.Magnetoelectric multiferroics are materials that have both a spontaneous ferroelectric polarisation (P) and magnetic magnetisation (M). Polarisation may be manipulated by an electric field and magnetisation by a magnetic field, hence the potential of multiferroics lies in the coupling between the two degrees of freedom and the manipulation of magnetisation by an applied electric field and vice versa. The properties of a magnetic device could be altered "on-the-fly" by applying an electric pulse, and in the context of the examples provided this would greatly diversify the logic elements in spintronic circuits. Furthermore, with both polarisation and magnetisation a multiferroic domain can take on one of four states (M ↑ P ↑, M ↑ P ↓, M ↓ P ↑, or M ↓ P ↓) dramatically increasing data storage density over the current binary system.Lutetium ferrite (LuFe2O4) is a multiferroic material in which both the magn

There is significant interest in identifying and manipulating new materials that have unique magnetic, dielectric, and transport properties. Single phase and artificially structured materials for which more that one electromagnetic functionality coexists are particularly interesting. In this project, the synthesis of epitaxial thin films and multilayered structure with potential to display these attributed were examined.

Magnetoelectric coupling and magnetic correlations are shown to affect coupled order parameters, magnetisation M and electric polarisation P, in a multiferroic material. A ferromagnetic ferroelectric system is considered with two types of magnetoelectric coupling (anisotropic and isotropic) which couple P to different magnetic correlations. Using a mean field calculation for anisotropic coupling, it is shown that magnetic fluctuations h(Sz i )2i alter P even for temperatures above the magnetic Curie temperature. For isotropic coupling, a Green's function technique with a Random Phase Approximation is used to calculate how transverse correlations between neighbouring magnetic spins at sites i and j, such as hSx i Sx j i, affect P. The ferroelectric transition temperature and even the order of the transition is altered by the inclusion of magnetic correlations. Thermal hysteresis of the electric polarisation is also shown to exist in this model system. The methods presented can be extended to treat multiferroic systems with more complicated magnetic orderings. Another consequence of magnetoelectric coupling is the hybridisation of magnetic/electric excitations, known as "electromagnons." We show that for a ferroelectric canted antiferromagnet (with a weak ferromagnetic moment) with isotropic magnetoelectric coupling an electromagnon mode exists. When such a material, together with a ferromagnet, form a thin film heterostructure, dipolar coupling between the films allows for the electromagnon frequency to be tuned in the microwave regime. We use an effective medium method that takes into account electromagnetic boundary conditions to find the high frequency susceptibility of such composites and identify the type of resonant modes. However, the magnetoelectric coupling is usually weak through dipolar coupling. Magnetostrictive/piezoelectric composites have been shown to have a much larger strain-mediated magnetoelectric coupling so we detail how the effective medium met, Thesis (Ph.D.)--University of Western Australia, 2009

[EN] Bismuth titanates of Aurivillius layer-structure (BiFeO3)nBi4Ti3O12, are of great technological interest because of their applications as non-volatile ferroelectric memories and high-temperature piezoelectric materials. The synthesis and crystallographic characterization of a new family of compounds (LnFeO3)nBi4Ti3O12 was recently reported, in which the layers consist of LnFeO3 perovskites with a lanthanide Ln3+ substituting diamagnetic Bi3+. We report herein the magnetic properties of bulk samples, with Ln = Nd, Eu, Gd and Tb, and n = 1 and 2. Single-layer materials are paramagnetic, similar to non-substituted bismuth titanate Bi5FeTi3O15, and show crystal field effects due to the crystallographic environment of Eu3+ and Tb3+. Several anomalies are detected in the magnetization M(T) of double-layer (LnFeO3)2Bi4Ti3O12 compounds, related to the strong magnetism of Tb and Gd, since they weakly appear for Nd and they are absent in the VanVleck Eu3+ ion and in the parent Bi6Fe2Ti3O18 compound., [ES] Los titanatos de hierro y bismuto con estructura laminar tipo Aurivillius, (BiFeO3)nBi4Ti3O12, tienen un gran interés tecnológico debido a sus aplicaciones como memorias ferroeléctricas no volátiles y como piezoeléctrico cerámico de alta temperatura. La síntesis y la caracterización cristalina de una nueva familia de compuestos (LnFeO3)nBi4Ti3O12 han sido recientemente reportadas, en la que el catión diamagnético Bi3+ ha sido sustituido por los paramagnéticos Ln3+ en los bloques de perovskita. Se estudian las propiedades magnéticas de muestras cerámicas en volumen con Ln = Nd, Eu, Gd y Tb, y n = 1 y 2. Los materiales con n=1 son paramagnéticos y similares al no sustituido Bi5FeTi3O15, y muestran efectos de campo cristalino debido al entorno cristalino de Eu3+ y Tb3+. Se han detectado algunas anomalías en la magnetización M(T) de los compuestos n=2 (LnFeO3)2Bi4Ti3O12 que están relacionadas con el fuerte magnetismo de Tb y Gd, que aparecen débilmente para Nd y que no aparecen para el ión VanVleck Eu3+ ni en el compuesto pariente Bi6Fe2Ti3O18.