Search

Valence change memories are novel data storage devices in which the resistance is determined by a reversible redox reaction triggered by voltage. The oxygen content and mobility within the active materials of these devices play a crucial role in their performance. Therefore, materials which present fast oxygen migration properties and can accommodate variable oxygen stoichiometry are promising candidates. In this work, the perovskite La0.5Sr0.5MnO3‐δ (LSM50) as memristive material is studied, which presents a more facile oxygen vacancy formation and faster oxygen migration compared to other strontium‐substituted manganites. For the first time reproducible resistive switching is reported in epitaxial LSM50‐based devices with active Ti electrodes, which show large operating window and stable multilevel states. Based on the structural, chemical, and electrical results, a simple phenomenological description of the resistive switching phenomena taking place in these novel LSM50‐based memristive devices is proposed. [ABSTRACT FROM AUTHOR]

The inclusion of La-Mn vacancies in LaMnO3 nanoparticles leads to a noticeable change in conductivity behavior. The sample retains its overall insulator characteristic, with a typical thermal activation mechanism at high temperatures, but it presents high magnetoconductivity below 200 K. The activation energy decreases linearly with the square of the reduced magnetization and vanishes when the sample is magnetized at saturation. Therefore, it turns out that electron hopping between Mn3+ and Mn4+ largely contributes to the conductivity below the Curie temperature. The influence of the applied magnetic field on conductivity also supports the hypothesis of hopping contribution, and the electric behavior can be explained as being due to an increase in the hopping probability via spin alignment. [ABSTRACT FROM AUTHOR]

The magnetocaloric effect of the mixed manganites La0.7Ca SrxMnO3 (x = 0.05, 0.07, and 0.10) prepared by sol–gel method have been investigated. Using Hamad's phenomenological model, we have estimated the important magnetocaloric properties such as the magnetic entropy change and the relative cooling power. Our work shows that Curie temperature and the relative cooling power of La0.7Ca SrxMnO3 manganites increase with increasing Sr content, while maximum magnetic entropy change decrease slightly. Tunable Curie temperature and considerable magnetocaloric effect in these manganites are advantageous for applications in sub-room and near-room-temperature magnetic refrigeration. [ABSTRACT FROM AUTHOR]

Complex oxide thin films exhibit intriguing phenomena due to the coupling between multiple degrees of freedom through interfacial structural engineering. Atomic tailoring of structural parameters determines unique band structure and phonon modes, regulating emergent magnetic and electrical properties of oxide films. However, the construction of different strained and oriented domains in one intact oxide thin film is impossible using conventional means. Here we report the fabrication and quantitative structural analysis of La0.7Sr0.3MnO3 (LSMO) homostructures assisted by atomic-flat freestanding membranes. Pristine substrates and suspended membranes regulate the epitaxial strain and orientation of subsequently grown films. Our results demonstrate an ultrathin transition layer (∼ 4 atomic layers) between freestanding membranes and LSMO films is formed due to the strain relaxation. This work offers a simple and scalable methodology for fabricating unprecedented innovative functional oxide homostructures through artificially controlled synthesis routes. [ABSTRACT FROM AUTHOR]

Here we report investigations of bulk and nano-sized Pr0.65Sr(0.35−x)CaxMnO3 compounds (x ≤ 0.3). Solid-state reaction was implemented for polycrystalline compounds and a modified sol–gel method was used for nanocrystalline compounds. X-ray diffraction disclosed diminishing cell volume with increasing Ca substitution in Pbnm space group for all samples. Optical microscopy was used for bulk surface morphology and transmission electron microscopy was utilized for nano-sized samples. Iodometric titration showed oxygen deficiency for bulk compounds and oxygen excess for nano-sized particles. Measurements of resistivity of bulk samples revealed features at temperatures associated with grain boundary condition and with ferromagnetic (FM)/paramagnetic (PM) transition. All samples exhibited negative magnetoresistivity. Magnetic critical behavior analysis suggested the polycrystalline samples are governed by a tricritical mean field model while nanocrystalline samples are governed by a mean field model. Curie temperatures values lower with increasing Ca substitution from 295 K for the parent compound to 201 K for x = 0.2. Bulk compounds exhibit high entropy change, with the highest value of 9.21 J/kgK for x = 0.2. Magnetocaloric effect and the possibility of tuning the Curie temperature by Ca substitution of Sr make the investigated bulk polycrystalline compounds promising for application in magnetic refrigeration. Nano-sized samples possess wider effective entropy change temperature (ΔTfwhm) and lower entropy changes of around 4 J/kgK which, however, puts in doubt their straightforward potential for applications as magnetocaloric materials. [ABSTRACT FROM AUTHOR]

The structural, magnetic, and magnetocaloric properties of La0.7Sr0.3-xLixMnO3 (0 ≤ x ≤ 0.15) samples have been investigated. All our samples were synthesized using the sol–gel technique at low temperature. The Rietveld refinement of the X-ray powder diffraction shows that the La0.7Sr0.3-xLixMnO3 compounds crystallize in a rhombohedral system with R 3 ¯ c space group. Magnetization measurements versus temperature showed that all our investigated samples display a paramagnetic-ferromagnetic transition with decreasing temperature. The Curie temperature, TC, decreases with increasing Li content from 365 K for x = 0 to 194.4 K for x = 0.15. The maximum of the magnetic entropy change, Δ S M Max , under an applied magnetic field change of 2 T is found to be 1.97, 1.05, 1.07, and 1 J kg−1 K−1 for x = 0, 0.05, 0.1, and 0.15 respectively. The relative cooling power reaches 82.1, 80.6, 83.2, and 102.5 Jkg−1 for x = 0, 0.05, 0.1, and 0.15, respectively, under the same magnetic field change of 2 T. [ABSTRACT FROM AUTHOR]

The growth of Li1+xMn2O4 via detonation reaction was investigated with respect to the presence of energetic precursors, such as the metallic nitrates and the degree of confinement of the explosive charge. The detonation products were characterized by scanning electron microscopy. Powder X-ray diffraction and transmission electron microscopy were used to characterize the products. Li1+xMn2O4 with 1 ∼ 2μm spherical morphology and more uniform secondary particles, but with smaller primary particles of diameters from 20 to 60 nm and a variety of morphologies were found. The oxides produced by this cheap method affirmed the validity of detonation synthesis of nano-size powders. [ABSTRACT FROM AUTHOR]

Constructing artificial heterointerfaces in the complex oxides composites to enhance and explore new functionalities have been attracting extensive scientific attentions. It is of great significant to clarify the role of heterointerfaces on the functional properties. In this study, the La0.7Sr0.3MnO3/NiO heterointerfaces were constructed in self-assembled epitaxial (La0.7Sr0.3MnO3)0.5:NiO0.5 (LSMO:NiO) nanocomposite thin films by the method of pulsed laser deposition (PLD). The effect of LSMO/NiO heterointerfaces on the magnetic and electrical transport properties were studied by changing the geometric distribution of heterointerface and interfacial area. Interestingly, enhanced magnetoresistance (MR) with room temperature range at a low magnetic field of 1 T can be regulated by LSMO/NiO heterointerfaces. The mechanism of artificial electronic phase separation formed by LSMO/NiO heterointerfaces was proposed to explain the enhanced room temperature MR effect. This study provides an effective way to regulate the functional properties of oxides nanocomposite thin films by utilizing heterointerfaces. [ABSTRACT FROM AUTHOR]

The phase structure and magnetic and magnetocaloric effect (MCE) properties have been studied in La2/3-xEuxCa1/3MnO3 (0 ≤ x ≤ 0.08) manganites prepared by conventional solid-state reaction. There is no miscellaneous phase in the samples, which are analyzed by the Rietveld refinement method. The refinement data shows the lattice decreases gradually with smaller radius of Eu3+ substitution. The Curie temperature of the material is continuity distributed from 150 to 225 K. Large magnetic entropy changes (ΔSM) are obtained with Eu3+ substitution, the highest value of −ΔSM is 4.93 J·kg−1 K−1 in the magnetic field change at 2 T, and the relative cooling power (RCP) value is around ~ 110 J·kg−1. The samples undergo a first-order magnetic phase transition with small hysteresis, for x = 0.00, 0.02, when x ≥ 0.4, the samples under a second-order magnetic phase transition. The results show that the substitution of Eu3+ can be used to modify the Curie temperature and obtain a continuous operating temperature interval, without sacrificing the MCE. The large MCE performance, continuous working temperature interval, and high cooling efficiency make our samples be used for the application of magnetic refrigeration. [ABSTRACT FROM AUTHOR]

The polycrystalline ceramic manganite Pr 0.5 Ca 0.5 MnO 3 has been synthesized with conventional solid-state reaction route and its structural, magnetic and electrical behavior was explored experimentally in terms of change in thermoelectric and magnetic properties around a specific temperature. Structural investigation through XRD confirms that sample is single phase. Magnetic study reveals that the sample is charge ordered manganite (CO ≈ 250 K). The Magnetic behavior show alignment of Mn spins due to ferromagnetic behavior at 148 K (T C). Furthermore, in M(H) curve, a non-linear conduct was observed around 220 K and magnetic moment alignment is argued with respect to temperature. The response of applied frequency, using impedance spectroscopy, was studied for change in relaxation progressions. Equivalent circuit model (RQ) was implemented for getting impedance values. These impedance values helped us for determining the resistance and capacitance of grain and grain boundaries. Using calculated electrical features for a range of temperatures confirmed a variation in the conduction system at T C = 148 K, using small polaronic hopping model (SPH) and in light of Mott's variable range hopping model (MVRH). With MVRH, it is also discussed that carriers would hop to large distance with the help of activation energy below T C and above this these carriers are being facilitated by various trap centers via SPH. Dispersion around 148 K in dielectric relaxation and one sort of polarization is close to relaxation calculated from grain boundaries. [ABSTRACT FROM AUTHOR]

Structural, magnetic, and electrical properties of La0.5Ca0.5MnO3 samples prepared at two different sintering temperatures (800°C and 1350°C which were labeled S800 and S1350, respectively) have been investigated. The Reitveled refined XRD patterns indicate an orthorombic structure with Pnma space group for both samples. Ac susceptibility measurements show that, the fraction of ferromagnetic and antiferromagnetic phases could be controlled with sintering temperature. S800 sample has a ferromagnetic state while the antiferromagnetic phase is enhanced in S1350 sample which causes the increase of thermal hysteresis in this sample. Two distinct regions (T>ӨD/2 and T˂ӨD/2, ӨD is the Debye’s temperature) were noticed to investigate the electrical properties. Based on the resistivity data at T>ӨD/2 region, the S800 and S1350 samples follow the adiabatic and non-adiabatic small polaron hopping (SPH) models, respectively. At T˂ӨD/2 region, the 3-dementional variable range hopping (VRH) model displays a good correlation with the experimental data of both samples. The related parameters of both SPH and VRH models are extracted. The results show that these parameters are particle size dependent.

[Display omitted] • Effects of Bi-doping on MCE properties are investigated in La 0.7 Ca 0.27 Na 0.03 MnO 3. • T C is 310, 246, 235, and 232 K for x = 0.0, 0.005, 0.01 and 0.02, respectively. • - Δ S M max is calculated as 8.42 Jkg-1K−1 for x = 0.02 at 5 T. • All samples exhibited a second-order magnetic phase transition. In the present study, the effects of Bi doping on the Mn-site of La 0.7 Ca 0.27 Na 0.03 Mn 1-x Bi x O 3 perovskite manganites on their structural, magnetic, and magnetocaloric properties were investigated. From XRD analyses, the main La 0.7 Ca 0.27 Na 0.03 MnO 3 manganite has a rhombohedral structure with space group R 3 ¯ c. In contrast, manganites obtained by doping with x = 0.005, 0.01, and 0.02 Bi were found to have an orthorhombic structure with space group Pnma. The Curie temperature (T C) is 310, 246, 235, and 232 K for x = 0.0, 0.005, 0.01 and 0.02, respectively. The maximum entropy change (- Δ S M max ) values were calculated as 6.34, 8.24, 8.33, and 8.42 Jkg-1K−1 for x = 0.0, 0.005, 0.01 and 0.02, respectively. The highest Relative Cooling Power (RCP) value was computed as 271 Jkg−1 for the sample with a Bi concentration of 0.01. It was determined from the H/M versus M2 curves that all samples displayed a second-order magnetic phase transition. [ABSTRACT FROM AUTHOR]

The thermoelectric properties of ceramics with composition A0.98La0.02MnO3-δ are anticipated to vary with the basicity and atomic portion of the alkaline earth metal, A. In the present investigation ceramic powder precursors with composition A0.98La0.02MnO3-δ (A = Ca, Ba) were synthesized by the solid-state method and sintered in air at 1400°C. Seebeck coefficient, electrical and thermal conductivities were characterized for both materials from 100 to 900°C in air. The highest zT of 0.10 at 900°C was reached for Ca0.98La0.02MnO3-δ. The high zT is attributed to the enhanced electronic conductivity (∼90 S/cm at 900°C) due to La doping. zT for Ba0.98La0.02MnO3-δ reached its highest value (0.02) at 800°C corresponding to a low electronic conductivity (∼2 S/cm), while the thermal conductivity was significantly reduced compared to Ca0.98La0.02MnO3-δ reaching ∼1W/(m•K) combined with a high Seebeck coefficient, −290 μV/K. The present data represent a valuable basis for further development of these materials with respect to applications in thermoelectric devices.

Abstract Valence change memories are novel data storage devices in which the resistance is determined by a reversible redox reaction triggered by voltage. The oxygen content and mobility within the active materials of these devices play a crucial role in their performance. Therefore, materials which present fast oxygen migration properties and can accommodate variable oxygen stoichiometry are promising candidates. In this work, the perovskite La0.5Sr0.5MnO3‐δ (LSM50) as memristive material is studied, which presents a more facile oxygen vacancy formation and faster oxygen migration compared to other strontium‐substituted manganites. For the first time reproducible resistive switching is reported in epitaxial LSM50‐based devices with active Ti electrodes, which show large operating window and stable multilevel states. Based on the structural, chemical, and electrical results, a simple phenomenological description of the resistive switching phenomena taking place in these novel LSM50‐based memristive devices is proposed.

In this study, the effect of Ru additive on the crystallographic, magnetic, and electrocatalytic properties of LaBiBaMnO 3 manganite was investigated for magnetic refrigeration and energy storage systems. XRD analysis of La 0·62 Bi 0·05 Ba 0·33 MnO 3 (LBBM) and La 0·62 Bi 0·05 Ba 0·33 Mn 0·90 Ru 0·10 O 3 (LBBMR) manganites prepared by solid state method reveals that the samples have a rhombohedral structure and the average grain size of perovskites are found to be below 3 μm. From the temperature-dependent magnetization measurements, it was observed that the value of Curie temperature (T C) decreased from 334 K for LBBM to 327 K for LBBMR with the substitution of Mn with 10% Ru. In parallel with the decrease observed in T C with Ru contribution, a decrease in magnetic entropy value was also observed. The substitution of 10% Ru reduced the onset potential 51 mV for oxygen evolution reaction (OER) and 74 mV for oxygen reduction reaction (ORR). The electrode activity of OER at 10 mA cm−2 was obtained at around 0.976 V. Ru substitution and increasing scan rate to 100 mV s−1 increased the current density and lowered onset potential for OER. The ORR efficiency of LBBM catalysts boosted up to 60% with the durability test for 6 h. The increased cathodic Tafel slope from −255.1 mV dec−1 to −120.4 mV dec−1 with 10% Ru substitution suggests that a fast charge transfer for ORR activity due to Ru-occupied Mn-sites in the structure. LBBMR sample achieved bifunctional properties as ORR/OER performance and magnetic cooling applications by substituting Mn with 10% Ru. • The effect of Ru additive on the magnetic and electrocatalytic properties of LaBaBiMnO 3 manganite was investigated. • T c temperatures are determined as 334 K for LBBM and 327 K for LBBMR. • The addition of 10% Ru reduced the onset potential 51 mV for OER and 74 mV for ORR. • Anodic and cathodic slopes were improved to −74.2 mV dec−1 and 92 mV dec−1 with the addition of 10% Ru for ORR/OER. • Replacing of 10% Ru in LBBMR catalyst kept its 60% efficiency up to 6 h while RuO 2 exhibited 90% efficiency. [ABSTRACT FROM AUTHOR]

Bipyramidal distortions play the governing role in physical properties of RMnO3 multiferroics. We have carried out angle-dispersive high pressure synchrotron X-ray diffraction experiments on hexagonal HoMnO3 to find the evolution of bipyramidal parameters upon external compression. It is found that the average bond-lengths and bond-angles between Mn and O atoms depict significant reduction, which results in the distortion in MnO5 bipyramidal structure at high pressures. Further, since subtle variations in these distortions are used to compare the properties of hexagonal RMnO3 manganite series, in-situ low temperature infrared spectroscopic studies of 10% Dy-doped HoMnO3 have been carried out to study the effect of further increase in R-site cationic radius on MnO5 deformations. It is found that the phonon modes specifically related to the motion of Mn and O atoms in ab-plane display irregularities near magnetic transition temperatures (TN and TSR) via spin phonon coupling. Even a small increase in average ionic radius of rare-earth atom suppresses this anomalous behaviour and follows the trend of undoped hexagonal rare-earth manganites. [ABSTRACT FROM AUTHOR]

The features of the formation of crystalline and magnetic subsystems in the Eu1-xMn1+xO3 solid solution depending on the superstoichiometric manganese content were studied by X-ray diffraction analysis (XRD) and magnetometry. XRD showed that an increase in x led to significant decrease in the unit cell volume, which was accounted for by the occurrence of the manganese disproportionation reaction and subsequent redistribution of different valence manganese ions in nonequivalent positions as follows: Eu 1 - x 3 + Mn x 2 + Mn 1 - x 3 + Mn x 4 + O 3 2 - . The degree of orthorhombic and Jahn–Teller distortions decreased with increasing x. The temperatures of magnetic transitions due to manganese ions ordering in the B- and A-sublattices, high-temperature (T1) one and low-temperature (T2) one, respectively, were determined based on the temperature dependences of dM/dT. It was established that an increase in the Mn content reduced the temperature T1. Magnetization measurements performed in Eu0.94Mn1.06O3 in a low applied magnetic field demonstrated magnetization reversal at temperatures below T1. [ABSTRACT FROM AUTHOR]

Here, we report synthesis and investigations of bulk and nano-sized La(0.7−x)EuxBa0.3MnO3 (x ≤ 0.4) compounds. The study presents a comparison between the structural and magnetic properties of the nano- and polycrystalline manganites La(0.7−x)EuxBa0.3MnO3, which are potential magnetocaloric materials to be used in domestic magnetic refrigeration close to room temperature. The parent compound, La0.7Ba0.3MnO3, has Curie temperature TC = 340 K. The magnetocaloric effect is at its maximum around TC. To reduce this temperature below 300 K, we partially replaced the La ions with Eu ions. A solid-state reaction was used to prepare bulk polycrystalline materials, and a sol-gel method was used for the nanoparticles. X-ray diffraction was used for the structural characterization of the compounds. Transmission electron spectroscopy (TEM) evidenced nanoparticle sizes in the range of 40–80 nm. Iodometry and inductively coupled plasma optical emission spectrometry (ICP-OES) was used to investigate the oxygen content of the studied compounds. Critical exponents were calculated for all samples, with bulk samples being governed by tricritical mean field model and nanocrystalline samples governed by the 3D Heisenberg model. The bulk sample with x = 0.05 shows room temperature phase transition TC = 297 K, which decreases with increasing x for the other samples. All nano-sized compounds show lower TC values compared to the same bulk samples. The magnetocaloric effect in bulk samples revealed a greater magnetic entropy change in a relatively narrow temperature range, while nanoparticles show lower values, but in a temperature range several times larger. The relative cooling power for bulk and nano-sized samples exhibit approximately equal values for the same substitution level, and this fact can substantially contribute to applications in magnetic refrigeration near room temperature. By combining the magnetic properties of the nano- and polycrystalline manganites, better magnetocaloric materials can be obtained. [ABSTRACT FROM AUTHOR]

The features of the formation of crystalline and magnetic subsystems in the Eu1-xMn1+xO3 solid solution depending on the superstoichiometric manganese content were studied by X-ray diffraction analysis (XRD) and magnetometry. XRD showed that an increase in x led to significant decrease in the unit cell volume, which was accounted for by the occurrence of the manganese disproportionation reaction and subsequent redistribution of different valence manganese ions in nonequivalent positions as follows: Eu 1 - x 3 + Mn x 2 + Mn 1 - x 3 + Mn x 4 + O 3 2 - . The degree of orthorhombic and Jahn–Teller distortions decreased with increasing x. The temperatures of magnetic transitions due to manganese ions ordering in the B- and A-sublattices, high-temperature (T1) one and low-temperature (T2) one, respectively, were determined based on the temperature dependences of dM/dT. It was established that an increase in the Mn content reduced the temperature T1. Magnetization measurements performed in Eu0.94Mn1.06O3 in a low applied magnetic field demonstrated magnetization reversal at temperatures below T1. [ABSTRACT FROM AUTHOR]

Doped lanthanum manganites La1−xCaxMnO3 with five different concentrations of Ca (x = 0.1, 0.2, 0.3, 0.4 and 0.5) were synthesized by high-temperature solid-state reaction method and characterized. The prepared samples were experimentally analyzed by X-ray diffraction (XRD), ultraviolet-visible spectroscopy (UV-Vis), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) and vibrating sample magnetometer (VSM) measurements. Orthorhombic structure is confirmed for this compound from powder X-ray diffraction data. The XRD data confirm the shrinkage in the unit cell of synthesized samples for increasing calcium concentration. The UV–Visible analysis for the estimation of optical band gap (Eg) reveals that the band gap decreases with the incorporation of Ca. The synthesized samples were investigated for charge density distribution using maximum entropy method, utilizing the XRD data sets. From the charge density analysis, it is found that the bond lengths for La–O and Mn–O bonds decrease with the addition of Ca. The ionic nature between La and O atoms and covalent nature between Mn and O atoms are enhanced for 40% of calcium-doping. For 50% of Ca-doped sample, ionic nature between La and O atoms and covalent nature between Mn and O atoms decrease. All the prepared samples exhibit ferromagnetism at 20 K and paramagnetism at 300 K. [ABSTRACT FROM AUTHOR]

Epitaxial strain modifies the physical properties of thin films deposited on single-crystal substrates. In a previous work, we demonstrated that in the case of La2/3Ca1/3MnO3 thin films the strain induced by the substrate can produce the segregation of a non-ferromagnetic layer (NFL) at the top surface of ferromagnetic epitaxial La2/3Ca1/3MnO3 for a critical value of the tetragonality τ, defined as τ = |c − a|a, of τC ≈ 0.024. Although preliminary analysis suggested its antiferromagnetic nature, to date a complete characterization of the magnetic state of such an NFL has not been performed. Here, we present a comprehensive magnetic characterization of the strain-induced segregated NFL. The field-cooled magnetic hysteresis loops exhibit an exchange bias mechanism below T ≈ 80 K, which is well below the Curie temperature of the ferromagnetic La2/3Ca1/3MnO3 layer. The exchange bias and coercive fields decay exponentially with temperature, which is commonly accepted to describe spin-glass (SG) behavior. The signatures of slow dynamics were confirmed by slow spin relaxation over a wide temperature regime. Low-energy muon spectroscopy experiments directly evidence the slowing down of the magnetic moments below ~100 K in the NFL. The experimental results indicate the SG nature of the NFL. This SG state can be understood within the context of the competing ferromagnetic and antiferromagnetic interactions of similar energies. [ABSTRACT FROM AUTHOR]

In this letter, we report the related transport properties of (La1-yPry)0.5Sr0.5MnO3 (y = 0.1, 0.2, 0.3, 0.4 and 0.5) films prepared by pulsed laser deposition. The transport properties of the films with different fields applied, such as optical field, magnetic field and strain field, were investigated. The results show that the resistance increases and the metal–insulator transition temperature shifts to lower temperature with the increasing of Pr doping. However, the film with the doping amount of 0.5 exhibits an insulation characteristic, and showed transient photoconductivity effect, with a relative change of photo-resistance of 7.2% at low temperature. Otherwise, a negative magneto-resistance of 54% was observed at 80 K and 1 T magnetic field. Further, by using an external electric field to modulate the films grown on the PbMg1/3Nb2/3O3-PbTiO3 substrate, it was found that the electric field reduced the resistance and had a faking effect on the relative change of the photo-resistance. It is mainly attributed to the influence of the external field on lattice distortion and charge carrier delocalization. [ABSTRACT FROM AUTHOR]

As a redox material, perovskite is considered one of the most efficient photovoltaic materials for producing hydrogen via water splitting reactions. This study used a wet chemical method to synthesize La1−xAgxMnO3 nanoparticles (0.00 ≤ x ≤ 0.09). The formation, chemical composition, and morphology of samples were examined by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and high-resolution transmission electron microscopy (HR-TEM). The optical properties were examined by ultraviolet visible (UV) diffuse reflectance spectroscopy. XRD demonstrated that the samples had a rhombohedral hexagonal structure with a space group R 3 ¯ c. The pore volume, pore size, and surface area were calculated and examined. The prepared samples were used as a photoanode in alkaline media for water splitting, and the photocurrent was measured. The photocurrent density recorded (14.01, 12.02, 11.67, and 10.28 μA/cm2) for x = (0.09, 0.06, 0.03, and 0.00) at 1 V vs. Ag/AgCl, respectively. The smaller impedance of the sample (x = 0.09) photoanodes than the sample (x = 0.00), which displayed a considerable decrease in charge transfer resistance. The electron lifetime (τ) increased with increasing Ag concentration, where x = 0.09 has the largest electron lifetime (τ) = 10.04 ms. Therefore, the electron–hole recombination rate of La0.91Ag0.09MnO3 is lower than LaMnO3. The samples demonstrated long-term stability for 1 h and enhanced photoelectrochemical performance. [ABSTRACT FROM AUTHOR]

Half-doped manganites has proven itself to be candid material for different magnetic-based devises. Various physical properties of pellet size half-doped La0.5Sr0.5MnO3 manganite treated with oxygen plasma and its comparison with the pristine samples is reported in this article. The impact of plasma irradiation on different properties such as structure, thermoelectric power and electrical resistivity were studied and compared before exposure. The crystal structure or symmetry remains unaffected due to plasma exposure. The Mn4+/Mn3+ ratio decreases after exposing to plasma compared to the pristine samples. It is observed that La0.5Sr0.5MnO3 samples exhibit insulating behaviour before and after plasma exposure. The resistivity increases appreciably after plasma treatment which may be credited to the reduction in the bond angle of Mn–O–Mn as well as the creation of oxygen vacancies. The negativity of the thermopower is seen at all temperatures attributing it to the electrons behaving as the dominant charge carriers and also shows that the magnitude is temperature dependent. [ABSTRACT FROM AUTHOR]

The influence of non-magnetic Ti doping at the Mn site on magnetoresistance and thermopower properties of Sm0.6Sr0.4MnO3 (Sm0.6Sr0.4Mn1-xTixO3, x = 0, 0.1 and 0.15 denoted by SMO, STMO0.1 and STMO0.15 respectively) prepared by solid-state reaction method is investigated. All three samples exhibit metal–insulator transition. STMO0.15 has a sudden drop of resistivity around 100 K. At low temperatures, the magnetoresistances of the samples get increased with Ti doping. All samples have negative thermoelectric power (TEP) at room temperature. Both high-temperature resistivity and TEP of the samples are successfully analyzed using the Variable Range hopping (VRH) model. Power factor, which determines the quality of a TEP material, is comparatively high for 15% Ti-doped sample STMO0.15 at room temperature. [ABSTRACT FROM AUTHOR]

The effects of K+ substitution at the Ba-site on the structural, magnetic, and electrical properties and magnetoresistance (MR) of La0.7Ba0.3−xKxMnO3 (x = 0 and 0.04) manganites prepared via the solid-state method were investigated. Rietveld refinement of X-ray diffraction data confirmed that both samples were crystallized in the rhombohedral structure with the R3 c ¯ space group. In addition, the unit cell volume, V, and the average grain size also increased with K+ ions. Magnetization versus applied field (M–H) measurement was carried out, and the saturation magnetization (Ms) was found to increase from 1.81 μ B /f.u. (x = 0) to 4.11 μ B /f.u. (x = 0.04), implying that K+ ions strengthened the ferromagnetic (FM) interaction. Furthermore, the metal–insulator transition temperature, TMI, increased from 257 K (x = 0) to 271 K (x = 0.04). The observed behaviour may be related to the enhancement of double-exchange (DE) interaction due to the increase in Mn-O-Mn bond angle and electronic bandwidth (W), favouring the increasing rate of the eg electron hopping process. The fitting of the electrical resistivity data in the metallic region describes the significance of residual resistivity, electron–electron and electron–magnon scattering processes to elucidate the electronic transport properties. Within the insulating region, variable range hopping (VRH) and small polaron hopping (SPH) models are proposed to describe the conduction mechanism. [ABSTRACT FROM AUTHOR]

Half-doped manganites has proven itself to be candid material for different magnetic-based devises. Various physical properties of pellet size half-doped La0.5Sr0.5MnO3 manganite treated with oxygen plasma and its comparison with the pristine samples is reported in this article. The impact of plasma irradiation on different properties such as structure, thermoelectric power and electrical resistivity were studied and compared before exposure. The crystal structure or symmetry remains unaffected due to plasma exposure. The Mn4+/Mn3+ ratio decreases after exposing to plasma compared to the pristine samples. It is observed that La0.5Sr0.5MnO3 samples exhibit insulating behaviour before and after plasma exposure. The resistivity increases appreciably after plasma treatment which may be credited to the reduction in the bond angle of Mn–O–Mn as well as the creation of oxygen vacancies. The negativity of the thermopower is seen at all temperatures attributing it to the electrons behaving as the dominant charge carriers and also shows that the magnitude is temperature dependent. [ABSTRACT FROM AUTHOR]

The influence of non-magnetic Ti doping at the Mn site on magnetoresistance and thermopower properties of Sm0.6Sr0.4MnO3 (Sm0.6Sr0.4Mn1-xTixO3, x = 0, 0.1 and 0.15 denoted by SMO, STMO0.1 and STMO0.15 respectively) prepared by solid-state reaction method is investigated. All three samples exhibit metal–insulator transition. STMO0.15 has a sudden drop of resistivity around 100 K. At low temperatures, the magnetoresistances of the samples get increased with Ti doping. All samples have negative thermoelectric power (TEP) at room temperature. Both high-temperature resistivity and TEP of the samples are successfully analyzed using the Variable Range hopping (VRH) model. Power factor, which determines the quality of a TEP material, is comparatively high for 15% Ti-doped sample STMO0.15 at room temperature. [ABSTRACT FROM AUTHOR]

Here we report the synthesis and investigation of bulk and nano-sized La0.7Ba0.3−xCaxMnO3 (x = 0, 0.15, 0.2 and 0.25) compounds that are promising candidates for magnetic refrigeration applications. We compare the structural and magnetic properties of bulk and nano-scale polycrystalline La0.7Ba0.3−xCaxMnO3 for potential use in magnetic cooling systems. Solid-state reactions were implemented for bulk materials, while the sol–gel method was used for nano-sized particles. Structurally and morphologically, the samples were investigated by X-ray diffraction (XRD), optical microscopy and transmission electron microscopy (TEM). Oxygen stoichiometry was investigated by iodometry. Bulk compounds exhibit oxygen deficiency, while nano-sized particles show excess oxygen. Critical magnetic behavior was revealed for all samples using the modified Arrott plot (MAP) method and confirmed by the Kouvel–Fisher (KF) method. The bulk polycrystalline compound behavior was better described by the tricritical field model, while the nanocrystalline samples were governed by the mean-field model. Resistivity in bulk material showed a peak at a temperature Tp1 attributed to grain boundary conditions and at Tp2 associated with a Curie temperature of Tc. Parent polycrystalline sample La0.7Ba0.3MnO3 has Tc at 340 K. Substitution of x = 0.15 of Ca brings Tc to 308 K, and x = 0.2 brings it to 279 K. Nanocrystalline samples exhibit a very wide effective temperature range in the magnetocaloric effect, up to 100 K. Bulk compounds exhibit a high and sharp peak in magnetic entropy change, up to 7 J/kgK at 4 T at Tc for x = 0.25. To compare the magnetocaloric performances of the studied compounds, both relative cooling power (RCP) and temperature-averaged entropy change (TEC) figures of merit were used. RCP is comparable for bulk polycrystalline and nano-sized samples of the same substitution level, while TEC shows a large difference between the two systems. The combination of bulk and nanocrystalline materials can contribute to the effectiveness and improvement of magnetocaloric materials.

• The ferromagnetic-paramagnetic transition in La 0.5 Sm 0.2 Sr 0.3 MnO 3 and La 0.5 Sm 0.2 Sr 0.3 Mn 0.95 Fe 0.05 O 3 was studied. • (μ 0 Critical exponents for LaSmSrMnO and LaSmSrMnFeO were determined. • Isotherms M(μ 0 H,T) and -∆S M (μ 0 H,T) were simulated using the Landau model for both compounds. In this paper, an effective calculation method resulting from the combination of Landau theory and the Arrott–Noakes equation is reported. The magnetic entropy change, - Δ S M , in a random ferromagnetic system is simulated. A phase shift from paramagnetic to ferromagnetic behavior is observed in these complex magnetic systems. Their critical behavior was ascertained by magnetization measurements and magnetocaloric computations. The critical exponents of the La 0.5 Sm 0.2 Sr 0.3 MnO 3 and La 0.5 Sm 0.2 Sr 0.3 Mn 0.95 Fe 0.05 O 3 manganites were discovered to be (0.43; 1.19; 3.78), and (0.52; 1.45; 3.78), respectively, using modified Arrott plots (applied to Tricritical mean-field, 3D-Ising, and 3D-Heisenberg models) and the Kouvel-Fisher technique. The critical exponents obtained from the study were used to investigate the Landau model by employing the Gibbs free energy to simulate isotherms of magnetization M (μ 0 H , T) and the curves of - Δ S M (μ 0 H , T). The findings indicate that short-range interactions are critical in determining the magnetic characteristics and are consistent with the 3D Heisenberg and 3D Ising models. [ABSTRACT FROM AUTHOR]

Interface‐type valence change memories (VCMs) are exciting candidates for multilevel storage in resistive random access memories (RRAM) and as artificial synapses for neuromorphic computing. Several materials have been proposed as VCM candidates and, depending on the materials and electrodes of choice, different switching mechanisms take place leading to the change in resistance. Here, the focus is on La0.8Sr0.2MnO3–δ (LSM) perovskite and, particularly, the role of its nanostructure on the memristive device performance. The nanostructural details of the layers are modified by growing LSM epitaxial thin films on different substrates, i.e., SrTiO3 (STO) and LaAlO3 (LAO), by metal‐organic chemical vapor deposition (MOCVD). An interface‐type memristive response is observed using Ti as active electrode and Pt as inert electrode. The modifications in the nanostructure of LSM (strain and dislocations) determine the memristive performance, leading to differences in cycle to cycle reproducibility and multilevel capabilities by the modification of the LSM's oxygen migration properties. The results show that nanostructure engineering is a promising approach for optimizing the performance of memristive devices, an approach which can also be extended and applied to other nanoionic electrochemical devices. [ABSTRACT FROM AUTHOR]

With an aim to improve the magnetocaloric effect above the room temperature for practical applications, La0.67Sr0.33MnO3 manganite with different manganese concentrations has been synthesized using a novel technique with oxalic acid as a fuel material. The X-ray photoelectron spectroscopy studies confirm the presence of mixed valence states of Mn3+ and Mn4+ with varying manganese content. Studies of the magnetic properties reveal that the samples undergo ferromagnetic to paramagnetic transitions well above room temperatures, i.e., around 383 K. The second order of magnetic phase transition has been confirmed from Arrott plots. The entropy change (ΔSM) over a wide temperature range was observed around magnetic transition temperature in all the samples which can be considered as a suitable material for magnetic refrigeration applications. A reduction in magnetic entropy in deficiency and excess of manganese samples has been attributed to the magnetic inhomogeneity. [ABSTRACT FROM AUTHOR]

This work investigated the effect of lightly Cr 3 + -doped ions on the structural, magnetic, and critical behavior of the nominal composition La 0.7 Ca 0.3 Mn 1 - x Cr x O 3 (0 ≤ x ≤ 0.09 ). Our compounds adopted a single phase with a rhombohedral structure (space group R 3 ¯ c). Furthermore, a comparative study of the crystallite sizes of compounds by X-ray powder diffraction, XRD, is reported. Individual contributions of the crystallite sizes and lattice microstrains were examined through the Williamson–Hall (W–H) analysis, the size-strain plot (SSP), and Halder–Wagner (H–W) methods. Greater accuracy was noted with W–H, where points were more likely to contact the fitting line than those using other methods. From magnetic data analyzed, a second-order paramagnetic–ferromagnetic phase transition appears at Curie temperatures, T C , between 212 and 261 K . A deviation from the Curie Weiss law is due to the presence of local inhomogeneities in our samples for T ≥ T C characteristic of a Griffiths Phase. The critical coefficient values obtained for β, γ, and δ from different methods are fairly close to the theoretical prediction of the tricritical mean-field model for x = 0.0 and 0.03 and which are in accordance with those predicted by the 3D Ising model for x = 0.06 . The values found for β, γ, and δ are close to those expected for the mean field x = 0.09 . In our samples, we found that two types of interactions exist, long-range exchange interactions for x = 0.0 , 0.03 and 0.06 and short-range exchange interactions for x = 0.09 . [ABSTRACT FROM AUTHOR]

Herein, the influence of Zn nonmagnetic doping on magnetic properties above the Curie temperature of La1−xDyxMn1−yZnyO3 diluted samples is studied. The Zn doping effect on the structure and distortions of MnO6 octahedra are observed, suggesting a relaxation effect of the tilting on the stress introduced by Zn doping. Magnetization versus temperature shows a transition from paramagnetic (PM) to ferromagnetic (FM) phase in the temperature range from 130 to 190 K. In the PM regime, FM spin clusters are observed by dc susceptibility and electron PM resonance (EPR). The effective moment of the clusters is obtained from the high‐temperature dc susceptibility. The effective exchange constants, J, are estimated using an open‐source program for the high‐temperature expansion (HTE) for inverse susceptibility χ−1 of the spin Heisenberg model. [ABSTRACT FROM AUTHOR]

A review of the basic concepts and mechanisms related to the electronic phase separation and the formation of nanoscale inhomogeneities in magnetic materials is given. We put the main emphasis onto strongly correlated electron systems such as manganites, where phase separation occurs due to the competition of ferro- and antiferromagnetic states, as well as cobaltites, where the spin-state transitions play an important role. Special attention is paid to the mechanism of phase separation related to the imperfect nesting of sheets of the Fermi surface, which is especially important for systems with spin density waves, a striking example of which are iron-containing pnictides. [ABSTRACT FROM AUTHOR]

Several phase separated manganites present a peculiar effect each time they go through a phase transition within the range characterized by phase separation. The effect is known as the thermal cycling effect (TCE) and is characterized by monotonous changes in the relative content of the coexisting phases. In this work, we analyze a way to control the effects induced by TCE, performing thermal treatments at high temperature. Our results revealed a complex interplay between the dynamic and static characteristics of the phase separated state, which can be analyzed in terms of three simple parameters. One related to the static properties, another to the dynamic properties and a last one that acts as a link between both features. [ABSTRACT FROM AUTHOR]

To study the effect of manganese non-stoichiometry at B-site, a series of manganites with compositional formula La 0.67 Sr 0.33 Mn 1±x O 3 (where x = 0, 0.05 and 0.1) was synthesized by oxalate precursor method. X-ray diffraction data confirm the rhombohedral structure of La 0.67 Sr 0.33 Mn 1±x O 3 along with minor phases of Mn 3 O 4. The average grain size is found to be 266 nm for x = 0 whereas its magnitude decreases with excess or deficiency in manganese concentration. An increase in the manganese non-stoichiometry leads to the coexistence of ferromagnetic and antiferromagnetic interactions. The effect of Mn 1±x on the magnetotransport properties could be understood on the basis of collective behaviour of magnetic spins, double exchange mechanism and ratio of Mn4+/Mn3+ ions. A crossover from negative to positive magnetoresistance behavior above metal-insulator transition temperature was observed for LSP-0.95 sample, whereas a positive magnetoresistance over the entire temperature region was noticed for LSP-1.10 sample. [ABSTRACT FROM AUTHOR]

We report the structural, magnetic, electrical and broadband microwave absorption in La 0.7 Na 0.3 MnO 3 sample synthesized by microwave (MW) irradiation (Na 0.3 LMO_MW) and compare them to the sample synthesized by solid-state (SS) reaction method (Na 0.3 LMO_SS). Single phase Na 0.3 LMO_MW was synthesized at 800 °C in 30 min, whereas, Na 0.3 LMO_SS sample was obtained by sintering at 1200 °C for 48 h. Although both these samples show ferromagnetic transition at T C ∼324.8 K, the MW-synthesized sample shows distinct physical properties: broad ferromagnetic transition, smaller saturation magnetization, a large difference between the magnetic ordering and metal-insulator transition temperatures, a large high-field magnetoresistance, a table top-like magnetocaloric effect, and a large low-field microwave absorption compared to the solid state synthesized sample. These differences are suggested to arise from magnetic heterogeneity induced by smaller grain size and surface spin disorder in the MW synthesized La 0.7 Na 0.3 MnO 3. [ABSTRACT FROM AUTHOR]

Among transition metal oxides, manganites have attracted significant attention because of colossal magnetoresistance (CMR)—a magnetic field-induced metal–insulator transition close to the Curie temperature. CMR is closely related to the ferromagnetic (FM) metallic phase which strongly competes with the antiferromagnetic (AFM) charge ordered (CO) phase, where conducting electrons localize and create a long range order giving rise to insulator-like behavior. One of the major open questions in manganites is the exact origin of this insulating behavior. Here we report a dc resistivity and magnetization study on manganite La 1 − x Ca x MnO 3 ceramic samples with different grain size, at the very boundary between CO/AFM insulating and FM metallic phases x = 0.5 . Clear signatures of variable range hopping (VRH) are discerned in resistivity, implying the disorder-induced (Anderson) localization of conducting electrons. A significant increase of disorder associated with the reduction in grain size, however, pushes the system in the opposite direction from the Anderson localization scenario, resulting in a drastic decrease of resistivity, collapse of the VRH, suppression of the CO/AFM phase and growth of an FM contribution. These contradictory results are interpreted within the standard core-shell model and recent theories of Anderson localization of interacting particles. [ABSTRACT FROM AUTHOR]

We explore the structural and magnetic tunability of polycrystalline La 0.2 Sr 0.8 MnO 3 sample by varying preparation conditions using solid state reaction method. Low temperature X-ray diffraction studies indicate that one of the samples shows a transition from cubic Pm-3m structure to dominant tetragonal I4/mcm structure below ∼ 260K, while the other sample exhibits transformation from cubic structure to a dominant phase with fundamental reflections corresponding to P4/mmm tetragonal symmetry below ∼ 260K, followed by an incomplete transition from this phase to tetragonal I4/mcm phase below ∼ 225K. The Rietveld analysis at 260K indicates that the former sample exhibits co-existence of cubic Pm-3m, tetragonal I4/mcm and P4/mmm phases while the latter sample shows the co-existence of Pm-3m and tetragonal P4/mmm phases. Density functional theory(DFT+U) calculations suggest C-type antiferromagnetic and d 3 z 2 - r 2 orbital ordering in I4/mcm structure and A-type antiferromagnetic with d x 2 - y 2 orbital ordering in P4/mmm structure. The results show a possibility of tuning magnetic structure from C-type antiferromagnetic to A-type antiferromagnetic in an intermediate temperature range at fixed concentration, by varying the strain fields. At high temperatures(300–518K), the resistivity follows small polaron hopping model in both the samples and change in preparation conditions leads to a change over from adibatic to non-adibatic limit.Above 518K, both the samples exhibit metallic resistivity. Our results open a new avenue for tuning magnetic and structural phases in phase separated systems, and such tunability may be useful in antiferromagnetic spintronic, designing correlated electron-based electronic devices. [ABSTRACT FROM AUTHOR]

In Pr0.7Ca0.15Ba0.15MnO3 manganite, the spin dynamics is studied by the method of electron paramagnetic resonance (EPR). The analysis showed that at the temperature ТL ≈Tmin, which corresponds to the minimal width of the EPR line, the shape of lines of absorption spectra is perfectly described by a single Lorentzian. At other temperatures (not ТL), the absorption lines of the EPR spectra are asymmetric, which is caused by their exchange broadening at Т < ТL or hopping conductivity at T > ТL. [ABSTRACT FROM AUTHOR]