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In this work, we have studied the effect of annealing temperature (T S) on the structural properties and the critical behavior of Nd 0.5 Sr 0.5 MnO 3 compound prepared by sol-gel method (T S = 600, 700, 800 and 1000 °C). Scanning electronic microscope images indicate that all the samples are nanosized. The refinement of X-ray diffraction data shows that the samples sintered at 600 °C and 700 °C have three structural phases and that the monoclinic phase is linked to a superstructure (superlattice). The samples sintered at 800 °C and 1000 °C crystallize in the orthorhombic structure with Pnma space group, indicating the occurrence of a structural transition with increasing particle size. The magnetic measurements reveal that all our samples exhibit a paramagnetic to ferromagnetic transition with the absence of the long-range charge-ordered antiferromagnetic state. Through the magnetic field dependent magnetization, the observed overlapping between the isotherms suggests the persistence of some antiferromagnetic ordering in the nanosized samples. The reduction of saturation magnetization at 5 K for the sample sintered at 1000 °C indicates the beginning of the establishment of long-range charge ordering. The study of critical exponents has shown that the mean-field model is the most suitable model to describe the magnetic interactions for all our samples, suggesting the presence of long-range ferromagnetic interactions. [ABSTRACT FROM AUTHOR]

In this study, La0.63Sr0.2Nd0.17MnO3 was synthesized by the sol-gel method, and its structural, morphological, magnetic, and magnetocaloric effects were investigated. The structural properties were analyzed by x-ray diffraction (XRD), and scanning electron microscopy (SEM) was used to characterize the morphology. An integrated magnetic measurement system was used to determine the magnetic properties. La0.63Sr0.2Nd0.17MnO3 crystallized in a hexagonal crystal system with space group R-3c. This was also confirmed by Rietveld refinement of the x-ray data from La0.63Sr0.2Nd0.17MnO3. With the doping of Nd3+, the cell volume decreases, which can be explained by the angles and bond lengths of the bonds between the Mn and O ions and the distortion of the lattice. Near the Curie temperature, La0.63Sr0.2Nd0.17MnO3 exhibits significant magnetocaloric effects. The magnetic study of La0.63Sr0.2Nd0.17MnO3 suggests that the transition from the paramagnetic to ferromagnetic phase can occur near the Curie temperature. The maximum magnetic entropy change and relative cooling power (RCP) of La0.63Sr0.2Nd0.17MnO3 (298 K, 3 T) are 2.70 J/(kg K) and 135 (J/kg), respectively. The effect of f-orbitals on the magnetic properties of La0.8Sr0.2MnO3 was investigated by first-principles calculations in density functional theory. Thus, it can be concluded that the magnetocaloric effects of the material after doping with f-orbital ions (Nd3+) is enhanced compared to La0.8Sr0.2MnO3. [ABSTRACT FROM AUTHOR]

This study investigates the critical behavior analysis and predicting magnetocaloric properties of La0.6-xGdxSr0.4MnO3 manganites, focusing on the impact of varying gadolinium (Gd) doping levels (x = 0, 0.0125, 0.05, and 0.10). Using modified Arrott plots (MAPs) and the Kouvel–Fisher method, the critical exponents γ and β were determined, yielding values that indicate how Gd content influences magnetic phase transitions. Additionally, the magnetocaloric effect (MCE) was analyzed to assess the magnetic entropy change − Δ S M (T) and the Temperature-Averaged Entropy Change (TEC) across different magnetic field changes. The results show that increasing Gd doping enhances the magnetocaloric response, suggesting potential applications in magnetic refrigeration. Overall, the study provides insights into optimizing these manganites for applications in high-temperature sensors, spintronics, and magnetic cooling. Highlights: Using modified Arrott plots (MAPs) and the Kouvel–Fisher method, the critical exponents γ and β of La0.6-xGdxSr0.4MnO3 (x = 0, 0.0125, 0.05, and 0.10) manganites were derived. The magnetocaloric effect (MCE) was analyzed to assess the magnetic entropy change − Δ S M (T) and the Temperature-Averaged Entropy Change (TEC) across different magnetic field changes. [ABSTRACT FROM AUTHOR]

The crystal structure of La 0.7 Sr 0.3 Mn 1−x Al x O 3 (LSMAO) with x = 0,0.025,0.05,0.075 and 0.10 ceramics prepared by the Sol-gel method were investigated by employing the XRD technique. The XRD patterns analysis by employing the Rietveld analysis confirms the nonexistence of secondary crystal phase/s and, materials are crystallized to the R-3c(167) space group. The microstructural properties were studied using scanning electron microscope (SEM) and transmission electron microscope (TEM). The dielectric property, AC conductivity, complex impedance spectroscopy and complex electrical modulus have been studied over a wide range of frequencies (103 Hz–107 Hz) at different temperatures from 313K to 393K. The Non-Debye type relaxation process has been observed in synthesized samples confirmed from the Nyquist plot. The Cole-Cole plots were employed to estimate the individual grain and grain boundary contributions. The Maxwell-Wagner (MW) polarization model provides an explanation for the greater values of dielectric constant at the lower frequencies. The present study explores the technological usefulness of LSMAO. [ABSTRACT FROM AUTHOR]

Swift heavy ion (SHI) irradiated LaMnO 3 /La 0.7 Ca 0.3 MnO 3 (LMO/LCMO) interfaces have been investigated to understand their charge transport and conduction mechanisms. Prominent effect of ion fluence has been discussed for the interface resistivity behaviors and metal to insulator phase transition for LMO/LCMO interfaces on the bases of lattice strain, crystalline granular state, crystalline imperfection, average grain size, grain boundary density and rms surface roughness. Field effect configuration (FEC) has been employed to study the interesting modifications in the irradiated LMO/LCMO interface resistivity under different applied electric fields to understand the depletion region, free charge carrier density, ferromagnetic phase fractions and crystal field based distortions. A Phenomenological percolation model has been employed to understand the role of phase separation in governing the charge conduction processes across LMO/LCMO interfaces. [ABSTRACT FROM AUTHOR]

Doped calcium manganite perovskites CaMnO 3− δ which constitute some of the most promising materials for all-oxide thermoelectric generators for conversion of heat to electricity operating in air over a wide temperature range have been modified by Sr–Pb–Ho co-doping. The Ca 0.5 Sr 0.45− x Pb 0.05 Ho x MnO 3− δ samples (x = 0.05, 0.10, 0.15 and 0.20) synthesized by the citrate-nitrate method of precursor preparation are shown to have an orthorhombic crystal structure (space group Pbnm) at room temperature. The relationship between temperature, concentration of donor ions, Jahn-Teller distortions and energy parameters of carrier formation and mobility that affect the thermoelectric properties of electron-doped manganites has been studied. The temperature-activated nature of the electrical conductivity (σ) was revealed to be consistent with the adiabatic mechanism of small-radius polaron transport. The increase in the absolute values of the Seebeck coefficient (S) with rising temperature and the decrease in σ above 500 K are attributed to the decrease in the concentration of Mn3+ ions due to their disproportionation into Mn2+ and Mn4+ ions. The equilibrium constants of the disproportionation reaction, carrier concentration and mobility have been derived from the temperature dependencies of S (T) and σ (T) in the conditions when δ ≈ 0. The best values of power factor (S 2 σ) and dimensionless thermoelectric figure-of-merit (ZT = S 2 σT / κ) have been found at the substitution level x = 0.15. [ABSTRACT FROM AUTHOR]

One of the most fascinating aspects of condensed matter is its ability to conduct electricity, which is particularly pronounced in conventional metals such as copper or silver. Such behavior stems from a strong tendency of valence electrons to delocalize in a periodic potential created by ions in the crystal lattice of a given material. In many advanced materials, however, this basic delocalization process of the valence electrons competes with various processes that tend to localize these very same valence electrons, thus driving the insulating behavior. The two such most important processes are the Mott localization, driven by strong correlation effects among the valence electrons, and the Anderson localization, driven by the interaction of the valence electrons with a strong disorder potential. These two localization processes are almost exclusively considered separately from both an experimental and a theoretical standpoint. Here, we offer an overview of our long-standing research on selected organic conductors and manganites, that clearly show the presence of both these localization processes. We discuss these results within existing theories of Mott–Anderson localization and argue that such behavior could be a common feature of many advanced materials. [ABSTRACT FROM AUTHOR]

Here, we report the results of a Mössbauer study on hyperfine electrical and magnetic interactions in quadruple perovskite BiMn7O12 doped with 57Fe probes. Measurements were performed in the temperature range of 10 K < T < 670 K, wherein BiMn6.9657Fe0.04O12 undergoes a cascade of structural (T1 ≈ 590 K, T2 ≈ 442 K, and T3 ≈ 240 K) and magnetic (TN1 ≈ 57 K, TN2 ≈ 50 K, and TN3 ≈ 24 K) phase transitions. The analysis of the electric field gradient (EFG) parameters, including the dipole contribution from Bi3+ ions, confirmed the presence of the local dipole moments pBi, which are randomly oriented in the paraelectric cubic phase (T > T1). The unusual behavior of the parameters of hyperfine interactions between T1 and T2 was attributed to the dynamic Jahn–Teller effect that leads to the softening of the orbital mode of Mn3+ ions. The parameters of the hyperfine interactions of 57Fe in the phases with non-zero spontaneous electrical polarization (Ps), including the P1 ↔ Im transition at T3, were analyzed. On the basis of the structural data and the quadrupole splitting Δ(T) derived from the 57Fe Mössbauer spectra, the algorithm, based on the Born effective charge model, is proposed to describe Ps(T) dependence. The Ps(T) dependence around the Im ↔ I2/m phase transition at T2 is analyzed using the effective field approach. Possible reasons for the complex relaxation behavior of the spectra in the magnetically ordered states (T < TN1) are also discussed. [ABSTRACT FROM AUTHOR]

In this work, we present a detailed investigation of the electrical and optical properties of Pr0.5−xGdxSr0.5MnO3 (0 ≤ x ≤ 0.1) polycrystalline compounds synthesized via a solid-state route. Impedance spectroscopy measurements were performed in the frequency and temperature ranges of 40 Hz–10 MHz and from 80 to 320 K, respectively. DC conductance (GDC) results evince the semiconductor behavior of all studied compounds in the whole temperature range of study. Doping with gadolinium causes a lowering in GDC values and an increase in the activation energies. GDC is shown to be governed by the small polaron hopping model (SPH) at high temperatures and by the variable range hopping (VRH) model at lower ones. The AC measurements show a metallic behavior which can be described by the simplified Drude model. The optical properties were characterized using UV–Vis absorption spectroscopy. By analyzing the UV absorption measurements and based on the Tauc model, we have determined the optical bandgap of each compound evaluated at around 4.5 eV which is a high value compared with those obtained in the literature. The Urbach energy, the optical extinction coefficient, and the refractive index were calculated on the basis of absorbance measurements. In addition, the skin depth, optical conductivity, and the dispersion energy parameters as a function of the wavelength of the incident photon were determined and discussed. These compounds may be interesting in optoelectronic applications. [ABSTRACT FROM AUTHOR]

Phase equilibria in the Li–Mn–Eu–O system were studied for the first time in the temperature range 700–1000°С, and a concentration diagram was constructed within the Li–Mn–Eu triangle at an oxygen partial pressure of 21 kPa. It was shown that the LiEuO2–Li2MnO3 section can be represented as quasi-binary, unlike the sections LiEuO2–LiMnO2 and LiEuO2–LiMn2O4. It was determined that the isomorphic substitution of Eu for Mn in spinel LiMn2O4 (Fd m) does not exceed 2 mol %, while Li2MnO3 (C2/m) loses its single-phase character. [ABSTRACT FROM AUTHOR]

The structural, transport and magnetic properties of La0.67Sr0.33MnO3 with additives of ZrO2 and Bi2O3 were investigated and the results are presented. The studied samples showed a ferromagnetic-to-paramagnetic phase transition around 383 K, and a sharp transition was observed for the La0.67Sr0.33MnO3/Bi2O3 sample. A soft ferromagnetic behavior at room temperature was observed in all the La0.67Sr0.33MnO3 samples: however, the sample with bismuth oxide additive exhibited higher saturation magnetization and an enhanced magnetocaloric effect. In addition to a metal–insulator transition, a broad peak was observed in the ferromagnetic metallic region due to inhomogeneities arising from the antiferromagnetic interactions in the samples, and this disappeared with the application of a magnetic field. Positive magnetoresistance was observed for ZrO2 added to La0.67Sr0.33MnO3 for temperatures greater than 200 K. [ABSTRACT FROM AUTHOR]

This study investigates the interaction between Co and Mn ions on spin state behaviour and magnetic frustration in hole-doped La0.6Dy0.1Ca0.3Co1-xMnxO3 (x = 0-1.0) cobaltite. It aims to understand the inducement of the double-exchange (DE) mechanism in this system. The samples were synthesised using the conventional solid-state synthesis method, and their crystal structure and magnetic properties were analysed. Rietveld refinement of X-ray diffraction data confirmed that La0.6Dy0.1Ca0.3Co1-xO3 (x = 0.0 and 0.2) samples had rhombohedral crystals structure with R c - 3 space group. An orthorhombic (Pbnm) perovskite structure was observed for the La0.6Dy0.1Ca0.3Co1-xMnxO3 (x = 0.4 -1.0) accompanied by increasing cell volume in the samples as the substitution level of Mn increases. Temperature dependence of resistivity p(T) shows semiconducting behaviour in the whole temperature and composition range of x = 0.0-0.8. However, further substitution with x = 0.9 and x = 1.0 give remarkable induced MI behaviour where the MI transition temperature (TMI) increased from 75 K and 132 K, respectively. The result suggests that Mn ions play an important role in the magnetic properties of the cobaltite, and the system can be tuned by adjusting the Mn concentration. [ABSTRACT FROM AUTHOR]

Pr0.67Ba0.33Mn1-xTixO3 (x = 0 and 0.02) were synthesized using a conventional solid-state synthesis method to investigate the effect of Ti substitution on their magnetic and electrical transport properties. All the samples were structurally evaluated by XRD diffraction Rietveld refinement method which showed an increase in unit cell volume with increasing Ni content, indicating that Ti is partially substituted at Mn. Scanning electron microscopy (SEM) and energy dispersive X-tray (EDX) are used to examine the surface morphology and identified elements in the samples' compounds. Fourier transform infrared spectroscopy (FTIR) reveals that all the samples exhibit a transmission band in the range of 590 cm-1 - 610 cm-1. For x = 0, magnetization measurements showed paramagnetic (PM) to ferromagnetic (FM) transition at the transition temperature, TC ~ 213 K. For Ti-substituted samples, ferromagnetic (FM) to PM transition was reduced with Curie temperature (TC), decreasing from 213 K (x = 0) to 205 K (x = 0.02). On the other hand, the M(H) showed the presence of a linear graph for x = 0 and x = 0.02 which may be related to the presence of paramagnetic at room temperature. [ABSTRACT FROM AUTHOR]

The present report focuses on the successful synthesis of La0.7Ca0.3−xAgxMnO3 (x = 0, 0.10, 0.15, 0.20, and 0.30) polycrystalline manganite samples through a soft chemical polymeric precursor route and subsequent impact of Ag doping and grain size on their magnetotransport features. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses reveal that Ag doping leads to a phase transformation from the orthorhombic phase to the rhombohedral phase (for x ≥ 15%). Furthermore, it shows that the insulator–metal transition temperature (TIM) and paramagnetic–ferromagnetic (PM-FM) transition temperature (TC) increase with Ag doping concentration and also with the sintering temperature. The prime factors leading to the enhancement with Ag doping are the well-known oxygenation effect by metallic Ag, which helps to improve the transport properties of La1−xCaxMnO3 (LCMO) manganite, and the increase in the tolerance factor (τ), which in turn leads to the Mn-O-Mn bond angle and the structural disorder near the grain boundaries that weaken the double exchange. The room temperature magnetoresistance values are found to be higher for Ag-doped LCMO samples than for the pristine LCMO. The enhanced ferromagnetic ordering temperature along with low-field magnetoresistance (LFMR) of the as-synthesized Ag-doped LCMO polycrystalline ceramic indicate its potential for device fabrication. [ABSTRACT FROM AUTHOR]

Highlights: Orthorhombic Eu1−xYxMnO3 (0.0 ≤ x ≤ 0.5) thin films were successfully synthesized by spin-coating. The film phase evidences an additional contribution to lattice strain with increasing Y-content. Appreciable modification of magnetic response from bulks to films due to structural distortions. The effects on the structure and magnetic properties of Eu1−xYxMnO3 (0.0 ≤ x ≤ 0.5) thin films due to lattice strain were investigated and compared with those obtained in equivalent composition ceramics. The films were deposited by spin-coating chemical solution onto Pt\TiO2\SiO2\Si (100) standard substrates. X-ray diffraction and Raman spectroscopy measurements revealed that all films crystallize in orthorhombic structure with space group Pnma, observing an added contraction of the unit cell with increasing Y-substitution ou Eu, corresponding to a broadening of the Mn-O1-Mn angle and a gradual decrease in magnetic order response. [ABSTRACT FROM AUTHOR]

A La0.7Sr0.3−xSmxMn0.95Ni0.05O3 (x = 0, 0.05, 0.10, 0.15) series of manganites were synthesized by the Pechini sol–gel method. The changes in structure and magnetic properties caused by the substitution of Sr2+ by Sm3+ were systematically investigated. The results of Rietveld refinement using GSAS (General Structure Analysis System) software showed that the samples had a rhombohedral structure before and after the substitution, but the cell volume increased. In addition, the Curie temperature (TC) of the samples gradually decreased with increased doping, and the magnetic entropy change ( Δ S M ) showed the same trend. This can be attributed to the weakening of the ferromagnetic coupling between Mn4+ and Mn3+. For the doped sample with x = 0.05, the TC was 300 K, and the maximum magnetic entropy change ( - Δ S M max ) reached 4.59 J kg−1 K−1 at an applied magnetic field of 5 T. The relative cooling power (RCP) also reached 297.39 J kg−1, which is certainly promising for meeting the need for room-temperature magnetic refrigeration. [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. [ABSTRACT FROM AUTHOR]

Industrial effluents containing dyes are a serious threat to the ecosystems and the removal of these dyes is most often conducted by adsorption. In this study, Sr-doped lanthanum (LSMO), calcium (CSMO) and barium (BSMO) manganites were prepared by the autocombustion and co-precipitation synthesis, and tested as the adsorbents for the removal of methyl orange (MO) as a model pollutant from the aqueous solution. The highest adsorption of the MO solution obtained by CSMO adsorbents was 83.5%, while the highest adsorption of 95% was achieved using LSMO and BSMO due to their large specific surface area and average pore diameter promoting the diffusion of the MO to the adsorbent surface. The oxygen nonstoichiometry of these materials also contributed to their adsorption properties due to the presence of unpaired electrons, which affected their electronic structure and lowered the adsorption energy. Kinetic studies showed that Ho’s pseudo-second-order model exhibited the best fit with the experimental data, indicating that chemisorption is the rate-limiting step. The rate constants were the highest for BSMO adsorbents ranging from 3.35 to 3.50 × 10–2 g mg–1·min–1. The adsorbents can be fully regenerated by calcining at 700°C for 2 h with no loss of their activity. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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]

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]

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.

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.

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]

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]

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]

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]

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.

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]

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]

A polycrystalline ceramic of Pr0.7Ca0.1Sr0.2MnO3 was prosperously synthesized by the solid-state reaction method. The M(T, H) data measured at different temperatures in applied magnetic fields H = 0–30 kOe showed that the material has a ferromagnetic-paramagnetic phase transition and exhibits the crossover of the first and second order magnetic phase transitions. Above Curie temperature (TC), the χ−1(T) curve witnesses a downturn in the paramagnetic region, suggesting an appearance of the Griffiths phase. Using the modified Arrott plots and the Kouvel-Fisher methods, we have identified a set of the critical parameters (TC, β, γ, and δ) for Pr0.7Ca0.1Sr0.2MnO3 compound. The results are found to be TC = 205.5 ± 0.1 K, β = 0.191 ± 0.010, γ = 0.856 ± 0.016 and δ = 5.649 ± 0.030. It can be seen that the values of the exponents β, γ, and δ are quite close to the exponents of the tricritical mean field model. Additionally, they ​​satisfy the scaling theory well, proving their reliability. Based on obtained results, we suggest that the ferromagnetic-paramagnetic transition in Pr0.7Ca0.1Sr0.2MnO3 compound is located at a crossover of the first and second order transformations.

La0,7Ca0,3MnO3 polycrystalline were synthesized from La2O3, CaO and MnO2 powder mixture using a solid state reaction technique. The compound powders were obtained through the free sintering method at different temperatures and sintering times in order to study the influence of technological conditions on Ca doped La manganites. The most important physical features as structure, microstructure and morphology were described after X-ray diffraction investigation. Photographs of the specimen fractures were taken with SEM (scanning electron microscope) and they revealed high porosity of the tested material and great tendency for its grains to create agglomerates. Influence of doping and technological conditions on lattice parameters were studied by means of Rietvield analysis. The XRD measurements reveal that La0,7Ca0,3MnO3 has orthorhombic symmetry with Pnma space group.