Half-doped Manganites Research Articles

Role of Griffiths phase on magnetocaloric properties and critical behavior of half-doped manganites [La0.5Sr0.5-xCaxMnO3(x = 0, 0.25, 0.5)

In this article, a comprehensive study of the effect of calcium substitution on structural magnetic, magnetocaloric properties, and role of Griffiths phase on magnetocaloric properties of half-doped manganites La0.5Sr0.5-xCaxMnO3(x=0,0.25,0.5)] have been presented. A structural phase transition I4/mcm - Pbnm is observed in the whole series. The system exhibits two magnetic transitions for Ca-00 and Ca-25, whereas only one phase transition is observed for Ca-50. Paramagnetic (PM)-ferromagnetic (FM) transition is observed in the whole series and ferromagnetic (FM) - antiferromagnetic (AFM) phase transition is only observed in Ca-25. Interestingly, short range FM clusters exist in the PM matrix of Ca-50, which is a signature of Griffiths phase (GP). The confirmation of GP has been carried out by performing iso-field magnetization measurements at 200 Oe, 500 Oe, and 1 kOe, respectively. Further magnetocaloric properties have been discussed on the basis of isothermal magnetization curves recorded at certain temperature intervals. Calcium substitution favors the enhancement of magnetic entropy change in the series, especially for Ca-50, a magnetic entropy change of 4.5J/(kgK) with relative cooling power of 180J/kg under 5T magnetic field was observed. Iso-field magnetization and magnetocaloric effect along with critical behavior analysis have been carried out to probe the GP and other inhomogeneities present in the respective series.

Exploring complex magnetic phase transitions and magnetocaloric effect in the nanocrystalline Nd0.5Ca0.5MnO[formula omitted] compound

In this article, we have investigated the temperature and magnetic field-induced complex magnetic phases and magnetocaloric properties of the nanocrystalline Nd0.5Ca0.5MnO3 (NCMO) compound in detail. The compound exhibits charge-ordered antiferromagnetic (CO-AFM) and charge-orbital-ordered antiferromagnetic (AFM) phases at TCO≃ 225 K and TN= 160 K respectively. At the intermediate temperature range, i.e. TN≤ T ≤ TCO, an unfamiliar magnetic state is present where the orbital ordering of manganese ions sets progressively. Both the short-range and zig-zag ordered ferromagnetic (FM) chains and an AFM phase coexist within this temperature window. The effect of magnetic field and temperature drastically modifies the fragile magnetic ground state and this change is also reflected in the magnetocaloric properties of the studied compound. At the low-temperature region, an inverse magnetocaloric effect (MCE) is noticed due to the kinetically arrested magnetic moments of the AFM phase. However, a conventional MCE is noticed in the high-temperature regime. The anomalous behavior of the MCE in the nanocrystalline NCMO compound is analyzed by considering the effect of the reduction of particle size and the meta-stable nature of the magnetic ground state. In addition, we have paid attention to the hysteretic losses associated with the first-order phase transition (FOPT) in order to estimate the magnetic cooling capacity for the material. These studies carry notable importance for the usefulness of nanocrystalline half-doped manganite compounds not only for basic scientific research purpose, but also having possible practical utilization in the magnetic refrigeration industries.

Magnetocaloric Effect in Half-Doped and Self-Doped Manganites: A Study to Green Refrigeration

Magnetic refrigeration has emerged as a promising and environmentally friendly technology due to its high efficiency and eco-friendly nature. It is becoming a strong competitor to traditional gas refrigeration and is often referred to as a green refrigeration technique. This technique utilizes the magnetocaloric effect (MCE) or inverse magnetocaloric effect (IMCE) to achieve a change in temperature after adiabatic demagnetization. Among various magnetocaloric materials, perovskite manganites have drawn significant attention due to their abundance and low field MCE. In this paper, a comparative study of MCE has been presented for two different types of manganites. The self-doped La0.9MnO3 shows a considerable amount of MCE (2.5 J/Kg-K) at an applied magnetic field of 10 kOe around 255 K. In contrast, Pr0.5Sr0.5Mn0.98Cr0.02O3another manganite, shows a comparably lower value of MCE (0.55 J/Kg-K) around 220 K under the same magnetic field, but it exhibits a large IMCE (1.26 J/Kg-K) around 150 K. This comparative study provides insights into the magnetocaloric properties of these manganites, which could have potential applications in green refrigeration.

Ground-state structure, orbital ordering and metal-insulator transition in double-perovskite PrBaMn2O6

In recent years, A-site ordered half-doped double-perovskite manganites $\rm RBaMn_2O_6$ (R=rare earth) have attracted much attention due to their remarkable physical properties and a prospect of application as magnetoresistance, multiferroic, and oxygen storage materials. The nature of the ground state in ${\rm RBaMn_2O_6}$ as well as sequence of phase transitions taking place at cooling are not yet well understood due to complexity in both experimental and theoretical studies. Here we address the origin of the ground-state structure in PrBaMn$_2$O$_6$ as well as its electronic and magnetic properties. Utilizing GGA+U approach and specially designed strategy to perform structural optimization, we show that the system has two competing AFM-A and AFM-CE magnetic structures with very close energies. The AFM-A structure is a metal, while AFM-CE is an insulator and the transition to the insulating state is accompanied by the charge Mn$^{3+}$/Mn$^{4+}$, and orbital $3x^2-r^2$/$3y^2-r^2$ orderings. This orbital ordering results in strong cooperative Jahn-Teller (JT) distortions, which lower the crystal symmetry. Our findings give a key to understanding contradictions in available experimental data on ${\rm PrBaMn_2O_6}$ and opens up the prospects to theoretical refinements of ground-state structures in other ${\rm RBaMn_2O_6}$ compounds.

Electronic and magnetic properties of epitaxial thin film of Nd0.5Ba0.5MnO3

Contradictory reports about the electronic and magnetic behavior of bulk Nd0.5Ba0.5MnO3 with uniform disorder exist in the literature. In this work, we investigated single crystalline, A-site disordered thin films of Nd0.5Ba0.5MnO3, grown on the SrTiO3 substrate by pulsed laser deposition. The epitaxial growth of these films in the layer-by-layer fashion has been confirmed by in situ reflection high energy electron diffraction, atomic force microscopy, x-ray reflectivity, and x-ray diffraction measurements. Interestingly, these films are found to be electrically insulating and exhibit spin-glass behavior at low temperatures. This offers a distinct approach to study Ba based half-doped manganites and an opportunity to further manipulate competing charge/orbital ordering and ferromagnetism in such systems, through heterostructure engineering.

Structural and Magnetic Properties of Nanosized Half-Doped Rare-Earth Ho0.5Ca0.5MnO3 Manganite

We investigated the structural and magnetic properties of 20 nm-sized nanoparticles of the half-doped manganite Ho0.5Ca0.5MnO3 prepared by sol-gel approach. Neutron powder diffraction patterns show Pbnm orthorhombic symmetry for 10 K < T < 290 K, with lattice parameters a, b, and c in the relationship c/√2 < a < b, indicating a cooperative Jahn–Teller effect, i.e., orbital ordering OO, from below room temperature. In contrast with the bulk samples, in the interval 250 < T < 300 K, the fingerprint of charge ordering (CO) does not manifest itself in the temperature dependence of lattice parameters. However, there are signs of CO in the temperature dependence of magnetization. Accordingly, below 100 K superlattice magnetic Bragg reflections arise, which are consistent with an antiferromagnetic phase strictly related to the bulk Mn ordering of a charge exchange-type (CE-type), but characterized by an increased fraction of ferromagnetic couplings between manganese species themselves. Our results show that in this narrow band half-doped manganite, size reduction only modifies the balance between the Anderson superexchange and Zener double exchange interactions, without destabilizing an overall very robust antiferromagnetic state.

Emergent perpendicular magnetic anisotropy at the interface of an oxide heterostructure

Controlling the magnetic anisotropy in magnetic oxides is critical for the development of oxide spintronics. Here we report on the finding of an emergent interfacial magnetism, with unique perpendicular magnetic anisotropy (PMA), at the interface of a half-doped manganite ${\mathrm{La}}_{1/2}{\mathrm{Sr}}_{1/2}\mathrm{Mn}{\mathrm{O}}_{3}$ film on $\mathrm{LaAl}{\mathrm{O}}_{3}$ substrate, through combined electron spin resonance measurements and density functional theory calculations. The interfacial magnetism can be explained by the $3{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbital-mediated ferromagnetic exchange interaction between Mn ions near the interface, which is enhanced by the compressive strain from the substrate. Our results provide a useful PMA system for possible spintronic applications, as well as atomistic-level insight to the magnetic anisotropy in strained manganite oxide interfaces.

Charge order textures induced by non-linear couplings in a half-doped manganite

The self-organization of strongly interacting electrons into superlattice structures underlies the properties of many quantum materials. How these electrons arrange within the superlattice dictates what symmetries are broken and what ground states are stabilized. Here we show that cryogenic scanning transmission electron microscopy (cryo-STEM) enables direct mapping of local symmetries and order at the intra-unit-cell level in the model charge-ordered system Nd1/2Sr1/2MnO3. In addition to imaging the prototypical site-centered charge order, we discover the nanoscale coexistence of an exotic intermediate state which mixes site and bond order and breaks inversion symmetry. We further show that nonlinear coupling of distinct lattice modes controls the selection between competing ground states. The results demonstrate the importance of lattice coupling for understanding and manipulating the character of electronic self-organization and that cryo-STEM can reveal local order in strongly correlated systems at the atomic scale.

Effect of structural deformations on the transport properties of half-doped manganite nanowires: An insight through high resolution image simulation

Study of structural deformations in manganite nanowires (NWs) are strongly required as in one dimensional (1D) NWs the structural deformations contribute hugely in controlling the electronic properties of the NWs. High resolution image simulation technique has been applied to resolve the atomic structure of a complex layered crystal, (La1−xCaxMnO3; x = 0.3 and 0.5), which comprises a high density of distorted lattice interfaces. This technique allows us to study lattice imperfections, like stacking disorder, twinning and layer undulations, with atomic scale resolution. Aberration corrected High Resolution Electron Microscopy (HREM) images directly reveals the projected crystal structure. Here it is obtained by exploring the strong suppression of image delocalization and using certain spherical-aberration coefficient (Cs) imaging conditions. It has been explained and correlated that the disordered system formation in NWs is due to the defects (termination of sites) in atomic column.

Interpretable, calibrated neural networks for analysis and understanding of inelastic neutron scattering data

Deep neural networks (NNs) provide flexible frameworks for learning data representations and functions relating data to other properties and are often claimed to achieve ‘super-human’ performance in inferring relationships between input data and desired property. In the context of inelastic neutron scattering experiments, however, as in many other scientific scenarios, a number of issues arise: (i) scarcity of labelled experimental data, (ii) lack of uncertainty quantification on results, and (iii) lack of interpretability of the deep NNs. In this work we examine approaches to all three issues. We use simulated data to train a deep NN to distinguish between two possible magnetic exchange models of a half-doped manganite. We apply the recently developed deterministic uncertainty quantification method to provide error estimates for the classification, demonstrating in the process how important realistic representations of instrument resolution in the training data are for reliable estimates on experimental data. Finally we use class activation maps to determine which regions of the spectra are most important for the final classification result reached by the network.

Designing reversible multi-level resistance states in a half-doped manganite

We design reversible multi-level resistance states in a half-doped charge-ordered manganite, (SCSMO). By exploiting the electronic phase separation in SCSMO at 10 K, we show that the system can be stabilized into several metastable states, against thermal cycling, up to 62 K. The magnetization and the resistivity remain unaltered in each metastable states during the thermal cycling. Monte Carlo calculations using a two-band double-exchange model, including super-exchange, electron-phonon coupling, and quenched disorder, show that the system freezes into a phase-separated metastable state due to the disorder during the thermal cycling. We outline pathway to control the multi-level switching between four reversible metastable states. Our results may open the door for future investigations to engineer memory devices based on multi-level resistance switching using phase-separated transition metal oxides.

Monte Carlo study of the effect of charge ordering on the electrical and magnetic properties of half-doped manganites

The Monte Carlo Antiferromagnetic Ising model was used to study the electrical properties and magnetoresistance of manganese oxides due to the charge ordering phase occurring at half doping, x = 0.5. The half-doped manganites have an insulator antiferromagnetic ground state. The internal energy, specific heat, resistivity, the magneto-resistance and magnetization have been investigated with different applied magnetic fields. Our simulation reveals that the resistivity decreases exponentially as the free charge increase to transport from the insulator to conductor phase. The magnetoresistance has positive high values with magnetic field. The obtained insulator–metal transition behavior candidates the half-doped manganites to be very good semiconductors diode junctions.

Correlations between electronic order and structural distortions and their ultrafast dynamics in the single-layer manganite Pr0.5Ca1.5MnO4

Time-resolved x-ray diffraction experiments on the half-doped single-layered manganite $\mathrm{P}{\mathrm{r}}_{0.5}\mathrm{C}{\mathrm{a}}_{1.5}\mathrm{Mn}{\mathrm{O}}_{4}$ are used to monitor the ultrafast photoinduced dynamics of the structural distortion associated with the charge and orbital ordering (CO/OO). As in the nonlayered three-dimensional counterpart, the ordered phase melts in less than 100 fs after 800-nm photoexcitation and subsequently partially recovers due to thermal equilibration of electronic and vibrational systems. Photoexciting $\mathrm{P}{\mathrm{r}}_{0.5}\mathrm{C}{\mathrm{a}}_{1.5}\mathrm{Mn}{\mathrm{O}}_{4}$ below the transition temperature of a second structural phase transition that occurs around 146 K (deep inside the CO/OO phase) releases this structural transition, but progresses on a much slower timescale. This additional reduction of crystal symmetry, which we ascribe to a further tilt of the oxygen octahedra, can thus be considered to be only weakly coupled to CO/OO. Furthermore, static hard-x-ray and resonant soft-x-ray diffraction at the $\mathrm{Mn}\phantom{\rule{0.16em}{0ex}}{L}_{2,3}$ edges experiments identify correlations between structural distortions and electronic order in thermal equilibrium.

Study of phase separation phenomena in half-doped manganites with isovalent substitution of rare-earth cations on example ofSm0.32Pr0.18Sr0.5MnO3

The effect of isovalent substitution of rare-earth cations on the phase separation in half-doped manganites was studied on the example of ${\mathrm{Sm}}_{0.32}{\mathrm{Pr}}_{0.18}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$ by high-resolution neutron powder diffraction, neutron beam depolarization, second-harmonic magnetic response, and magnetization and resistivity measurements from 4 K up to room temperature and higher. A structural phase transition from the orthorhombic $Pbnm$ phase to a mixture of $Pbnm$ and monoclinic $P{2}_{1}/m$ phases was observed upon cooling. The magnetic ground state was found to be phase-separated into three magnetic phases emerging at different temperatures, viz., ferromagnetic (FM) and antiferromagnetic A and charge-ordered CE types. FM clusters arise far above room temperature in the orthorhombic phase and coalesce upon cooling to produce the long-range FM order below 250 K and to arrive at the percolative FM phase below 120 K. The A- and CE-type orders form in the monoclinic phase at the temperatures 200 K and 120 K, respectively. The Sm/Pr isovalent substitution qualitatively changes the phase separation and significantly increases its temperature range compared to the parent compounds. The results obtained give us knowledge of phase separation occurring in systems with strong electron correlations and extend opportunities for fine-tuning of their properties.

Orbital dependence of superexchange interaction in charge-ordered manganites

The current investigation is devoted to the theoretical study of orbital structure influence upon magnetic subsystem in half-doped charge-ordered manganites. The main interaction of magnetic subsystem is superexchange interaction. It forms spin-wave dispersion dependencies. Because of charge and orbital ordering, there are a lot of superexchange parameters in these compounds. That makes the dispersion rather complicated. This work clarifies some features of dispersion in terms of orbitally-dependent superexchange interactions.

Experimental evidence of magnetic training effect of electronic phase in Nd–Ca half-doped manganite

Cooling and heating modes are shown to be able to significantly modify the magnetic and electrical properties of the half-doped perovskite manganite. The present paper reports on a precise investigation of this phenomenon (the so-called training effect) carried out on Nd0.5Ca0.5MnO3 manganite, which allow a fine tuning of the magnetic ground state. Refinement of the X-ray diffraction pattern shows that the synthesized sample is single phase and crystallizes in the orthorhombic structure with Pnma space group. Using magnetometry measurements, we have found that the sample is the seat of interesting phenomena like charge ordering, magnetic phase separation, spin-glass and inverse magnetocaloric effect. Magnetic-field-driven magnetization (M(μ0H)) measurements evidence the metamagnetic transition which not only depends on field value, but also on the thermal process (cooling or heating). Metamagnetic irreversibility in the magnetic field range (±5 T) and memory effect are observed at low temperatures due to the kinetic arrest phenomenon. It is worthwhile to mention that the coercive field increases with decreasing temperature and reaches 955 Oe at 20 K, which is sufficiently large compared with that one in the soft magnetic and makes the material quite interesting for spintronic applications. The electrical resistivity in a zero field was measured on both cooling and warming modes, and the data obtained were fitted by using different theoretical models. At low temperatures, the resistivity shows the presence of insulator-metal transition and is found to be in conformity with the magnetization data. A magnetic training effect due to the thermo-magnetic history dependent behavior is observed, where the resistivity is consistently irreversible.

Manifestation of charge/orbital order and charge transfer in temperature-dependent optical conductivity of single-layered Pr0.5Ca1.5MnO4

Half-doped single-layered manganite, has shown a charge/orbital order of the eg electrons and CE-type spin order of the t2g electrons of the Mn ions. A previous experimental study on that system, supported by a simple modelling, has suggested that the charge/orbital ordering play an important role in governing the temperature dependence of optical conductivity of a broad peak around 0.7–0.8 eV. In addition, another peak around 3.5 eV, which is less sensitive to temperature, has been attributed to the charge transfer from O-p to Mn-eg orbitals. Nevertheless, the theoretical explanation was incomplete as the role of O-p orbitals was not considered in the model. In this paper, we propose to improve the model by incorporating both Mn-eg and O-p orbitals. We assume the existence of charge/orbital ordering and investigate how this ordering as well as the charge-transfer phenomenon control the temperature dependence of the optical conductivity. Our results reveal the charge/orbital-ordering peak in the region 0.7–1.2 eV, which is blue-shifted with decreasing temperature, and the charge-transfer peak around 3.5 eV, which is less sensitive to temperature. The capability of our model to capture the general profile and temperature dependence of the optical conductivity suggests the validity of our theory.

Critical Behavior Near the Ferromagnetic Transition in Phase-Separated $$\hbox {La}_{0.5}\hbox {Ca}_{0.2}\hbox {Sr}_{0.3}\hbox {MnO}_{3}$$ La 0.5 Ca 0.2 Sr 0.3 MnO 3 Manganite

The critical behavior of $$\hbox {La}_{0.5}\hbox {Ca}_{0.2}\hbox {Sr}_{0.3}\hbox {MnO}_{3}$$ manganite has been investigated by measuring the magnetization around the Curie temperature $$T_\mathrm{C}$$ . The experimental results indicate that this sample exhibits a second-order phase transition. The critical exponents ( $$\beta $$ , $$\gamma $$ and $$\delta $$ ) are estimated using the Kouvel–Fisher technique and critical isotherm analysis. The reliability of obtained exponents was confirmed by using the universal scaling hypothesis. The obtained $$\beta $$ value ( $$=0.47$$ ) is close to that expected for the mean-field model ( $$=0.5$$ ), while the $$\gamma $$ value ( $$=1.28$$ ) lies between the 3D Ising ( $$=1.24$$ ) and 3D Heisenberg ( $$=1.336$$ ) models. This reveals that the critical exponents of this half-doped manganite do not correspond to the conventional universality classes. In addition, the results show that the ferromagnetic interaction is of itinerant type.

Crystal Structure, Magnetic and Electrical Properties of Half-Doped Chromium Manganite La0.5Sr0.5Mn0.5Cr0.5O3

Polycrystalline sample La0.5Sr0.5Mn0.5Cr0.5O3 (LSMCO) was prepared by a conventional solid-state reaction method in the air, and its structure was determined by Rietveld refinement using consistent resolution time-of-flight X-ray diffraction. Different structural models have been used for analysis of X-ray diffraction data recorded at room temperature, but the best fits have been found using a mixture of rhombohedral and an orthorhombic structural phases (42(1)%/57(1)%). The rhombohedral, centro-symmetric space group R $$ \overline{3} $$ c $$ \left(\mathrm{No}.167,a\approx \sqrt{2}{a}_p,c\approx 2\sqrt{3}{a}_p,Z=6\right) $$ with octahedral tilting scheme a−a−a− leading to a three identical out-of-phase tilt angle of (Mn/Cr)-O6 octahedron along three perovskite main directions: x, y, and z-axes. The orthorhombic space group Pbnm $$ \left(\mathrm{No}.62,a\approx b\approx \sqrt{2}{a}_p,c\approx 2{a}_p,Z=4\right) $$ with octahedral tilting scheme a−a−c+, leads to an anti-phase tilts of the same magnitude about [100] and [010] directions of the elementary specimen cell and in-phase tilt of different magnitude about the [001] direction. At low temperature 10 K, Rietveld refinement of powder diffraction data revealed that the crystal structure of sample is distorted with a single orthorhombic symmetry (space group Pbnm). Magnetic measurements indicate that our investigated sample exhibits an overall anti-ferromagnetic behavior mainly attributed to the frustration induced by both competing AFM Cr3+-O-Cr3+, Mn4+-O-Mn4+ networks, and the mixed one Mn4+-O-Cr3+exchange magnetic interactions. The temperature dependence of the resistivity measured under zero field shows a semi-conducting behavior in the whole temperature range, which probably could be described by the adiabatic small polaron hoping model with relatively high polaron activation energy.

Surface Ferromagnetism in Pr0.5Ca0.5MnO3 Nanoparticles as a Consequence of Local Imbalance in Mn3+:Mn4+ Ratio

Half-doped praseodymium manganites, Pr0.5Ca0.5MnO3, synthesized by the sol–gel method produce highly crystalline nanoparticles with no structural disorder at the surface. As for most half-doped (Mn3+:Mn4+ = 1) antiferromagnetic (AFM) charge-ordered manganites, ferromagnetism (FM) appears as particle size decreases. A possible origin of FM phase development is the lacking oxygen ligands at the surface that increases Mn3+ at the expense of Mn4+ in order to maintain electronic neutrality. In this work, we show that Mn3+:Mn4+ = 1 ratio is preserved in the whole crystalline particle with the exception of the surface, where this ratio changes to Mn3+:Mn4+ > 1. This Mn3+ excess makes a double exchange Mn3+ → O2– → Mn4+ prevail over the superexchange, giving rise to the FM interactions. We show that a shell thickness of only one unit cell, with acell = 0.38 nm, is enough to explain the onset of FM at the surface, whereas the volume remains AFM. Furthermore, the FM to AFM ratio fits to the increase of surface-to-v...