Charge-ordered Manganites Research Articles

Cationic mismatch effect on structural and magnetic behavior of La0.5-xEuxCa0.5-yBayMnO3 charge-ordered manganites

In this research, we have tried to investigate the effect of the cationic disorder on charge ordering and magnetic phase separation in La0.5-xEuxCa0.5-yBayMnO3 (x = 0, 0.05, and 0.1) compounds synthesized by using the sol-gel method. All the samples possess the same average ionic radius at A site <rA> and the same Mn3+/Mn4+ ratio. Our analysis confirmed the chemical composition by energy dispersive X-ray spectroscopy and verified the structural integrity and purity of the phase by X-ray diffraction, revealing an orthorhombic structure with Pnma space group, which is in agreement with the tolerance factor. The important mismatch effect observed in the substituted specimens induces drastic effects on the structural and microstructural properties. Magnetic measurements indicate a great reduction of Curie temperature for substituted samples (from 250 K for x = 0 to 90 K for x = 0.05 and 85 K for x = 0.1), which indicates a weakening of ferromagnetic double exchange interactions induced by cationic disorder. The Arrott plots and the magnetocaloric study of the Eu−based samples indicate the persistence of the antiferromagnetic charge-ordered state characterizing the pristine compound (x = 0) as well as the presence of a first-ordered metamagnetic transition for all the studied samples, which is ascribed to the presence of antiferromagnetic domains. Such transition induced the appearance of a shoulder peak near 100 K in the magnetic entropy change curves for high magnetic field values, resulting in an extra cooling efficiency.

Coherent acoustic modulation and defect-sensitive ultrafast carrier dynamics of Pr0.5Ca0.5MnO3 thin films investigated by time-resolved terahertz spectroscopy

In transition metal oxides, the potential of competing energetics of interacting fundamental entities is best displayed in perovskite manganites via the formation of a variety of exotic phases; however, there are limitations of extreme sensitivity to extrinsic and intrinsic defects and the slightest of structural modulations. Here, we report the effect of oxygen annealing and epitaxial strain on the ultrafast carrier excitation and relaxation mechanism in charge-ordered (CO) manganite Pr0.5Ca0.5MnO3 (PCMO) thin films of 60 and 150 nm thicknesses, both as-grown and oxygen annealed, as investigated by optical pump–terahertz (THz) probe measurements. Transient THz transmittance is negative for both films. Bi-exponential relaxation behavior accompanied with acoustic modulations was observed that varies along with strain and oxygen content of the films. As fitted by the sum of exponentials, the fast relaxation time constant is found to be fluence independent, while the slow relaxation time constant decreases with pump fluence for both films and is less for the annealed film suggesting that the relaxation in PCMO strongly depends on strain and oxygen content. This study on non-equilibrium carrier dynamics depicting the sensitivity of defects and subtle structural modifications is unprecedented in demonstrating the ultrafast control of CO manganites.

Correlating X-ray absorption spectra and ultraviolet photoelectron spectra to understand magnetic and transport properties of charge-ordered perovskite manganites

X-ray absorption spectra and ultraviolet photoelectron spectra have been used to compare the electronic structures of (La0.6Pr0.4)0.65Ca0.35MnO3 and (La0.4Pr0.6)0.65Ca0.35MnO3 systems to understand their magnetic and transport properties. Structural analysis showed that increased Pr-substitution triggers expansion in apical Mn-O bond lengths, which eventually causes narrowing of the eg band width. (La0.6Pr0.4)0.65Ca0.35MnO3 exhibits ferromagnetic ordering below 215 K and shows semiconductor to metal transition around 212 K, while (La0.4Pr0.6)0.65Ca0.35MnO3 exhibit complex magnetic behaviour and also show local semiconductor to metal transition around 95 K before re-entering into semiconducting region again below 80 K. A slight change in the slope of resistivity around 220 K and four orders increase in resistivity indicates presence of charge-ordering in (La0.4Pr0.6)0.65Ca0.35MnO3. Moreover, (La0.4Pr0.6)0.65Ca0.35MnO3 exhibits ~ 100% magneto-resistance. X-ray absorption studies of Mn-L2,3 edge and O-K edge reveal the onset of Mn3+-Mn4+ charge-ordering in (La0.4Pr0.6)0.65Ca0.35MnO3 at room temperature. O-K edge spectra exhibits an increased spectral intensity of O2p-Mn3d(eg) states near Fermi-level for (La0.4Pr0.6)0.65Ca0.35MnO3, which reflects the number of unoccupied eg states. Valence band ultraviolet photoemission spectroscopy study revealed that Mn3d(eg) states of (La0.6Pr0.4)0.65Ca0.35MnO3 extends into conduction band by crossing-over the Fermi-level. On the other hand, (La0.4Pr0.6)0.65Ca0.35MnO3 exhibits no electronic state at Fermi-level explaining stronger semiconducting nature of x = 0.6 system than x = 0.4 near room temperature.

Atomic-Resolution Cryogenic Scanning Transmission Electron Microscopy for Quantum Materials.

ConspectusThe rich physics permeating the phase diagrams of quantum materials have commanded the attention of the solid-state chemistry, materials science, and condensed-matter physics communities, sparking immense research into quantum phase transitions including superconducting, ferroic, and charge-order transitions. Many of these transitions occur at low temperatures and involve electronic, magnetic, or lattice order, which emerges on the atomic to mesoscopic scales. The complex interplay of these states and the heterogeneity that arises due to competition and intertwining of phases, however, is not fully understood and requires probes that capture ordering over multiple length scales down to the local atomic symmetries. Advances in scanning transmission electron microscopy (STEM) have enabled atomic-resolution imaging as well as mapping of functional picometer-scale atomic displacements inside materials. In this Account, we discuss our group's work to expand the reach of atomic-resolution STEM to cryogenic temperatures (cryo-STEM) to study quantum materials with focus on charge-ordered systems.Charge-ordered phases, in which electrons as well as the atomic lattice form periodic patterns that lift the translational symmetries of the crystal, are not only intertwined with superconductivity but also underlie other exotic electronic phenomena such as colossal magnetoresistance and metal-insulator transitions. The periodic lattice distortions (PLDs) modulate the positions of the crystal's nuclei, which can be readily probed by electron microscopy. In a set of examples, we demonstrate cryo-STEM as a powerful technique for probing local order, nanometer-scale heterogeneities, and topological defects in charge-ordered manganites and in transition metal dichalcogenide charge density wave (CDW) systems.With the nearly commensurate-to-commensurate CDW transition upon cooling in 1T-TaS2, we show that nanoscale lattice textures in CDW phases can be revealed through direct imaging. These early atomic-resolution results, however, also highlighted the need for improvements in cryo-STEM imaging, which led to a push to advance data collection and analysis for direct spatial mapping and quantification of PLDs. By introducing an image registration algorithm developed specifically to accommodate fast, low signal-to-noise image acquisitions of crystalline lattices, we address previous limitations due to sample drift in cryo-STEM experiments. This has enabled subangstrom cryo-STEM imaging with sufficient signal-to-noise to reveal the low temperature structure of 1T'-TaTe2. Furthermore, it allows mapping and quantification of PLD atomic displacements in the charge-ordered manganites Bi0.35Sr0.18Ca0.47MnO3 and Nd0.5Sr0.5MnO3 with picometer precision at ∼95 K to resolve not only distinct ordered phases (i.e., site- and bond-centered charge order) but also their nanoscale coexistence within the same sample.Atomic-resolution cryo-STEM opens new opportunities for understanding the microscopic underpinnings of quantum phases. In this Account, we focus on spatial mapping of lattice degrees of freedom in phases that are present at temperatures down to liquid nitrogen. Further advances in instrumentation are needed to expand the temperature range and to also enable atomic-resolution measurements that rely on weaker signals such as electron energy loss spectroscopy (EELS) for probing of electronic structure or 4D-STEM approaches to map electric and magnetic fields.

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.

Charge Ordering in Manganite and Ferrite Systems

Charge Ordering in Manganite and Ferrite Systems

Structural, magnetic, and dielectric properties of charge-order phases in manganite La(Ca0.8Sr0.2)2Mn2O7

Charge-ordered layered manganites ReA2Mn2O7 (Re = rare-earth species and A = Ca, Sr, Ba, etc.) are believed to offer a number of fascinating electronic and magnetic properties, including the long-time claimed but not yet confirmed ferroelectricity associated with charge-ordering. Experimental observations of the charge-order induced transport and electrically polar behaviors have been insufficient. In this work, we synthesize the La(Ca0.8Sr0.2)2Mn2O7 (LCSMO) single crystal and investigate its structural, magnetic, and dielectric properties. It is revealed that LCSMO undergoes two consecutive charge-ordering transitions upon decreasing temperature T before entering an antiferromagnetic state in the low-T range. The first charge-order transition occurs at temperature TCO1 ∼ 314 K from the high-T paramagnetic state. This charge-order state (CO1 state) is transferred into another charge-order state (CO2 state) by a sequence starting from ∼290 K, and the resultant CO2 state is dynamic and polar-like. The dynamic behaviors of this polar-like CO2 state is confirmed by the remarkable dielectric relaxation associated with this state. The present work provides a connection between the charge-ordering and electrically polar response in LCSMO, while ferroelectricity remains yet to be an issue.

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.

Orbital Ordering Effect on Magnetic Resonance Spectra in Charge-Ordered and Frustrated Manganites

A theoretical study of the interrelation of the crystal structure, charge, orbital, and magnetic subsystems in R1 – xAxMnO3 charge-ordered manganites has been carried out (where R3+ is the rare earth ion, A2+ is the alkaline earth metal ion, x = 0.5, 2/3). The model of orbital-dependent exchange interactions and single-ion anisotropy is used. The presence of quasi-low-dimensional magnetic structures is exhibited. The spin waves spectra and antiferromagnetic resonance spectra are calculated.

Влияние орбитального упорядочения на спектры магнитного резонанса в зарядово-упорядоченных и фрустрированных манганитах

A theoretical study of the interrelation of the crystal structure, charge, orbital, and magnetic subsystems in R1–xAxMnO3 charge-ordered manganites has been carried out (where R3+ is the rare earth ion, A2+ is the alkaline earth metal ion, x = 0.5, 2/3). The model of orbital-dependent exchange interactions and single-ion anisotropy is used. The presence of quasi-low-dimensional magnetic structures is exhibited. The spin waves spectra and antiferromagnetic resonance spectra are calculated.

Charge orbital and spin ordering transitions in La1-xSrxMnO3+δ (x = 0.67 & 0.71)

Structural and magnetic phase-transitions in La1-xSrxMnO3+δ (x = 0.67 & 0.71) have been studied employing x-ray diffraction, neutron diffraction and transmission electron microscopy (TEM) in conjunction with electrical-transport and magnetic studies. These studies reveal a cubic to incommensurate (IC) charge-ordered (CO) tetragonal phase-transition in both the samples with k-vector varying with temperature and thus indicates charge density wave-like nature. Followed by magnetic transition from paramagnetic phase to incommensurate antiferromagnetic phase. The nearly equal expansion of the a,b axes in the ab plane and the contraction of the c-axis (i.e. type-I Jahn-Teller distortion (JTD)) indicate that the CO is accompanied by a planer ferro-ordering of 3dx2−y2 orbitals in the ab plan, which is different from haring-bone type orderingof3d3x2−r2/3d3y2−r2 orbitals, commonly observed in narrow bandwidth CO manganites. For x = 0.71, a second phase-transition from type-I JTD-CO phase (c < a) to charge disorder type-II JTD tetragonal phase (c > a) was observed. Through EDAX analysis in TEM, a nano-scale marginal composition difference has been observed to lead to nanoscale phase-coexistence. These studies have revealed first microscopic evidence of CO and nano-scale multiphase coexistence in La1-xSrxMnO3-δ with x = 0.67 & 0.71.

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...

Nature and evolution of incommensurate charge order in manganites visualized with cryogenic scanning transmission electron microscopy

Incommensurate charge order in hole-doped oxides is intertwined with exotic phenomena such as colossal magnetoresistance, high-temperature superconductivity, and electronic nematicity. Here, we map, at atomic resolution, the nature of incommensurate charge-lattice order in a manganite using scanning transmission electron microscopy at room temperature and cryogenic temperature ([Formula: see text]93 K). In diffraction, the ordering wave vector changes upon cooling, a behavior typically associated with incommensurate order. However, using real space measurements, we discover that the ordered state forms lattice-locked regions over a few wavelengths interspersed with phase defects and changing periodicity. The cations undergo picometer-scale ([Formula: see text]6 pm to 11 pm) transverse displacements, suggesting that charge-lattice coupling is strong. We further unearth phase inhomogeneity in the periodic lattice displacements at room temperature, and emergent phase coherence at 93 K. Such local phase variations govern the long-range correlations of the charge-ordered state and locally change the periodicity of the modulations, resulting in wave vector shifts in reciprocal space. These atomically resolved observations underscore the importance of lattice coupling and phase inhomogeneity, and provide a microscopic explanation for putative "incommensurate" order in hole-doped oxides.

Bending and breaking of stripes in a charge ordered manganite

In charge-ordered phases, broken translational symmetry emerges from couplings between charge, spin, lattice, or orbital degrees of freedom, giving rise to remarkable phenomena such as colossal magnetoresistance and metal–insulator transitions. The role of the lattice in charge-ordered states remains particularly enigmatic, soliciting characterization of the microscopic lattice behavior. Here we directly map picometer scale periodic lattice displacements at individual atomic columns in the room temperature charge-ordered manganite Bi0.35Sr0.18Ca0.47MnO3 using aberration-corrected scanning transmission electron microscopy. We measure transverse, displacive lattice modulations of the cations, distinct from existing manganite charge-order models. We reveal locally unidirectional striped domains as small as ~5 nm, despite apparent bidirectionality over larger length scales. Further, we observe a direct link between disorder in one lattice modulation, in the form of dislocations and shear deformations, and nascent order in the perpendicular modulation. By examining the defects and symmetries of periodic lattice displacements near the charge ordering phase transition, we directly visualize the local competition underpinning spatial heterogeneity in a complex oxide.

Role of the stability of charge ordering in exchange bias effect in doped manganites

In this work we have carried out an elaborate study on the magnetic properties and investigated the exchange bias phenomena of some charge-ordered (CO) manganites. The detailed study of Sm1−xCaxMnO3 (x = 0.5, 0.55, 0.6, 0.65, 0.7) compounds shows that Sm0.4Ca0.6MnO3, which is the most robust charge ordered material studied here, shows significantly large exchange bias field (HE) as compared to the other compounds. Our experimental results and analysis indicate that TCO, which reflects the stability of the charge-ordered state, is one of the key parameters for the exchange bias effect. Similar behaviour is found in other rare-earth analogues, viz., La1−xCaxMnO3 and Pr1−xCaxMnO3 compounds as well. We also found that with increasing stability of CO states in Sm1−xCaxMnO3 compounds, HE enhances due to increase in number and reduction in size of ferromagnetic clusters.

Particle size dependence on the structural, transport and optical properties of charge-ordered Pr0.6Ca0.4MnO3

Structural, transport and optical properties of nano-crystalline Pr0.6Ca0.4MnO3 have been investigated to emphasize on the semiconducting properties of charge-ordered manganite. Rietveld refinement of X-ray diffraction pattern of Pr0.6Ca0.4MnO3 nanoparticles show that due to increase in sintering temperature, MnO6 octahedra elongated along z-direction and compressed in x-y plane. Both Mn–O–Mn angles are found to decrease with increasing sintering temperature. Fourier transform infrared (FTIR) spectroscopy measurements reveal that the stretching and bending vibration of Mn–O–Mn is responsible for the change in Mn–O–Mn bond length and bond angle respectively. With increasing sintering temperature, these vibrations tend to increase, which resulted in the further distortion of MnO6 octahedra. Magnetic measurements suggest that charge ordering is established and system becomes antiferromagnetic with increasing particle size. Resistivity behavior of Pr0.6Ca0.4MnO3 nanoparticles clearly exhibit semiconducting nature of these systems, which is due to the formation of charge-ordered state of Mn3+ and Mn4+. Estimated optical band-gap of ∼3.7 eV for Pr0.6Ca0.4MnO3 nanocrystals, makes it a potential candidate for wide band-gap magnetic semiconductors.

ChemInform Abstract: Magnetic and Charge Ordering in Nanosized Manganites

AbstractReview: 107 refs.

Magnetic and charge ordering in nanosized manganites

Perovskite manganites exhibit a wide range of functional properties, such as colossal magneto-resistance, magnetocaloric effect, multiferroic property, and some interesting physical phenomena including spin, charge, and orbital ordering. Recent advances in science and technology associated with perovskite oxides have resulted in the feature sizes of microelectronic devices down-scaling into nanoscale dimensions. The nanoscale perovskite manganites display novel magnetic and electronic properties that are different from their bulk and film counterparts. Understanding the size effects of perovskite manganites at the nanoscale is of importance not only for the fundamental scientific research but also for developing next generation of electronic and magnetic nanodevices. In this paper, the current understanding and the fundamental issues related to the size effects on the magnetic properties and charge ordering in manganites are reviewed, which covers lattice structure, magnetic and electronic properties in both ferromagnetic and antiferromagnetic based manganites. In addition to review the literatures, this article identifies the promising avenues for the future research in this area.

Possible Existence of Two Charge-Ordered Phases in Pr1−xCaxMnO3 for 0.40 ≤ x ≤ 0.50

We report the Ca-concentration dependence of crystal structure, electrical transport and magnetic properties in the charge-ordered manganite Pr1−xCaxMnO3 for 0.40 ≤ x ≤ 0.50. We have observed an abrupt change in the thermoelectric power at x ∼ 0.425, which implies that the charge-ordered phase discontinuously changes at this composition. We further find that the lattice distortion and ferromagnetic volume fraction also change at that particular concentration. These results indicate that the checker-board-type charge order near x = 0.50 is unstable below x = 0.425. A possible origin for this phenomenon is discussed.

Study of Ferromagnetism Through Electron Self-Energy of Charge Ordered Manganites

Study of Ferromagnetism Through Electron Self-Energy of Charge Ordered Manganites