Charge-orbital Ordering Research Articles

Effects of strain on the magnetic and transport properties of the epitaxial La0.5Ca0.5MnO3 thin films

The epitaxial strain can considerably modify the physical properties of thin films compared to the bulk. This paper reports the effects of substrate-induced strain on La0.5Ca0.5MnO3 (LCMO) thin films, grown on (100) SrTiO3 (STO) and LaAlO3 (LAO) substrates by pulsed laser deposition technique. Transport and magnetic properties were found to be strongly dependent on strain type. It is also shown that compressive (tensile) strain leads to the increase (decrease) in the magnetization of the films. Moreover, it was observed that all LCMO films deposited on both LAO and STO substrates behave as an insulator, but LCMO/LAO thin films with compressive strain have lower resistivity than LCMO/STO thin films with tensile strain. Applying magnetic field to LCMO/STO thin films with thickness of 25 and 50nm leads to very small change in the resistivity, while the effects of magnetic field on the sample with thickness of 125nm leads to an insulator–metal transition. For LCMO/LAO thin films, the magnetic field has a strong impact on the resistivity of samples. The results show that the magnetoresistance (MR) is enhanced by increasing film thickness for LCMO/LAO samples, due to the relatively stronger phase separation. For LCMO/STO thin films MR is drastically decreased by reduction of film thickness, which is attributed to the enhancement of the charge–orbital order (CO–O) accompanying the complex spin order (the so-called CE type). The changes of the antiferromagnetic structure from the CE to C type and the enhancement of the CE type could be attributed to the in-plane compressive and tensile strain, respectively.

Magnetoelectric and magneto­resistive properties of the СexMn1-xS semiconductors

The effect of electron doping on the magnetoelectric and magnetoresistivity properties of cation-substituted antiferromagnetic СexMn1–xS (x ≤ 0.05) semiconductors at temperatures of 77–500 K in magnetic fields of up to 10 kOe was investigated. For all the compositions shifts of the temperature of the dielectric loss maximum toward higher temperature (80–500 K) with increasing magnetic field up to 10 kOe are found. The current dependence of magnetoresistance from the I–V characteristic measured in a magnetic field was established. The increase of the magnetoresistive effect with increase of the cerium ions concentration in the СexMn1–xS is found. The type of current carriers was determined on the basis of the Hall coefficient measurement. For explanation of experimental data the model of charge-orbital ordering, including orbital glass is used.

Gate control of ferromagnetic insulating phase in lightly-doped La0.875Sr0.125MnO3−δ film

The electric field effect on the lightly doped La0.875Sr0.125MnO3−δ (LSMO) thin film in electric double-layer transistors was investigated by measuring transport properties of the film under various gate voltages. It was found that the positive gate bias leads to an increase of the charge-orbital ordering (COO) transition temperature and a decrease of the Curie temperature TC, indicating the suppression of ferromagnetic metal (FMM) phases and preference of COO/ferromagnetic insulator (FMI) with the hole depletion by gate bias. Such different electric field effects can be ascribed to the weakening of the ferromagnetic interaction and enhancement of Jahn-Teller (JT) distortion caused by the transformation of JT inactive Mn4+-ions to JT active Mn3+-ions. Moreover, a step-like increase in the high temperature region of the ρ-T curve, which is related to the transition of cooperative JT distortion, was found to develop with increasing the positive bias, indicating that the cooperative JT distorted phase is stabilized by the depletion of holes in LSMO film. These results demonstrate that the modulation of holes via electric field strongly affects the balance between energy gains of different interactions and thus produce different effects on the competing FMI, FMM, and cooperative JT distorted phases in LSMO film.

Metal-insulator transition in a spin-orbital-lattice coupled Mott system:K2V8O16

We have explored the underlying mechanism of the metal-insulator transition (MIT) in hollandite-type vanadate, ${\mathrm{K}}_{2}{\mathrm{V}}_{8}{\mathrm{O}}_{16}$, which has a quasi-one-dimensional chain structure and undergoes the MIT and Peierls-like structural transition upon cooling. For this purpose, we have investigated its electronic and magnetic properties in comparison to those of ${\mathrm{Rb}}_{2}{\mathrm{V}}_{8}{\mathrm{O}}_{16}$ that also undergoes the MIT but without the Peierls-like structural distortion. We have found that ${\mathrm{K}}_{2}{\mathrm{V}}_{8}{\mathrm{O}}_{16}$ is a spin-orbital-lattice coupled Mott system and manifests the orbital-selective Mott transition. The interplay of on-site Coulomb interaction, the magnetic-exchange interaction, and the Jahn-Teller-type tetragonal distortion plays an essential role in driving the MIT of ${\mathrm{K}}_{2}{\mathrm{V}}_{8}{\mathrm{O}}_{16}$, inducing the the charge ordering (CO) and orbital ordering of V ${t}_{2g}$ bands. The CO of $\mathrm{V}^{3+}$ and $\mathrm{V}^{4+}$ occurs in separate chains, preserving the inversion symmetry of the crystal. The ${d}_{xy}$ orbitals form the spin-singlet state along the chain direction. The Peierls-like distortion does not play an essential role in the MIT.

The influence of magnetic and structural heterogeneity on thermopower, magnetothermopower, electrical resistivity, and magnetoresistivity of Nd0.5Sr0.5MnO3 manganite

The thermopower, S, magnetothermopower, ΔS/S, resistivity, ρ, and magnetoresistivity, Δρ/ρ, depending on the temperature T and magnetic field H, have been studied in an Nd0.5Sr0.5MnO3 single crystal consisting of three types of clusters: an antiferromagnetic CE-type with charge-orbital ordering (below the Neel temperature TNCE ~ 145 K) and an A-type with TNA ~ 220 K; a ferromagnetic at 234 ≤ T ≤ 252 K, and a ferromagnetic metal phase below the Curie temperature TC = 248 K. The thermopower was found to be negative, indicating the dominance of the electronic type of conductivity. In the S(T) curves, a sharp minimum is observed in the temperature range of 100 K ≤ T ≤ 133 K, close to TNCE, where the absolute S value attains 53 μV/K. With a further increase in temperature, the absolute S value decreases rapidly; at 200 K it is equal to 7 μV/K. It then slightly increases, reaching its maximum value of 15 μV/K at a temperature of 254 K, which is close to TC. The absolute thermopower decreased under the influence of the magnetic field; i.e., a negative magnetothermopower occurs. In {ΔS/S}(T) curves, a sharp minimum is observed at T = 130 K close to TNCE, where the magnetothermopower reaches a huge value of ~45% at H = 13.23 kOe. A broad minimum in the {ΔS/S}(T) curves is observed near the Curie temperature and its value is also high, viz., ~15% in the maximum measuring magnetic field of 13.23 kOe. The extremely high magnetothermopower values mean that the charge-orbital ordered nanoclusters or ferron type make the main contribution to the thermopower of the entire sample. The behavior of the ρ(T) and {Δρ/ρ}(T) curves is similar to that of the S(T) and {ΔS/S}(T) dependencies, which is in agreement with this conclusion.

Electronic structure of 5d transition-metal compounds

In this contribution, we describe fundamental electronic structures and spin–charge–orbital orderings of 5d transition-metal compounds. By reviewing photoemission spectroscopy studies on CuIr2S4, IrTe2, Cs2Au2Br6, and AuTe2, we discuss the difference between the t2g and eg systems with respect to the spin–orbit splitting versus the Jahn–Teller splitting. In contrast to the Ir oxides with Mott-Hubbard character, the smallness of the charge-transfer energy plays essential roles in the exotic spin–charge–orbital orderings of the Ir and Au chalcogenides and halides. In particular, the Te 5p hole provides an interesting interplay between the charge–orbital order and the spin–orbit interaction in IrTe2.

Influence of Lattice Strain on Phase Separation and Percolative Behaviors in La<sub>0.325</sub>Pr<sub>0.3</sub>Ca<sub>0.375</sub> MnO<sub>3</sub> Thin Films

The compressive and tensile strains induced by different substrates on La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.325</sub> Pr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.375</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (LPCMO) thin films have been investigated. It is found that the in-plane compressive strain favors the ferromagnetic metallic phase by increasing the hopping amplitude between neighboring elections and decreasing the tilt of the MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> octahedron, leading to the weakening of the phase separation. On the other hand, the in-plane tensile strain in films prefers to increase the stability of long-range charge/orbital ordering (COO) phase. Under 5 T magnetic field, the 30 nm LPCMO films on NGO and LSAT substrates show an obvious metal-insulator transition, indicating that the COO phase can be melted by the magnetic field at low temperature in the film with small in-plane tensile strain. The LPCMO film on STO substrate keeps insulating in the entire measuring temperature range even under 5 T magnetic field, suggesting that the COO phase in the film with a large tensile strain is more robust.

Connection of thermopower and giant magnetothermopower with magnetic and structural heterogeneity in Sm0.55Sr0.45MnO3 manganite

It was first shown that the thermoelectric power in a magnetic semiconductor Sm0.55Sr0.45MnO3 is caused by ferromagnetic (FM) ferron-type nanoclusters and antiferromagnetic (AFM) CE-type nanoclusters with charge-orbital (CO) ordering. The presence of these clusters is caused by heavily Sr doping of a SmMnO3 compound, which replaced Sm ions. Thermoelectric power S and magnetothermopower ΔS/S has been studied in three samples: ceramics and two single-crystal samples. One of single-crystals was cooled in the air, another one was cooled in an oxygen atmosphere. The annealing in oxygen closes oxygen vacancies and, thus, increases the fraction of the CE-type AFM phase with the CO ordering which displaces the oxygen ions. S(T) curves of single-crystal sample cooled in the air and ceramic sample have a sharp increase starting from Curie temperature (TC) region and a slow decline till 325K. At the same time {ΔS/S}(T) curves have a sharp minimum near TC=126K, reaching the giant absolute value of 87% in the magnetic field H=14.17kOe. This means that thermopower almost vanish with thermal destruction of FM clusters, i.e. thermopower is caused by these clusters in which crystal lattice is compressed. Oxygen cooled single-crystal's S(T) curves have a broad maximum near 270K, including Neel temperature of CE-type clusters TNCE=240K. {ΔS/S}(T) curve has a sharp minimum at the TNCE reaching the absolute value of 50% in H=13.2kOe. The decrease of S is caused by destruction of CO order displacing oxygen ions in CE-type AFM clusters. Thus changed crystal lattice in nanoclusters of ferron type or AFM of CE-type makes the main contribution in thermopower in these three samples.

Direct Observations of Photoexcitation Induced Dynamics of Charge Density Wave and Charge-Orbital Ordered State Using 2.8 MeV Ultrafast Electron Diffraction

Direct Observations of Photoexcitation Induced Dynamics of Charge Density Wave and Charge-Orbital Ordered State Using 2.8 MeV Ultrafast Electron Diffraction

Structural and magnetic phase separation of complex half-doped manganites by the example of Sm0.32Pr0.18Sr0.5MnO3

High-resolution neutron powder diffraction is used to investigate the phase separation and microscopic nature of the magnetoresistance in complex half-doped manganites. Investigations are performed by the example of Sm0.32Pr0.18Sr0.5MnO3 exhibiting the colossal magnetoresistance effect. The structural transition from a high-temperature orthorhombic Pbnm phase to a mixed phase containing two (orthorhombic Pbnm and monoclinic P21/m) phases is revealed. A gradual decrease in the amount of the orthorhombic phase and, consequently, an increase in the amount of the monoclinic phase are observed with decreasing temperature. Redistribution of the two phases ceases below T ≈ 100 K. Low-temperature analysis of the magnetic contribution to the experimental neutron-diffraction patterns indicates that the magnetic ground state of the compound under study is phase-separated and corresponds to a mixed state involving three magnetic phases formed at different temperatures: a ferromagnetic phase, A- type antiferromagnetic, and CE- type charge ordered antiferromagnetic phases with charge-orbital ordering. Ferromagnetic ordering arises near room temperature in the orthorhombic Pbnm phase and manifests itself only until helium temperatures. The A- and CE-types antiferromagnetic states (T N ≈ 170 and 120 K, respectively) appear in the monoclinic crystallographic phase.

Unusual anisotropic magnetoresistance in charge-orbital ordered Nd0.5Sr0.5MnO3 polycrystals

Due to its potential application in magnetic recording and sensing technologies, the anisotropic magnetoresistance (AMR) effect has attracted lasting attention. Despite the long history, AMR effect has not been fully understood especially in the unconventional materials, such as perovskite manganites. Here, we report an unusual AMR effect in the charge-orbital ordered (COO) Nd0.5Sr0.5MnO3 polycrystals, which is observed when the magnetic field rotates in the plane that is perpendicular to the current (out-of-plane AMR). Despite being a polycrystalline sample where no anisotropy is expected, the resistivity shows a large irreversible drop with rotating magnetic field. A model has been proposed based on anisotropic magnetic field induced the melting of COO phase to explain the unusual out-of-plane AMR successfully. Our results demonstrate a new way for understanding the close relationship between phase separation and AMR effect in COO manganites.

Frequency effects on charge ordering in Y0.5Ca0.5MnO3 by impedance spectroscopy

Abstract In this work, structural and electrical properties of Y 0.5 Ca 0.5 MnO 3 are investigated by employing X-ray diffraction and impedance spectroscopy, respectively. Applied ac electric field showed the charge ordering transition temperature around 265 K and below this temperature the heteromorphic behavior of the sample is discussed in the proximity of T CO . With frequency effects the volume of robust charge orbital ordering (COO) domains diminishes due to different competing phases along with Jahn Teller distortions. Comprehensive melting and collapse of charge orbital ordering occurs below T N (125 K), where a colossal drop in the value of impedance is observed. The change in profile of modulus plane plots determines the spreading of relaxation time of intermingled phases. Hopping mechanism is elaborated in terms of strong electron phonon coupling. Variable range hopping model and Arrhenius model are used to discuss the short and long range hopping between Mn 3+ and Mn 4+ channels assessing the activation energy E a .

Orbital correlations and dimensional crossover in epitaxial Pr0.5Ca0.5MnO3/La0.5Sr0.5MnO3 superlattices

We studied the charge-orbital ordering in the superlattice of charge-ordered insulating Pr0.5Ca0.5MnO3 and ferromagnetic metallic La0.5Sr0.5MnO3 by resonant soft x-ray diffraction (RSXD) and hard x-ray photoemission spectroscopy (HXPES). A temperature-dependent incommensurability is found in the orbital ordering by RSXD. In addition, a large hysteresis is observed that is caused by phase competition between the insulating charge ordered and metallic ferromagnetic states. No magnetic phase transitions are observed in contrast to pure Pr0.5Ca0.5MnO3 thin films, confirming the unique character of the superlattice. Mn HXPES spectra revealed a hysteresis in the metalicity, supporting the picture of phase competition. The deviation from the commensurate orbital order can be directly related to the decrease of ordered-layer thickness that leads to dimensional crossover from three-dimensional to two-dimensional orbital ordering.

Influence of the magnetic and structural heterogeneities on the thermopower, magnetothermpower, and spontaneous generation of electric voltage in the manganite Sm0.55Sr0.45MnO3

The thermopower α and magnetothermopower Δα/α have been studied on single-crystal Sm0.55Sr0.45MnO3 samples consisting of three types of magnetic clusters (ferromagnetic clusters with the Curie temperature TC = 134 K, the A-type antiferromagnetic clusters with the Neel temperature TNA ≥ TC, and the CE-type antiferromagnetic clusters with TNCE = 240 K). The temperature dependences of α and Δα/α have extremes in the vicinity of TNCE, namely, a wide maximum in curve α(T) and a sharp minimum in curve{Δα/α}(T). The negative magnetothermopower in the minimum has a giant value of 50% in the magnetic field of 13.2 kOe. The thermopower is shown to be mainly due to the existence of the CE-type antiferromagnetic clusters, in which there is a charge-orbital ordering that displaces the oxygen atoms. The changed crystal lattice inside these clusters changes the value of the thermopower inside them. This thermopower influences the voltage drop across the sample during measuring the thermopower and, thus, the effective value of α of the whole sample. The application of a magnetic field near TNCE accelerating the destruction of the CE-type antiferromagnetic order causes the sharp decrease in the thermopower of the whole sample. This implies that the CE-type antiferromagnetic clusters with the charge-orbital ordering make main contribution to the thermopower of the whole sample.

Influence of lattice strain on charge/orbital ordering and phase separation in Pr0.7(Ca0.6Sr0.4)0.3MnO3 thin films

The static and dynamic lattice strain effects on the competition between ferromagnetic and charge/orbital ordering (COO) phase, phase separation (PS) and transport properties were studied in Pr0.7(Ca0.6Sr0.4)0.3MnO3 (PCSMO) films. It is found that the tensile strained films show pronounced percolative transport behaviors with increased hysteresis, indicating that the stability of the long-range COO is enhanced by the tensile strain. On the other hand, a nearly reversible insulator-metal transition was observed in the compressive strained films, suggesting a strong suppression of the long-range COO. The experiment of dynamic strain effect induced by the bias electric field further verifies the conclusion. Moreover, coactions of the ferroelectric polarization of the substrate and the dynamic strain effect on the PS were found in present PCSMO/PMN-PT film.

Dopant-dependence on charge-orbital order in impurity doped layered manganites

In this study we have investigated dopant effects on a typical charge-orbital ordered state in a single layered manganite La0.5Sr1.5MnO4 using magnetization measurement and resonant x-ray scattering technique. We have studied how the charge-orbital ordered state is affected by the 3% substitution of Cr, Fe and Ga ions for Mn ions. It is revealed that the impurity doping suppresses the long-range charge-orbital order, in which the degree of suppression strongly depends on dopant ions, and eventually forms a phase-separated ground state between charge-orbital order and ferromagnetic states.

Spin–Charge–Orbital Ordering in Hollandite-Type Manganites Studied by Model Hartree–Fock Calculation

We have studied spin–charge–orbital orderings in a Mn3+/Mn4+ mixed valence state on a hollandite-type lattice using unrestricted Hartree–Fock calculation on a multi-band Mn \(3d\)–O \(2p\) lattice model. The calculations show that all the Mn3+–Mn4+, Mn3+–Mn3+, and Mn4+–Mn4+ superexchange interactions are ferromagnetic and play important roles to stabilize the charge and orbital ordering patterns. The most stable charge and orbital ordering pattern is consistent with the \(1\times 1\times 1\) orthorhombic or monoclinic structure of K1.6Mn8O16.

Anisotropy-induced crossover from Drude conductivity to charge-density-wave excitations in a stripe-type charge-ordered manganite

The half-doped Pr${}_{0.5}$Sr${}_{0.5}$MnO${}_{3}$ (PSMO) manganite represents a unique stripe type of charge orbital order that induces transport and magnetic anisotropy. Low-energy charge dynamics in PSMO (110) epitaxial film were investigated along and across two dissimilar in-plane orthogonal axes, [1-10] and [001], by recording the complex refractive index in the energy range of 1--7 meV. We report a novel observation of a gradual crossover from Drude-like metallic conductivity to charge-density-wave (CDW)--like collective excitations as the polarized terahertz excitation field is swept across the orthogonal in-plane axes. This is a rare manifestation of a CDW mode in a highly conducting stripe-type charge-ordered (CO) system, which is fundamentally different from the CDW modes of checkerboard CO. Interestingly, the contrasting charge dynamics along orthogonal in-plane axes form an intrinsic electronic switch, apparently a consequence of the unique CO of PSMO.

Magnetization States of All-Oxide Spin Valves Controlled by Charge-orbital Ordering of Coupled Ferromagnets

Charge-orbital ordering is commonly present in complex transition metal oxides and offers interesting opportunities for novel electronic devices. In this work, we demonstrate for the first time that the magnetization states of the spin valve can be directly manipulated by charge-orbital ordering. We investigate the interlayer exchange coupling (IEC) between two epitaxial magnetite layers separated by a nonmagnetic epitaxial MgO dielectric. We find that the state of the charge-orbital ordering in magnetite defines the strength, and even the sign of the IEC. First-principles calculations further show that the charge-orbital ordering modifies the spin polarized electronic states at the Fe3O4/MgO interfaces and results in a sufficiently large phase shift of wave function which are responsible for the observed IEC sign change across Verwey temperature. Our findings may open new interesting avenues for the electric field control of the magnetization states of spin valves via charge-orbital ordering driven IEC sign change.

Effects of charge-orbital order-disorder phenomena on the unoccupied electronic states in the single-layered half-doped Pr0.5Ca1.5MnO4

Here we report a study on the unoccupied states of the half-doped Pr${}_{0.5}$Ca${}_{1.5}$MnO${}_{4}$ (PCMO). Our investigation, based on temperature dependent x-ray absorption linear dichroism (XLD) and density-functional theory discloses the role of the charge-orbital ordering-disordering mechanisms on the unoccupied density of states. In particular, the lowest unoccupied band has a Mn ${e}_{g}$ ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ character, proving that the physical properties of the two-dimensional (2D) PCMO are also determined by the out-of-plane orbital. Yet, the difference in energy between the ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ states is observed to increase when a charge-orbital ordering is established, hence revealing that the Mn $3d$ electronic hopping is frustrated when the MnO${}_{6}$ cluster orthorhombic strain is increased. This finding addresses the question concerning the complex interplay between the in-plane and out-of plane orbitals in these 2D half-doped single layered manganites.