Electronic Phase Separation Research Articles

Features of electronic phase separation in multiferroic ErMn2O5

The effect of rare-earth ions Er3+, which has a large orbital contribution to the magnetic moment, on the phase transitions and states of the phase separation nanoregions has been studied in multiferroic ErMn2O5. These nanoregions are semiconductor heterostructures formed due to self-organization processes in the ErMn2O5 matrix. A significant effect of Er3+ ions, the moments of which are rigidly oriented along the c axis of the crystal, on the magnetic dynamics, heat capacity, and multiferroic properties of superlattices layers in a wide temperature range of 5 ÷ 300 K has been revealed.

Nanoscale Antiferromagnetic Domain Imaging using Full-Field Resonant X-ray Magnetic Diffraction Microscopy.

The physical properties of magnetic materials frequently depend not only on the microscopic spin and electronic structures, but also on the structures of mesoscopic length scales that emerge, for instance, from domain formations, or chemical and/or electronic phase separations. However, experimental access to such mesoscopic structures is currently limited, especially for antiferromagnets with net zero magnetization. Here, full-field microscopy and resonant magnetic X-ray diffraction are combined to visualize antiferromagnetic (AF) domains of the spin-orbit Mott insulator Sr2 IrO4 with area over ≈0.1 mm2 and with spatial resolution as high as ≈150nm. With the unprecedented wide field of views and high spatial resolution, an intertwining of two AF domains on a length comparable to the measured average AF domain wall width of 545nm is revealed. This mesoscopic structure comprises a substantial portion of the sample surface, and thus can result in a macroscopic response unexpected from its microscopic magnetic structure. In particular, the symmetry analysis presented in this work shows that the inversion symmetry, which is preserved by the microscopic AF order, becomes ill-defined at the mesoscopic length scale. This result underscores the importance of this novel technique for a thorough understanding of the physical properties of antiferromagnets.

Effect of A‐Site Cation Ordering on the Electrical and Magnetic Properties of Manganite Films

(111)‐oriented [(SrMnO3)1/(LaMnO3)2]n superlattices with artificial A‐site cation ordering with respect to the compositionally equivalent La2/3Sr1/3MnO3 films with a random distribution of the A‐site dopants are fabricated. It is found that the ferromagnetism and electrical transport properties of La2/3Sr1/3MnO3 films are primarily independent of the degree of A‐site cation ordering, as evidenced by their identical critical temperatures of phase transition. These results may reveal the spontaneous nature of the electronic phase separation, which is key to the colossal magnetoresistance effect in manganites. In contrast, compared to the La2/3Sr1/3MnO3 films, the A‐site ordered (111) superlattices have a tendency to preserve oxygen vacancies inside, making them extremely difficult to be compensated by the post‐annealing process.

Electronic phase separation in the hexagonal perovskite Ba3SrMo2O9

Competition between exotic magnetic and electronic ground states underpins the properties of strongly correlated transition metal oxides and can result in electronic phase separation (EPS). The 6H-perovskite ${\mathrm{Ba}}_{3}{\mathrm{SrMo}}_{2}{\mathrm{O}}_{9}$ exhibits no magnetic order down to 1.6 K and EPS is observed at 230 K. The ground state of this material contains a complex mixture of spin-singlet ${\mathrm{Mo}}_{2}{\mathrm{O}}_{9}$ dimers and quasimolecular ${\mathrm{Mo}}_{2}{\mathrm{O}}_{9}$ clusters. Segregation in ${\mathrm{Ba}}_{3}{\mathrm{SrMo}}_{2}{\mathrm{O}}_{9}$ emerges due to competition between direct Mo--Mo bonding and Mo--O--Mo magnetic superexchange, comprising an unusual mechanism of EPS in transition-metal oxides.

The effects of substrate temperature on the magnetic and magnetotransport properties of Cr1-xTe epitaxial films

Cr1-xTe epitaxial films have been grown on SrTiO3 (111) single-crystal substrates using molecular beam epitaxy. The effects of substrate temperature on the structural, electronic, and magnetic properties of the Cr1-xTe films are studied. As the substrate temperature increases, the Cr content increases, which results in the increase in the Curie temperature and in-plane magnetization and decrease in the out-of-plane magnetization and coercive field as well as significant changes in magnetoresistance and perpendicular magnetic anisotropy of the Cr1-xTe films. All Cr1-xTe films show metal-to-semiconductor-like transitions which are coupled with paramagnetic-to-ferromagnetic transitions whose temperatures depend on the Cr concentration x. Upon the application of magnetic fields, all samples show negative magnetoresistance effect which is strongest near the transitions and can be understood within the framework of electronic phase separation. These results demonstrate that the substrate temperature have significant effects on the magnetic and magnetotransport properties of Cr1-xTe films and should be carefully controlled in order to obtain desired electronic and magnetic properties.

Anomalous Hall effect and anisotropic magnetoresistance of molecular beam epitaxy grown Cr2Te3 thin films

High-quality Cr2Te3 epitaxial thin films with thicknesses ranging from 2.5 to 30 nm have been epitaxially grown on SrTiO3(111) single-crystal substrates using molecular beam epitaxy. The optimal growth temperature of the films was found to be 275 °C. All Cr2Te3 films show semiconductor-to-metal transition which is coupled with the paramagnetic-to-ferromagnetic transition at TC = 175 K. Consequently, all Cr2Te3 films show negative magnetoresistance effect which is strongest near TC and can be understood within the framework of electronic phase separation scenario. Both anomalous Hall effect and out-of-plane magnetic hysteresis loops show that all Cr2Te3 films have perpendicular magnetic anisotropy with the easy magnetization axis along the c-axis of the films. With decreasing film thickness the coercivity increases and reaches 10 kOe for the thinnest 2.5-nm film. Magnetoresistance measurements at different angles between the direction of the magnetic field and the easy magnetization axis reveal that Cr2Te3 films show anisotropic magnetoresistance whose out-of-plane values are larger than those of in-plane ones.

Компьютерное моделирование наноскопических фазово-неоднородных состояний и фазовых диаграмм ВТСП купратов и никелатов

More than 35 years of experience in the study of cuprate superconductors shows that the main characteristics of the phase diagram can only be obtained by taking into account mesoscopic static/dynamic phase inhomogeneity as a key property of these materials. Within a minimal model for the CuO2 planes with the on-site Hilbert space reduced to only three effective valence centers [CuO4]7-,6-,5- (nominally Cu1+,2+,3+) with different conventional spin and different orbital symmetry we propose a unified non-BCS model that allows one to describe the main features of the phase diagrams of doped cuprates within the framework of a simple effective field theory. Using Maxwell's construction, the global nature of the electronic phase separation in the CuO2 planes of HTSC cuprates is established, which makes it possible to understand and explain many fundamental features of the physics of the normal and superconducting state of cuprates, including the mechanism of formation of the HTSC and pseudogap phase. The features of phase-inhomogeneous states and their evolution with temperature and doping degree, including the special role of the impurity potential in cuprates/nickelates with nonisovalent substitution, are considered for particular examples of the charge triplet model in the framework of the classical Monte Carlo method.

Research of spinterface in organic spintronic devices

Spintronics are attractive to the utilization in next-generation quantum-computing and memory. Compared with inorganic spintronics, organic spintronics not only controls the spin degree-of-freedom but also possesses advantages such as chemical tailorability, flexibility, and low-cost fabrication process. Besides, the organic spin valve with a sandwich configuration that is composed of two ferromagnetic electrodes and an organic space layer is one of the classical devices in organic spintronics. Greatly enhanced or inversed magnetoresistance (MR) sign appearing in organic spin valve is induced by the unique interfacial effect an organic semiconductor/ferromagnetic interface. The significant enhancement or inversion of MR is later proved to be caused by the spin-dependent hybridization between molecular and ferromagnetic interface, <i>i.e.</i>, the spinterface. The hybridization is ascribed to spin-dependent broadening and shifting of molecular orbitals. The spinterface takes place at one molecular layer when attaching to the surface of ferromagnetic metal. It indicates that the MR response can be modulated artificially in a specific device by converting the nature of spinterface. Despite lots of researches aiming at exploring the mechanism of spinterface, several questions need urgently to be resolved. For instance, the spin polarization, which is difficult to identify and observe with the surface sensitive technique and the inversion or enhancement of MR signal, which is also hard to explain accurately. The solid evidence of spinterface existing in real spintronic device also needs to be further testified. Besides, the precise manipulation of the MR sign by changing the nature of spinterface is quite difficult. According to the above background, this review summarizes the advance in spinterface and prospects future controllable utilization of spinterface. In Section 2, we introduce the basic principle of spintronic device and spinterface. The formation of unique spinterface in organic spin valve is clarified by using the difference in energy level alignment between inorganic and organic materials. Enhancement and inversion of MR sign are related to the broadening and shifting of the molecular level. In Section 3, several examples about identification of spinterface are listed, containing characterization by surface sensitive techniques and identification in real working devices. In Section 4 some methods about the manipulation of spinterface are exhibited, including modulation of ferroelectric organic barrier, interface engineering, regulation of electronic phase separation in ferromagnetic electrodes, etc. Finally, in this review some unresolved questions in spintronics are given, such as multi-functional and room-temperature organic spin valve and improvement of the spin injection efficiency. Spinterface is of great importance for both scientific research and future industrial interest in organic spintronics. The present study paves the way for the further development of novel excellent organic spin valves.

Computer simulation of nanoscopic phase-inhomogeneous states and phase diagrams of HTSC cuprates and nickelates

More than 35 years of experience in the study of cuprate superconductors shows that the main characteristics of the phase diagram can only be obtained by taking into account mesoscopic static/dynamic phase inhomogeneity as a key property of these materials. Within a minimal model for the CuO2/NiO2 planes with the on-site Hilbert space reduced to only three effective valence centers [CuO4]7-,6-,5- (nominally Cu1+,2+,3+) with different conventional spin and different orbital symmetry we propose a unified non-BCS model that allows one to describe the main features of the phase diagrams of doped cuprates within the framework of a simple effective field theory. Using Maxwell's construction, the global nature of the electronic phase separation in the CuO2 planes of HTSC cuprates is established, which makes it possible to understand and explain many fundamental features of the physics of the normal and superconducting state of cuprates, including the mechanism of formation of the HTSC and pseudogap phase. The features of phase-inhomogeneous states and their evolution with temperature and doping degree, including the special role of the impurity potential in cuprates/nickelates with nonisovalent substitution, are considered for particular examples of the charge triplet model in the framework of the classical Monte Carlo method. Keywords: cuprates, nickelates, effective field theory, phase diagram, phase separation, Monte-Carlo.

Crystal-field potential and short-range order effects in inelastic neutron scattering, magnetization, and heat capacity of the cage-glass compound HoB12

The strongly correlated system Ho11B12 with boron sublattice Jahn-Teller instability and nanoscale electronic phase separation (dynamic charge stripes) was studied in detail by inelastic neutron scattering (INS), magnetometry and heat capacity measurements at temperatures in the range 3-300 K. From the analysis of registered INS spectra, we determined parameters of the cubic crystal field at holmium sites, B4=- 0.333 meV and B6= -2.003 meV (in Stevens notations), with an unconventional large ratio B6/B4 pointing on the dominant role of conduction electrons in the formation of a crystal field potential. The molecular field in the antiferromagnetic state, Bloc = (1.75+- 0.1) T has been directly determined from the INS spectra together with short-range order effects detected in the paramagnetic state. A comparison of measured magnetization in diluted Lu0.99Ho0.01B12 and concentrated HoB12 single crystals showed a strong suppression of Ho magnetic moments by antiferromagnetic exchange interactions in holmium dodecaboride. To account explicitly for the short-range antiferromagnetic correlations, a self-consistent holmium dimer model was developed that allowed us to reproduce successfully field and temperature variations of the magnetization and heat capacity in the cage-glass phase of HoB12 in external magnetic fields.

Emergence of large thermal noise close to a temperature-driven metal-insulator transition

We report that close to a Mott transition there is an emergence of large thermal noise (${S}_{\mathrm{th}}$) which occurs concomitantly with large correlated flicker noise ($\frac{1}{f}$ noise) with significant non-Gaussian content. This was observed in films of ${\text{NdNiO}}_{3}$ (thickness 15 nm) grown on crystalline ${\text{SrTiO}}_{3}$ substrates with different crystallographic orientations that show a hysteretic transition from a high temperature metallic phase to a low temperature insulating phase in the temperature range 160 to 211 K depending on the substrate orientation and the heating and cooling cycle. The thermal noise, which is distinct from the flicker noise, deviates from the canonical Johnson-Nyquist value of $4{k}_{B}TR$ as measured through the ratio $\ensuremath{\zeta}(T)(\ensuremath{\equiv}\frac{{S}_{\mathrm{th}}(T)}{4{k}_{B}TR})$. The ratio $\ensuremath{\zeta}$ reaches a maximum value of ${\ensuremath{\zeta}}_{M}$ at a temperature ${T}^{*}$ that is close to but distinct from the metal-insulator transition (MIT) temperature ${T}_{\mathrm{MI}}$. In all the films near ${T}^{*}$, the scaled thermal noise maxima ${\ensuremath{\zeta}}_{M}\ensuremath{\gg}1$. The films were found to be largely strain relaxed with residual in-plane and out-of-plane strain as measured by x-ray reciprocal space mapping. It has been observed that the ratio $\frac{{T}^{*}}{{T}_{\mathrm{MI}}}$ as well as ${\ensuremath{\zeta}}_{M}$ have a close dependence on the in-plane-strain in the film. The enhanced thermal noise that occurs along with large correlated flicker noise both arise from slow kinetics of relaxation as established from temperature dependence of the correlation time ($\ensuremath{\tau}$) that gets significantly larger in the temperature range around ${T}^{*}$, reaching a maxima at $T={T}^{*}$. It has been proposed that the existence of large noise (both thermal and flicker noise) owes its origin to electronic phase separation (EPS) that exists near the MIT. A physical model has been suggested that EPS near MIT temperature can give rise to a sparse phase of nanometric small pockets of metallic phases (nanopuddles) that are surrounded by and embedded within the minority insulating phase. The nanopuddles act as a source of charge fluctuations and are coupled weakly to the majority metallic phase by tunneling through the layer of the minority insulating phase. Such isolated metallic nanopuddles can be Coulomb charged if the charging energy ${E}_{C}\ensuremath{\ge}{k}_{B}T$ and can have slow relaxation of fluctuations acting as a source of large noise. It has been argued that the size distribution of the nanopuddles, their average size $\ensuremath{\langle}d\ensuremath{\rangle}$, as well as the temperature dependence of their number density ${N}_{d}$ can determine the temperature ${T}^{*}$.

Arrested Phase Separation in Double-Exchange Models: Large-Scale Simulation Enabled by Machine Learning.

We present large-scale dynamical simulations of electronic phase separation in the single-band double-exchange model based on deep-learning neural-network potentials trained from small-size exact diagonalization solutions. We uncover an intriguing correlation-induced freezing behavior as doped holes are segregated from half filled insulating background during equilibration. While the aggregation of holes is stabilized by the formation of ferromagnetic clusters through Hund's coupling between charge carriers and local magnetic moments, this stabilization also creates confining potentials for holes when antiferromagnetic spin-spin correlation is well developed in the background. The dramatically reduced mobility of the self-trapped holes prematurely disrupts further growth of the ferromagnetic clusters, leading to an arrested phase separation. Implications of our findings for phase separation dynamics in materials that exhibit colossal magnetoresistance effect are discussed.

Manipulating the anisotropic phase separation in strained VO2 epitaxial films by nanoscale ion-implantation

Manipulating the strain induced poly-domains and phase transition in a correlated oxide material is important for high performance device fabrication. Though the electronic transport in the strained oxide film at macroscopic scales can be directly measured, the anisotropic electronic state and the controllable phase separation cross the insulator-to-metal transition within nanoscale size are still elusive. Here, we selected a VO2 crystal film as prototypical oxide and achieved the manipulation of anisotropy electronic phase separation via injecting a He+ nanobeam into the VO2 film at room temperature. In addition, this nanoscale phase separation was directly visualized by infrared near-field imaging measurements, showing the pronounced and unique cR-axis dependent anisotropy on the VO2 surface. Our results offered insight toward understanding the anisotropic nanoscale phase separation in strained metal oxide films.

Growth and characterization of off-stoichiometric LaVO3 thin films

$\mathrm{La}{\mathrm{VO}}_{3}$ (LVO) has been proposed as a promising material for photovoltaics because its strongly correlated $d$ electrons can facilitate the creation of multiple electron-hole pairs per incoming photon, which would lead to increased device efficiency. In this paper, we intentionally grow off-stoichiometric LVO films by changing the growth conditions such as laser fluence. Our aim is to study how deviating La:V stoichiometries affect the electronic properties of LVO thin films. We find that the off-stoichiometry clearly alters the physical properties of the films. Structural characterization shows that both La-rich and V-rich films have different levels of structural distortion, with La-rich (V-rich) films showing a larger (smaller) out-of-plane lattice parameter compared to what one would expect from epitaxial strain effects alone. Both types of films show deviation from the behavior of bulk LVO in optical measurement, i.e., they do not show signatures of the expected long range orbital order, which can be a result of the structural distortions or the presence of structural domains. In transport measurements, La-rich films display clear signatures of electronic phase separation accompanying a temperature induced metal-insulator transition, while V-rich films behave as Mott insulators. The out-of-plane lattice parameter plays a crucial role in determining the transport properties, as the crossover from Mott-insulating to disorder-induced phase-separated behavior occurs around a lattice parameter value of 3.96 \AA{}, quite different from what has been previously reported.

Atomistic origin of metal versus charge-density-wave phase separation in indium atomic wires on Si(111)

We investigate in atomic scale the electronic phase separation occurring in the well known quasi 1D charge-density wave (CDW) phase of the In atomic wire array on a Si(111) surface. The characteristic atomic scale defects, originated from excess In atoms, are found to be actively involved in the formation of the phase boundary between the metallic and the CDW phases by extensive analysis of scanning tunneling microscopy images at various temperatures. These particular defects flip the phase of the quasi 1D CDW to impose strong local constraints in the CDW correlation. We show that such local constraints and the substantial interwire CDW interaction induce local condensates of CDW and the phase separation between the metallic and the CDW phases. This work unveils the atomistic origin of the electronic phase separation, highlighting the importance of atomic scale structures of defects and their collective interaction in electronically inhomogeneous materials.

Two-component electronic phase separation in the doped Mott insulator Y1−xCaxTiO3

The Mott insulator-metal transition has been the subject of intense research in condensed matter physics. Here, the authors study this transition in the prototypical perovskite oxide Y(Ca)TiO${}_{3}$, using a combination of x-ray absorption spectroscopy measurements and density functional theory calculations. They uncover electronic phase separation into hole-poor Mott-insulating and hole-rich metallic regions, i.e., an inherent first-order and percolative nature of the transition that occurs at nonzero hole doping.

Optical Control of Superlattices States Formed Due to Electronic Phase Separation in Multiferroic Eu0.8Ce0.2Mn2O5.

The effect of optical pumping and magnetic field on properties of the electronic phase separation regions, which are the multiferroic semiconductor heterostructures in the form of superlattices, have been studied in Eu0.8Ce0.2Mn2O5. These superlattices are formed due to self-organization in a dielectric crystal matrix as a result of the competing internal interactions balance and occupy a small crystal volume. The dynamical equilibrium states of superlattices are initially formed during cycling of as-grown samples in a magnetic field. The superlattices in such states are ferromagnetic and electrically neutral. Sets of ferromagnetic resonances were observed from individual layers of superlattices. Their features give rise information on properties of these layers and of a superlattice as a whole. The differences in the parameters of these resonances were due to different distributions of Mn3+ and Mn4+ ions in individual superlattices layers. It has been found that optical pumping having different powers allows us to control of multiferroic properties of superlattices layers by changing their magnetic and electric properties. It is shown that, under certain conditions, it is possible to significantly increase the temperatures at which multiferroic heterostructures exist.

Electronic phase separation induced non-volatile bi-polar resistive switching in spatially confined manganite microbridges

Correlated manganite films exhibit functional transport properties due to the co-existence of the competing electronic phase domains which are energetically similar. Here, we investigate very large bi-polar resistive switching (RS) in spatially confined La0.3Pr0.4Ca0.3MnO3 films. In this system, non-volatile bi-polar RS (up to ∼2× 106%) takes place via electric field induced expansion/shrinkage of metallic phase domains, which are separated by an insulating phase domain. These effects are observed without the need of a pre-forming process. We suggest the modification of a memristor model for phase separated systems to explain the observed non-volatile bi-polar I–V characteristics. Investigations of the endurance of the RS over many switching cycles (more than 2.7 × 104 switching) show that it does not decay and full switching occurs with a high success rate. Furthermore, the ability to carry out switching between a number of distinct resistance levels is demonstrated.

Magnetotransport Effects and Electronic Phase Separation in Manganese Sulfides with Electron–Hole Doping

Magnetotransport Effects and Electronic Phase Separation in Manganese Sulfides with Electron–Hole Doping

Large Tuning of Electroresistance in an Electromagnetic Device Structure Based on the Ferromagnetic-Piezoelectric Interface.

The electrical control of the conducting state through phase transition and/or resistivity switching in heterostructures of strongly correlated oxides is at the core of the large on-going research activity of fundamental and applied interest. In an electromechanical device made of a ferromagnetic-piezoelectric heterostructure, we observe an anomalous negative electroresistance of ∼-282% and a significant tuning of the metal-to-insulator transition temperature when an electric field is applied across the piezoelectric. Supported by finite-element simulations, we identify the electric field applied along the conducting bridge of the device as the plausible origin: stretching the underlying piezoelectric substrate gives rise to a lattice distortion of the ferromagnetic manganite overlayer through epitaxial strain. Large modulations of the resistance are also observed by applying static dc voltages across the thickness of the piezoelectric substrate. These results indicate that the emergent electronic phase separation in the manganites can be selectively manipulated when interfacing with a piezoelectric material, which offers great opportunities in designing oxide-based electromechanical devices.