Phase Separation In Manganites Research Articles

Direct experimental evidence of physical origin of electronic phase separation in manganites

Electronic phase separation in complex oxides is the inhomogeneous spatial distribution of electronic phases, involving length scales much larger than those of structural defects or nonuniform distribution of chemical dopants. While experimental efforts focused on phase separation and established its correlation with nonlinear responses under external stimuli, it remains controversial whether phase separation requires quenched disorder for its realization. Early theory predicted that if perfectly "clean" samples could be grown, both phase separation and nonlinearities would be replaced by a bicritical-like phase diagram. Here, using a layer-by-layer superlattice growth technique we fabricate a fully chemically ordered "tricolor" manganite superlattice, and compare its properties with those of isovalent alloyed manganite films. Remarkably, the fully ordered manganite does not exhibit phase separation, while its presence is pronounced in the alloy. This suggests that chemical-doping-induced disorder is crucial to stabilize the potentially useful nonlinear responses of manganites, as theory predicted.

Electric-field modulation of photoinduced effect in phase-separated Pr0.65 (Ca0.75Sr0.25)0.35MnO3/PMN-PT heterostructure

In this letter, we report the photoinduced effect modulated by different electric fields in the Pr0.65 (Ca0.75Sr0.25)0.35MnO3/0.7PbMg1/3Nb2/3O3-0.3PbTiO3 heterostructure. The film exhibits a decrease in the resistance up to five orders of magnitude by enhancing applied electric fields, combined with an electric-field-induced insulator-to-metal transition. More interestingly, a reversible bistability arises in the photoinduced change in resistance at T < 80 K as the voltages are increased. The results can be attributed to the phase separation in manganites, which provides a prototype of photoelectric conversion for electric-field modulation of all-oxide heterostructures.

Coupled magnetic and elastic properties in LaPr(CaSr)MnO manganites

We investigate a series of manganese oxides, the La0.225Pr0.4(Ca1−xSrx)0.375MnO3 system. The x=0 sample is a prototype compound for the study of phase separation in manganites, where ferromagnetic and charge ordered antiferromagnetic phases coexist. Replacing Ca2+ by Sr2+ gradually turns the system into a homogeneous ferromagnet. Our results show that the material structure plays a major role in the observed magnetic properties. On cooling, at temperatures below ∼100 K, a strong contraction of the lattice is followed by an increase in the magnetization. This is observed both through thermal expansion and magnetostriction measurements, providing distinct evidence of magneto-elastic coupling in these phase separated compounds.

Manipulating electronic phase separation in strongly correlated oxides with an ordered array of antidots

The interesting transport and magnetic properties in manganites depend sensitively on the nucleation and growth of electronic phase-separated domains. By fabricating antidot arrays in La0.325Pr0.3Ca0.375MnO3 (LPCMO) epitaxial thin films, we create ordered arrays of micrometer-sized ferromagnetic metallic (FMM) rings in the LPCMO films that lead to dramatically increased metal-insulator transition temperatures and reduced resistances. The FMM rings emerge from the edges of the antidots where the lattice symmetry is broken. Based on our Monte Carlo simulation, these FMM rings assist the nucleation and growth of FMM phase domains increasing the metal-insulator transition with decreasing temperature or increasing magnetic field. This study points to a way in which electronic phase separation in manganites can be artificially controlled without changing chemical composition or applying external field.

Exchange bias in a singleLaMnO3film induced by vertical electronic phase separation

We verify that the exchange bias effect unexpectedly emerges in a single ${\mathrm{LaMnO}}_{3\ensuremath{-}\ensuremath{\delta}}$ film, one of the most studied correlated oxides. We combine x-ray absorption spectroscopy results, which serves as a fingerprint of the electronic structure, with microstructure characterizations and magnetization data to explore the origin of the exchange bias behavior. Taken together, these measurements provide compelling evidence that the formation of a Mn${}^{2+}$ component associated with the double exchange between Mn${}^{2+}$-O-Mn${}^{3+}$ produces robust ferromagnetism in the upper part of the film, which is exchanged coupled with the antiferromagnetic bottom part that is dominated by Mn${}^{3+}$. Thus, the Mn ions are found to be distributed unevenly in the depth profile, with vertical electronic phase separation, in contrast to the often accepted view of the lateral phase separation in manganites. The observation of exchange bias in a chemically single film paves the way for interface engineering induced by vertical electronic phase separation towards technological applications.

Probing magnetic phase separation in manganites by nonlinear susceptibility

At low frequencies ν≤300Hz, nonlinear magnetic susceptibility of polycrystalline La0.7Pb0.3Mn0.8(Co, Ni)0.2O3 samples near the ferromagnetic (FM)–paramagnetic (PM) phase transition reinforces the earlier observation, based on linear ac susceptibility, that these systems behave as a three-dimensional isotropic dipolar ferromagnet in the asymptotic critical regime and exhibit an isotropic dipolar to isotropic Heisenberg crossover. In addition, the nonlinear susceptibility results reveal the phase segregation of the conducting and insulating FM phases (clusters) within the conducting FM regime and existence of conducting FM clusters in the insulating PM matrix at high frequencies.

Non-linear current–voltage characteristics of (La 0.5Eu 0.5) 0.7Pb 0.3MnO 3 single crystals: Possible manifestation of the internal heating of chargecarriers

Temperature evolution of the current–voltage characteristics (CVCs) of a single-crystal lanthanum manganite (La 0.5Eu 0.5) 0.7Pb 0.3MnO 3 is investigated in a wide (up to 1 А) range of instrumental currents. Effects of a transport current and an applied electric field on the resistance of the material are studied in view of possible implementation of the charge ordering break in dielectric regions occurring due to phase separation in manganites. A negative differential resistance portion observed in the CVCs suggests the presence of a current switching effect. Below the temperature of the metal–dielectric transition in (La 0.5Eu 0.5) 0.7Pb 0.3MnO 3, the hysteresis is observed in the CVC. The detailed analysis of the internal sample heating on the basis of experimental data on thermal conductivity showed, however, that these CVC features can be explained within the concept of non-equilibrium heating of electron gas.

Anisotropic-strain-induced antiferromagnetic-insulating state with strong phase instability in epitaxial (La0.8Pr0.2)0.67Ca0.33MnO3 films

We grow epitaxial (La0.8Pr0.2)0.67Ca0.33MnO3 films simultaneously on the lattice-closely-matched substrates, cubic (LaAlO3)0.3(Sr2AlTaO6)0.7 [LSAT(001)] and orthorhombic NdGaO3 [NGO(001) and NGO(110)]. While all as-grown films show a ferromagnetic-metallic (FM) ground state as observed for the bulk target, the annealed films show quite different magnetotransport behavior as follows: on NGO(110) they show a robust FM ground state, on LSAT(001) and NGO(001) however, they show surprisingly a coexisted antiferromagnetic insulating state with high phase instability in a wide temperature range. The phase coexistence being easily induced via the control of anisotropic epitaxial strain suggests that the phase separation in manganites could be elastically driven, and thus can be strain-engineered for devices applications.

Recent developments in the theoretical study of phase separation in manganites andunderdoped cuprates

This paper is a brief review of the status of ‘phase separation’ ideas in manganites andcuprates, mainly focused on the recent efforts by the authors. It is argued that in the lastyear considerable progress has been made in the understanding of manganites, since thefamous colossal magnetoresistance peak in the resistivity versus temperature has beennumerically observed in unbiased Monte Carlo simulations using realistic models(namely, including double exchange, phonons, and quenched disorder). It is alsoconjectured that a phenomenology similar to the one found in manganites could bepresent in the underdoped regime of the cuprates. It is predicted that a state withsuperconducting patches exists above the critical temperature in the underdopedregime, in agreement with recent scanning tunneling microscopy experiments.

Magnetic-field-controlled phase separation in manganites: Electron magnetic resonance study

The electron magnetic resonance spectra of Sm{sub 1-x}Sr{sub x}MnO{sub 3} (x = 0.30, 0.40, 0.45) manganites have been studied. At temperatures that are higher than the Curie point by several tens of kelvins, samples with x = 0.40 and 0.45 exhibit a ferromagnetic resonance (FMR) spectrum imposed on their usual EPR spectrum. The FMR spectrum appears as the applied magnetic field H exceeds a certain critical field H{sub c}, which decreases upon cooling and becomes zero at T = T{sub C}. These results agree with the magnetic-measurement data and indicate the magnetic-field-induced nucleation and growth of ferromagnetic domains in a paramagnetic matrix. In the initial growth stage, the volume of the ferromagnetic domains is proportional to (H - H{sub c}){sup {beta}}, where {beta} = 4.0 {+-} 0.3, and it changes in phase with magnetic field modulation up to a frequency of 100 kHz. In the same field and temperature ranges, hysteretic phenomena and narrow unstable spectral lines are detected; these lines indicate a dynamic character of the phase separation. The results obtained are interpreted in terms of the competition of different types of magnetic and charge ordering.

Anomalous first-to-zero sound crossover inLa1−xCaxMnO3

The elastic properties of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ $(x\ensuremath{\approx}1∕3)$ are anomalous, particularly in the paramagnetic phase, where values of the longitudinal sound velocity at $\ensuremath{\sim}{10}^{10}\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}$, in the zero-sound regime, are more than 50% larger than those from ultrasound experiments at ${10}^{3}--{10}^{6}\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}$. Such a behavior, indicative of coupling to relaxation processes, is similar to that of liquids and soft viscoelastic matter, but quite unusual for a hard solid. These results suggest that the physics of phase separation in manganites is more complex than that of a stationary mixture of competing phases.

Magnetic Phase Separation in Manganites

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Thermodynamic modeling of phase separation in manganites

We present a phenomenological model based on the thermodynamics of the phase separated state of manganites, accounting for its static and dynamic properties. Through calorimetric measurements on La$_{0.225}$Pr$_{0.40}$Ca$ _{0.375}$MnO$_{3}$ the low temperature free energies of the coexisting ferromagnetic and charge ordered phases are evaluated. The phase separated state is modeled by free energy densities uniformly spread over the sample volume. The calculations contemplate the out of equilibrium features of the coexisting phase regime, to allow a comparison between magnetic measurements and the predictions of the model. A phase diagram including the static and dynamic properties of the system is constructed, showing the existence of blocked and unblocked regimes which are characteristics of the phase separated state in manganites.

Two types of magnetic polarons localized at impurities in an antiferromagnetic chain

The possibility of different types of magnetic polarons (ferrons) is discussed for an antiferromagnetic (AF) chain doped by non-magnetic donor impurities. The conduction electrons are assumed to be bound by the impurities. It is shown that two types of ferrons can exist in such a system: ferrons with the sizes of the order of the localization length of the electron near the impurity, and those with more extended distortions of the AF order in the chain. The ferrons of the first type correspond to the ground state, whereas ferrons of the second type can be treated as metastable excitations. The energy of the excited states decreases with the growth in density of impurities. This is a signature of an attractive interaction between ferrons, which could provide a possible mechanism for a large-scale phase separation in manganites and related compounds.

Electrical noise as evidence for phase separation in manganites

We review measurements of the electrical noise conducted to study the dynamics of the percolation-like metal–insulator transition in mixed valence manganites. We observed mainly 1 / f noise, but in few cases significant random telegraph noise (RTN) in La 0.7 Ca 0.3 MnO 3 films. The latter originates from two-level (TL) fluctuators which are not related to magnetic domains but exhibit large differences in resistance. The life times of the two states experimentally observed over a broad temperature and field range are analyzed by thermal activation and the net magnetization differences of the states are estimated. These results are related to measurements on Pr 0.7 Ca 0.3 MnO 3 .

Phase separation in manganites induced by orbital-ordering strains.

Doped manganite perovskites AMnO(3) exhibit a rich variety of electronic properties, resulting from the interplay of charge (Mn(3+)/Mn(4+)), spin (Mn magnetic moment) and orbital (Mn(3+) Jahn-Teller distortion) degrees of freedom. Magnetisation measurements and ESR spectra have been used to study a series of eight AMnO(3) perovskites, in which the A cation sites are occupied by a distribution of 70% trivalent lanthanide and 30% divalent Ca, Sr or Ba ions. These all have a mean A cation radius of 1.20 Angstrom but different values of the cation size variance sigma(2). A change from orbital disorder to order (cooperative Jahn-Teller distortions) was previously found in the insulating regime at sigma(2) = approximately 0.005 Angstrom(2). This work has shown that co-existence of the orbitally ordered and disordered phases is found in sigma(2)= 0.0016-0.0040 Angstrom(2) samples, with a difference of 40 K between their Curie temperatures. This is ascribed to competition between orbital ordering and microstructural lattice strains. At larger sigma(2) > 0.005 Angstrom(2), the orbital ordering strains are dominant and only this phase is observed. This intermediate temperature phase segregation is one of many strain-driven separation phenomena in manganites.

Electron instability in doped-manganites-based heterojunctions

Electrodynamic and thermal properties of doped-manganites-based heterojunctions are numerically calculated for a model description of electron instability effects revealed in them under a strong current injection. The calculations are performed by jointly solving the thermal conduction and Poisson equations. Comparison of experimental and calculated data suggests that the effect of transport current on resistive properties of manganites-based heterojunctions is indicative of impurity-assisted phase separation in manganites. The motion of charged oxygen ions in the surface layer with high density of vacancies under the action of an electric field catalyzes phase separation.

Nanoscale phase separation in manganites

We study the possibility of nanoscale phase separation in manganites in the framework of the double exchange model. The homogeneous canted state of this model is proved to be unstable toward the formation of small ferromagnetic droplets inside an antiferromagnetic insulating matrix. For the ferromagnetic polaronic state we analyze the quantum effects related to the tails of electronic wave function and a possibility of electron hopping in the antiferromagnetic background. We find that these effects lead to the formation of the threshold for the polaronic state.

Magnetic phase diagram and electronic phase separation in manganites

A detailed study on magnetic phase diagram and electronic phase separation has been performed in a mean field approximation for low carrier density manganites. It is shown that the phase separation is rather dependent of the values of Hund coupling, and it washes away the canted G-type AF. The C-type AF and CE-type AF, however, are stable against the phase separation. Observed electron-density dependence of magnetization may well be explained by taking into account weaker Hund coupling or electron-density dependence of J.

Surface-induced phase separation in manganites: A microscopic origin for powder magnetoresistance

Through the analysis of the magnetic properties and of the nuclear magnetic resonance response of La2/3Ca1/3MnO3 ceramics with different grain sizes, we have found that poorly conducting regions, some ferromagnetic and some weakly magnetic, are located at the surface of the grains. We state that these regions constitute the tunnel barrier responsible for the low-field magnetoresistance usually observed in powders of half-metallic oxides. In addition, the spin disorder accompanying the coexistence of phases with different magnetoelectronic character could contribute to the large high-field magnetoresistance also typical of such ceramic samples. From a more general perspective, these findings can be of relevance to understand the microscopic origin of phase separation in manganites.