State Of Manganites Research Articles

Grain size effect on the critical behavior of the ferromagnetic transition in electronically phase separated La0.5Ca0.5MnO3

Mixed valence manganese oxides (manganites) are known to exhibit intrinsic inhomogeneous magnetic states, which are influenced by a great variety of factors, including oxygen stoichiometry, nanostructuration and ceramic grain size. In this context, the prototypical half-doped compound La0.5Ca0.5MnO3was extensively studied. Here we reloaded the phase separated scenario, focusing on the thermodynamical critical behavior of this system at the paramagnetic - ferromagnetic transition and its correlation with the evolution of its ordering capabilities controlled through the ceramic grain size.Our findings establish a clear connection between the correlation length and the grain size, shedding light on the role of this parameter in the control of inhomogeneous magnetic ground states in manganites. Therefore it is possible to tailor fundamental physical properties as the scaling laws of the system just tuning the ceramic grain size.

Disordered Jahn–Teller-Polaron States in the Simple Perovskite Manganite Ca1−xLaxMnO3with 0.15 ≦x≦ 0.28

Among orbital-ordered states in manganites with strongly correlated electronic systems, both the C-type orbital-ordered (Ctype-OO) state and the charge- and orbital-ordered (COO) state have been re...

A-site ordered state in manganites with perovskite-like structure based on optimally doped compounds Ln0.70Ba0.30MnO3 (Ln = Pr, Nd)

In this paper, we report on the crystal structure and magnetic properties of the nanostructured Ba-ordered phases of rare-earth manganites obtained from the optimally doped solid solutions Ln0.70Ba0.30MnO3 (Ln = Pr, Nd). The materials were studied by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and SQUID-magnetometry techniques. It is found that states with different degrees of cation ordering in the A-sublattice of the ABO3 perovskite can be obtained by employing special conditions of chemical treatment. In particular, reduction of the parent compounds results in the formation of a nanocomposite containing ferrimagnetic anion-deficient ordered phase LnBaMn2O5. Oxidation of the composite does not change an average size of the nanocrystallites, but drastically alters their phase composition to stabilize ferromagnetic stoichiometric ordered phase LnBaMn2O6 and ferromagnetic superstoichiometric disordered phase Ln0.90Ba0.10MnO3+δ. It is shown that the magnetic properties of the materials are determined by the joint action of chemical (cation ordering) and external (surface tension) pressures.

Thermal cycling effects on static and dynamic properties of a phase separated manganite

In this work we address the interplay between two phenomena which are signatures of the out-of-equilibrium state in phase separated manganites: irreversibility against thermal cycling and aging/rejuvenation process. The sample investigated is La0.5Ca0.5MnO3, a prototypical manganite exhibiting phase separation. Two regimes for isothermal relaxation were observed according to the temperature range: for T > 100 K, aging/rejuvenation effects are observed, while for T < 100 K an irreversible aging was found. Our results show that thermal cycles act as a tool to unveil the dynamical behavior of the phase separated state in manganites, revealing the close interplay between static and dynamic properties of phase separated manganites.

Reversibility of magnetic field driven transition from electronic phase separation state to single-phase state in manganites: A microscopic view

Electronic phase separation (EPS) is a common phenomenon in strongly correlated oxides. For colossal magnetoresistive (CMR) manganites, the EPS is so pronounced that not only does it govern the CMR behavior, but also raises a question whether EPS exists as a ground state for systems or a metastable state. While it has been well known that a magnetic field can drive the transition of the EPS state into a single-phase state in manganites, the reversibility of this transition is not well studied. In this work we use magnetic force microscopy (MFM) to directly visualize the reversibility of the field driven transition between the EPS state and the single-phase state at different temperatures. The MFM images correspond well with the global magnetic and transport property measurements, uncovering the underlying mechanism of the field driven transition between the EPS state and the single-phase state. We argue that EPS state is a consequence of system quenching whose response to an external magnetic field is governed by a local energy landscape.

Phase separation and locally induced states in manganites

ABSTRACTLocal charged states have been induced at the surface of lanthanum strontium manganite single crystals as a result of the local bias application by a conducting tip of atomic force microscope. These results show the dependence of the surface potential of induced area on the writing time similar to that for ferroelectrics. Phenomenon of phase separation with charge segregation is considered for low-doped manganites. It indicates the tendency of manganites toward charge segregation stimulated by the magnetic ordering.

Effect of Fe substitution on charge order state and magnetocaloric effect in Pr0.5Sr0.5Mn1−xFexO3 (0.00 ≤ x ≤ 0.08) manganites

The charge order state and magnetocaloric effect have been studied in Pr0.5Sr0.5Mn1−xFexO3 (0.00 ≤ x ≤ 0.08) prepared by conventional solid-state reaction. Charge order state is weak in parent Pr0.5Sr0.5MnO3 and absent with increasing doping level. The long range charge order antiferromagnetic state (CO/AFM) reemerges for x = 0.06 and subsequently weak accompanied with rapid collapse in double exchange ferromagnetic order for x = 0.08. Magnetic measurements indicate that Fe does not participate in the double exchange interaction, the Curie temperature (TC) is systematically depressed with increasing Fe content from 265 K (x = 0.00) down to 185 K (x = 0.06). Negative magnetic entropy change (ΔSMmax) of −0.96 J/kg K and large positive value of 2.57 J/kg K in Pr0.5Sr0.5Mn0.94Fe0.06O3 are achieved when it undergoes paramagnetic to ferromagnetic (PM-FM) and ferromagnetic to antiferromagnetic (FM-AFM) magnetic phase transition in a magnetic change of 15.0 kOe. Though the materials with considerable magnetic hysteresis in FM-CO/AFM because of first order magnetic phase transition, it gives a strategy to obtain change order state in manganites.

Influence of magnetic field on electric-field-induced local polar states in manganites

It is shown that creation of local charged states at the surface of the lanthanum-strontium manganite single crystals by means of bias application via a conducting atomic force microscope tip is strongly affected by magnetic field. Both a charge and a size of created structures increase significantly on application of the magnetic field during the induction. We argue that the observed phenomenon originates from a known tendency of manganites toward charge segregation and its intimate relation to magnetic ordering.

Visualization of a ferromagnetic metallic edge state in manganite strips.

Recently, broken symmetry effect induced edge states in two-dimensional electronic systems have attracted great attention. However, whether edge states may exist in strongly correlated oxides is not yet known. In this work, using perovskite manganites as prototype systems, we demonstrate that edge states do exist in strongly correlated oxides. Distinct appearance of ferromagnetic metallic phase is observed along the edge of manganite strips by magnetic force microscopy. The edge states have strong influence on the transport properties of the strips, leading to higher metal-insulator transition temperatures and lower resistivity in narrower strips. Model calculations show that the edge states are associated with the broken symmetry effect of the antiferromagnetic charge-ordered states in manganites. Besides providing a new understanding of the broken symmetry effect in complex oxides, our discoveries indicate that novel edge state physics may exist in strongly correlated oxides beyond the current two-dimensional electronic systems.

Phase Separation and Magnetoelectric Properties of Locally Induced States in Manganites

Phenomenon of phase separation with charge segregation is considered for low-doped manganites. The effects of giant dielectric permittivity and colossal magnetocapacitance due to phase separation accompanied by charged inhomogeneities are discussed. Long-range Coulomb interaction is responsible for the formation of inhomogeneous charged states and determines their characteristic length scales. Polar nanoscale areas are also formed at room temperature by applying electric field via the tip of scanning probe microscope to the surface of La0.9Sr0.1MnO3 and La0.89Sr0.11MnO3 single crystals. These results show the dependence of surface potential of induced area on the writing time, which is similar to that for ferroelectrics.

Orbital Order and Electronic Correlations in a Planar Model for Manganites

Understanding orbital ordered (OO) Mott insulating states lies at the heart of a consistent resolution of the colossal magneto-resistance (CMR) observed in manganites, where its melting induces a low-$T$ insulator-metal transition upon hole doping for $0.25 \le x\le 0.45$. Motivated thereby, we study the OO states in a planar model for bilayer manganites using dynamical-mean-field-theory (DMFT) and finite-size diagonalisation methods. We derive the OO ground states in manganites, for $x=0,\frac{1}{2},\frac{2}{3},\frac{3}{4}$ in agreement with observations, including the charge-orbital-magnetic ordered stripe phases for $x>\frac{1}{2}$. These OO states are shown to be associated with an alloy ordering of the $d_{3x^{2}-r^{2}}/d_{3y^{2}-r^{2}}$ orbitals on each $Mn^{3+}$ site.

Asymmetric electroresistance of cluster glass state in manganites

We report the electrostatic modulation of transport in strained Pr0.65(Ca0.75Sr0.25)0.35MnO3 thin films grown on SrTiO3 by gating with ionic liquid in electric double layer transistors (EDLT). In such manganite films with strong phase separation, a cluster glass magnetic state emerges at low temperatures with a spin freezing temperature of about 99 K, which is accompanied by the reentrant insulating state with high resistance below 30 K. In the EDLT, we observe bipolar and asymmetric modulation of the channel resistance, as well as an enhanced electroresistance up to 200% at positive gate bias. Our results provide insights on the carrier-density-dependent correlated electron physics of cluster glass systems.

Ultrafast structural and electronic dynamics of the metallic phase in a layered manganite.

The transition between different states in manganites can be driven by various external stimuli. Controlling these transitions with light opens the possibility to investigate the microscopic path through which they evolve. We performed femtosecond (fs) transmission electron microscopy on a bi-layered manganite to study its response to ultrafast photoexcitation. We show that a photoinduced temperature jump launches a pressure wave that provokes coherent oscillations of the lattice parameters, detected via ultrafast electron diffraction. Their impact on the electronic structure are monitored via ultrafast electron energy loss spectroscopy, revealing the dynamics of the different orbitals in response to specific structural distortions.

Voltage-dependent domain evolution in La0.89Sr0.11MnO3 single crystals by Piezoresponse Force Microscopy

Bias voltage dependent domain dynamics have been investigated on the surface of La0.89Sr0.11MnO3 (LSMO-0.11) single crystals by Piezoresponse Force Microscopy (PFM). The created domain size increases with both the amplitude and duration of the bias voltage pulse. It is observed that domain growth takes place following an activated process wherein the domain wall interacts with the defects (e.g. oxygen vacancies) resulting from the high electric field under the PFM tip. Fractal analysis, based on the interaction of the domain boundary with the defects, provides the Hausdorff fractal dimension value ∼1.3, lower than that usually observed for solid-state crystalline ferroelectrics indicating a smaller correlation length value for LSMO-0.11 crystal. These studies reveal a clear potential of LSMO for new memory devices based on ferroelectric-like domain states in manganites.

Electroresistive and magnetoresistive properties of Nd 0.7Sr 0.3MnO 3 after quenching under pressure of 9 GPa

Polycrystalline Nd 0.7Sr 0.3MnO 3 was quenched from 1300 K to 300 K and 80 K after it had been subjected to a high quasihydrostatic pressure of 9 GPa. Such high pressure and high temperature treatment (HPT) results in significant changes of the crystallochemical parameters—Mn–O lengths and Mn–O–Mn angles within unchanged lattice symmetry of the Pnma-type. A strong increase of the resistivity and a large decrease of the FM–PI transition temperature were detected for the Nd 0.7Sr 0.3MnO 3 HPT treated samples. The intrinsic characteristic T MI( T C) ( T MI is the metal–insulator and T C is the ferromagnetic–paramagnetic transition temperature) correlates with the change of the Mn–O(1)–Mn angle, which is consistent with the double exchange model of the ferromagnetic metallic state in manganites. Remarkable electroresistive (ER) and magnetoresistive (MR) effects appear after HPT treatment, which were not present in the starting Nd 0.7Sr 0.3MnO 3 sample. The structure sensitive properties such as resistivity, MR and ER effects correlate with the change of the nanograin sizes after HPT treatment. Nonlinear current–voltage characteristics showing a hysteresis appear for HPT treated samples at low temperatures. The transport in granular Nd 0.7Sr 0.3MnO 3 samples is likely defined by spin-dependent scattering of charge carriers inside the ferromagnetic metallic grains with embedded small charged isolating islands and by jumping over charged insulating barriers at the intergrain boundaries, which can be strongly affected by the external electric and magnetic fields.

Locally induced charged polarization states in manganites

Spatially inhomogeneous states were induced on the surface of single-crystal lanthanum-strontium manganites by the local application of an external electric field. The dynamics of the growth and relaxation of these states were studied by atomic force microscopy. It is shown that the induced states possess a charge and reveal piezoelectric properties.

Hole and electron attractor model: An explanation of clustered states in manganites

A previously proposed model, based on the attractor role of doping cations, is extended to account for the electric and magnetic behavior on La1-xCaxMnO3 manganites in the whole compositional range (0 < x < 1). From this model, the spontaneous magnetization value is predicted quantitatively over the entire compositional range of the solid solution. By further including as basic parameters the tolerance factor and the band-width, the model is expanded to give an overall description of other Ln1-xTxMnO3 systems (Ln = lanthanide, T = alkaline-earth metal) with special emphasis in explaining the asymmetry and complexity of the magnetic phase diagrams. Those basic parameters are accounted for, in turn, by compositional variations, x, as well as by effects due to the different size of substitutional atoms. The model also sheds some light on the current issue of explaining the phase segregation and allowing the prediction of the spontaneous magnetization values in Ln1-xTxMnO3 systems.

Magnetic properties of pseudomorphic epitaxial films ofPr0.7Ca0.3MnO3under different biaxial tensile stresses

In order to analyse the effect of strain on the magnetic properties of narrow-band manganites, the temperature and field dependent susceptibilities of about 8.5 nm thick epitaxial Pr0.7Ca0.3MnO3 films, respectively grown on (001) and (110) SrTiO3 substrates, have been compared. For ultrathin samples grown on (001) SrTiO3, a bulk-like cluster-glass magnetic behaviour is found, indicative of the possible coexistence of antiferromagnetic and ferromagnetic phases. On the contrary, ultrathin films grown on (110) substrates show a robust ferromagnetism, with a strong spontaneous magnetization of about 3.4 mB /Mn atom along the easy axis. On the base of high resolution reciprocal space mapping analyses performed by x-ray diffraction, the different behaviours are discussed in terms of the crystallographic constraints imposed by the epitaxy of Pr0.7Ca0.3MnO3 on SrTiO3. We suggest that for growth on (110) SrTiO3, the tensile strain on the film c-axis, lying within the substrate plane, favours the ferromagnetic phase, possibly by allowing a mixed occupancy and hybridization of both in-plane and out-of-plane eg orbitals. Our data allow to shed some physics of inhomogeneous states in manganites and on the nature of their ferromagnetic insulating state.

Charge density waves enhance the electronic noise of manganites

The transport and noise properties of Pr_{0.7}Ca_{0.3}MnO_{3} epitaxial thin films in the temperature range from room temperature to 160 K are reported. It is shown that both the broadband 1/f noise properties and the dependence of resistance on electric field are consistent with the idea of a collective electrical transport, as in the classical model of sliding charge density waves. On the other hand, the observations cannot be reconciled with standard models of charge ordering and charge melting. Methodologically, it is proposed to consider noise-spectra analysis as a unique tool for the identification of the transport mechanism in such highly correlated systems. On the basis of the results, the electrical transport is envisaged as one of the most effective ways to understand the nature of the insulating, charge-modulated ground states in manganites.

Atomic-resolution imaging of oxidation states in manganites

Aberration corrected electron optics allows routine acquisition of high spatial resolution spectroscopic images in the scanning transmission electron microscope, which is important when trying to understand the physics of transition-metal oxides such as manganites. The physical properties of these perovskites are intimately related to the occupancies of the partially filled $3d$ bands, which define their oxidation state. In this work, we review procedures to obtain this electronic property in ${\text{La}}_{x}{\text{Ca}}_{1\ensuremath{-}x}{\text{MnO}}_{3}$ from atomic-column-resolved electron energy-loss spectra measured in the aberration corrected scanning transmission electron microscope. In bulk samples, several features of both the average $\text{Mn}\text{ }{L}_{2,3}$ edge and the $\text{O}\text{ }K$ edge fine structure change linearly with Mn nominal valence. These linear correlations are extracted and used as a calibration to quantify oxidation states from atomic resolution spectroscopic images. In such images, the same fine-structure features exhibit further changes, commensurate with the underlying atomic lattice. Mn valence values calculated from those images show unexpected oscillations. The combination of experiment with density-functional theory and dynamical scattering simulations allows detailed interpretation of these maps, distinguishing dynamical scattering effects from actual changes in electronic properties related to the local atomic structure. Specifically, in ${\text{LaMnO}}_{3}$, the two nonequivalent O sites can be distinguished by these methods.