Doped Manganese Perovskites Research Articles

Inhomogeneous Magnet NaMnIII[BP2O7(OH)3]: Ferromagnetic Clusters Inserted in a Metamagnetic Matrix

AbstractNaMnIII[BP2O7(OH)3] (1) has been hydrothermally synthesized and characterized by single‐crystal X‐ray diffraction. The manganese(III) cations are coordinated in a tetragonal‐elongated octahedral environment, indicating the possible large magnetic anisotropy. The MnO6 octahedra are interconnected by BO4/PO4 groups, forming a 3D triclinic net of MnIII backbone. Detailed dc and ac magnetic measurements have been conducted, and a preliminary H–T phase diagram is deduced accordingly. Compound 1 adopts an unusual magnetic inhomogeneous model, consisting of ferromagnetic clusters and a metamagnetic background: At the paramagnetic temperature region, the Curie–Weiss fit shows that the manganese atoms are all in the high spin state (S = 2), and the predominant interaction is ferromagnetic. An antiferromagnetic ground state forms at approximately 10 K, the isothermal magnetizations between 6.5 and 10 K show the typical metamagnetism. At approximately 6.5 K, a short‐range ordering of the ferromagnetic cluster occurs. Below 3.5 K, the ferromagnetic clusters tend to be frozen at low external field, and the inhomogeneous magnetism can be clearly indicated by the isothermal magnetizations below 3 K. This unusual magnetism cannot be understood by the popular mechanism in doped manganese perovskites, which are well known to be magnetically inhomogeneous because of the mixed valence of manganese. In compound 1, one of the possible reasons for this unusual behaviour is the competition between magnetic superexchange interactions and the large magnetic anisotropy of MnIII.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Critical behavior of the field-dependent susceptibility in single-crystalLa0.73Ba0.27MnO3

Detailed field- and temperature-dependent ac susceptibility data on single-crystal ${\mathrm{La}}_{0.73}{\mathrm{Ba}}_{0.27}\mathrm{Mn}{\mathrm{O}}_{3}$, while confirming the applicability of Heisenberg model exponents, indicate that the associated scaling law extends over a wide region of the $(H,T)$ plane around ${T}_{c}$, with no evidence of crossover effects. From this it can be inferred that the system is magnetically homogeneous (at least in applied fields exceeding any anisotropy/coercive field), an unanticipated result in view of the existence of various sources of disorder (double- and superexchange coupling, for example), and reports of spontaneous electronic phase separation and Griffiths singularities in doped manganese perovskites.

Quantitative structural refinement of Mn K edge XANES in LaMnO3 and CaMnO3 perovskites

The special magnetotransport properties of hole doped manganese perovskites originate from a complex interplay among structural, magnetic and electronic degree of freedom. In this picture the local atomic structure around Mn ions plays a special role and this is the reason why short range order techniques like X-ray absorption spectroscopy (XAS) have been deeply exploited for studying these compounds. The analysis of near edge region features (XANES) of XAS spectra can provide very fine details of the local structure around Mn, complementary to the EXAFS, so contributing to the full understanding of the peculiar physical properties of these materials. Nevertheless the XANES analysis is complicated by the large amount of structural and electronic details involved making difficult the quantitative interpretation.This work exploits the recently developed MXAN code to achieve a full structural refinement of the MnK edge XANES of LaMnO3 and CaMnO3 compounds; they are the end compounds of the doped manganite series LaxCa1−xMnO3, in which the Mn ions are present only in one charge state as Mn3+ and Mn4+ respectively. The good agreement between the results derived from the analysis of near edge and extended region of the XAS spectra demonstrates that a quantitative picture of the local structure can be obtained from structural refinement of MnK edge XANES data in these crystalline compounds. The XANES analysis offers, in addition, the possibility to directly achieve information on the topology of local atomic structure around the absorber not directly achievable from EXAFS.

Simulation of the temperature‐dependent resistivity of La1–xTexMnO3

AbstractThe resistivity ρ of La1–xTexMnO3 (x = 0.1, 0.14) is studied using a random resistor network, based on phase separation between ferromagnetic (FM) and paramagnetic (PM) domains. By considering the irregular shape of the domains, a revised method, which is used to find the shortest paths across the sample, leads to a good agreement between the simulated results and experiment data. Moreover, it is found that FM components increase with Te doping, leading to a reduction of the resistivity and a shift of the transition temperature. This method is proved to be valid and the phase separation scenario is shown theoretically to be good enough in describing the electrical conductivity of the doped manganese perovskites. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Percolation Model of the Temperature Dependence of ExoticMagnetic Field in Doped Manganese Perovskites

We investigate the magnetic transitions in a(La1−xBx)2/3Ca1/3MnO3 system, whichconsists of paramagnetic and ferromagnetic domains, based on a magnetictheoretical percolation model. In the mean-field approximation, theresistance as a function of temperature and magnetic field has beenderived analytically and simulated numerically. It is found that thedependence of the critical temperature on magnetic field is linear whenapplied magnetic field is not too strong. Our theoretical predicationsare in good agreement with recent experimental observations.

Percolation model of the temperature dependence of resistance in doped manganese perovskites

Assuming the sample to consist of ferromagnetic metallic (FMM) particles with a volume fraction (f ) randomly distributed over the paramagnetic insulating background, Monte Carlo simulations of electrical conductivity show excellent fits to the data measured in (La1−xYx)2/3Ca1/3MnO3 (x=0, 0.1 and 0.2). We find that the transition to metallic state occurs as the f reaches a percolation threshold, suggesting that the percolation of FMM domains is responsible for the observed insulator–metal transition.

Phenomenological model for colossal magnetoresistance in optimally doped manganese perovskites

Modeling the conductive channel as a series of ferromagnetic (FM) domain with resistivity of ${\ensuremath{\rho}}_{\mathrm{FM}}$ and paramagnetic (PM) region with resistivity of ${\ensuremath{\rho}}_{\mathrm{PM}},$ we propose a phenomenological equation for resistivity as $\ensuremath{\rho}=[1\ensuremath{-}f(T,H)]{\ensuremath{\rho}}_{\mathrm{PM}}+f(T,H){\ensuremath{\rho}}_{\mathrm{FM}},$ where $f(T,H)$ is the volume fraction of FM domains. This allows us to account quantitatively for reports of CMR in the optimally doped manganese perovskites. We present temperature-dependent magnetoresistance data measured in $({\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Y}}_{x}{)}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_{3}$ $(x=0.195)$ and demonstrate that the model yields excellent agreement with experiments over the whole range covering the high-temperature insulating and the low-temperature metallic regimes. The physical basis for this model is discussed on the basis of dynamic phase segregation and the CMR is argued to be originated mainly from the application of magnetic fields that accelerates the growth of the more conductive FM domains.

Origins of both insulator–metal transition and colossal magnetoresistance in doped manganese perovskites

Based on experiments of both transport and paramagnetic resonance for (La1−xYx)2/3Ca1/3MnO3, the ground state above Tc and possible origins of both insulator–metal transition and colossal magnetoresistance near Tc are discussed. Modeling the system as a network of junctions, each consisting of a paramagnetic region sandwiched between two ferromagnetic domains, a phenomenological expression is proposed for resistance as a function of temperature and magnetic field. We show that the observed transport and magnetotransport phenomena can be quantitatively explained by the present model for the whole temperature range studied.

Phase diagram for Ca 1− xY xMnO 3 type crystals

We present a simple model to study the electron doped manganese perovskites. The model considers the competition between double exchange mechanism for itinerant electrons and antiferromagnetic superexchange interaction for localized electrons. It represents each Mn 4+ ion by a spin 1/2, on which an electron can be added to produce Mn 3+; we include a hopping energy t, a strong intratomic ferromagnetic interaction exchange J (in the limit J/ t→∞), and an interatomic antiferromagnetic interaction K between the local spins. Using the Renormalized Perturbation Expansion and a Mean Field Approximation on the hopping terms and on the superexchange interaction, we calculate the free energy. From it, the stability of the antiferromagnetic, canted, ferromagnetic, and novel spin glass phases can be determined as functions of the parameters characterizing the system. The model results can be expressed in terms of t and K for each value of the doping x in phase diagrams. The magnetization m and canting angle θ can also be calculated as fuctions of temperature for fixed values of doping and model parameters.

Interplay of charge and orbital ordering in manganese perovskites

A model of localized classical electrons coupled to lattice degrees of freedom and, via the Coulomb interaction, to each other, has been studied to gain insight into the charge and orbital ordering observed in lightly doped manganese perovskites. Expressions are obtained for the minimum energy and ionic displacements caused by given hole and electron orbital configurations. The expressions are analyzed for several hole configurations, including that experimentally observed by Yamada et al. in La_{7/8}Sr_{1/8}MnO_3. We find that, although the preferred charge and orbital ordering depend sensitively on parameters, there are ranges of the parameters in which the experimentally observed hole configuration has the lowest energy. For these parameter values we also find that the energy differences between different hole configurations are on the order of the observed charge ordering transition temperature. The effects of additional strains are also studied. Some results for La_{1/2}Ca_{1/2}MnO_3 are presented, although our model may not adequately describe this material because the high temperature phase is metallic.

EPR evidence for Jahn-Teller polarons in La 1− xCa xMnO 3+ y

We present the results of the electron paramagnetic resonance (EPR) study in the mixed valence perovskite La 1− x Ca x MnO 3+ y for 16O and 18O isotope substituted compounds. The characteristic differences observed in EPR intensity and linewidth for the two isotope samples can be explained by a model in which a bottlenecked spin relaxation takes place from the exchange-coupled constituent Mn 4+ ions via the Mn 3+ Jahn-Teller ions to the lattice. For x=0.2 the ferromagnetic exchange energy J exhibits a 16O/ 18O oxygen isotope effect of ∼ t-10%. The observed isotope effects provide the microscopic evidence for the formation of Jahn-Teller polarons in doped manganese perovskites.

Spin waves in the antiferromagnet perovskite LaMnO3: A neutron-scattering study.

As part of a general work on doped manganese perovskites, we have carried out detailed neutron-scattering experiments on powder and single crystals of the othorhombic phase of undoped LaMn${\mathrm{O}}_{3}$. The temperature dependence of the sublattice magnetization has been determined in the antiferromagnetic phase (${T}_{N}=139.5$ K), and the critical exponent is $\ensuremath{\beta}=0.28$, well below that corresponding to a pure three-dimensional Heisenberg antiferromagnet. We have measured the dispersion of the spin waves propagating in the highest symmetry directions solving the problems related to twinning. The whole spin wave spectrum is well accounted for with a Heisenberg Hamiltonian and a single ion anisotropy term responsible for the easy magnetization direction (b axis). This term induces a gap of 2.7 meV at low temperature in the spin wave dispersion curve. An important result is that the ferromagnetic exchange integral (${J}_{1}\ensuremath{\approx}0.83$ meV), coupling the spins within the ferromagnetic basal plane (a,b), is larger by a factor 1.4 than the antiferromagnetic exchange integral (${J}_{2}\ensuremath{\approx}\ensuremath{-}0.58$ meV) coupling spins belonging to adjacent Mn${\mathrm{O}}_{2}$ planes along c.

Lattice effects on the magnetoresistance in doped manganese perovskites (abstract)

A detailed study of doped LaMnO3 with fixed carrier concentration has revealed a direct relationship between the Curie temperature Tc and the average ionic radius of the La site 〈rA〉, which is varied by substituting rare earths of different ionic radii for La. With decreasing 〈rA〉 magnetic order and significant magnetoresistance occur at lower temperatures with increasing temperature hysteresis, and the magnitude of the magnetoresistance increases dramatically. The predominant structural effect of decreasing 〈rA〉 is to decrease the Mn–O–Mn bond angle, which is accompanied by slight variations in the Mn–O bond distance. These results demonstrate that the notion of ‘‘double exchange’’ must be generalized to include changes in the Mn–Mn electronic hopping element as a result of microstructural changes induced by composition, temperature and pressure variations.

Pressure effects on the magnetoresistance in doped manganese perovskites.

We present a detailed study on the effects of applied hydrostatic pressure on the magnetoresistance in doped LaMn${\mathrm{O}}_{3}$ at fixed doping level. In all cases, the application of external pressure monotonically increases the Curie temperature. This is compared with the application of "internal" pressure, which is varied by substituting different rare-earth ions for La. Both effects can be understood in one simple picture that relates the structural modifications to the variation of the Mn-Mn electronic transfer integral. Thus a general phase diagram has been derived with the transfer integral as the implicit microscopic parameter dominating the magnetic and transport properties of doped LaMn${\mathrm{O}}_{3}$.