Reentrant orbital order and the true ground state of LaSr2Mn2O7

Contrary to conventional wisdom, our purified La 2-2x Sr 1+2x Mn2O7 crystals exhibit CE-type orbital and charge order as the low-temperature ground state for a hole doping level h=0.5. For small deviations from h=0.5, the high-temperature CE phase is replaced at low temperatures by an A-type antiferromagnet without coexistence. Larger deviations result in a lack of CE order at any temperature. Thus, small inhomogeneities in cation or oxygen composition could explain why others commonly see this reentrance with coexistence.     

Influence of grain size on the electrical properties of the double-layered LaSr2Mn2O7 manganite

The double-layered LaSr₂Mn₂O₇ manganite was synthesized by the sol–gel process. Two samples with the average grain size from ～150 nm to ～1 μm were prepared by controlling the sintering temperature. Both samples had a tetragonal structure, with a small fraction of impurity phase in the S-1250 sample. In order to investigate the probable influence of grain size on the conduction mechanism, resistivity of the samples was measured as a function of temperature, and the data obtained was analyzed by different conduction mechanisms. It was found that with increase in the grain size, resistivity decreased at all temperature ranges. The results show that the adiabatic small polaron hopping (SPH) model is probably responsible for conduction at high temperature range, and that the 3D variable range hopping (VRH) model shows a better correlation with the experimental data for low temperature range. These analyses indicate the influence of grain size on the parameters obtained from fitting the data by both models.     

Highly degenerate canted spin structure in bilayer manganite La1.1Sr1.9Mn2O7

We have observed magnetic diffuse signals of the bilayer manganite La1.1Sr1.9Mn2O7 by neutron scattering. Even at the low temperature, 20 K, there is a diffuse signal, which is much stronger than that observed in the high temperatures, 144 and 288 K. This signal cannot be characterized by a bilayer structure as observed at 144 K nor by a monolayer structure as at 288 K, but by a high degeneracy of a canted spin structure. Thus, an observation of this diffuse signal is the direct evidence for the establishment of the canted spin structure in La1.1Sr1.9Mn2O7.     

Fragile magnetic ground state in half-doped LaSr2Mn2O7

We investigated the orbital and antiferromagnetic ordering behaviors of the half-doped bilayer manganite La(2-2x)Sr(1+2x)Mn2O7 (x ? 0.5) by using Mn L(2,3)-edge resonant soft x-ray scattering. Resonant soft x-ray scattering reveals the CE-type orbital order below T(oo) ? 220 K, which shows partial melting behavior below T(m) ? 165 K. We also found coexistence CE- and A-type antiferromagnetic orders. Both orders involve the CE-type orbital order with nearly the same orbital character and are coupled with each other. These results manifest that the ground state with the CE-type antiferromagnetic order is easily susceptible to destabilization into the A-type one even with a small fluctuation of the doping level, as suggested by the extremely narrow magnetic phase boundaries at x ? 0.5±0.005.     

Effect of doping Ca on polaron hopping in LaSr2Mn2O7

From the transport studies in the bilayer manganites LaSr_2−x Ca _x Mn₂O₇, we have found the variable-range hopping model proposed by Viret et al to be inadequate to describe the transport of charge in these materials. The polarons appear to hop to their nearest neighbors with an activation energy, which in part is dependent on the magnetic interactions in the lattice.     

Charge and orbital order in frustrated Pb3Mn7O15

The candidate magnetoelectric Pb(3)Mn(7)O(15) has a structure consisting of one-third filled kagome layers linked by ribbons of edge sharing octahedra in the stacking direction. Previous reports have indicated a complex hexagonal-orthorhombic structural transition upon cooling through ~335 K, although its origins are uncertain. Here both structures are revisited using a combination of neutron and synchrotron x-ray diffraction data. Large shifts of oxygen positions are detected, which show that the interlayer sites and those which occupy voids in the kagome lattice are trivially charge ordered in both phases. The symmetry breaking is found to occur due to Mn(3+) orbital ordering on the ribbon sites and charge ordering of the subset of layer sites which make up a kagome network.     

Evidence for charge orbital and spin stripe order in an overdoped manganite

Overdoped La0.42Sr1.58MnO4 exhibits a complex ordering of charges, orbitals, and spins. Neutron diffraction experiments reveal three incommensurate and one commensurate order parameters to be tightly coupled. The position and the shape of the distinct superstructure scattering as well as higher-order signals are inconsistent with a harmonic charge and spin-density-wave picture but point to a stripe arrangement in which ferromagnetic zigzag chains are disrupted by excess Mn(4+).     

Enhanced charge gap in the bilayer manganite La2-2xSr1+2xMn2O7 near x = 0.4

We have investigated the optical conductivity spectra of La2-2xSr1+2xMn2O7 (0.3相似文献   

Field-induced ferromagnetic metallic state in the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7, probed by neutron scattering

The bilayer manganite La1.2Sr1.8Mn2O7 exhibits a phase transition from a paramagnetic insulating (PI) to a ferromagnetic metallic (FM) state with a colossal magnetoresistance (CMR) effect. Upon 60% Pr substitution, magnetic order and PI to FM transition are suppressed. Application of a moderate magnetic field restores an FM state with a CMR effect. Neutron scattering by a single crystal of (La0.4Pr0.6)1.2Sr1.8Mn2O7, under a magnetic field of 5 T, has revealed a long-range and homogeneous ferromagnetic order. In the PI phase, under zero field, correlated lattice polarons have been detected. At 28 K, under 5 T, the spin wave dispersion curve determines an in-plane isotropic spin wave stiffness constant of 146 meV A(2). So the magnetic field not only generates a homogeneous ferromagnetic ground state, but also restores a magnetic coupling characteristic of FM CMR manganites.     

Room temperature magnetocaloric effect in Pr1.75Sr1.25Mn2O7 double-layered perovskite manganite system

Abstract

In this work, we have studied on double-layered perovskite (Ruddlesden–Popper) manganite structure in Pr_1.75Sr_1.25Mn₂O₇ synthesised by sol–gel method. The crystal structure of the double-layered perovskite is found as tetragonal from the X-ray diffraction analysis with I4/mmm space group. A high Curie temperature, T_C = 305 K is observed from the temperature dependence of magnetisation measurement. The isothermal magnetisation curves showed that magnetic phase transition is second order due to the positive slope of the Arrott plots. Maximum magnetic entropy change (ΔS_M) and adiabatic temperature change (ΔT_ad) values are calculated as 3.99 J kg^?1 K^?1 and 2.1 K under external magnetic field of 70 kOe, respectively. Since our double-layered perovskite manganite sample has desired T_C value and relatively high ΔS_M, it can be a potential candidate as a magnetocaloric material for room temperature magnetic cooling systems.     

Field-induced ferromagnetic metallic state of bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7: a polarized neutron diffraction study

Unpolarized and polarized neutron diffraction measurements have been carried out on the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7 which undergoes simultaneous semiconductor-metal paraferromagnetic transitions under magnetic field. Maximum entropy magnetization density reconstruction and multipole refinement on flipping ratios evidence the existence of two distinct field-induced states. The field-induced ferromagnetic state where the field is parallel to the c axis is characterized by the presence of magnetic moment on the Sr site of 0.48(2)mu(B), due to the Pr substitution. It also shows a high population of the d3(z(2)-r(2)) orbitals of Mn3+. For the field-induced state where the field is parallel to the a or b axes no magnetization density was found at the Sr site and the dx(2)(-y(2)) orbital is slightly more populated than the d3(z(2)-r(2)) one.     

Investigation of magnetic transitions through ultrasonic measurements in double-layered CMR manganite La1.2Sr1.8Mn2O7

A polycrystalline, double-layered, colossal magnetoresistive manganite La_1.2Sr_1.8Mn₂O₇ is synthesized by sol-gel process and its magnetic and ultrasonic properties were investigated in the temperature range 80–300 K. The sample has Curie temperature at 124 K, where the sample exhibits a transition from paramagnetic insulator to ferromagnetic metallic state. The longitudinal sound velocity measurements show a significant hardening of sound velocity below T_C, which may be attributed to the coupling between ferromagnetic spins and longitudinal acoustic phonons. The magnetization and ultrasonic studies reveal the presence of secondary transition at ≈ 260 K in this sample. The present sound velocity measurement results confirm the reliability of ultrasonic investigations as an independent tool to probe magnetic transitions in manganites.     

Photo-induced effect in the layered perovskite manganite La_(1.2)Sr_(1.8)Mn_(1.8)Co_(0.2)O_7

1IntroductionWiththediscoveryofcolossalmagnetoresistance(CMR)effectinmanganites,hole-dopingperovskitemanganiteswithunusualelectronictransportandmagneticpropertieshaveattractedconsiderableattention.Thesepropertiesresultfromanintrinsicinteractionbetweencharge,spin,orbitalandlatticedegreesoffreedomthatarestronglycoupledtoeachother[1—6].DoubleexchangemodelcombinedwithJohn-Tellereffectwasusedtoexplainthesepropertiespartly[7—9].InordertogetbetterunderstandingofthemechanismofCMReffect,externals…     

Pressure-induced colossal piezoresistance effect and the collapse of the polaronic state in the bilayer manganite (La(0.4)Pr(0.6))(1.2)Sr(1.8)Mn2O7

We have investigated the effect of hydrostatic pressure as a function of temperature on the resistivity of a single crystal of the bilayer manganite (La(0.4)Pr(0.6))(1.2)Sr(1.8)Mn(2)O(7). Whereas a strong insulating behaviour is observed at all temperatures at ambient pressure, a clear transition into a metallic-like behaviour is induced when the sample is subjected to a pressure (P) of ~1.0 GPa at T < 70 K. A huge negative piezoresistance ~10(6) in the low temperature region at moderate pressures is observed. When the pressure is increased further (5.5 GPa), the high temperature polaronic state disappears and a metallic behaviour is observed. The insulator to metal transition temperature exponentially increases with pressure and the distinct peak in the resistivity that is observed at 1.0 GPa almost vanishes for P > 7.0 GPa. A modification in the orbital occupation of the e(g) electron between 3d(x(2)-y(2)) and 3d(z(2)-r(2)) states, as proposed earlier, leading to a ferromagnetic double-exchange phenomenon, can qualitatively account for our data.     

Pressure tuning of magnetic interactions in layered (La0.6Nd0.4)1. 2Sr1.8Mn2O7 manganite

The effect of pressure of up to 9 GPa on the magnetic and structural properties of layered (La0.6Nd0.4)1.2Sr1.8Mn2O7 manganite has been investigated. Pressure shifts the Mn atoms from the center of each bilayer towards the adjacent bilayer. The observed shift of the Mn atoms involves mainly a charge redistribution within the d(3z(2)-r(2)) axial orbital. The decrease in the interbilayer spacing and the shift of the Mn atoms away from the center of each bilayer result in increased magnetic and electronic coupling between the adjacent bilayers along the c axis.     

类钙钛矿化合物Ca（Mn2Cu1）Mn4O12的磁性与磁电阻效应

利用固相反应法制备了名义成分为Ca(Mn2Cu1)Mn4O12的类钙钛矿锰氧化物.x射线衍射表明，为了获得较为致密的样品和减小杂相含量，可以采用高温烧结再在1073K长时间空气中退火的制备方法.样品在低温下同时存在铁磁相和反铁磁相，由于反铁磁相的存在导致样品在4.5K时的磁化强度显著降低，并在8T的高磁场下仍未达到饱和.样品呈半导体导电性质，在85K和6T磁场下磁电阻比的最大值可达-46%. 关键词： ［AC3］(B4)O12类钙钛矿锰氧化物 庞磁电阻效应 铁磁性 反铁磁性