Yutaka Moritomo

A First-Order Phase Transition Induced by a Magnetic Field

An electronic (metal-to-insulator) phase transition of the first order, which can be caused by an external magnetic field, was discovered in Nd1/2Sr1/2MnO3. A clear hysteresis was observed during the increase and decrease of an external magnetic field at a fixed temperature. The hysteretic field region was observed to depend critically on temperature and to drastically expand with a decrease of temperature, perhaps as a result of suppression of the effect of thermal fluctuations on the first-order phase transition. Although it has seldom been observed, this is thought to be a generic feature of the first-order phase transition at low temperatures near 0 kelvin.

Giant Magnetotransport Phenomena in Filling-Controlled Kondo Lattice System: La1-xSrxMnO3

Giant magnetotransport phenomena including the field-induced nonmetal-metal transition have been found in single crystals of La 1- x Sr x MnO 3 near the critical composition ( x ≈0.17) for the nonmetal-metal transition and in the temperature region around the magnetic phase transition. Change of the resistivity shows a universal curve as a function of the magnitude of temperature- or field-induced magnetization, the most of which agrees with the prediction by the D =∞ and S =∞ Kondo lattice model with strong ferromagnetic (Hund) coupling.

MAGNETISM AND STRUCTURAL DISTORTION IN THE LA0.7SR0.3MNO3 METALLIC FERROMAGNET

Neutron scattering studies on a single crystal of the highly correlated electron system, La{sub 1{minus}{ital x}}Sr{sub {ital x}}MnO{sub 3} with {ital x}{approx_equal}0.3, have been carried out elucidating both the spin and lattice dynamics of this metallic ferromagnet. We report a large measured value of the spin wave stiffness constant, which directly shows that the electron transfer energy of the {ital d} band is large. The spin dynamics, including magnetic critical scattering, demonstrate that this material behaves similar to other typical metallic ferromagnets such as Fe or Ni. The crystal structure is rhombohedral, as previously reported, for all temperatures studied (below {approximately}425 K). We have observed superlattice peaks which show that the primary rhombohedral lattice distortion arises from oxygen octahedra rotations resulting in an {ital R}{bar 3}{ital c} structure. The superlattice reflection intensities, which are very sensitive to structural changes, are independent of temperature demonstrating that there is no primary lattice distortion anomaly at the magnetic transition temperature {ital T}{sub {ital C}}=378.1 K; however, there is a lattice contraction. {copyright} {ital 1996 The American Physical Society.}

ORIGINS OF COLOSSAL MAGNETORESISTANCE IN PEROVSKITE-TYPE MANGANESE OXIDES (INVITED)

Phenomena of colossal magnetoresistance (MR) or magnetic field induced insulator–metal (I–M) transitions have been investigated for single crystals of perovskite‐type manganese oxides with controlled carrier density and one‐electron bandwidth. In addition to the canonical MR behavior near the Curie temperature, the first‐order phase transition accompanying several orders of magnitude change in resistivity has been observed under an external magnetic field for many of the composition‐controlled crystals as an intrinsic bulk phenomenon. It was proved by the systematic experimental investigations that the field‐induced destruction of the charge‐ordered state accompanying the lattice structural as well as metamagnetic transition is a major origin of such a colossal MR. Versatile MR phenomena and I–M phase diagrams in the T–H plane are presented with their interpretation.

Structural basis for the fast phase change of Ge2Sb2Te5: Ring statistics analogy between the crystal and amorphous states

The three-dimensional atomic configuration of amorphous Ge2Sb2Te5 and GeTe were derived by reverse Monte Carlo simulation with synchrotron-radiation x-ray diffraction data. The authors found that amorphous Ge2Sb2Te5 can be regarded as “even-numbered ring structure,” because the ring statistics is dominated by four- and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge–Ge homopolar bonds in amorphous GeTe constructs both odd- and even-numbered rings. They believe that the unusual ring statistics of amorphous Ge2Sb2Te5 is the key for the fast crystallization speed of the material.

Anomalous Magnetotransport Properties of Pr1-xCaxMnO3

Magnetotransport properties were investigated for a crystal of perovskite-type manganese oxide, Pr 1- x Ca x MnO 3 ( x =0.3). The single-crystal compound is insulating and below 220 K shows the charge-ordering of Mn 3+ and Mn 4+ species. Application of a magnetic field (>20 kOe) changes a canted antiferromagnetic insulating state into a ferromagnetic metallic state accompanied by change of resistivity by several orders of magnitude. At low temperatures, e.g. , <50 K, the hysteresis of the transition field is pronounced and the field-induced insulator-to-metal transition becomes irreversible.

A sodium manganese ferrocyanide thin film for Na-ion batteries

A thin film of sodium manganese ferrocyanide, Na1.32Mn[Fe(CN)6]0.83·3.5H2O, exhibits discharge capacity (= 109 mA h g(-1)) and discharge voltage (3.4 V in average) at 0.5 C against Na in aprotic solvent. The ex situ XRD experiments reveal that the host framework remains cubic without showing any structural phase transition during the charge process. The discharge property is discernible up to 40 C.

Striction-Coupled Magnetoresistance in Perovskite-Type Manganese Oxides

Magnetoresistance resulting in a drop in resistivity of more than three orders of magnitude that is strongly coupled to lattice striction has been observed under a relatively low magnetic field (0.4 tesla at 115 kelvin) for a single crystal of perovskite-type manganese oxide with finely controlled ionic radii of the A sites, (Nd,Sm)1/2Sr1/2MnO3. The colossal magnetoresistance phenomena are viewed as a first-order insulator-to-metal phase transition induced by a magnetic field, which accompanies a metamagnetic (antiferromagnetic-to-ferromagnetic) transition and a structural change in the lattice. Clear hystereses and abrupt changes in magnetization, striction, and resistivity were observed in increasing and decreasing magnetic fields at temperatures (113 to 150 kelvin) just above the Curie temperature.

Relation between crystal and magnetic structures of layered manganite La2-2xSr1+2xMn2O7 (0.30 ≤ x ≤ 0.50)

The magnetic phase diagram of La 2-2 x Sr 1+2 x Mn 2 O 7 (0.30 ≤ x ≤ 0.50) was reexamined from the point of view of the crystal structure, particularly the Jahn-Teller (JT) distortion Δ J T of Mn-O 6 octahedra, i.e., the ratio of the averaged apical Mn-O bond length to the equatorial Mn-O bond length. We have found that Δ J T decreases with increasing hole concentration x , while the canting angle between neighboring planes continuously increases from 0° (planar ferromagnet: 0.32 ≤ x ≤0.38) to 180° (A-type antiferromagnet (AFM): x =0.48). The dominance of the A-type AFM structure with the decrease of Δ J T can be ascribed to the change in the e g orbital state from d 3 z 2 - r 2 to d x 2 - y 2 .

Nanomagnetic droplets and implications to orbital ordering in La1-xSrxCoO3

Inelastic cold-neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic correlations that are dynamic follow the activation to the excited state, identified as the intermediate S = 1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La(1-x)Sr(x)CoO3 , the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.