A. Asamitsu

Current switching of resistive states in magnetoresistive manganites

Magnetoresistive devices (based on, for example, magnetic multilayers) exhibit large changes in electrical resistance in response to a magnetic field, which has led to dramatic improvements in the data density and reading speed of magnetic recording systems. Manganese oxides having a perovskite structure (the so-called manganites) can exhibit a magnetoresistive response that is many orders of magnitude larger than that found for other materials, and there is therefore hope that these compounds might similarly be exploited for recording applications. Here we show that the switching of resistive states in the manganites can be achieved not only by a magnetic field, but also by an electric field. For manganites of the form Pr1−xCaxMnO3, we find that an electrical current (and by implication a static electric field) triggers the collapse of the low-temperature, electrically insulating charge-ordered state to a metallic ferromagnetic state. We suggest that such a phenomenon could be exploited to pattern conducting ferromagnetic domains within an insulating antiferromagnetic matrix, and so provide a route for fabricating micrometre- or nanometre-scale electromagnets.

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.

Interplane Tunneling Magnetoresistance in a Layered Manganite Crystal

The current-perpendicular-to-plane magnetoresistance (CPP-MR) has been investigated for the layered manganite, La2−2xSr1+2xMn2O7 (x = 0.3), which is composed of the ferromagnetic-metallic MnO2 bilayers separated by nonmagnetic insulating block layers. The CPP-MR is extremely large (104 percent at 50 kilo-oersted) at temperatures near above the three-dimensional ordering temperature (Tc ≈ 90 kelvin) because of the field-induced coherent motion between planes of the spin-polarized electrons. Below Tc, the interplane magnetic domain boundary on the insulating block layer serves as the charge-transport barrier, but it can be removed by a low saturation field, which gives rise to the low-field tunneling MR as large as 240 percent.

The Anomalous Hall Effect and Magnetic Monopoles in Momentum Space

Efforts to find the magnetic monopole in real space have been made in cosmic rays and in particle accelerators, but there has not yet been any firm evidence for its existence because of its very heavy mass, ∼1016 giga–electron volts. We show that the magnetic monopole can appear in the crystal momentum space of solids in the accessible low-energy region (∼0.1 to 1 electron volts) in the context of the anomalous Hall effect. We report experimental results together with first-principles calculations on the ferromagnetic crystal SrRuO3 that provide evidence for the magnetic monopole in the crystal momentum space.

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.

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.

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.

Crossover behavior of the anomalous hall effect and anomalous nernst effect in itinerant ferromagnets

The anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) are experimentally investigated in a variety of ferromagnetic metals including pure transition metals, oxides, and chalcogenides, whose resistivities range over 5 orders of magnitude. For these ferromagnets, the transverse conductivity sigma{xy} versus the longitudinal conductivity sigma{xx} shows a crossover behavior with three distinct regimes in accordance qualitatively with a recent unified theory of the intrinsic and extrinsic AHE. We also found that the transverse Peltier coefficient alpha{xy} for the ANE obeys the Mott rule. These results offer a coherent and semiquantitative understanding of the AHE and ANE to an issue of controversy for many decades.

Critical change of magnetoresistance with bandwidth and doping in perovskite manganites

To explore the optimized colossal magnetoresistance (MR), i.e., higher MR with lower field, magnetotransport properties of single-crystalline perovskite manganites have systematically been investigated. Near x=1/2 with intrinsic instability of charge ordering (a 1/1 ordering of Mn3+/Mn4+), the one-electron bandwidth (W) is varied by reducing the radius of R-site cation in R1−xSrxMnO3. For R=Nd, the MR behavior is rather canonical, while for R=Sm, the field-induced nonmetal-to-metal transition of the first order shows up accompanying a change in resistivity by several orders of magnitude as a result of an enhanced antiferromagnetic interaction.