Kiyoyuki Terakura

Room-temperature magnetoresistance in an oxide material with an ordered double-perovskite structure

Colossal magnetoresistance—a huge decrease in resistance in response to a magnetic field—has recently been observed in manganese oxides with perovskite structure. This effect is attracting considerable interest from both fundamental and practical points of view. In the context of using this effect in practical devices, a noteworthy feature of these materials is the high degree of spin polarization of the charge carriers, caused by the half-metallic nature of these materials,; this in principle allows spin-dependent carrier scattering processes, and hence the resistance, to be strongly influenced by low magnetic fields. This type of field control has been demonstrated for charge-carrier scattering at tunnelling junctions, and at crystal-twin or ceramic grain boundaries,, although the operating temperature of such structures is still too low (⩽150 K) for most applications. Here we report a material—Sr2FeMoO6, an ordered double perovskite—exhibiting intrinsic tunnelling-type magnetoresistance at room temperature. We explain the origin of this behaviour with electronic-structure calculations that indicate the material to be half-metallic. Our results show promise for the development of ordered perovskite magnetoresistive devices that are operable at room temperature.

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.

Orbital-State-Mediated Phase-Control of Manganites

Using the epitaxial strain, the magnetic and electronic phases can be controlled for thin films of the manganites, La1-xSrxMnO3, grown on perovskite substrates with various lattice parameters. The strain-induced orbital-ordering (disordering) via coupling

Strong ferromagnetism and weak antiferromagnetism in double perovskites: Sr2FeMO6 (M = Mo, W, and Re)

Double perovskites Sr2FeMO6 (M = Mo and Re) exhibit significant colossal magnetoresistance even at room temperature due to the high Curie temperatures (419 and 401 K). However, such a high Curie temperature is puzzling, given the large separation between

Active Sites and Mechanisms for Oxygen Reduction Reaction on Nitrogen-Doped Carbon Alloy Catalysts: Stone–Wales Defect and Curvature Effect

Carbon alloy catalysts (CACs) are promising oxygen reduction reaction (ORR) catalysts to substitute platinum. However, despite extensive studies on CACs, the reaction sites and mechanisms for ORR are still in controversy. Herein, we present rather general consideration on possible ORR mechanisms for various structures in nitrogen doped CACs based on the first-principles calculations. Our study indicates that only a particular structure of a nitrogen pair doped Stone-Wales defect provides a good active site. The ORR activity of this structure can be tuned by the curvature around the active site, which makes its limiting potential approaching the maximum limiting potential (0.80 V) in the volcano plot for the ORR activity of CACs. The calculated results can be compared with the recent experimental ones of the half-wave potential for CAC systems that range from 0.60 to 0.80 V in the reversible-hydrogen-electrode (RHE) scale.

Hydration of alkali ions from first principles molecular dynamics revisited

Structural and dynamical properties of the hydration of Li(+), Na(+), and K(+) in liquid water at ambient conditions were studied by first principles molecular dynamics. Our simulations successfully captured the different hydration behavior shown by the three alkali ions as observed in experiments. The present analyses of the dependence of the self-diffusion coefficient and rotational correlation time of water on the ion concentration suggest that Li(+) (K(+)) is certainly categorized as a structure maker (breaker), whereas Na(+) acts as a weak structure breaker. An analysis of the relevant electronic structures, based on maximally localized Wannier functions, revealed that the dipole moment of H(2)O molecules in the first solvation shell of Na(+) and K(+) decreases by about 0.1 D compared to that in the bulk, due to a contraction of the oxygen lone pair orbital pointing toward the metal ion.

A Possible Ground State and Its Electronic Structure of a Mother Material (LaOFeAs) of New Superconductors

The electronic and magnetic properties of the mother material LaOFeAs of new superconductors have been carefully studied using first-principles electronic structure calculations based on the generalized gradient approximation in the density functional theory. The present calculation predicts that the ground state of LaOFeAs is antiferromagnetic with a stripe type magnetic moment alignment leading to orthorhombic symmetry of the crystal. In this particular magnetic state, the density of states at the Fermi level is very small. On the other hand, LaOFeP has turned out to be paramagnetic and a good metal. Implications of the results regarding the experimental observations will also be presented.

Water at supercritical conditions: A first principles study

We analyze, via first principles molecular dynamics, the structural and electronic properties of water close to and above the critical point. Contrary to the ordinary liquid state, at supercritical conditions the hydrogen bond network is destabilized to various extents and the continuous breaking and reformation of hydrogen bonded structures allow large density and dipole fluctuations that, in turn, can significantly affect the dielectric properties of the solvent. Close to the critical point, where the density is very low, small clusters, mainly dimers and trimers, are the dominant features, but many molecules exhibit no H-bond. On the other hand, at higher densities, more extended structures appear, but still a continuous network cannot form. In both cases, H-bond configurations that are anomalous with respect to the normal liquid phase appear. These features strongly affect the solvent properties of supercritical water with respect to those of ambient water. They most likely vary continuously as a func...

A general mechanism underlying ferromagnetism in transition metal compounds

Among various magnetic orderings exhibited by transition-metal compounds, ferromagnetism is the most important particularly in technological aspects. Recent intensive studies on the colossal magnetoresistance (CMR) have given even another reason for the importance of ferromagnetism. It would be very useful if one could elucidate mechanisms for the stability of ferromagnetism. Zener’s mechanism based on the s-d model and the double exchange mechanism originally proposed also by Zener are such examples. In the present article, we point out that a new general mechanism underlies robust ferromagnetism in a group of transition metal compounds which include MnAs, ordered double perovskites such as A2FeMO6 with A=Ca,Sr and Ba and M=Mo and Re, and organic compound V(TCNE)2· 12CH2Cl2. For some of these materials, the Curie temperature Tc is quite high despite the fact that the magnetic ions are far separated by nonmagnetic ions. The stability of ferromagnetism for these materials can be predicted by the band-structure calculations 9) based on the density functional theory, which automatically take into account several mechanisms for exchange interactions, such as RKKY (including Zener’s mechanism), double exchange and even superexchange. This fact, however, does not mean that the mechanism for the stability of ferromagnetism is clarified. Below we present a physical picture for a new mechanism which may lead to the stability of ferromagnetism. We start with a simple case where the mechanism may be most clearly understood. We assume that the typical elements with the p states as the valence states intervene between the transition-metal elements and that the d states coming from the latter elements split energetically into fully occupied majority-spin state and empty minority-spin one due to the strong intraatomic exchange interaction irrespective of the magnetic ordering. This assumption is reasonable even in the metallic substances of present interest where the d states make bands, because the large inter-atomic distance between transition-metal atoms in these systems may make the d band width smaller than the exchange Fig. 1. A schematic illustration of the situation where ferromagnetism is strongly stabilized.The solid lines denote the density of states when the p-d mixing is switched off. The broken lines denote the valence p density of states with the p-d mixing. The electrons in the shaded area in the majority spin state are transferred to the shaded area in the minority spin state, leading to negative spin polarization in the valence p state and stabilization of ferromagnetic state with respect to antiferromagnetic state.

Hydration properties of magnesium and calcium ions from constrained first principles molecular dynamics

We studied the solvation structures of the divalent metal cations Mg(2+) and Ca(2+) in ambient water by applying a Car-Parrinello-based constrained molecular dynamics method. By employing the metal-water oxygen coordination number as a reaction coordinate, we could identify distinct aqua complexes characterized by structural variations of the first coordination shell. In particular, our estimated free-energy profile clearly shows that the global minimum for Mg(2+) is represented by a rather stable sixfold coordination in the octahedral arrangement, in agreement with experiments. Conversely, for Ca(2+) the free-energy curve shows several shallow local minima, suggesting that the hydration structure of Ca(2+) is highly variable. Implications for water exchange reactions are also discussed.