Mitsuo Kataoka

The origin of the helical spin density wave in MnSi

Abstract By using a symmetry consideration, it is shown that an itinerant electron ferromagnet with no inversion symmetry is unstable against a helical spin density wave (HSDW) if a magnetic anisotropy energy is sufficiently small. The spin-orbit interaction is important to the stability of this HSDW. Only one of the clockwise and counterclockwise HSDWs is realized. The long and isotropic period of the HSDW in MnSi is explained successfully on this standpoint.

Helical Spin Density Wave Due to Antisymmetric Exchange Interaction

For the spin systems with both the symmetric exchange interaction (SEI) and the antisymmetric exchange interaction (AEI) (the Dzyaloshinsky-Moriya interaction), the relative stability between the ferromagnetic state (FMS) and the helical spin density wave (HSDW) is discussed phenomenologically. The arguments are made to the spin systems of which the spin structures are described by the spin density S ( r ) varying slowly with the position r and the SEI is favorable to the FMS. For the 21 types of crystal symmetry with the isogonal point groups having no inversion symmetry, the characteristics of the instability of the FMS are investigated. By use of the Landau free energy obtained for crystals with the isogonal point groups T and O , the magnetization process, the magnetic phase diagram, and the intensity of the neutron diffuse scattering are calculated. A comparison between the calculated results and the experiments on MnSi is also given.

Spin Waves in Systems with Long Period Helical Spin Density Waves Due to the Antisymmetric and Symmetric Exchange Interactions

In order to elucidate the different behaviours of the helical spin density waves (HSDW) due to the antisymmetric and symmetric exchange interactions, spin waves in the conical spin density wave (CSDW) states and the induced ferromagnetic states (IFMS) under external magnetic fields are calculated by use of phenomenological Hamiltonians with and without the antisymmetric exchange interaction. It is found that the spin wave spectra for the two types of HSDW are strongly different from each other, especially in the magnetic field dependences, enough to be able to distinguish them. The ESR frequencies are also investigated theoretically for bothtypes. In the light of these results, the observed magnetic field dependencies of the ESR frequencies in MnSi and Fe 1- x Co x Si are discussed to prove the HSDW due to the antisymmetric exchange interaction in these compounds.

Interference between Superconductivity and Martensitic Transition in A15 Compounds

An interference effect between the high- T c superconductivity and martensitic transition in A15 compounds such as V 3 Si and Nb 3 Sn has been studied. It is found from the measurements of the sound velocity, sound attenuation, thermal dilation and the electrical resistance, that an occurrence of superconductivity interferes drastically with the developing tetragonal distortions caused by the martensitic transformation, but the high- T c superconductivity in these compounds is only weakly affected by the presence of the martensitic transition. This unilateral interference effect is qualitatively consistent with the second-order band Jahn-Teller mechanism which means the Fermi level crossing the upper split band of the doubly degenerate \(\varGamma_{12}\) subbands. Discussions on T c are given on the basis of the Fermi surface effect of \(\varGamma_{12}\) subbands.

Antisymmetric Spin Interaction in Metals

The bilinear term of the spin-polarization density in the spin-dependent energy of metals is separated into the terms of the symmetric spin interaction (SSI) and of the antisymmetric spin interaction (ASI), whose properties are first investigated phenomenologically. A microscopic derivation of the SSI and ASI is given for a metal which has no inversion center. It is shown that the spin-orbit coupling which splits electronic bands in the absence of inversion center produces an antisymmetric component of the wave-vector-dependent susceptibility and the ASI is proportional to the antisymmetric component to the first order of the latter quantity. The obtained ASI energy is concluded to be not always small compared with the SSI energy.

Theory of the Antiferroorbital Ordering Due to the Cooperative Jahn–Teller Effect

The antiferroorbital ordering, i.e., the antiparallel ordering of electronic orbitals, in compounds is studied theoretically on the basis of the cooperative Jahn–Teller (JT) effect by use of a stat...

Theory of the Acoustic de Haas-van Alphen Effect

The de Haas-van Alphen (dHvA) oscillations of the magnetostrictions and the elastic constants are investigated theoretically. The electron band with orthorhombic symmetry is treated in the effective mass approximation. For ω s τ≪1 with the sound-wave angular frequency ω s and the electron life time τ, Landau orbitals are formed in a deformed metal without affecting local ion motions. In this static approximation to the ion motion, the free energy of the electron-lattice system is calculated as a function of strains. This free energy reveals the acoustic dHvA oscillations and proves that their amplitudes strongly depend on both the direction of an applied magnetic field and the strain mode. It is emphasized that the present acoustic dHvA effect can be a good probe into the electronic properties of metals such as the Fermi surface and the electron-lattice coupling.

Displacement waves in long-period superlattice alloys

The displacement of atoms from their normal positions, which is often observed in long-period superlattice (LPS) alloys, is investigated theoretically using the virtual crystal model. It is shown that the charge density wave (CDW) which exists in the LPS is the possible cause of these atomic displacements. If atomic potentials in binary alloys are divided into (i) the mean potential of the two kinds of atom and (ii) the potential difference, then the displacement waves of the optical mode are produced by the interaction between the CDW and the mean potential, while those of the acoustical mode are produced by the interaction between the CDW and the potential difference. The theory, when applied to the specific cases of L10-s and L12-s type of LPS alloys, gives results which are in qualitative agreement with the atomic displacements observed for CuAu and Au3Cd.

The martensitic transition in A15 compounds— the elastic softening in the low temperature phase

Abstract By taking into account the recent electronic band calculations for A15 compounds, the two bands evolving from the Γ 12 states are investigated by the k - p perturbation theory in order to clarify the origin of the martensitic transition. The second order martensitic transition and the arrest of the elastic softening in ( c 11 - c 12 ) are obtained.

Acoustic de Haas-van Alphen effect in LaAs

Acoustic de Haas-van Alphen effect of a semi-metallic compound LaAs has been investigated. The electron α Fermi surfaces and the hole β, γ surfaces in LaAs have been observed. The carrier number of LaAs was n = 0.0024 per La atom. The oscillation intensity of the longitudinal C 11 mode revealed enhanced values |g ii | ≅ 8−16 of the electron-strain coupling constants for the α and β surfaces in LaAs