Kunitomo Hirai

Lattice Distortion and Multiple Spin Density Wave State in γMn Alloys

The relation between tetragonal lattice distortion observed in Mn-rich γMn alloys such as MnNi, MnFe and MnCu and their magnetic structures is investigated on the basis of the Landau expansion of free energy. By taking into account a coupling between lattice distortions and three magnetic order parameters specifying the first-kind antiferromagnetic ordering on the fcc lattice, possible phase diagrams in the temperature-composition plane are discussed. Observed various types of phase diagram for lattice distortion including the orthorhombic phase in γMnNi alloys are shown to be well explained and the type of multiple spin density wave state for each phase is predicted.

Real Space Approach to the Electronic Structure of Transition Metals

The role of d symmetry of atomic orbitals in determining the electronic structure of transition metals is discussed by use of a real space expansion of the Green function. Terms which are sensitive to the crystal structures are separated from those corresponding to the path integrals on a Bethe type lattice. On the basis of this discussion a simple method is developed for calculating the electronic structure of transition metals. By use of the method the correspondence between the band theory and atomic interaction models can be established. It is found that the density of states for bcc and fcc are satisfactorily reproduced with the information from the interactions among near neighboring atoms, and that the difference between bcc and fcc arises mostly from three or four atoms interactions, while distant neighbors contribute mostly to the structure insensitive self-energy.

Triple-Q and Single-Q States in Antiferromagnetic fcc Transition Metals with the First-Kind Ordering

By using a real space expansion approach to calculate the Green function of d -electrons, the four-spin interaction is derived and the relative stability among various multiple spin density wave states is discussed for antiferromagnetic fcc transition metals with the first-kind ordering. The transition from a triple- Q state to a single- Q state is shown to occur with increasing or decreasing the number of d -electrons from the nearly half-filled band region.

Electronic Structure of Sinusoidal Spin Density Wave State in Chromium

The spin density wave in chromium is discussed on the basis of electronic structure calculation. The electronic structure of sinusoidal spin density wave states with finite local magnetic moments is calculated for chromium by means of a tight-binding model Hamiltonian with a minimal basis set, which is obtained through some simplification to the first-principle LMTO method. The wave vector of the sinusoidal spin density wave state of lowest energy is found to be in good agreement with experiments for chromium and its alloys. Not only the amplitude of the fundamental wave of the spin density wave, but also that of the concomitant charge density wave and that of the third harmonic of the spin density wave are discussed.

Magnetism in Spin-Density-Wave Chromium from First-Principles Calculation

The electronic structure of the spin-density-wave state in chromium is calculated by means of the first-principles Korringa-Kohn-Rostoker Green function method within the framework of the local spin density functional formalism. Results of the calculation are found to be in good agreement with those of experiments concerning the magnetism in spin-density-wave chromium, that is, the amplitude of the fundamental wave and the ordering wave vector. The amplitudes of the harmonics of the spin density wave and concomitant charge density wave as well as the hyperfine field at a chromium nucleus are furthermore investigated.

Magnetic Phase Diagrams at Zero Temperature of bcc and fcc Transition Metals

The method for calculating the electronic structure of transition metals which was developed before is applied to ferromagnetic, antiferromagnetic and helical spin density wave states. By use of the method, the magnitude of local moments, energy, etc. are calculated within the Hartree-Fock approximation. The relative stability of those states is investigated for a given valence, and phase diagrams of bcc and fcc transition metals are constructed. It is found that the most stable state changes continuously from an antiferromagnetic one to a ferromagnetic one via a helical spin density wave one when the valence changes from five to ten. The correspondence between the obtained phase diagrams and experimentally observed ones for 3 d metals is discussed.

Spin-Density-Wave Order and Interlayer Magnetic Coupling in Fe/Cr Superlattices

A first-principles electronic structure calculation for Fe/Cr superlattices with layered structures is performed by means of the Korringa-Kohn-Rostoker Green function method within the framework of the local spin density functional formalism. The calculation is carried out for periodic superlattices which consist of ferromagnetic Fe layers and antiferromagnetic or spin-density-wave Cr layers, with magnetizations of two successive Fe layers being aligned parallel or antiparallel, and interlayer magnetic coupling is evaluated by the total energy difference between the parallel and antiparallel couplings. It is shown that the interlayer magnetic coupling oscillates between parallel and antiparallel with a two-monolayer period of the spacer thickness of the Cr layer but the oscillation involves a phase slip around a critical thickness of 16 monolayers, at which the magnetic order in the Cr layer changes from the antiferromagnetic one to the spin-density-wave one. Characteristics of the spin-density-wave order...

Electronic Structure of Helical Spin Density Wave State in fcc Iron

The spin density wave in fcc iron is discussed on the basis of electronic structure calculation. The electronic structure of helical spin density wave states with finite local magnetic moments is calculated for fcc iron by a tight-binding model Hamiltonian with a minimal basis set, which is obtained through some simplification to the first-principle LMTO method. The wave vector of the helical spin density wave state of lowest energy is found to be in good agreement with recent neutron diffraction experiments for fcc iron precipitates. The calculation is also carried out for chromium, and difference in characteristics of the spin density wave between fcc iron and chromium is discussed.

Multiple Spin Density Wave State and Symmetry in fcc Antiferromagnets

Relative stabilities among various multiple spin density wave (MSDW) states specified by equivalent wave vectors are discussed on the basis of symmetry considerations for the first-, the second- and the third-kind antiferromagnetic structures in the fcc lattice. The energy of the MSDW state is expressed as the expansion with respect to normalized amplitudes for constituent single- Q states. The degeneracy among MSDW states, which is not lifted in the Heisenberg model, is shown to be removed by more than or equal to the fourth-order perturbation with respect to transfer integrals between atoms. Within the lowest order expression to remove the degeneracy, possible stable structures are discussed. The effect of higher order perturbations and the prospect to determine the most stable MSDW state in transition metals with the first-kind ordering are also discussed.

Total Energy Calculation for Spin-Density-Wave Chromium

The total energy of the spin-density-wave state in chromium is calculated by means of the first-principles Korringa-Kohn-Rostoker Green function method within the framework of the local spin density functional formalism. The calculation for the case of the experimental lattice constant shows that the total energy per atom becomes the lowest at a wave vector close to the observed ordering wave vector of spin-density-wave chromium. The calculation with varying the lattice constant shows that the total energy of the spin-density-wave states does not become minimum against the change of the lattice constant, and it may be concluded that the theoretical ground state at the total energy minimum for chromium is a nonmagnetic state, though on the verge of a transition to a spin-density-wave state. The effect of the periodic lattice distortion due to the charge density wave, which usually accompanies the spin density wave, on the total energy is further investigated.