Shinpei Fujii

A Half-Metallic Band Structure and Fe2MnZ (Z=Al, Si, P)

Band calculation results show that Heusler compounds Fe 2 MnZ (Z=Al,Si,P) have a band gap near the Fermi energy for one spin direction. Except for Fe 2 MnSi, the compounds does not have a half-metallic character because there is a slight density of states at the Fermi energy in the minority-spin state. An analysis of density of states indicates that there exists sufficiently strong d - d hybridization between metal atoms and weak p - d hybridization between Si and metal atoms. Due to this weakness, the replacement of Si by Al or P does not cause considerable change in the DOS structure. However, the weak hybridization defines the electronic character of the system.

Electronic Structure and Lattice Transformation in Ni2MnGa and Co2NbSn

It is known experimentally that Heusler alloys, Ni 2 MnGa and Co 2 NbSn undergo the cubic-to-tetragonal lattice transformation, in a ferromagnetic state and a paramagnetic one, respectively. We calculated electronic structures of Co 2 NbSn and Ni 2 MnGa for both cubic and tetragonal structures by KKR method. Comparing the density-of-state of the cubic and tetragonal structures for each alloy, it is expected that the band Jahn-Teller effect cause the lattice transformation in both alloys.

Electronic and Magnetic Properties of X2Mn1-xVxSi (X=Fe and Co)

We have performed energy-band calculations of X 2 Mn 1- x V x Si (X=Fe and Co) at x =0, 0.25, 0.5, 0.75, 1.0. In ferromagnetic ordering, we obtain excellent agreement between calculated and experimental data for the magnetic moment per formula unit and the lattice constant, at each Mn content. We have obtained the result that state densities in Fe 2 Mn 1- x V x Si have a large peak at the Fermi level, while they do not in Co 2 Mn 1- x V x Si. We have also estimated Heisenberg exchange constants from total energy differences between ferromagnetic and antiferromagnetic orderings. The Curie temperature roughly calculated from the exchange constants increases from x =0 to x =0.5 and then decreases rapidly in Fe 2 Mn 1- x V x Si, though it monotonically decreases from x =0 to x =0.75 in Co 2 Mn 1- x V x Si. The behavior of calculated Curie temperatures in Fe 2 Mn 1- x V x Si is consistent with the observed one.

Band Calculations for Non-Collinear Spin Arrangements in Gamma-Phase Manganese-Iron Alloys

We perfomed total-energy band calculations for Mn, FeMn, Fe 3 Mn and Fe in gamma phase. In the calculations, we dealt with not only a collinear spin arrangement but also non-collinear spin arrangements in the local density approximation. Apart from gamma-Fe and FeMn, we obtained reasonable results in the ground-state spin arrangement and magnetic moments.

MAGNETIC STATES IN BCC MANGANESE

We have carried out the total-energy band calculations of bcc Mn as a function of lattice constant for paramagnetic, ferromagnetic and antiferromagnetic states. We have found that bcc Mn undergoes a first-order transition from a low-spin state to a high-spin state, not only in the ferromagnetic state but also in the antiferromagnetic state. The obtained results of the total-energy calculations indicate that bcc Mn prefers a paramagnetic state under compression, a low-spin ferromagnetic state under zero pressure and a high-spin antiferromagnetic state under expansion.

Roles of Excess Atoms in Electronic and Magnetic Properties of Fe2+xV1-xZ (Z = Al, Ga)

To expand our knowledge of Fe 2+ x V 1- x Z (Z = Al, Ga), band calculations (LMTO-ASA) have been performed for several x values ( x =0, ±0.125, ±0.25) with a supercell method. Obtained density of states and magnetic moments indicate that excess atoms, which are introduced instead of the original Fe or V atom in Fe 2 VZ, bring a drastic change in electronic and magnetic properties of Fe 2 VZ. The existence of those atoms causes the collapse of the gap (or pseudogap) and the change from nonmagnetic to magnetic states. In the magnetic state, those atoms carry 2.5–2.6µ B for x >0 and 0.7–1.1µ B for x <0.

Antiferromagnetism and Atomic Disorder in Fe2VSi

To expand our knowledge of Fe 2 VSi, band calculations (LMTO-ASA) have been performed for paramagnetic (P), ferromagnetic (F) and antiferromagnetic (AF) orderings. The total-energy results show the...

High Spin Polarizaion of Ferrimagnetic Heusler-type Alloys in Mn-Cr-Z System (Z = IIIb, IVb, Vb Elements)

First-principle total-energy calculations show that a ferrimagnetic state is most stable in Mn 2 YZ (Y = Cr and Mn) with the Heusler structure and that among them there are highly spin-polarized ones with magnetic moments of 0, 1, and 2µ B per formula unit. Electronic structures of nonstoichiometric alloys suggest that partial replacement of constituent atoms in the highly spin-polarized alloys can be made without a decrease in spin polarizaion. Our results predict that in Mn–Cr–Z alloys we may get highly spin-polarized alloys with magnetic moments per formula unit ranging from -0.8 to 2µ B .

Half-Metallicity of (001) Film of Fe2CrSi

On the basis of the electronic structures obtained by first-principles calculations, we have studied the half-metallicity of a (001) film of the Heusler compound Fe 2 CrSi, which is predicted to be a half-metallic ferromagnet in its bulk. The result shows that a film with a surface consisting of Si and Cr atoms holds a high spin polarization (nearly 100%) and that the atomic disorder between Cr and Si atoms reduces spin polarization, but that the film holds a comparatively high spin polarization of 75%. We also show that, in a film with a surface consisting of Fe atoms, spin polarization decreases drastically from that of the bulk.

Spin Arrangement and Electronic Structure of Mn3GaX (X=C and N)

The electronic structures of Mn3GaX (X=C, N) were calculated for the ferromagnetic, antiferromagnetic and triangular spin arrangements. It is found that the difference of the hybridization between the spin states in the three magnetic states produces differences in the magnetic moments and the total energies.