Tadashi Kai

The possibility of ferromagnetic BCC ruthenium

Band-structure calculations based on the augmented-plane-wave method and the rigid-band model are used to investigate the ferromagnetic 4d transition metals in the constrained crystal structure. The densities of states of 4d transition metals at the Fermi energy are estimated from those calculated for BCC and FCC Rh in paramagnetism. The results show that BCC Ru seems to be ferromagnetic using the Stoner criterion because of the high density of states. The band structure of Ru is studied in detail using the first-principles total-energy-band calculation by the Korringa-Kohn-Rostoker method based on the local-density approximation, whether ferromagnetic BCC Ru exists or not. It is concluded that BCC Ru has a magnetic moment of about 1mu B at 5% expanded lattice constant.

Effect of copious vacancies on magnetims of Pd

The electronic structures and magnetic moments of Pd with copious vacancies up to 15% have been calculated using all electron self-consistent-field Korringa-Kohn-Rostoker coherent-potential-approximation (SCF-KKR-CPA) method. The result of the calculation predicts that the magnetic transition occurs at about 10% vacancy concentration. This phenomena results from the charge transfer from Pd to vacancy.

Study of giant magnetoresistance in Co/X superlattices (X=Cu, Ru, Rh and Pd) by first-principle band calculation

Abstract First-principle band structure calculation based on linear-muffin-tin-orbital atomic-sphere-approximation method is used to study the giant magnetoresistance (MR) effect and the electronic structure of Co/X superlattices (X=Cu, Ru, Rh and Pd). The MR ratio is estimated as the ratio of the D(eF)vF2 between the anti-parallel and the parallel magnetic configurations, where D(eF) and vF are the density of states at Fermi energy and the Fermi velocity. The non-magnetic layer thickness dependence of the MR ratio is also calculated. These results are in good agreement with experiments.

Influence of spin-dependent scattering at rough interface on the giant magnetoresistance in magnetic multilayers

Spin-dependent scattering in Co/Cu and Co/Ru multilayers with interface roughness is estimated. The electronic band structures of Co/Cu and Co/Ru multilayers with smooth interfaces for parallel and antiparallel spin configurations are calculated self-consistently using linear muffin-tin orbital atomic sphere approximation (LMTO-ASA) band structure calculations based on density functional theory. The spin-dependent scattering caused by interface roughness is calculated from the transition probabilities obtained from self-consistent wave functions and potential differences found from these band structure calculations. Our results indicate that spin-dependent scattering caused by interface roughness plays an important role in the giant magnetoresistance (GMR) effect for magnetic multilayers, as predicted by calculations using a simple model. Furthermore, it is concluded that the scattering depends on differences in the band structure for different spin configurations, as well as for interface roughness. The GMR difference between Co/Cu and Co/Ru can be explained qualitatively by difference in spin-dependent scattering due to interface roughness, which is caused by the band structure with different spin configuration.

Effect of Nb addition to Ni and Fe on electronic structure and magnetic properties

Abstract The electronic structure and magnetic properties of fcc NiNb and bcc FeNb alloys within the Nb concentration of 20 at% are studied by band structure calculations based on the Korringa-Kohn-Rostoker coherent-potential approximation method. The results show that niobium has a magnetic moment in Ni and Fe with antiparallel coupling. The density of states of Nb in fcc NiNb and bcc FeNb calculated in the paramagnetic state shows a minimum at the Fermi energy which leads to the antiparallel coupling of Nb. The local magnetic moments of Nb and Ni drop abruptly in fcc NiNb alloy as the Nb concentration increases, although the local magnetic moment of the Nb hardly decreases in bcc FeNb alloy.

A theoretical study of interfacial structure of Co/Cu and Co/Pd multilayers

The electronic structures and energies of Co/Cu and Co/Pd multilayers with abrupt or mixed interfaces have been calculated using the linear muffin-tin orbital method in the atomic sphere approximation. The mixed interface is modelled by exchanging atoms between Co layers and Cu (Pd) layers. The calculated total energies of Co/Cu multilayers with mixed interfaces are higher than those of Co/Cu multilayers with abrupt interfaces. The calculated total energies of Co/Pd multilayers with mixed interfaces are lower than those of Co/Pd multilayers with abrupt interfaces. Stable interface structure appears to be abrupt in Co/Cu multilayers and mixed in Co/Pd multilayers, as revealed by experimental observations. Total-energy differences between the multilayer with abrupt interfaces and the multilayer with mixed interfaces are caused by Co 3d-band narrowing for Co/Cu multilayers and Pd spin polarizations for Co/Pd multilayers.

The effect of addition of Ru to Fe on the electronic structure and magnetic properties

Band structure calculations based on the Korringa-Kohn-Rostoker coherent-potential approximation method are used to study the electronic structure and magnetic properties of BCC FeRu alloys within a Ru concentration of 20 at.%, although it is experimentally known that FeRu alloys include the FCC and HCP structures beyond a 5 at.% Ru concentration. The results show that the ruthenium is ferromagnetic in alpha -Fe with a moment of about 0.5mu B and that the ruthenium addition to alpha -Fe enhances the magnetic moment at Fe sites by about 0.02mu B/Ru at.% because of the magnetovolume effect and large spin splitting by Ru addition. The density of states calculated in the paramagnetic state shows that the strong d-d hybridization between Fe and Ru causes the spin splitting of Ru. The BCC structure is important in the process of Ru becoming ferromagnetic. We have clarified the reason why the average magnetic moment of the FeRu BCC alloy scarcely decreases with the Ru addition.

Effect of charge neutrality condition in KKR-CPA method

Abstract The electronic structure of ferromagnetic binary bcc Fe 0.98 Ni 0.02 and Fe 0.8 Ni 0.2 alloys are calculated by the KKR-CPA method using two charge neutrality conditions. The two charge neutrality conditions are considered to be satisfied by each site and configuration averaged site. To study ferromagnetic states, spin polarized calculation is introduced to these methods. The numerical results show that the effect is small for magnetic moment. It is found that the individual interstitial electron density is not suitable for the cohesive problem of the alloys.

Effect of 4d transition metals on the electronic structure and magnetic properties of

The electronic structure and magnetic properties of bcc random alloys with Fe and 4d transition metals were studied using the first-principles Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method. The average magnetic moments of the alloys agreed well with the experimental results. The local magnetic moment at the Fe site was enhanced in all alloys.

Effect of Ru addition to Ni on the electronic structure and magnetic properties

Band-structure calculations based on the Korringa - Kohn - Rostoker coherent-potential approximation method are used to study the electronic structure and magnetic properties of FCC Ni - Ru alloy with a Ru concentration of 20 at.%. The results show that the ruthenium is ferromagnetic in Ni with a moment of about in the low-Ru-concentration region. The density of states calculated in the paramagnetic state shows that the strong d - d hybridization between Ni and Ru causes spin splitting of Ru, which is similar to the BCC Fe - Ru alloy. However, the local magnetic moment of Ru in the FCC Ni - Fu alloy drops abruptly as the Ru concentration increases. It is due to the non-magnetic character of FCC Ru, although BCC Ru is ferromagnetic. We clarified the reason why the average magnetic moment of the FCC Ni - Ru alloy drastically decreases on Ru addition.