Yohei Kota

Gilbert magnetic damping constant of epitaxially grown Co-based Heusler alloy thin films

The magnetic damping constant in a series of Co2MnAlxSi1−x and Co2FexMn1−xSi Heusler alloy epitaxial films were systematically investigated by using ferromagnetic resonance technique. The determined magnetic damping constant is roughly proportional to the density of states at the Fermi energy of the first principle calculation. The result is consistent with the theoretical prediction when taking spin-orbit interaction into account. The small Gilbert damping constant for the fabricated films other than the Co2FexMn1−xSi film with x>0.6 can be originated in the half-metallic electronic structure of Heusler alloys.

Relationship between Magnetocrystalline Anisotropy and Orbital Magnetic Moment in L10-Type Ordered and Disordered Alloys

The magnetocrystalline anisotropy energy and orbital magnetic moment in L 1 0 -type transition metal alloys such as FePt, FePd, FeNi, CoPt, CoPd, and MnAl are evaluated while continuously varying the degree of order. The electronic structure with spin–orbit interaction is calculated by employing the tight-binding linear muffin-tin orbital method based on the local spin-density approximation. To control the degree of order, we consider a substitutional disorder and then adopt the coherent potential approximation. The magnetocrystalline anisotropy energy Δ E is roughly proportional to the power of the long-range order parameter S , i.e., Δ E ∝ S n ( n ∼1.6–2.4). We also discuss the relationship between the magnetocrystalline anisotropy energy and the orbital magnetic moment. In the same compositional system with different degrees of order, the difference between the orbital magnetic moment in the magnetic easy axis and that in the hard one is proportional to Δ E . However, the coefficient corresponding to t...

Mechanism of Uniaxial Magnetocrystalline Anisotropy in Transition Metal Alloys

Magnetocrystalline anisotropy in transition metal alloys (FePt, CoPt, FePd, MnAl, MnGa, and FeCo) was studied using first-principles calculations to elucidate its specific mechanism. The tight-binding linear muffin-tin orbital method in the local spin-density approximation was employed to calculate the electronic structure of each compound, and the anisotropy energy was evaluated using the magnetic force theorem and the second-order perturbation theory in terms of spin–orbit interactions. We systematically describe the mechanism of uniaxial magnetocrystalline anisotropy in real materials and present the conditions under which the anisotropy energy can be increased. The large magnetocrystalline anisotropy energy in FePt and CoPt arises from the strong spin–orbit interaction of Pt. In contrast, even though the spin–orbit interaction in MnAl, MnGa, and FeCo is weak, the anisotropy energies of these compounds are comparable to that of FePd. We found that MnAl, MnGa, and FeCo have an electronic structure that ...

Magnetocrystalline anisotropy in FePt with L10 ordering and tetragonal distortion

We evaluate the magnetocrystalline anisotropy energy of L10 type FePt alloys with the lattice distortion and atomic disorder by using the first-principles calculation, which adopts the tight-binding linear muffin-tin orbital method in conjunction with the coherent potential approximation techniques. The calculated result indicates that the magnetocrystalline anisotropy energy is quite sensitive to the mentioned factors. In particular, it is drastically decreased with the degree of ordering compared with the expected value from the completely ordered structure. We will suggest that the improvement of the chemical ordering of the L10 crystal is one of the significant points to obtain a large magnetocrystalline anisotropy from FePt compounds.

Degree of Order Dependence on Magnetocrystalline Anisotropy in Body-Centered Tetragonal FeCo Alloys

We investigate the magnetocrystalline anisotropy (MCA) energy of tetragonal distorted FeCo alloys depending on the degree of order by first-principles electronic structure calculation combined with the coherent potential approximation. The obtained results indicate that the MCA energy of FeCo alloys strongly depends on the degree of order under optimal conditions, where the axial ratio of the body-centered tetragonal (bct) structure is 1.25 and the composition is Fe0.5Co0.5. We find that the modification of the electronic structure resulting from electron scattering by chemical disorder has a considerable influence on the MCA under these conditions.

Uncompensated antiferromagnetic moments in Mn-Ir/FM (FM = Ni-Co, Co-Fe, Fe-Ni) bilayers: Compositional dependence and its origin

Ferromagnetic (FM) material dependence of the uncompensated (UC) antiferromagnetic (AF) moments in AF/FM exchange biased bilayers has been studied using the x-ray magnetic circular dichroism technique in the AF/FM (AF = γ-Mn-Ir, FM = Ni-Co, Co-Fe, Fe-Ni) bilayers. The direction and magnitude of the UC-Mn moment change significantly when the composition of the FM layer changes. The crystal structure of the FM layer affects the magnitude of the UC-Mn moments. The UC-Mn moments and the FM moments of Fe-rich alloys prefer the anti-parallel alignment. Conversely, the UC-Mn moments align parallel to the FM moments in Co-rich or Ni-rich regions. A first-principles calculation pertaining to the L12-Mn3Ir/FM (FM = Ni4-nCon, Co4-nFen, Fe4-nNin; n = 0, 1, 2, 3) bilayer system was carried out to characterize the UC-Mn moments near the interface. It was found that the UC-Mn moments originate from the reorientation of the magnetic moments of Mn and other ferromagnetic atoms near the AF/FM interface. The calculated resu...

First Principles Calculation of Magnetocrystalline Anisotropy Energy of MnBi and MnBi1-xSnx

We calculated the magnetic anisotropy constant Ku of MnBi using a first principles approach, and obtained a negative Ku in agreeable with experimental results. Furthermore, we also found a band filling dependence indicating that a slight decrease in the valence electron number will change Ku from negative to positive. When some of the Bi is replaced with Sn to decrease the valence electron number, the Ku value of MnBi1-xSnx drastically changes to a positive value, Ku~2 MJ/m3, for x > 0.05.

Effect of lattice deformation on exchange coupling constants in Cr2O3

We studied lattice deformation effect on exchange interaction in the corundum-type Cr2O3 theoretically. First-principles electronic structure calculations were performed to evaluate the total energy and exchange coupling constants of Cr2O3 under lattice deformation. We found that a few percent elastic deformation is expected via misfit strain and that the first- and second-nearest neighbor exchange coupling constants of Cr2O3 strongly depend on the lattice deformation. These results imply a possibility for improving the thermal stability of Cr2O3 based magnetoelectric devices by lattice deformation.

Spin-polarized electronic structures and transport properties of Fe-Co alloys

The electrical resistivities of Fe-Co alloys owing to random alloy disorder are calculated using the Kubo-Greenwood formula. The obtained electrical resistivities agree well with experimental data quantitatively at low temperature. The spin polarization of Fe50Co50 estimated from the conductivity (86%) has opposite sign to that from the densities of the states at the Fermi level (−73%). It is found that the conductivity is governed mainly by s electrons, and the s electrons in the minority spin states are less conductive due to strong scattering by the large densities of the states of d electrons than the majority spin electrons.

First-principles study for electronic structure and physical property of Co-based Heusler alloys

The statistical investigation of the physical property of Co-based Heusler alloys with the atomic composition or configuration dependence are carried out in first-principles approach. In particular, the electronic structure, magnetic moment and electrical resistivity due to disorder alloy effects of Co2MnAl1−zSiz (0.0 < z < 1.0) and Co2Mn1−yFeySi (0.0 < y < 1.0) with the L21, B2 and A2 structure are calculated by using the tight-binding linear muffin-tin orbital method combined with the coherent potential approximation based on the local spin-density functional approximation and by using the Kubo-Greenwood formula, respectively. The obtained results indicate that these properties of the Co-based Heusler alloys significantly depend on the atomic configuration and composition, especially, they are sensitive to the existence of the half-metallic property.