N. Fukatani

Stabilization of Antiferromagnetism in Fe2VSi Epitaxial Thin Films

The structural, electrical, and magnetic properties of epitaxial thin films of a Heusler compound Fe2VSi on (001) MgAl2O4 and MgO substrates were investigated. Structural characterization revealed that the films on MgAl2O4 (the lattice mismatch to Fe2VSi of 0.7%) were in tetragonally strained states, while the films on MgO (the lattice mismatch = 4.9%) were in relaxed strain states with a cubic lattice. Unstrained films on MgO exhibited an antiferromagnetic transition at 129 K, which is close to the bulk data. In contrast, strained films on MgAl2O4 showed a significant enhancement in the antiferromagnetic transition temperature up to 193 K. These results could be attributed to the stabilization of antiferromagnetic ordering induced by epitaxial strain.

Epitaxial strain and antiferromagnetism in Heusler Fe2VSi thin films

The effects of biaxial strain on the electrical and magnetic properties of an antiferromagnetic Heusler compound Fe2VSi were systematically investigated. A series of epitaxial Fe2VSi thin films on MgAl2O4 and MgO substrates were fabricated with different tensile strains by varying the substrate-lattice mismatch and the film thickness. The strain was characterized by the ratio of the out-of-plane lattice parameter c to the in-plane lattice parameter a; this ratio c/a varied from 0.987 to 0.998 at room temperature. The tensile epitaxial strain was found to increase the antiferromagnetic Neel temperature TN to 193 K, which is 70 K higher than that for the unstrained bulk material. A clear dependence of TN on c/a was observed, which is consistent with theoretical predictions in which the band Jahn-Teller effect plays a significant role.

Structural and electrical properties of half-Heusler La-Pt-Bi thin films grown by 3-source magnetron co-sputtering

Half-Heusler La-Pt-Bi thin films have been deposited on YAlO3(001) substrate by 3-source magnetron co-sputtering. Control of the Bi content was the critical factor to obtain single phase, c-axis-oriented thin films. Generation of secondary phases was effectively prevented by precise control of the deposition rate for separate targets as well as adjustment of the deposition temperature. The realization of single-phase LaPtBi thin films will provide new potential applications to topological insulating devices based on Heusler alloys.

The effect of magnetocrystalline anisotropy on the domain structure of patterned Fe2CrSi Heusler alloy thin films

The effects of magnetic anisotropy on domain structures in half-metallic Heusler alloy Fe2CrSi thin film elements were investigated using high resolution x-ray magnetic circular dichroism photoemission electron microscopy. The transition of the dominating contribution from the magnetocrystalline anisotropy to the shape anisotropy is observed in square-shaped elements when reducing the size below 2.0–2.5 μm. In particular, we identify in disk-shaped Heusler elements the vortex state as the ground state. The shape-anisotropy dominated, well-defined magnetization configuration shows the potential of the Fe2CrSi Heusler alloy for applications in vortex-core- or domain-wall-devices, where the high spin polarization is desirable.

Structural and magnetic properties in Heusler-type ferromagnet/antiferromagnet bilayers

The structure and magnetic properties of Heusler ferromagnet/antiferromagnet (FM/AFM) bilayers were investigated. Structural characterization revealed that Fe2CrSi/Ru2MnGe bilayers were epitaxially grown with an L21 ordered structure when the Ru2MnGe growth temperature was 573 K. The magnetization curve indicates that exchange bias occurs in the Fe2CrSi/Ru2MnGe bilayers at 77 K. The appearance of an exchange-anisotropy field Hex depends on the growth of Ru2MnGe layer, indicating that the exchange bias originates from the epitaxial Fe2CrSi/Ru2MnGe interface. The exchange anisotropy energy Jk of the Fe2CrSi/Ru2MnGe and Co2MnGe/Ru2MnGe bilayer systems appears to follow a Heisenberg-like exchange coupling model. This suggests that the Cr and Mn interface atoms in the FM layers play significant roles in exchange coupling in the Fe2CrSi/Ru2MnGe and Co2MnGe/Ru2MnGe bilayer systems, respectively.

Inter-Layer Exchange Coupling in

In this study, we have fabricated Heusler alloy Fe₂CrSi/Fe₂VSi superlattice on MgAl₂O₄ substrate and have investigated inter-layer exchange coupling among Fe₂CrSi layers. Biquadratic coupling is dominated below the Neel temperature of Fe₂VSi. By comparing magnetic coupling energy at 77 K and room temperature, it is suggested that the inter-layer exchange coupling is dominated by the proximity effect, i.e. the antiferromagnetic alignment of magnetic moments in Fe₂VSi spacer layer.

\hbox{Fe}_{2}\hbox{CrSi/Fe}_{2}\hbox{VSi}

Epitaxial MgAl2O4 thin films were grown on Heusler alloy Fe2CrSi by reactive magnetron sputtering of a MgAl2 target in an O2+Ar atmosphere. To grow MgAl2O4 on Fe2CrSi, we inserted a protective layer of MgAl2 between Fe2CrSi and MgAl2O4 to prevent Fe2CrSi from being oxidized. Growth of MgAl2O4 was found to be very sensitive to the MgAl2 thickness and PO2 during deposition of MgAl2O4. A strong XRD peak of MgAl2O4 (004) was observed with an ultrathin (0.2 nm) MgAl2 layer. The saturation magnetic moment of Fe2CrSi was measured to be 370 emu/cm3 (1.84 µB/f.u.) at room temperature and it is expected to have a high spin polarization. The Fe2CrSi/MgAl2O4 heterostructure is promising for use in future spintronic devices.

Multilayers Fabricated on

Structural and electrical transport properties were investigated for Heusler-type alloy Ru₂ MnGe thin films. Ru₂ MnGe films on MgO substrate were subjected to an in-plane compressive strain due to their small lattice mismatch (-0.7%), and exhibited enhanced antiferromagnetic transition temperature (<i>T</i>_N) up to 353 K, which is much higher than that of the bulk material (<i>T</i>_N = 316 K). In contrast, the films on MgAl ₂O ₄ were almost in a relaxed-strain state, and showed <i>T</i>_N close to the bulk value (304 K). It was also found that the <i>T</i>_N of Ru₂MnGe thin films on MgO exhibited oscillating behavior depending on <i>c</i>/<i>a</i> ratio. We argued that the next-nearest neighbor magnetic interactions (<i>J</i>₂) of Mn-Mn atoms has oscillated depending on the degree of strain in Ru₂MnGe, which contribute to the oscillating behavior of <i>T</i>_N against <i>c</i>/<i>a</i>.

\hbox{MgAl}_{2}\hbox{O}_{4}

The structural and transport properties in Fe2CrSi/MgAl2Ox/Fe2CrSi were studied. Full-epitaxial Fe2CrSi/MgAl2Ox/Fe2CrSi structures were obtained. From the differential conductance measurements, the combination of protective MgAl2-oxidation layer and reactively sputtered MgAl2Ox layer show potential properties for suitable tunnel barriers for MTJs.