Keisuke Yamada

Electrical detection of magnetic states in crossed nanowires using the topological Hall effect

We used micromagnetic simulations to investigate the spatial distributions of the effective magnetic fields induced by spin chirality in crossed nanowires with three characteristic magnetic structures: a radiated-shape, an antivortex, and a uniform-like states. Our results indicate that, unlike the anomalous Hall effect, the topological Hall effect (which is related to the spin chirality) depends on both the polarity and the vorticity. Therefore, measuring the topological Hall effect can detect both the polarity and the vorticity simultaneously in crossed nanowires. This approach may be suitable for use as an elemental technique in the quest for a next-generation multi-value memory.

Ferromagnetic resonance of Ni wires fabricated on ferroelectric LiNbO3 substrate for studying magnetic anisotropy induced by the heterojunction

The electrical ferromagnetic resonance of micro-scale Ni wires with magnetic anisotropy induced by the heterojunction between the Ni layer and ferroelectric single crystalline LiNbO3 substrate was demonstrated by using rectifying effect. The two resonance modes were observed in the Ni wire aligned parallel to the applied magnetic field in plane. The lower resonance frequency mode is considered to correspond to the normal resonance mode with domain resonance, while the higher resonance mode is attributed to the mode which is contributed by the heterojunction between the Ni layer and LiNbO3 substrate. Our results manifest that the rectifying electrical detections are very useful for understating and evaluating the magnetic properties induced by the heterojunction.

Control of Domain Structure in Artificial Ni Wires Fabricated on an LiNbO 3 Substrate

We demonstrated control of magnetic domain structure formation within micro-scale polycrystalline Ni wires fabricated on a single crystalline LiNbO3 substrate. We observed the domain structures using X-ray magnetic circular dichroism-photoemission electron microscopy at SPring-8 BL25SU and BL17SU. Both zebra-stripe domain and ordinary single domain structures can be formed in the Ni wire in control of alignment of the wire onto the substrate. The result suggests that magnetoelastic effects from pizoelectristic substrates can strongly influence the magnetization alignment, overwhelming their shape magnetic anisotropy. The formation of zebra-stripe domain structure can be qualitatively explained by the micromagnetic simulation under the assumption of the magnetic anisotropy perpendicular to the longitudinal axis of wire. Our finding provides a way to control the domain structure and magnetization reversal using the combination of shape magnetic anisotropy and strain-induced anisotropy.

Examination of the stability of skyrmion structures in perpendicularly-magnetized Co/Ni films

The conditions of skyrmion structures in Co/Ni films with perpendicular anisotropy and the Dzyaloshinskii–Moriya interaction (DMI) are reported using micromagnetic simulations. It was found that skyrmion structures existing around the DMI constant D ~ 1.0 erg/cm2 exhibit a perpendicular magnetic anisotropy (K u) in the range of 4.4 ≤ K u ≤ 6.0 Merg/cm3. This K u range is almost identical to the range obtained using analytical equations. However, skyrmion-annihilating conditions were obtained for simulations at small D values. Moreover, the D value required for skyrmion structures suitable for memory applications was (D c − 0.1) ≤ D ≤ D c erg/cm2 from the magnetic energy calculations.