Satoshi Sumi

Interference induced enhancement of magneto-optical Kerr effect in ultrathin magnetic films

We have studied the magneto-optical spectra of ultrathin magnetic films deposited on Si substrates coated with an oxide layer (SiOx). We find that the Kerr rotation angle and the ellipticity of ~1u2009nm thick CoFeB thin films, almost transparent to visible light, show a strong dependence on the thickness of the SiOx layer. The Kerr signal from the 1u2009nm CoFeB thin film can be larger than that of ~100u2009nm thick CoFeB films for a given SiOx thickness and light wavelength. The enhancement of the Kerr signal occurs when optical interference takes place within the SiOx layer. Interestingly, under such resonance condition, the measured Kerr signal is in some cases larger than the estimation despite the good agreement of the measured and calculated reflection amplitude. We infer the discrepancy originates from interface states that are distinct from the bulk characteristics. These results show that optical interference effect can be utilized to study the magneto-optical properties of ultrathin films.

Electric-field-induced on/off switching of the Faraday effect

We demonstrate that the Faraday effect can be electrically and reversibly switched on and off at room temperature without changing the external magnetic field. A Co ultrathin film deposited on a Pt underlayer was used for the demonstration. An electric field was applied to the surface of the Co film by forming an electric double layer using an ionic liquid. The switching of the Faraday effect was caused by the electric-field-induced ferromagnetic phase transition in the system. The magnitude of the electrical change in the Faraday rotation angle and the ellipticity are also discussed.

Thermal reduction of the threshold current density for current-induced domain wall motion in Tb-Co magnetic alloy wire

We investigated the current-induced domain wall motion (CIDWM) in Pt (3 nm)/Tb0.34Co0.66 (6 nm) alloy wire at various temperatures (T). The threshold current density (Jth) for the CIDWM in the Tb-Co alloy wire was 0.85 × 1011 A/m2 at room temperature, and drastically decreased as T was increased. Particularly, at T = 343 K, Jth (= 0.13 × 1011 A/m2) was almost 7 times lower than that at room temperature. The reduction in Jth can be attributed to thermally activated creep motion and a reduction in the depinning current density. These results indicate that devices using CIDWM can be thermally assisted.