Yuichiro Kurokawa

Enhancement of spin orbit torques in a Tb-Co alloy magnetic wire by controlling its Tb composition

We investigated the current-induced domain wall motion (CIDWM) in Pt(3 nm)/TbxCo1-x(6 nm) alloy wires with various Tb composition (x). We found that the threshold current density (Jth) for the CIDWM in the TbxCo1-x alloy wires decreases with increasing x. In particular, the Jth with x = 0.37 is almost 3 times smaller than that with x = 0.23. We estimated Dzyaloshinskii-Moriya interaction (DMI) effective field (HDMI) by measuring CIDWM in a longitudinal magnetic field. We found that DMI constant (D) estimated by the HDMI also strongly depends on x. The size of the DMI may be modified by changing electronegativity or local atomic arrangement in Tb-Co alloy. These results suggest that Tb can induce strong HDMI and effectively affect CIDWM in TbxCo1-x alloy wires.

Current-induced domain wall motion attributed to spin Hall effect and Dzyaloshinsky–Moriya interaction in Pt/GdFeCo (100 nm) magnetic wire

We investigated the current-induced domain wall motion (CIDWM) in Pt/GdFeCo bilayer wires where the thicknesses of the GdFeCo layer are 110 and 150 nm. We found that the direction of CIDWM in the Pt/GdFeCo wires is the same as the current flow direction. The velocity of the domain wall depends on the in-plane magnetic field. These results indicate that the CIDWM along the current direction in the Pt/GdFeCo wires is probably attributed to the spin Hall effect and Dzyaloshinsky–Moriya interaction. Generally, these effects do not appear in thick magnetic wires because they effectively occur at the interfaces of a heavy metal and magnetic layers. Therefore, these results are interesting phenomena because they probably suggest that the spin Hall effect and Dzyaloshinsky–Moriya interaction in the Pt/GdFeCo wire have an anomalously long-range influence.

Electrically conductive properties of Ag/Si composite nanoparticle assembled films prepared with a plasma–gas-condensation cluster source

Ag1−x/Six composite nanoparticle assembled films were prepared using a plasma–gas-condensation cluster beam deposition apparatus. The electrical conductivity σ and Hall coefficient |RH| of Ag1−x/Six nanoparticle assembled films obey a power law of the volume fraction of Ag pAg. The marked change at around pAg = pc indicates that the percolation of Ag nanoparticles takes place at the threshold value pc. Moreover, we found that the |RH| at T = 5 K of the Ag1−x/Six nanoparticle assembled film with the closest pAg to pc is 20 times higher than that of the bulk Ag.

Critical field of two-dimensional superconducting Sn1−x/Six bimetallic composite cluster assembled films with energetic cluster impact deposition

Sn1−x/Six cluster assembled films have been prepared by an energetic cluster impact deposition using a plasma-gas-condensation cluster beam deposition apparatus. Transmission electron microscope images indicated that individual clusters have composite morphologies, where Sn and Si were separated from each other. The superconducting critical magnetic fields, Hc, of Sn1−x/Six cluster assembled films were measured and found to be much higher than the critical magnetic field of the bulk Sn. We estimated the Hc values by using a theory of the superconducting thin film. The estimated values are in good agreement with the experiments, indicating that the Sn1−x/Six cluster assembled films can be regarded as a two-dimensional system although thickness, t, of Sn1−x/Six cluster assembled films (t ≈ 1000 nm) is thicker than conventional superconducting thin film (t < 100 nm).

Multilayered current-induced domain wall motion in Pt/Tb-Co/Ta/Tb-Co/Pt magnetic wire

We fabricated a Pt (3 nm)/Tb0.28Co0.72 (4 nm)/Ta (4 nm)/Tb0.26Co0.74 (4 nm)/Pt (3 nm) multilayered magnetic wire and investigated current-induced domain wall motion within it. We found that the Hall resistance of the Pt/Tb-Co/Ta/Tb-Co/Pt wire has four stable levels under a magnetic field of zero. Moreover, we observed that the domain walls in both the upper and lower ferromagnetic layers are controlled by a single current source. This indicates that Pt/Tb-Co/Ta/Tb-Co/Pt wire can be used in multi-level magnetic wire memory.

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.

Characterization of Critical Volume for Percolation in Sn/Si Bimetallic Cluster Assemblies with Various Packing Fractions

Sn1−x/Six cluster assembled films were prepared using a plasma-gas-condensation cluster beam deposition apparatus by energetic cluster impact deposition, and their conductivities were measured for various compositional ratios and packing fractions f. We found that the electrical conductivity σ of Sn1−x/Six cluster assembled films obeys a power law of the Sn concentration pSn. The marked change at approximately pSn = pc, indicates that the percolation of Sn clusters takes place at the threshold value pc. Moreover, we successfully demonstrated using Sn1−x/Six cluster assembled films with various f values that the critical volume fractions of a percolating object, vc = fpc, can be invariant with respect to f.

Resonant Hall effect under generation of a self-sustaining mode of spin current in nonmagnetic bipolar conductors with identical characters between holes and electrons

We have proposed an enhancement mechanism of the Hall effect, the signal of which is amplified due to the generation of a sustaining mode of spin current. Our analytic derivations of the Hall resistivity revealed the conditions indispensable for the observation of the effect: (i) the presence of the transverse component of an effective electric field due to spin splitting in chemical potential in addition to the longitudinal component; (ii) the simultaneous presence of holes and electrons each having approximately the same characteristics; (iii) spin-polarized current injection from magnetized electrodes; (iv) the boundary condition for the transverse current (J c, y = 0). The model proposed in this study was experimentally verified by using van der Pauw-type Hall devices consisting of the nonmagnetic bipolar conductor YH x (x 2) and TbFeCo electrodes. Replacing Au electrodes with TbFeCo electrodes alters the Hall resistivity from the ordinary Hall effect to the anomalous Hall-like effect with an enhancement factor of approximately 50 at 4 T. We interpreted the enhancement phenomenon in terms of the present model.

Possibility of Exciton Mediated Superconductivity in Nano-Sized Sn/Si Core–Shell Clusters: A Process Technology towards Heterogeneous Material in Nano-Scale

We have produced Sn/Si core–shell cluster assemblies by a plasma-gas-condensation cluster beam deposition apparatus. For the sample with Si content = 12 at. %, the temperature dependence of electrical resistivity exhibits a metallic behavior above 10 K and the onset of superconducting transition below 6.1 K. With decreasing temperature, the thermomagnetic curve for the sample with Si content = 8 at. % begins to decrease steadily toward negative value below 7.7 K, indicating the Meissner effect. An increase in the transition temperature, TC is attributable to exciton-type superconductivity.