Magnetoelectric switching of perpendicular exchange bias in Pt/Co/α-Cr₂O₃/Pt stacked films

We report the realization of magnetoelectric switching of the perpendicular exchange bias in Pt/Co/α-Cr₂O₃/Pt stacked films. The perpendicular exchange bias was switched isothermally by the simultaneous application of magnetic and electric fields. The threshold electric field required to switch the perpendicular exchange bias was found to be inversely proportional to the magnetic field, which confirmed the magnetoelectric mechanism of the process. The observed temperature dependence of the threshold electric field suggested that the energy barrier of the antiferromagnetic spin reversal was significantly lower than that assuming the coherent rotation. Pulse voltage measurements indicated that the antiferromagnetic domain propagation dominates the switching process. These results suggest an analogy of the electric-field-induced magnetization with a simple ferromagnet.Kentaro Toyoki, Yu Shiratsuchi, Atsushi Kobane, Chiharu Mitsumata, Yoshinori Kotani, Tetsuya Nakamura, and Ryoichi Nakatani, Appl. Phys. Lett. 106, 162404 (2015); https://doi.org/10.1063/1.4918940

ヒト胃癌患者におけるtripartite motif 32の過剰発現は予後不良に関連する

Tripartite motif protein (TRIM) 32 belongs to the TRIM family, which is composed of RING finger, B-box and coiled-coil domains. TRIM32 has been reported to function as an enzyme 3 ubiquitin ligase and is overexpressed in numerous types of cancer. The present study evaluated the clinical significance of TRIM32 expression levels in gastric cancer. The current study also investigated the TRIM32 expression levels in 142 patients with gastric cancer using immunohistochemistry and examined its clinical importance and potential as a prognostic marker. Furthermore, the function of TRIM32 was examined in vitro. High TRIM32 expression levels were detected in gastric cancer tissues. The postoperative overall and relapse-free survival rates were significantly reduced in patients with tumors with high levels of TRIM32 expression compared with those with tumors expressing low levels of TRIM32. Tumors expressing high levels of TRIM32 were associated with an increased risk of postoperative recurrence, particularly hematogenous recurrence. Multivariate analysis identified TRIM32 status as an independent prognostic factor. Furthermore, TRIM32 gene silencing induced apoptosis and inhibited the proliferation of gastric cancer cells in vitro. Therefore, TRIM32 expression levels may be of potential prognostic value in gastric cancer.博士（医学）・乙第1402号・平成29年6月28日Copyright © Spandidos Publications 2017. All rights reserved.The Spandidos Publications link is available at " http://dx.doi.org/10.3892/ol.2017.5806

Robust magnetic domain of Pt/Co/Au/Cr₂O₃/Pt stacked films with a perpendicular exchange bias

Magnetic domain pattern and magnetic domain wall motion are particularly important to understand the magnetization process. Here, we investigated the magnetization process of perpendicularly exchange-biased Pt/Co/Au/Cr₂O₃/Pt stacked films based on observations of the magnetic domain. In particular, in contrast to previous studies which use fully exchange-biased state, we used the bi-exchange-biased state. We found that the magnetic domain pattern at the remanent state was robust against magnetic-field cycling, which is relevant to the absence of the training effect. The magnetization process was followed by domain wall propagation in the increasing branch of the magnetization curve. In the decreasing branch, both nucleation of the reversed domain and domain wall propagation were involved. The former was accompanied by latency, suggesting that thermal activation played a significant role in the nucleation of the reversed domain.Yu Shiratsuchi, Saori Yoshida, Hiroaki Yoshida, Yoshinori Kotani, Kentaro Toyoki, Ryoichi Nakatani, Chiharu Mitsumata, and Tetsuya Nakamura, Journal of Applied Physics 127, 153902 (2020); https://doi.org/10.1063/5.0002240

Antiferromagnetic domain wall creep driven by magnetoelectric effect

We observed the magnetoelectric induced domain wall propagation in a Pt/Co/Au/Cr₂O₃/Pt stacked thin film based on magnetic domain observations using scanning soft X-ray magnetic circular dichroism microscopy. The antiferromagnetic (Cr₂O₃) domain wall velocity was estimated by a quasi-static approach using a pulsed voltage. At a pulse voltage amplitude of -12 V, corresponding to an electric field of -8.0 × 10²kV/cm, the domain wall velocity was very low, at 0.3 m/s. The domain wall velocity increased with increasing voltage amplitude, reaching 22 m/s at -20 V (-1.3 × 10³kV/cm). The change in the domain wall velocity with the applied voltage amplitude indicates the creep motion of the domain wall. Using a phenomenological model, we estimated the domain wall depinning energy, and found that the bulk and interface terms of the magnetic anisotropy affect the effective magnetic field to the same degree, suggesting that the magnetic domain wall motion may be controllable by the antiferromagnetic layer thickness.Yu Shiratsuchi, Hiroaki Yoshida, Yoshinori Kotani, Kentaro Toyoki, Thi Van Anh Nguyen, Tetsuya Nakamura, and Ryoichi Nakatani, APL Materials 6, 121104 (2018); https://doi.org/10.1063/1.5053928

Observation of the magnetoelectric reversal process of the antiferromagnetic domain

We investigated the switching process of the perpendicular exchange bias, which is driven by the magnetoelectric effect, by conducting magnetic domain observations using scanning soft X-ray magnetic circular dichroism microscopy. Isothermal and simultaneous application of magnetic and electric fields switches the perpendicular exchange bias polarity. The switching process proceeds by the nucleation and growth of reversed domains. The correspondence among the ferromagnetic/antiferromagnetic domains and exchange bias polarity indicates that interfacial antiferromagnetic spin/domain reversal is responsible for the magnetoelectric switching of the perpendicular exchange bias polarity.Yu Shiratsuchi, Shunsuke Watanabe, Hiroaki Yoshida, Noriaki Kishida, Ryoichi Nakatani, Yoshinori Kotani, Kentaro Toyoki, and Tetsuya Nakamura, Appl. Phys. Lett. 113, 242404 (2018); https://doi.org/10.1063/1.5053925

Simultaneous achievement of high perpendicular exchange bias and low coercivity by controlling ferromagnetic/antiferromagnetic interfacial magnetic anisotropy

This study investigates the influence of Pt and Au spacer layers on the perpendicular exchange bias field and coercivity of Pt/Co/(Pt or Au)/Cr₂O₃/Pt films. When using a Pt-spacer, the perpendicular exchange bias was highly degraded to less than 0.1 erg/cm², which was about half that of the Au-spacer system. The Au spacer also suppressed the enhancement in coercivity that usually occurs at around room temperature when using Pt. It is suggested that this difference in exchange bias field is due to in-plane interfacial magnetic anisotropy at the Pt/Cr₂O₃ interface, which cants the interfacial Cr spin from the surface normal and results in degradation in the perpendicular exchange bias.Yu Shiratsuchi, Wataru Kuroda, Thi Van Anh Nguyen, Yoshinori Kotani, Kentaro Toyoki, Tetsuya Nakamura, Motohiro Suzuki, Kohji Nakamura, and Ryoichi Nakatani, Journal of Applied Physics 121, 073902 (2017); https://doi.org/10.1063/1.4976568

Giant Anomalous Hall Conductivity at the Pt/Cr₂O₃ Interface

The interface between a magnetic material and a heavy metal that has a large spin-orbit interaction is at the root of various spin-related phenomena. In this paper, we address the peculiar spin-dependent transport at a Pt/Cr₂O₃ interface by exploring the origin of the nonlinear anomalous Hall effect (AHE) in Pt/Cr₂O₃ bilayers. X-ray magnetic circular dichroism (XMCD) measurements show no appreciable magnetic moment at the interface originating from Cr 3d and Pt 5d orbitals, which could be associated with the AHE response. A possible interfacial magnetic moment M at the Pt/Cr₂O₃ interface, assumed from the detection limit of the XMCD measurements, yields an anomalous Hall conductivity (σAHE) per unit net magnetic moment (M),-σAHE/M, of 0.57 V-1, which is extraordinary large compared with that for general magnetic materials. Together with first-principles calculations, the results suggest the possibility of an intrinsic AHE in the Pt/Cr₂O₃ interface that does not rely on the net magnetic moment.T.Moriyama, Y.Shiratsuchi, T.Iino, et al. Giant Anomalous Hall Conductivity at the Pt/Cr₂O₃ Interface. Physical Review Applied 13, 034052 (2020); https://doi.org/10.1103/PhysRevApplied.13.034052