Magnetoelectric induced switching of perpendicular exchange bias using 30-nm-thick Cr₂O₃ thin film

Magnetoelectric (ME) effect is a result of the interplay between magnetism and electric field and now, it is regarded as a principle that can be applied to the technique of controlling the antiferromagnetic (AFM) domain state. The ME-controlled AFM domain state can be read out by the magnetization of the adjacent ferromagnetic layer coupled with the ME AFM layer via exchange bias. In this technique, the reduction in the ME layer thickness is an ongoing challenge. In this paper, we demonstrate the ME-induced switching of exchange bias polarity using the 30-nm thick ME Cr₂O₃ thin film. Two typical switching processes, the ME field cooling (MEFC) and isothermal modes, are both explored. The required ME field for the switching in the MEFC mode suggests that the ME susceptibility (α₃₃) is not deteriorated at 30 nm thickness regime. The isothermal change of the exchange bias shows the hysteresis with respect to the electric field, and there is an asymmetry of the switching field depending on the switching direction. The quantitative analysis of this asymmetry yields α₃₃ at 273 K of 3.7 ± 0.5 ps/m, which is comparable to the reported value for the bulk Cr₂O₃.Y.Shiratsuchi, Y.Tao, K.Toyoki, R.Nakatani, Magnetoelectric induced switching of perpendicular exchange bias using 30-nm-thick Cr₂O₃ thin film. Magnetochemistry 7, 36 (2021); https://doi.org/10.3390/magnetochemistry7030036

Temperature lag with the onset of exchange bias, superparamagnetic blocking, and antiferromagnetic ordering in ultrathin ferromagnet/antiferromagnet thin film

The magnetization of a nanosized magnet, such as an ultrathin film, thermally fluctuates and can become superparamagnetic. In ferromagnetic/antiferromagnetic thin films, superparamagnetism can be suppressed in accordance with antiferromagnetic ordering. The exchange bias can also be induced at the ferromagnetic/antiferromagnetic interface, and it is nontrivial whether the superparamagnetic blocking temperature (TB_SPM) can match either the onset temperature of the exchange bias (TB_EB) or the Néel temperature (TN). In this study, we investigated the temperature dependence of parameters such as coercivity, exchange bias field, magneto-optic Kerr rotation (θK), and AC magnetization (MAC) to elucidate the matching of TB_EB, TB_SPM, and TN in a Pt/Co/Au/Cr₂O₃/Pt thin film. Based on the temperature dependences of MAC, TB_SPM was yielded as about 283 K. TB_EB and TN, which were determined based on the temperature dependence of θK, were 278 and 282 K, respectively. TB_SPM was almost equal to TN but TB_EB was smaller. This temperature lag was caused by the difference in the magnetic anisotropy energy required to induce the exchange bias and suppress superparamagnetism.Yu Shiratsuchi, Yiran Tao, Rou Tsutsumi, Kentaro Toyoki, and Ryoichi Nakatani, Journal of Applied Physics 130, 193902 (2021); https://doi.org/10.1063/5.0060606

Magnetic properties and magnetic domain observation of ndfeb sintered magnets treated by grain boundary diffusion process with dyal co-sorption

Effective grain boundary diffusion (GBD) process with DyAl co-sorption is applied to enhance the coercivity of NdFeB sintered magnets. The coercivity of the magnet (HcJ = 1789 kA m⁻¹) subjected to the present GBD treatment was observed to be superior to that of the untreated magnet (HcJ = 1003 kA m⁻¹) and the conventional GBD magnet (HcJ = 1661 kA m⁻¹) treated with DyAl alloy. In the present GBD magnet, the DyAl co-sorption process facilitated Dy diffusion into the center region of the magnet (thickness: 3.5 mm), resulting in high coercivity. Further, magnetic domain observations were made using magnetic force microscopy (MFM) to observe the thermal demagnetization behavior of the present GBD magnet. The present GBD magnet suppressed the continuous domain reversal of adjacent grains; thus, the partially persistent single-domain structure remained, even at 453 K.M.Uenohara, H.Nishio, K.Toyoki, et al. Magnetic properties and magnetic domain observation of ndfeb sintered magnets treated by grain boundary diffusion process with dyal co-sorption. Materials Transactions 62, 1216 (2021); https://doi.org/10.2320/matertrans.MT-M2020389

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

Switching of perpendicular exchange bias in Pt/Co/Pt/α-Cr₂O₃/Pt layered structure using magneto-electric effect

Switching of the perpendicular exchange bias polarity using a magneto-electric (ME) effect of α-Cr₂O₃ was investigated. From the change in the exchange bias field with the electric field during the ME field cooling, i.e., the simultaneous application of both magnetic and electric fields during the cooling, we determined the threshold electric field to switch the perpendicular exchange bias polarity. It was found that the threshold electric field was inversely proportional to the magnetic field indicating that the EH product was constant. The high EH product was required to switch the exchange bias for the film possessing the high exchange anisotropy energy density, which suggests that the energy gain by the ME effect has to overcome the interfacial exchange coupling energy to reverse the interfacial antiferromagnetic spin.Kentaro Toyoki, Yu Shiratsuchia, Atsushi Kobane, Shotaro Harimoto, Satoshi Onoue, Hikaru Nomura, and Ryoichi Nakatani, Journal of Applied Physics 117, 17D902 (2015); https://doi.org/10.1063/1.4906322