Magnetic coupling at interface of ultrathin Co film and antiferromagnetic Cr₂O₃ (0001) film

The magnetic coupling at the interface of Co and Cr has been investigated using ultrathin Co film on Cr₂O₃ (0001) film, which is expected to be a promising system to show the magnetoelectronic effect. We mainly examined the temperature dependence of magnetization, the exchange bias at different field directions relative to the Cr spin direction, and the temperature dependence of exchange bias. The temperature dependence of magnetization under 50 Oe indicates the collinear coupling of Co spin and Cr spin at the interface. Furthermore, the exchange bias effect is observed in the coupled direction of Co and Cr, namely, parallel to the Cr spin direction. However, the changes in the magnetic behavior with changing field directions and Co thicknesses imply the existence of biquadratic coupling of Co spin and Cr spin, as well. In agreement with the numerical calculation under the coexistence of collinear and biquadratic couplings, the exchange bias field decreases nonmonotonically with increasing temperature.Yu Shiratsuchia, Toshihiro Nakatani, Shin-ichi Kawahara, and Ryoichi Nakatani, Journal of Applied Physics 106, 033903 (2009); https://doi.org/10.1063/1.3182802

Contribution of Langevin behavior to the low temperature maximum in zero-field cooled magnetization of the discontinuous Fe films

The dominant factor of low temperature maximum of zero-field cooled (ZFC) magnetization has been investigated using a discontinuous ultrathin Fe film which is superparamagnetic at room temperature. The peak temperature of ZFC magnetization increases with increasing magnetic field strength, contrary to the Ǹel-Brown model. It is explained by the Langevin behavior of superparamagnetic particles dominating the low temperature maximum of ZFC magnetization due to the wide energy barrier distribution. For the case, the peak temperature should not be a good estimation of the blocking temperature since thermal activation of magnetization does not have a significant role in the low temperature maximum of ZFC magnetization.Yu Shiratsuchi, Ryoichi Nakatani, and Masahiko Yamamoto, Journal of Applied Physics 103, 07B503 (2008); https://doi.org/10.1063/1.2830948

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

Frustration and relaxation of antiferromagnetic domains reversed by magneto-electric field cooling in a Pt/Co/Au/Cr₂O₃/Pt-stacked film

Using magnetic domain observations, we investigated the reversal process of the perpendicular exchange bias polarity resulting from the antiferromagnetic Cr₂O₃ domain reversal driven by magneto-electric field cooling (MEFC). The exchange bias polarity changed from negative to positive with increasing electric field during MEFC. The relevant change in the magnetic domain revealed the stochastic appearance of the reversed magnetic domains that exhibit the positive exchange bias. The local magnetization curves suggest that the antiferromagnetic domain state after MEFC was frustrated because of energy competition between the interfacial exchange coupling and the bulk magneto-electric effect. The frustrated nature of the magnetic domain structure is supported by the training effect of the exchange bias after MEFC.Yu Shiratsuchi, Shunsuke Watanabe, Shogo Yonemura, Tatsuo Shibata, and Ryoichi Nakatani, AIP Advances 8, 125313 (2018); https://doi.org/10.1063/1.5053136

Competition of perpendicular magnetic anisotropy and exchange magnetic anisotropy in a Pt/Co/α-Cr₂O₃(0001) thin film

We investigated perpendicular magnetic anisotropy and exchange magnetic anisotropy in a Pt/Co/α-Cr₂O₃(0001) thin film grown on an α-Al₂O₃(0001) substrate. The film exhibits perpendicular magnetic anisotropy below a Co thickness of 1.2 nm at room temperature. Independent of the magnetic easy direction of the Co layer, the perpendicular exchange bias (PEB) appears in a direction perpendicular to the film below 80 K. The maximum unidirectional magnetic anisotropy energy estimated from the exchange bias field is 0.33 erg/cm², which is higher than the reported PEB strength. The perpendicular exchange bias is accompanied by the in-plane remanent magnetization and an increase in the in-plane coercivity. We speculate that the increases in the in-plane remanent magnetization and the in-plane coercivity are caused by the spin canting of Cr³⁺ in the α-Cr₂O₃(0001) layer.Yu Shiratsuchia, Hayato Noutomi, Hiroto Oikawa, Toshiaki Fujita, and Ryoichi Nakatani, Journal of Applied Physics 109, 07C101 (2011); https://doi.org/10.1063/1.3535555

Energy condition of isothermal magnetoelectric switching of perpendicular exchange bias in Pt/Co/Au/Cr₂O₃/Pt stacked film

Energy condition for isothermal reversible magnetoelectric switching of exchange bias was investigated using Pt/Co/Au/Cr₂O₃/Pt stacked films with different thicknesses of the antiferromagnetic layer (tAFM). We discussed the effective magnetic anisotropy energy of the antiferromagnetic layer (K eff AFM), the interface exchange coupling energy (J INT), and the offset electric field (E 0). The dependence of K effAFM on tAFM suggested that the magnetic domain wall motion significantly influenced the switching of the electric-field-induced magnetization similar to an ordinal ferromagnet. Below 0.025 mJ/m², J INT was equal to the exchange anisotropy energy (JK), and above 0.025 mJ/m², J INT exceeded JK, suggesting that JK is restricted by the magnetic domain wall energy. The dependence of E0 on tAFM revealed that E0 mainly arose from the interfacial uncompensated antiferromagnetic moment. The obtained results suggest that the energetic interpretation of static switching of electric-field-induced magnetization in Cr₂O₃ was similar to that of the ordinary ferromagnetic materials.Thi Van Anh Nguyen, Yu Shiratsuchi, Shogo Yonemura, Tatsuo Shibata, and Ryoichi Nakatani, Journal of Applied Physics 124, 233902 (2018); https://doi.org/10.1063/1.5047563

Magnetic field dependence of threshold electric field for magnetoelectric switching of exchange-bias polarity

We report the magnetic field dependence of the threshold electric field Eth for the magnetoelectric switching of the perpendicular exchange bias in Pt/Co/Au/Cr₂O₃/Pt stacked films using a reversible isothermal electric tuning approach. The Eth values for the positive-to-negative and negative-to-positive switching are different because of the unidirectional nature of the interfacial exchange coupling. The Eth values are inversely proportional to the magnetic-field strength, and the quantitative analysis of this relationship suggests that the switching is driven by the nucleation and growth of the antiferromagnetic domain. We also find that the magnetic-field dependence of Eth exhibits an offset electric field that might be related to the uncompensated antiferromagnetic moments located mainly at the interface.Thi Van Anh Nguyen, Yu Shiratsuchi, Atsushi Kobane, Saori Yoshida, and Ryoichi Nakatani, Journal of Applied Physics 122, 073905 (2017); https://doi.org/10.1063/1.4991053

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