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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., 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.

Magnetoelectric (ME) effect is a type of cross-coupling between unconjugated physical quantities, such as the interplay between magnetization and electric field. The ME effect requires simultaneous breaking of spatial and time inversion symmetries, and it sometimes appears in specific antiferromagnetic (AFM) insulators. In recent years, there has been a growing interest for applying the ME effect to spintronic devices, where the effect is utilized as an input method for the digital information. In this article, we review the recent progress of this scheme mainly based on our own achievements. We particularly focus on several fundamental issues, including the ME control of the AFM domain state, which is detectable through the perpendicular exchange bias polarity. The progress made in understanding the switching mechanism, interpretation of the switching energy, switching dynamics, and finally, the future prospects are included.

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.

The proximity effect in a heavy metal attached to a ferromagnetic (FM) layer results in varied interface magnetism. In this study, we investigated the critical behavior of the perpendicular exchange bias in a Ru/Pd/Co/Pt/Cr2O3/Pd stacked film, with focus on the influence of the Pt spacer layer. The apparent critical exponent β increases with increasing Pt thickness. We discussed the possible mechanism from the perspective of the induced magnetic moment of Pt and interfacial exchange coupling. The former was explored based on X-ray magnetic circular dichroism, which revealed that the magnetic moment of Pt was almost temperature-independent within the studied temperature regime. This suggests that the interfacial FM moment is stable. The latter, i.e., the reduction of the interfacial exchange coupling by the Pt spacer layer, is a possible cause of the increase in β, as theoretically predicted.

We investigated the perpendicular exchange anisotropy of Pt(111)/Co1-x Ni x(111)/α-Cr2 O 3(0001) thin films. In order to address the inherent influence of the ferromagnetic layer composition, we also investigated the structure and the change in the perpendicular magnetic anisotropy (PMA) with the ferromagnetic layer composition. From the ferromagnetic layer thickness dependence of the PMA energy density, the interface contribution to the PMA was determined. The results suggest that the PMA is induced at the Co1-x Ni x(111)/α-Cr2 O 3(0001) interface as well as at the Pt(111)/Co 1-xNix(111) interface. Below the Ni composition of 0.5, the high perpendicular exchange anisotropy of about 0.34 erg/cm 2 was observed. The perpendicular exchange anisotropy energy is insensitive to the ferromagnetic layer composition, which should be explained by the interfacial uncompensated antiferromagnetic spins.

We investigated the control of the perpendicular exchange bias (PEB) in Pt/Co/α-Cr2O3 (0001) thin films that exhibit a unique temperature dependence of the PEB. Previously, we demonstrated that the onset of the PEB is explained by the competition between the interface coupling energy and the magnetic anisotropy energy in the α-Cr2O3 layer. Based on this finding, in this study, we investigated the influence of a Pt spacer layer at the Co/α-Cr2O3 interface to control the interface exchange coupling. By inserting a Pt spacer layer, the unidirectional magnetic anisotropy energy JK was reduced because of the weakened exchange coupling between Co and α-Cr2O3. The weakened direct exchange coupling caused an increase in the onset temperature of JK, resulting in a reduction in the α-Cr2O3 layer thickness necessary to maintain the PEB up to the Neel temperature. In addition, the change in the interface exchange coupling mechanism between Co and α-Cr2O3 is discussed with respect to the temperature dependence of JK.

The effect of the crystalline quality of ultrathin Co films on perpendicular exchange bias (PEB) has been investigated using a Au/Co/Au/α-Cr 2 O 3 thin film grown on a Ag-buffered Si(1 1 1) substrate. Our investigation is based on the effect of the Au spacer layer on the crystalline quality of the Co layer and the resultant changes in PEB. An α-Cr 2 O 3 (0 0 0 1)layer is fabricated by the thermal oxidization of a Cr(1 1 0) thin film. The structural properties of the α-Cr 2 O 3 (0 0 0 1) layer including the cross-sectional structure, lattice parameters, and valence state have been investigated. The fabricated α-Cr 2 O 3 (0 0 0 1) layer contains twin domains and has slightly smaller lattice parametersthan those of bulk-Cr 2 O 3 . The valence state of the Cr 2 O 3 (0 0 0 1) layer is similar to that of bulk Cr 2 O 3 . The ultrathin Co film directly grown on the α-Cr 2 O 3 (0 0 0 1) deposited by an e-beam evaporator is polycrystalline. The insertion of a Au spacer layer with a thickness below 0.5 nm improves the crystalline quality of Co, probably resulting in hcp-Co(0 0 0 1). Perpendicular magnetic anisotropy (PMA) appears below the Neel temperature of Cr 2 O 3 for all the investigated films. Although the PMA appears independently of the crystallinequality of Co, PEB is affected by the crystalline quality of Co. For the polycrystalline Co film, PEB is low, however, a high PEB is observed for the Co films whose in-plane atom arrangement is identical to that of Cr 3+ in Cr 2 O 3 (0 0 0 1). The results are qualitatively discussed on the basis of the direct exchange coupling between Cr and Co at the interface as the dominant coupling mechanism.

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., 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.

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.

We have investigated interface magnetism of Au-capped ultrathin Co film grown on antiferromagnetic Cr2O3(0001) thin film. In particular, perpendicular magnetic anisotropy and perpendicular exchange bias are investigated. To strengthen the analysis, structural investigations are also conducted. Ultrathin Co film shows both perpendicular magnetic anisotropy and perpendicular exchange bias. Perpendicular magnetic anisotropy is mainly caused by Au capping layer. However, Co/Cr2O3 interface also affects the perpendicular magnetic anisotropy through the collinear exchange coupling of Co spin and Cr spin. The collinear exchange coupling gives rise to the perpendicular exchange bias. This is explained by aligned Cr spin direction owing to epitaxial growth and small lattice misfit.

The structures of antiferromagnetic Cr2O3(0001) thin films with perpendicular exchange bias were investigated using reflection high-energy electron diffraction, X-ray reflectivity, and synchrotron X-ray diffraction. We mainly investigated the specific ion positions of Cr3+ and O2− in the corundum structure and discussed their relationship to the magnetic anisotropy of Cr2O3. The Cr2O3(0001) thin film grown on a Pt(111) buffer layer exhibited a perpendicular exchange anisotropy density of 0.42 mJ/m2, in which the Cr3+ position is the primary factor in the enhancement of magnetic anisotropy due to dipolar-interaction. In contrast, the single-crystalline Cr2O3(0001) film grown on a α-Al2O3(0001) substrate featured a low exchange magnetic anisotropy of 0.098 mJ/m2. In this film, the Cr3+ position parameter is an insignificant factor, leading to low magnetic anisotropy. The O2− ion position also differs between the two types of films, which can affect both the magnetic anisotropy energy originating from fine structures and the magneto-electric properties of Cr2O3.

We have investigated the fabrication of α-Cr2O3(0001) thin film on a Au(111) surface and the effect of the antiferromagnetic feature of α-Cr2O3(0001) on the magnetic properties of ultrathin Co film on α-Cr2O3(0001). Highly oriented α-Cr2O3(0001) thin film is fabricated by oxidizing Cr(110) thin film grown on a Au(111) surface. Upon the oxidization of threefold symmetric Cr(110) thin film, the α-Cr2O3(0001) thin film is produced. On α-Cr2O3(0001) thin film, ultrathin Co film is epitaxially grown at 473 K to form fcc-Co(111) film. The antiferromagnetic feature of α-Cr2O3 affects the magnetic properties of the ultrathin Co film. Exchange bias is only observed when the magnetic field is applied perpendicular to the film surface, i.e., the direction parallel to the Cr spin. Additionally, in the temperature dependence of magnetization, the clear signature of a Neel temperature of approximately 300 K is observed. The obtained results indicate the collinear coupling of Co and Cr spins.

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., 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.

In this study, the pulse-voltage-driven dynamical switching of perpendicular exchange bias in a Pt/Co/Au/Cr2O3/Pt thin film observed at pulse amplitudes exceeding the DC switching threshold is investigated. The switching time decreases with increasing pulse amplitude, reaching values below 10 ns at an amplitude two times higher than the above threshold. Conversely, several hundreds of nanoseconds is required for switching at amplitudes close to the threshold, suggesting that the switching mechanism is dominated by the motion of magnetic domains of the antiferromagnetic layer.

The effects of the dot size on the exchange bias in the Ni–Fe/Fe–Mn square dots have been investigated. The exchange bias field and the blocking temperature of the square dots decrease as the dot size decreases. We consider this behavior is because the spins are fluctuated by the thermal activation in the laterally constricted antiferromagnetic grains which are located at the edges of the dots.

Exchange couplings between MnIr antiferromagnetic layers and NiFe magnetic layers have been investigated in Hf (8 nm)/MnIr (16 nm)/NiFe (5 nm)/Hf (7 nm)/Si(10 0) films formed by ion-beam sputtering. When the acceleration voltage of the ion gun was higher than 600 V and the ion current was 50–100 mA, the FCC-(1 1 1) texture of the MnIr layer became strong and the MnIr crystallites were relatively large. The large crystallites caused strong exchange couplings between the MnIr layers and the NiFe layers. The highest exchange bias field was 14.8 kA/m. A Ta (7 nm)/MnIr (14 nm)/NiFe (5 nm)/Ta (6 nm)/Si(1 0 0) film was also prepared by RF sputtering. The RF-sputtered film had the larger crystallites than the ion-beam-sputtered films. The exchange bias field was 23.1 kA/m.

Magnetoresistance effects and film structures have been investigated in [Fe-Mn/Ni-Fe-Co/Co/Cu/Co/Ni-Fe-Co/NiO/Si] sandwiches. The NiO buffer layers increase exchange bias fields applied to the Ni-Fe-Co/Co magnetic bilayers neighboring with the Fe-Mn layers. However, the NiO buffer layers decrease the magnetoresistance ratios of the sandwiches. Therefore, we also inserted Hf buffer layers between the NiO buffer layers and the Si substrates. The Hf buffer layer, 1.0 nm in thickness, increases the magnetoresistance ratios of the sandwiches.

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., 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.

Y.Shiratsuchi, H.Noutomi, H.Oikawa, et al. Detection and in situ switching of unreversed interfacial antiferromagnetic spins in a perpendicular-exchange-biased system. Physical Review Letters 109, 077202 (2012); https://doi.org/10.1103/PhysRevLett.109.077202., By using the perpendicular-exchange-biased Pt/Co/α-Cr₂O₃ system, we provide experimental evidence that the unreversed uncompensated Cr spins exist at the Co/α-Cr₂O₃ interface. The unreversed uncompensated Cr spin manifests itself in both the vertical shift of an element-specific magnetization curve and the relative peak intensity of soft-x-ray magnetic circular dichroism spectrum. We also demonstrate an in situ switching of the interfacial Cr spins and correspondingly a reversal of the exchange bias without interfacial atomic diffusion. Such switching shows the direct relationship between the interfacial antiferromagnetic spins and origin of the exchange bias. The demonstrated switching of exchange bias would likely offer a new design of advanced spintronics devices, using the perpendicular-exchange-biased system, with low power consumption and ultrafast operation.

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.

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.

We investigated exchange couplings between magnetic layers and antiferromagnetic layers in (Mn-Ir, Fe-Mn)/Ni-Fe-Co and Ni-Fe-Co/(Mn-Ir, Fe-Mn) films formed by ion beam sputtering. We changed a stacking order between the magnetic layers and antiferromagnetic layers, and the thickness of the antiferromagnetic layers. In both the Fe-Mn/Ni-Fe-Co and the Ni-Fe-Co/Fe-Mn films, high exchange bias fields applied to the Ni-Fe-Co layers were observed, when the thickness of the Fe-Mn layer was 10-20 nm. When the Mn-Ir layers were formed on the Ni-Fe layers, the high exchange bias fields were also observed. However, the exchange bias fields were very low when the Mn-Ir layers were formed under the Ni-Fe layers. The decrease of the exchange bias field was caused by the deterioration of the (111) texture near the top surface which faces the Ni-Fe layer in the Mn-Ir layer.

We show experimental evidence of the equilibrium surface magnetization of α-Cr2O3(0001) by studying chromium magnetization at the Co(111)/α-Cr2O3(0001) interface. The soft X-ray magnetic circular dichroism intensity from uncompensated Cr spins was found to be almost independent of the interface roughness and exchange anisotropy energy. Moreover, no exchange bias training effect was found, except in the transition temperature regime, suggesting that the antiferromagnetic spin structure, including the interface spin arrangement, is in equilibrium. Furthermore, the exchange bias polarity was switched by magnetoelectric field cooling, which is also a salient feature of the surface magnetization of α-Cr2O3(0001).

We found the regeneration of perpendicular exchange bias at high temperature in the Pt-capping/Co/Pt-spacer/α-Cr2O3/Pt-buffer thin film with an ultrathin (0.2 nm) Pt-spacer layer after the disappearance at low temperature. Abrupt changes in the coercivity accompany the abrupt disappearance and regeneration of exchange bias. The direction of the regenerated exchange bias could be reversed by altering the ferromagnetic spin orientation during temperature rise. The exchange bias did not regenerate when the Pt spacer layer was grown at a slow growth rate, suggesting that the growth mode of Pt on the α-Cr2O3 layer affects the regeneration feature.

Yu Shiratsuchi, Kohei Wakatsu, Tetsuya Nakamura, Hiroto Oikawa, Satoru Maenou, Yasuo Narumi, Kou Tazoe, Chiharu Mitsumata, Toyohiko Kinoshita, Hiroyuki Nojiri, and Ryoichi Nakatani, Appl. Phys. Lett. 100, 262413 (2012); https://doi.org/10.1063/1.4731643., Isothermal switching of a perpendicular exchange bias by a strong pulsed magnetic field has been investigated using a Pt/Co/α-Cr₂O₃ thin film system. The switching of the perpendicular exchange bias is accompanied by the spin reversal of interfacial uncompensated antiferromagnetic Cr spins. We have also demonstrated that the switching of the exchange bias is reversible by changing the pulsed magnetic field direction. The mechanism of the demonstrated switching is discussed from the viewpoint of the spin flop transition of the α-Cr₂O₃ layer.

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.

Perpendicular exchange bias was investigated using Pt/Co/α-Cr2O3(0001) thin films prepared by a reactive DC magnetron sputtering method. On the α-Cr2O3(0001) layer, the Pt/Co thin film simultaneously showed perpendicular magnetic anisotropy (PMA) and perpendicular exchange bias (PEB). Although PEB was not observed at room temperature, it suddenly appeared, accompanied by a unique temperature dependence. The suddenly appearing PEB was as large as a unidirectional magnetic anisotropy energy, JK, of about 0.29 erg/cm2. The mechanism of the sudden appearance of JK is discussed on the basis of Meiklejohn and Bean's exchange anisotropy model.

Exchange couplings between antiferromagnetic layers and magnetic layers have been investigated in Hf/Ni–20 at.%Fe/Mn–22 at.%Ir/Fe–40 at.%Mn/Ni–20 at.%Fe/Hf/Si multilayers. The Mn–Ir antiferromagnetic layers formed on the Fe–Mn antiferromagnetic layers have γ-Mn crystal structures from the bottom surfaces to the top surfaces. The structures cause relatively high exchange bias fields applied to the Ni–Fe layers neighboring the Mn–Ir layers.