We report the realization of magnetoelectric switching of the perpendicular exchange bias in Pt/ Co/a-Cr 2 O 3 /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. V C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4918940]The manipulation of magnetization is a fundamental concept on which devices for spintronics applications are based. A straightforward technique to achieve this is the implementation of the current-induced Oersted field, which is used in high-density storage devices such as hard disk drives. However, the high energy consumption of the method can be problematic, particularly in micro/nano devices. The spin-polarized charge current has attracted attention as an alternative way to manipulate magnetization because it can directly interact with ferromagnetic (FM) spins via a spintransfer torque. 1,2 This technique requires high current densities, above 10 6 A/cm 2 , 3,4 which might generate undesirable heat dissipation in the devices. The electric-field control of magnetization is another candidate, which does not involve this problem. In this method, an electric field applied across a magnetoelectric (ME) insulator induces magnetization switching; several materials, such as TbMnO 2 (Ref. 5) and BaFeO 3 , 6 have been proposed as ME insulators. a-Cr 2 O 3 is one of the proposed ME insulators exhibiting antiferromagnetic (AFM) features; 7,8 because of these, an exchange bias is induced by coupling with the FM layer. In our previous papers, 9,10 we reported that a perpendicularly directed exchange bias above 0.4 erg/cm 2 could be induced in Pt/Co/ a-Cr 2 O 3 /Pt stacked films. This high perpendicular exchange bias is related to the ME-controllable boundary magnetization. 11-14 For bulk a-Cr 2 O 3 (Refs. 13 and 15) and a-Cr 2 O 3 thin films, 14,16 the electrical switching of the exchange bias has been achieved by the simultaneous application of magnetic and electric fields. Two switching modes have been proposed for this process: ME-field cooling 14-16 and isothermal switching. 13 While the former mode was reported for both bulk a-Cr 2 O 3 (Ref. 15) and a-Cr 2 O 3 thin films, 14,16 the latter mode using the a-Cr 2 O 3 thin film is still challenging. In this study, the isothermal switching of the exchange bias using a-Cr 2 O 3 thin films is demonstrated. We also address and discuss the charac...

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).

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.

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)/Cr 2 O 3 /Pt films. When using a Pt-spacer, the perpendicular exchange bias was highly degraded to less than 0.1 erg/cm 2 , which was about half that of the Auspacer 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/Cr 2 O 3 interface, which cants the interfacial Cr spin from the surface normal and results in degradation in the perpendicular exchange bias.