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...

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.

By using the perpendicular-exchange-biased Pt/Co/α-Cr(2)O(3) system, we provide experimental evidence that the unreversed uncompensated Cr spins exist at the Co/α-Cr(2)O(3) 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.

The giant magnetoresistance ͑GMR͒ effect in granular and multilayer thin films has been widely investigated because of possible device applications. Despite this intensive effort, the underlying mechanisms responsible for the effect have not been identified. We present measurements of the thermoelectric power ͑TEP͒ and thermal conductivity on a wide variety of granular and multilayer GMR systems. The strong magnetic field dependences of both the TEP and the thermal conductivity are found to be closely related to the magnetoresistance. The TEP measurements require that the high density of states in the ferromagnetic materials play a major role in the GMR effect. The thermal conductivity measurements indicate that the scattering mechanisms in granular samples are elastic while multilayer samples have a significant inelastic, spin-flip component. ͓S0163-1829͑96͒09745-7͔

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).