Magnetoelectric switching of perpendicular exchange bias in Pt/Co/α-Cr2O3/Pt stacked films

Abstract

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 and Magneto-Electric Control Using Cr₂O₃(0001) Thin Film

“…In this paper, we review the previous reports on ME-controlled exchange bias in all-thin-lm systems with Cr 2 O 3 layers, focusing mainly on our own achievements [13][14][15][16][17][18] . We also discuss the characteristics of the exchange bias achieved using Cr 2 O 3 thin lms because it has been hypothesized that exchange bias can be induced by using this material, and exchange bias itself is of signi cant interest in the eld of nano-magnetism.…”

“…In other words, by applying magnetic and electric elds simultaneously, i.e., if H and E are both non-zero, the AFM domains are switchable [11][12][13]16,17,19,20,51) . Since the polarity of the perpendicular exchange bias in a Pt/Co/Cr 2 O 3 /Pt system is determined by the interfacial AFM spin orientation, the exchange bias can be made switchable by using the ME effect.…”

“…In chapter 2, we show that the exchange bias induced by a Cr 2 O 3 (0001) thin lm exhibits (1) high, perpendicular exchange anisotropy above 0.4 mJ/m 2 and unique temperature dependence, with abrupt onset at a speci c temperature, as well as (2) a close relationship with the boundary magnetization, which is coupled with the ME effect. In chapter 3, we present two methods of switching the ME-controlled exchange bias: ME eld cooling (MEFC) [11][12][13] and isothermal switching 16,19,20) . In chapter 3, we also address dynamic exchange bias reversal by using a pulsed voltage.…”

Dispersion of Electric-Field-Induced Faraday Effect in MagnetoelectricCr2O3

“…Invariance of dΘ=dV with respect to thickness scaling makes this method suitable for thin-film investigations, provided the dielectric properties of the films allow maintaining voltages similar to those applied in the bulk single crystal. While high voltages at the nanoscale are out of reach, electric fields in excess of 250 kV=mm have been applied across chromia (0001) films corresponding to voltages >100 V for films of <500 nm thickness [17,18]. The squares and circles in Fig.…”

“…In ME devices, voltage-controlled nonlinear switching of boundary magnetization, a generic property of ME antiferromagnets [14][15][16], is mapped onto voltage-controlled switching between remnant magnetization of an adjacent ferromagnetic (FM) thin film through quantum-mechanical exchange at the AFM/FM interface. It gives rise to voltagecontrolled exchange bias [15,17,18] enabling, e.g., ultralow-power ME magnetic random-access memory, majority gates, and other ME variations of memory and logic device applications [4,5].…”