Voltage-Controlled Antiferromagnetism in Magnetic Tunnel Junctions

We demonstrate a voltage-controlled exchange bias effect in $\mathrm{CoFeB}/\mathrm{MgO}/\mathrm{CoFeB}$ magnetic tunnel junctions that is related to the interfacial $\mathrm{Fe}(\mathrm{Co}){\mathrm{O}}_x$ formed between the CoFeB electrodes and the MgO barrier. The unique combination of interfacial antiferromagnetism, giant tunneling magnetoresistance, and sharp switching of the perpendicularly magnetized CoFeB allows sensitive detection of the exchange bias. We find that the exchange bias field can be isothermally controlled by magnetic fields at low temperatures. More importantly, the exchange bias can also be effectively manipulated by the electric field applied to the MgO barrier due to the voltage-controlled antiferromagnetic anisotropy in this system.

Figure 1

(a) TMR curves of a p-MTJ measured at RT (black) and 30 K (red and light blue). While no exchange bias is observed at RT, after cooling down to 30 K under a perpendicular field of $\pm 3000\mathrm{Oe}$, obvious $H_{\mathrm{EB}}$ for the soft and hard FM layers can be observed. (b) Temperature dependence of $H_{\mathrm{EB}}$ measured for two samples in the Hall bar geometry, showing the observed exchange bias originates from the $\mathrm{FM}/\mathrm{MgO}$ interface.

Figure 2

(a) Representative TMR curves taken after the application of $-4500\mathrm{Oe}$ setting field (red) and after the application of $+4500\mathrm{Oe}$ setting field (black). (Inset) Reversible and deterministic control of $H_{\mathrm{EB}}$ by applying setting field isothermally. (b) $H_{\mathrm{EB}}$ dependence on external setting field for both FM layers. Dashed lines indicated their respective coercivities. All data are collected at the constant temperature of 30 K.

Figure 3

(a) Schematic picture of the exchange bias effect in AFM-FM bilayers, where the exchange coupling between the FM spins (brown) and the uncompensated interfacial spins (blue) of the AFM (black) leads to a shift of the hysteresis loop characterized by $H_{\mathrm{EB}}$. (b) Schematic diagram of the isothermal control of $H_{\mathrm{EB}}$ with magnetic field. $H_{C1}$ and $H_{C2}$ are the magnetization switching fields in negative and positive field direction. $H_{\mathrm{AF}}$ is the critical setting field above which $H_{\mathrm{EB}}$ polarity changes. (c) Setting field dependence of the hard layer $H_{\mathrm{EB}}$ in a p-MTJ under different voltages, in which the positive voltage increases $H_{\mathrm{AF}}$ and the negative voltage reduces $H_{\mathrm{AF}}$.

Figure 4

(a) Representative TMR curves measured under different bias voltages at 30 K. (b) The voltage dependence of magnetization switching fields $H_{C1}$ and $H_{C2}$ at RT, as a result of the VCMA effect. (c) The voltage dependence of $H_{C1}$ and $H_{C2}$ at 30 K, as a result of both the VCMA and voltage-controlled exchange bias effects. (d) Voltage dependence of the exchange bias field at 30 K. Experimental data are fitted using Eq. (1) assuming a linear voltage dependence for $K_{\mathrm{AF}}$.