Resonant Enhancement of Exchange Coupling for Voltage-Controlled Magnetic Switching

Abstract

We predict that it is possible to achieve a purely voltage-driven switching of a ferromagnet using spin-dependent resonant tunneling. In a configuration of two exchange-coupled magnets through a resonant-tunneling barrier, application of a voltage leads to a resonant enhancement and an oscillatory nature of the exchange coupling. The peak equivalent exchange field is strong enough to switch typical ferromagnets used in scaled magnetic memory devices. The switched configuration is retained once the electric field is removed since the equilibrium exchange coupling is negligible, suppressed by large barriers. Bidirectional switching is possible with the same polarity of the voltage, unlike conventional magnetic memory devices where a bidirectional current or a magnetic field is necessary. Further, the threshold of switching is decoupled from the speed, due to the conservative nature of the exerted torque. This is very different from the conventional spin-torque devices that exhibit a trade-off due to the nonconservative nature of the switching torque. We further show that the structure shows an oscillation in the magnetoresistance (MR) stemming from the resonant tunneling. Interestingly, the MR is higher for smaller voltages while the exchange field is higher for larger voltages-this is promising for efficient read and write operations in potential memory applications.