Materials Research for High Efficiency Spin Orbit Torque Plus Voltage Controlled Magnetic Anisotropy Magnetoresistive Random-Access Memory

Abstract

Current magnetoresistive random-access memory (MRAM) products utilize spin-transfer torque (STT) writing which requires large critical current densities, limiting the device lifetime as these large currents are forced through the tunneling barrier. Spin-orbit torque (SOT)-MRAM is a promising alternative to STT that circumvents many of STT’s issues by generating spin currents in a spin orbit torque channel under the free layer without interacting with the tunneling barrier. Decreasing the power consumption of SOT-MRAM requires materials with large spin torque efficiencies and low resistivities, such as heavy metals and topological semi-metals. A further decrease in power consumption can be realized by utilizing the voltage controlled magnetic anisotropy (VCMA) effect, which allows for a dynamic reduction in magnetic anisotropy with an applied voltage, lowering the switching energy while retaining high anisotropy for thermal stability. However, typical MTJ material stacks have shown minimal linear VCMA responses. Recent theoretical works have predicted a large and bidirectional VCMA effect at high levels of electron depletion, however, the required voltages to achieve these levels of depletion are beyond the dielectric breakdown of experimental gates. Inserting high work-function materials underneath the magnetic layer will deplete electrons from the magnetic layer, creating a built-in bias voltage. This can shift the gating window into the electron-depleted regime where the pJ/Vm and bidirectional VCMA effect was predicted.