Enhancement of Voltage Controlled Magnetic Anisotropy (VCMA) Through Electron Depletion

Abstract

Enhancement of switching perpendicular magnetic tunnel junctions (p-MTJs) with an electric-field has been a milestone for realizing ultra-low energy memory and computing devices. To integrate with current STT-MTJ and SOT-MTJ devices the typical linear fJ/Vm range voltage controlled magnetic anisotropy (VCMA) needs to be enhanced through stack engineering. We have investigated possible non-linear voltage controlled magnetic anisotropy (VCMA) in heavily electron depleted devices, where the magnetic anisotropy could be reduced with positive and negative voltages, differing with the typical linear VCMA voltage behavior. A possible bidirectional and 1.1 pJ/Vm VCMA effect can be realized based on theoretical predictions with heavily electron depleted Fe/MgO interfaces. To improve upon existing VCMA technology we have proposed inserting high work function materials under the magnetic layer which will deplete electrons from the magnetic layer biasing the gating window into the electron-depleted regime where the pJ/Vm and bidirectional VCMA effect was predicted. We have demonstrated tunable control of the under-layer’s work function by utilizing Ta/Pd(x)/Ta trilayers. By modulating the Pd thickness from 0-20 nm we have observed a tunable change in the Ta layer’s work function from 4.3eV to 4.8eV. To investigate the extent of the electron depletion as a function of the Pd thickness in the trilayer we have performed DFT calculations on supercells of Ta/Pd (X)/Ta /CoFe /MgO. We are working to improve the quality of our gate dielectric materials to avoid hysteretic ionic effects allowing for a reliable VCMA measurement of the Ta/Pd/Ta samples.  Enhancement of Voltage Controlled Magnetic Anisotropy (VCMA) Through Electron Depletion