An Ab initio study on the enhancement of tunneling magnetoresistance and spin injection in Ni/vacuum/Ni magnetic tunnel junctions by h-BN stacking

This study investigates the spin-dependent transport properties of two different magnetic tunnel junctions (MTJs) with a slab with three Ni atomic layers: Ni(111)/vacuum/Ni(111) and Ni(111)-h-BN/h-BN/Ni(111)-h-BN. By combining density functional calculations and a nonequilibrium Greens function technique, we found that the Ni/vacuum/Ni MTJ has a low TMR ratio of only 3.4%. However, when monolayers of h-BN are stacked on the Ni(111) surface, the TMR ratio can be increased to 136.07% at equilibrium state. When a bias of [0, 100] mV is applied, the TMR ratios are greater than 100% in a wide voltage range. Furthermore, the MTJ exhibits a spin injection efficiency of 88.53% in the parallel magnetic configuration, which remains stable above 80% throughout the entire bias range. All these results indicate that the TMR and spin injection of the MTJs with freestanding Ni as the leads and vacuum as the tunneling barrier can be dramatically improved by epitaxially growing monolayer h-BN on the surface of Ni(111). The length of the pure h-BN region can affect the TMR performance of the device. Specifically, the TMR tends to decrease with the length of the central barrier, but it can still be greater than 100% within a certain bias range. Meanwhile, the spin injection of the MTJs is hardly affected and can be maintained at above 80%, indicating high spin injection efficiency. The findings of this study have significant implications in the understanding of the TMR and spin injection mechanisms in MTJs and the design of TMR devices.