Topological semi-metal Na3Bi as efficient spin injector in current driven magnetic tunnel junction

Abstract

The free layer in the current driven magnetic tunnel junction (MTJ) can be switched by injecting spin-polarized current from an adjacent spin injector. A nonmagnetic efficient spin injector, a converter from charge current to spin current, has long been and is still being quested in the field of spintronics. The first discovered nonmagnetic spin injector was the heavy spin Hall metals (HMs) such as Pt and β-W. The HMs can only convert 2%-10% of the charge current to spin current. The rest of the charge current is wasted and has no contribution in MTJ switching. The waste of charge current during MTJ switching is one of the major sources of energy loss in MTJ operation. Later, it has been found that topological insulators (TIs) such as Bi 2 Se 3 can convert around 37% charge current to spin current. Nevertheless, the topological insulator has low conductivity compared with the free layer of an MTJ, which results in a large amount of shunting charge current loss through the free layer. Topological semimetals (TMs) such as Na 3 Bi provide us with a trade-off point between HM and TI as a nonmagnetic spin injector. TMs have higher charge current to spin current conversion efficiency than HMs and higher electrical conductivity than TIs. In this work, we first calculated the density functional theory band structure of Na 3 Bi and then modeled and matched the near-Fermi-level band structure with the 8 band k ⋅ p model. We have used the k ⋅ p Hamiltonian in quantum transport (nonequilibrium Green's function) formalism to determine the charge current to spin current conversion efficiency in Na 3 Bi. We have found that Na 3 Bi can convert around 27.33% of charge current to spin current, and its conductivity is ∼12.5 times more than that of Bi 2 Se 3. A CoFeB (fixed layer)-MgO (tunneling barrier)-CoFeB (free layer)- Na 3 Bi (spin injector) MTJ consumes almost 9.09 × and 655.57 × less electrical power during isospeed write operation compared with CoFeB-MgO-CoFeB-Pt and CoFeB-MgO-CoFeB- Bi 2 Se 3 MTJs, respectively. Application of isowrite voltage of 1V shows that CoFeB-MgO-CoFeB- Na 3 Bi MTJ switches 4.3 × faster than CoFeB-MgO-CoFeB-Pt MTJ, while CoFeB-MgO-CoFeB- Bi 2 Se 3 MTJ fails to switch and continues to oscillate.