Increasing spin-orbit torque efficiency by doping Pt: sub-monolayer insertions versus alloys

Abstract

Spin–orbit torque magnetoresisitive random-access memory (SOT-MRAM) is a promising candidate as a non-volatile SRAM replacement. However, one of the biggest challenges that limits the SOT-MRAM application, is the relatively high current that is needed to switch the free layer. Pt is a promising SOT track material due to its low resistivity and compatibility with back end of line (BEOL) processes, though it has relatively low SOT-efficiency. A possible route to increase the SOT-efficiency of Pt is to dope it with another material. In this work, a comparative study is conducted between Pt with sub-monolayer insertions (local doping) and Pt-alloys (global doping), referred to as PtX. The structural differences between them lead to a different SOT-efficiency trend with respect to the content of the PtX layer. It is shown that in the Pt/X-insertions, a SOT-efficiency increase is obtained up to three sub-monolayer insertions and is relatively independent of the material X. On the other hand, the SOT-efficiencies in Pt-alloys are highly dependent on the material X. Compared to pure Pt, our results show that the SOT-efficiency of PtCu-alloys increases up to about three times, whereas PtCr-alloys show no significant enhancement. Furthermore, PtCu is a promising route towards BEOL compatible SOT-MRAMs, since it is shown that it can maintain the perpendicular magnetic anisotropy of 1 nm Co after 400 ∘C annealing, whilst maintaining an increased SOT-efficiency.