Spin-orbit-torque (SOT) - magnetoresistance random access memory (MRAM) is considered the next generation of non-volatile memory. Towards this end large spin-orbit torques necessary for SOT-MRAM can be induced in topological insulator/ferromagnet bilayers. However, it is difficult to grow bilayers that adequately induce large spin-orbit torques for realistic SOT-MRAM. A Japanese group led by Pham Nam Hai reported the growth and characterization of a new bilayer system in the Journal of Applied Physics.
Hai studied the small band-gap BiSb alloy, utilizing its large electrical conductivity and spin-orbit interactions. The electronic engineers optimized crystal growth to create an interface between the mismatched hexagonal crystal structure of BiSb with three-fold symmetry and the tetragonal crystal structure of MnGa with four-fold symmetry. Hai described using molecular beam epitaxy to grow the 20nm thick BiSb layer on top of the GaAs substrate. The template method was then used to overcome the crystal structure and lattice mismatches, to add subsequently 8-nm thin films of MnGa with perpendicular magnetization on top of the BiSb.
Successful growth was confirmed by reflection high energy electron diffraction. An in-plane crystallographic relationship between the BiSb and MnGa layer was observed using X-ray diffraction. Good perpendicular magnetic anisotropy of the MnGa films was confirmed by magnetic circular dichroism intensity, anomalous Hall effect and superconducting quantum interference device.
Hai explained that the next step is to integrate these layers into realistic memory devices: “We have demonstrated that it is possible for one device, but we have to integrate one billion of these devices and master the process integrations.” Hai also pointed out that material integration will have to be demonstrated using industry friendly sputtering methods to take it closer to application.
Source: “Growth and characterization of MnGa thin films with perpendicular magnetic anisotropy on BiSb topological insulator,” by Nguyen Huynh Duy Khang, Yugo Ueda, Kenichiro Yao, and Pham Nam Hai, Journal of Applied Physics (2017). The article can be accessed at https://doi.org/10.1063/1.4999617.