Utilizing Vacancies to Investigate Unconventional Torque in the Nb-N System

Spin orbit torque (SOT) has long been plagued by difficulties switching perpendicular magnetization without an external magnetic field. Many solutions have been proposed and experimentally demonstrated but an irregular geometry, complicated synthesis path, or lack of suitable substrates often inhibits their upward scaling.

Nb₄N₅ is unique in this way; its structure is the same as rocksalt NbN, but an ordered array of Nb vacancies reduces its symmetry to space group 87, enabling unconventional spin current generation [1] while also lattice matching well to conventional substrates. With the advent of orbital torque switching Nb has received recent attention in SOT devices despite its weaker spin orbit coupling [2,3]. Several papers have also reported interesting changes to the damping and field-like torques below the superconducting transition [4,5]. However, orbital torque is less explored. Whether unconventional torque can be generated by symmetry breaking, and field-free switching obtained, is unclear.

I will detail our group’s efforts in synthesizing Nb₄N₅, for the first time, via molecular beam epitaxy (MBE). The phase diagram of Nb-N compounds is explored, and a growth window is obtained for high-purity Nb₄N₅. Experimental transport properties of Nb₄N₅ are compared to its rock-salt counterpart. Investigations of changes above/below its T_c, as well as tests of its unconventional torque, are ongoing.

[1] A. Roy, M. Phys. Rev. Mater. 2022 6

[2] F. Liu, B. Phys. Rev. B., 2023 107

[3] S. Dutta. Phys. Rev. B., 2022 106

[4] Wakamura, T. Nature Mater., 2015 14

[5] M. Müller. Phys. Rev. Lett., 2021 126