Utilizing Vacancies to Generate Unconventional Spin Torque with the Ta-N System

SOT-MRAM has long been plagued by difficulties with switching a magnet with perpendicular anisotropy without the assistance of an external magnetic field. Many solutions have been proposed and experimentally demonstrated, but they often suffer from an irregular geometry, a complicated synthesis path, or a lack of suitable substrates, that inhibits their ability to scale upward to wafer sizes.<br/><br/>Ta₄N₅ is a unique material in this way; its crystal structure is the same as that of cubic TaN, apart from an ordered array of Ta vacancies. This means its symmetry is reduced to that of space group 87, which enables unconventional spin current generation necessary for field-free switching [1], while also lattice matching well to several conventional substrates. Tantalum has long been a useful material for spin-orbit torque applications [2], and cubic TaN has been shown to have a modest SOT efficiency as well [3]. Additionally several papers have reported theoretical calculations that show changes in transport properties between the two structures from the full rock-salt structure [4,5] that suggest a more favorable performance as an SOT device than TaN.<br/><br/>I will detail our group's efforts in synthesizing epitaxial Ta₄N₅ for the first time, via molecular beam epitaxy (MBE). The Ta-N phase diagram is explored via MBE and, guided by theoretical works on its stability, a growth window is obtained for high-quality Ta₄N₅. Experimental transport properties of Ta₄N₅ compared to previous theoretical calculations as well as its rock-salt structure counterpart are discussed. Tests of SOT efficiency, as well as its capability for field-free switching, are ongoing.<br/><br/>[1]: A. Roy, M.H.D Guimarães, and J. Slawinska, "Unconventional spin Hall effects in nonmagnetic solids" <i>Physical Review Materials</i>, Vol. 6, Iss. 4, April 2022<br/><br/>[2]: L. Liu, C. Pai, Y. Li, H.W. Teng, D.C. Ralph, and R.A. Buhrman, "Spin-Torque Switching with the Giant Spin Hall Effect of Tantalum" <i>Science</i>, Vol. 336, Iss. 6081, May 2012<br/><br/>[3]: P.W. Water, X. Hang, Y. Fan, W. Jiang, H. Yun, D. Lyu, D. Zhang, T.J. Peterson, P. Sahu, O.J. Benally, Z. Cresswell, J. Liu, R. Pahari, D. Kukla, T. Low, K.A. Mkhoyan, and J.P. Wang, "Room temperature spin-orbit torque efficiency in sputtered low-temperature superconductor δ-TaN" <i>Physical Review Materials</i>, Vol. 6, July 2022<br/><br/>[4]: C. Stampfl, and A.J. Freeman, "Metallic to insulating nature of TaN_x: Role of Ta and N vacancies" <i>Physical Review B, </i>Vol. 67, February 2003<br/><br/>[5]: L. Yu, C. Stampfl, D. Marshall, T. Eshrich, V. Narayanan, J.M. Rowell, N. Newman, and A.J. Freeman, "Mechanism and control of the metal-to-insulator transition in rocksalt tantalum nitride" <i>Physical Review B</i>, Vol. 65, June 2022