Spin-Orbit Torque Phenomena in Complex Oxide Heterostructures

Energy-efficient magnetic spin orbit torque nano-oscillators and coupled oscillator arrays are being explored for low-power neuromorphic computing systems [1]. Commonly studied oscillator material systems are mostly based on metallic ferromagnet(FM)/ heavy metal(HM) (FM=CoFeB, Py, NM=Pt, Ta, W) bilayers with a relatively high intrinsic Gilbert damping of the order of 10^-2. I will discuss recent efforts to replace the metallic layers with complex oxides with coupled spin, electron and lattice degrees of freedom [2]. Large spin-charge conversion, low damping, and small resonance linewidth are essential constituents for development of energy efficient oscillators. In this regard half-metallic perovskite ferromagnet, La_0.67Sr_0.33MnO₃ (LSMO) is studied as the magnetic free layer combined with transition metal oxides such IrO₂, CaIrO₃.and NdNiO₃ as the spin-orbit torque layer provide potentially new functionality. LSMO has a very low damping that is anisotropic in the plane and nearly 100% spin polarization giving large spin torque signals. IrO₂ and CaIrO₃ have unique electronic structures, where the density of states near the Fermi level is dominated by only 5d electrons with strong spin orbit coupling and large charge to spin conversion [3]. NdNiO₃ (NNO) exhibits a first-order metal-insulator transition centered at 200K in bulk. The onset of the metal-insulator phase transition is also accompanied by a complex E’ type anti-ferromagnetic ordering in this material. Spin-orbit torque measurement of the constituent materials and progress towards an all oxide nano-oscillator will be discussed. The work is done in collaboration with Biswajit Sahoo, K. Akilan, Sarmistha Das, Sébastien Petit-Watelot, Carlos Rojas, and Alex Frano and supported by US Department of Energy under Grant No. DE-SC0019273.<br/><br/>[1] J. Grollier et al., Nature electronics <b>3</b> (7), 360 (2020).<br/>[2] Imada, Fujimori and Tokura, Rev. Mod. Phys. <b>70</b>, 1039 (1998)<br/>[3] Sahoo, Frano and Fullerton, Appl. Phys. Lett. 123, 032404 (2023).