Efficient charge to spin conversion in iridium oxide thin films

Abstract

Many 5d transition metal oxides have a unique electronic structure, where the density of states near the Fermi level is dominated by only 5d electrons with strong spin–orbit coupling. IrO2, a Dirac nodal line semi-metal, is the simplest of these oxides. The presence of 5d electrons and gap opening of Dirac nodal lines via strong spin–orbit coupling allows for the hybridization of the 5d electrons of the oxide with the itinerant d electrons of a ferromagnet, while simultaneously increasing the intrinsic spin Hall effect. We report large charge-to-spin conversion in thin films of this material using spin-torque ferromagnetic resonance experiments. By independently performing line shape analysis and linewidth modulation experiments, we conclusively determine the spin Hall angle of optimized IrO2 films to be ∼8 times larger than that of Pt.

Recent progress on topological semimetal IrO2: electronic structures, synthesis, and transport properties

“…The measurements occurred by spin-torque FMR to obtain an effective spin Hall conductivity. For (001) IrO 2, the spin Hall angles are determined to be θ SH = 0.45 at 300 K and θ SH = 0.65 at 30 K, among the highest θ SH measured on IrO 2 (figure 15) [140,141] and of opposite sign to some other results [124]. In (110) films, θ SH is much reduced and is anisotropic for different current directions; both effects ascribed to anisotropic strain in the (110) films contrasted with the isotropic strain in the (001) films (figure 15).…”

“…Epitaxial thin films were realized on singlecrystalline TiO 2 substrates either in a pure O 2 atmosphere [113] or in an O 2 /Ar gas mixture [139]. Heterostructures composed of spin-orbit-coupled IrO 2 and ferromagnetic permalloys, such as Ni 81 Fe 19 and Co 40 Fe 40 B 20 , were deposited in situ in DC or RF sputtering systems for spin transport studies [140][141][142][143][144]. The depositions were conducted either at room temperature followed by a post-annealing process [141] or directly at elevated temperatures [140,142].…”

“…Heterostructures composed of spin-orbit-coupled IrO 2 and ferromagnetic permalloys, such as Ni 81 Fe 19 and Co 40 Fe 40 B 20 , were deposited in situ in DC or RF sputtering systems for spin transport studies [140][141][142][143][144]. The depositions were conducted either at room temperature followed by a post-annealing process [141] or directly at elevated temperatures [140,142]. The substrates were Si/SiO 2 or TiO 2 , which again yields polycrystalline [140,141] or epitaxial IrO 2 layers [142], respectively.…”

“…The depositions were conducted either at room temperature followed by a post-annealing process [141] or directly at elevated temperatures [140,142]. The substrates were Si/SiO 2 or TiO 2 , which again yields polycrystalline [140,141] or epitaxial IrO 2 layers [142], respectively. Since the layer uniformity impacts the spin transport across the interface, the substrates were typically rotated during the deposition to ensure that each layer has a uniform thickness.…”

“…Tokura and Nagaosa [16] finds spin Hall angles up to 0.31 in the polycrystalline IrO 2 , about 4x larger than that found in Pt in comparable Pt/CoFeB bilayers. Reprinted from [141], with the permission of AIP Publishing. directions form a route for breaking the constraints inherent in conventional SOTs.…”