Effective Spin-Mixing Conductance of Heavy-Metal–Ferromagnet Interfaces

Abstract

The effective spin-mixing conductance ( eff ↑↓ ) of a heavy metal/ferromagnet (HM/FM) interface characterizes the efficiency of the interfacial spin transport. Accurately determining eff ↑↓ is critical to the quantitative understanding of measurements of direct and inverse spin Hall effects. eff ↑↓ is typically ascertained from the inverse dependence of magnetic damping on the FM thickness under the assumption that spin pumping is the dominant mechanism affecting this dependence. Here we report that, this assumption fails badly in many in-plane magnetized prototypical HM/FM systems in the nm-scale thickness regime. Instead, the majority of the damping is from two-magnon scattering at the FM interface, while spin-memory-loss scattering at the interface can also be significant. If these two effects are neglected, the results will be an unphysical "giant" apparent eff ↑↓ and hence considerable underestimation of both the spin Hall ratio and the spin Hall conductivity in inverse/direct spin Hall experiments.

Energy‐Efficient Ultrafast SOT‐MRAMs Based on Low‐Resistivity Spin Hall Metal Au0.25Pt0.75

“…In a SOT-MRAM, the spin current generated by the spin Hall effect (SHE) [10][11][12][13] of a heavy metal layer switches the magnetic free layer of a MTJ (see Figure 1a). [1] Recent harmonic response measurements on Au 1−x Pt x /Co bilayers [18,19] have indicated that the Au 0.25 Pt 0.75 alloy can be a particularly compelling spin Hall metal for energy-efficient SOT applications due to the combination of a relatively low resistivity (ρ xx ≈ 80 µΩ cm) with a strong antidamping SOT efficiency (ξ DL j ≈ 0.3-0. [7,[14][15][16][17] SOT-MRAMs based on a spin Hall metal that combines a giant spin Hall ratio (θ SH ) with a relatively low resistivity (ρ xx ) can also have unlimited endurance due to the suppression of Joule heating induced bursting and migration of the write line [2] as well as low values of write impedance that is compatible with superconducting circuits in cryogenic computing systems.…”

“…[15] Low value of 4πM eff reduces the critical current for antidamping switching. [22,23] All measurements were performed at room temperature. As determined by vibrating sample magnetometry, the saturation magnetization (M s ) of the Fe 0.6 Co 0.2 B 0.2 layers is 1240 emu cm −3 .…”

“…The SOT efficiency is similar to [Pt 0.6/ Hf 0.2] 6 /Pt 0.6 multilayers (ξ ≈ 0.29 DL j , ρ xx ≈ 140 µΩ cm [23] ), but the lower-resistivity Au 0.25 Pt 0.75 (ρ xx ≈ 85 µΩ cm) is more favorable for applications that require unlimited endurance [2] and low device impedance. [19] Adv. [19] Adv.…”

“…For example, for the pulse width of 200 ps (1 ns), the required switching current for the Au 0.25 Pt 0.75 device is 6I ∞ (2I ∞ ), markedly smaller than 90I ∞ (20I ∞ ) predicted by the macrospin simulation (see Figure 2b). There is also strong interfacial Dzyaloshinskii-Moriya interaction (DMI) [35] and magnetic roughness (variations of thickness and interfacial magnetic anisotropy field) [22] at the Pt (alloy)/ FM interfaces, which should enhance the magnetic nonuniformity. Here, we attribute the observed ultrafast switching mainly to the enhanced nonuniform micromagnetic dynamics within the free layer of our devices.…”

“…As schematically shown in Figure 2d, the SOT-MRAMs fabricated in our group have substantial tapering (see ref. [19] Finally, H eff in the short strong pulse region can exceed the value of H c (H c = 15 Oe for Device A, 40 Oe for Device B), which might be reminiscent of a switching driven directly by H eff . There is also strong interfacial Dzyaloshinskii-Moriya interaction (DMI) [35] and magnetic roughness (variations of thickness and interfacial magnetic anisotropy field) [22] at the Pt (alloy)/ FM interfaces, which should enhance the magnetic nonuniformity.…”