Spin-Torque Ferromagnetic Resonance Induced by the Spin Hall Effect

Abstract

We demonstrate that the spin Hall effect in a thin film with strong spin-orbit scattering can excite magnetic precession in an adjacent ferromagnetic film. The flow of alternating current through a Pt/NiFe bilayer generates an oscillating transverse spin current in the Pt, and the resultant transfer of spin angular momentum to the NiFe induces ferromagnetic resonance (FMR) dynamics. The Oersted field from the current also generates an FMR signal but with a different symmetry. The ratio of these two signals allows a quantitative determination of the spin current and the spin Hall angle. We study Pt/Permalloy bilayer films with a microwave-frequency (RF) charge current applied in the film plane (Permalloy = Py = Ni 81 Fe 19 ). An oscillating transverse spin current is generated in the Pt by the SHE and injected into the adjacent Py ( Fig. 1(a)), thereby exerting an oscillating spin torque (ST) on the Py that induces magnetization precession. This leads to an oscillation of the bilayer resistance due to the anisotropic magnetoresistance (AMR) of Py. A DC voltage signal is generated across the sample from the mixing of the RF current and the oscillating resistance, similar to the signal that arises from ST induced FMR in spin valves and magnetic tunnel junctions [12][13][14][15]. The resonance properties enable a quantitative measure of the spin current absorbed by the Py.Our measurement setup is shown in Fig. 1(c). Pt/Py bilayers were grown by DC magnetron sputter deposition. The starting material for the Pt was 99.95% pure. Highly resistive 3 Ta (1 nm) was employed as the capping layer to prevent oxidation of the Py. The bilayers were subsequently patterned into microstrips of 1 to 20 μm wide and 3 to 250 μm long. By using a bias tee, we were able to apply a microwave current and at the same time measure the DC voltage. A sweeping magnetic field H ext was applied in the film plane, with the angle θ between H ext and microstrip kept at 45° unless otherwise indicated. The output power of the microwave signal generator was varied from 0 to 20 dBm and the measured DC voltage was proportional to the applied power, indicating that the induced precession was in the small angle regime. All the measurements we present were performed at room temperature with a power of 10 dBm.We model the motion of the Py magnetic moment m by the Landau-Lifshitz-GilbertHere γ is the gyromagnetic ratio, α is the Gilbert damping coefficient, μ 0 is the permeability in vacuum, M s is the saturation magnetization of Py, t is the thickness of the Py layer, , / 2 S RF J erepresents the oscillating spin current density injected into Py, H RF is the Oersted field generated by the RF current, H eff is the sum of H ext and the demagnetization field 4π M eff , and σ is the direction of the injected spin moment. The third and fourth terms on the right hand side of Eq. (1) are the result of in-plane spin torque and the out-of-plane torque due to the Oersted field, respectively ( Fig. 1(a)). The mixing signal in response to a combination of in...

Room-Temperature Giant Charge-to-Spin Conversion at the SrTiO3–LaAlO3 Oxide Interface

“…This value is one or two orders of magnitude higher than that of the heavy metals, such as Pt, Ta and W, having spin Hall angle of ~0.06-0.3. 15,16,18,19 Moreover, the performance of spin current generation in the STO/LAO heterostructure is similar or even better than the emerging topological insulator Bi2Se3 with SOT efficiency of ~0.43-3.5. 22,25,26 Our observation further confirms the previous report, which shows a strong current induced Rashba effective field HR with the value of ~1.76 T for a charge current density of JC = 10 5 A/cm 2 .…”

Current-induced spin torque resonance of a magnetic insulator

“…Here, we report on the observation of spin torque-induced magnetization dynamics [18] in a magnetic insulator. Applying a microwave-frequency (GHz) charge current to the Pt layer of a YIG/Pt sample, we are able to drive ferromagnetic resonance by the combined action of Oersted fields and spin transfer torque.…”

Spin Torque Study of the Spin Hall Conductivity and Spin Diffusion Length in Platinum Thin Films with Varying Resistivity

“…Two common methods to quantify the strength of the SHE are to employ ferromagnet/normal metal (FM/NM) bilayers and either (1) detect the spin transfer torque that the SHE-induced spin current from the NM layer exerts on the magnetization of the adjacent FM layer [19,20], or (2) use spin pumping to inject a spin current from the FM to the NM and detect the electric current in the NM layer that is induced by the inverse SHE (ISHE) [21][22][23]. In the former case due to spin backflow (SBF) at the FM/NM interface [24,25] and/or enhanced spin scattering at the interface (spin memory loss or SML) [26], only a portion NM|FM [19,[27][28][29][30][31], beta-Ta [19] and beta-W [4].…”