Compositional effect on spin-wave auto-oscillation behavior of Ni100-xFex/Pt spin Hall nano-oscillators

Abstract

The recent demonstration of pure spin current-induced spin-transfer torque arising from the spin Hall effect (SHE) represents an efficient route to controlling the magnetization dynamics in magnetic nanostructures. In a ferromagnet /heavy metal bilayer, a pure spin current is generated when a longitudinal charge current passes through the HM and induces a transverse spin current due to the strong spin-orbit coupling in the HM [1, 2]. The conversion of the charge current density (JC) to pure spin current density (JS) is characterized by the spin Hall angle (θSHA) in these bilayers. The pure spin current may be sufficient to excite perpetual self-oscillations in the magnetization, which can further induce spin waves in these systems. This is of particular interest for spintronics applications and has led to a new class of microwave devices called spin Hall nano-oscillators (SHNOs) [3,4]. Generating higher spin current densities through a higher θSHA is necessary to improve the performance of SHNOs and to avoid higher charge current densities. So far, studies have focused on different HMs with various spin-orbit coupling strengths to generate higher spin current---for example, the β-phase of W, Ta, or NixCu1-x. In this work, we demonstrate the compositional effect on the magnetodynamic and auto-oscillations properties of Ni100-xFex/Pt (x= 10 to 40) nanoconstriction based spin Hall nano-oscillators [5].The devices are fabricated from Ni100-xFex(5) /Pt (6) bilayer (thicknesses in nanometers) deposited in a high vacuum magnetron sputtering chamber where Ni-Fe alloys were co-sputtered under the same conditions and from pure Ni and Fe targets, where the composition was established by varying the respective plasma powers. Two kinds of spin Hall devices were fabricated from each film: (1) 8 x 16 μm2 rectangular stripes and (2) nanoconstriction-based SHNOs with a width of 140 nm.We first discuss the ST-FMR spectra measured on rectangular stripes to determine the variation of the magnetodynamics with the Fe content. We measure an increase in the magnetization (μ0Ms) and a decrease in the Gilbert damping. In addition to that, we measure a reduction of the impact of spin-torque on the ST-FMR linewidth with the dc current (μ0ΔH/ I) as Fe content increases Fig. 1(a). This is translated into a reduction in the spin Hall angle with the increase of Fe content as shown in Fig. 1(b). The reduction in the spin Hall angle can be correlated primarily to the compositional effect: the spin Hall angle scales inversely proportional with the saturation magnetization. Note that the observed variation of spin Hall angle with increasing Fe content is also qualitatively consistent with the decrease of damping as a function of the Fe content, and can be well explained in terms of the spin transport model Ref. [6]. The suppression of spin pumping due to increased μ0Ms should result in lowering the effective spin mixing conductance and therefore spin-torque efficiency, which can be seen as a monotonic decrease of spin Hall angle with increasing Fe content in Fig. 1(b).Next, we turn to discuss the compositional effect on the characteristics of auto-oscillations in nanoconstriction-based SHNOs. We record the generated microwave power spectral density (PSD) as a function of a direct current (I) under an in-plane magnetic field μ0H = 0.05 T using a spectrum analyzer and a low noise amplifier with a +33 dB gain. The spectral characteristics of the auto-oscillations i.e. frequency, power, and linewidth were extracted from the PSD. It is interesting to note that the onset auto-oscillation frequency differs between devices, i.e. the onset frequency shifts up for Fe-rich devices due to higher magnetization. The integrated power varies with the dc current, showing a bell shape of amplitude around 1 pico-watt. A qualitative comparison between the power of devices shows that Fe-rich devices have lower output power. This is understood as the readout of devices depends on AMR and as the AMR drops in Fe-rich devices the power follows. Finally, the measured auto-oscillations have linewidths around 50 MHz.The threshold current for auto-oscillations, (Ith), is extracted from plots of p-1 vs. I, [7] as shown in the inset of Fig. 1(c). For Fe-rich devices, higher currents are required to excite auto-oscillations. The enhancement in threshold current densities for Fe-rich devices is a direct consequence of the reduction in the spin Hall angle.Our experimental results show that the dominant compositional effect is from the magnetization that plays a central role in determining the characteristics of the spin Hall devices. **