Spintronics utilizes the spin orbit coupling (SOC) of a heavy metal layer to generate spin currents by the spin Hall effect (SHE), which can then induce spin orbit torques (SOT) to alter the magnetization of an adjacent ferromagnet. SOT can be both fast and energy-efficient and enable technological applications in spintronic devices such as spin Hall nano-oscillators (SHNOs) [1-3], SOT based magnetic random access memory(SOT-MRAM), and spin logic devices [4,5]. However, spin currents generated by SOC also exist in magnetic materials. Recent studies evidenced very large emission of spin currents and strong SOT for magnetization switching in ferrimagnets near compensation [6,7].
In the present study nearly compensated GdFeCo, and Py, are used as spin Hall layer and ferromagnetic layer, respectively. We studied the tunable behaviour of both damping-like(DL) and field-like(FL) SOT efficiencies (θDL and θFL) with respect to a Cu insertion layer in Gd26.7(FeCo)73.3 (10 nm)/Cu(1-5 nm)/Py(3 nm) stack. We performed spin transfer torque ferromagnetic resonance (ST-FMR) measurements on microbars (6 ×12 μm2) to determine the magneto dynamical properties such as current induced DL and FL SOT efficiencies. Figure 1 (a) and (b) show the ST-FMR spectra in the frequency range of 10-15 GHz, and the corresponding linewidth vs. frequency plot of Gd26.7(FeCo)73.3 (10 nm)/Cu(4 nm)/Py (3 nm); (c) and (d) show the Cu thickness dependent SOT efficiencies at f=8 GHz. The DL-SOT efficiency is found to be maximum, θDL~ 20%, at Cu (2 nm) followed by a decreasing trend at higher Cu thicknesses. However, FL-SOT efficiency shows a minimum, θFL~ 10%(-7%), up to Cu (2 nm) followed by ~-53%(48%) at Cu (5 nm) for +ve(-ve) direction of applied magnetic field. The larger value of θFL at higher Cu thickness will be useful for ultrafast current induced magnetization switching as seen in other ferrimagnetic systems. 
**References**
1. H. Mazraati et al., Low operational current spin Hall nanooscillators based on NiFe/W bilayers, Appl. Phys. Lett. 109, 242402 (2016).
2. H. Fulara et al., Spin-orbit torque–driven propagating spin waves, Sci. Adv. 5, eaax8467 (2019).
3. M. Zahedinejad et al., Two-dimensional mutually synchronized spin Hall nano-oscillator arrays for neuromorphic computing. Nat. Nanotechnol. 15, 47–52 (2020).
4. K. Garello et al., Ultrafast magnetization switching by spin-orbit torques, Appl. Phys. Lett. 105, 212402 (2014).
5. M. Cubukcu et al., Spin-orbit torque magnetization switching of a three-terminal perpendicular magnetic tunnel junction, Appl. Phys. Lett. 104, 042406 (2014).
6. D. Céspedes-Berrocal et al., Current-induced spin torques on single GdFeCo magnetic layers, Adv. Mater., 33, 2007047(2021).
7. R. Mishra et al., Anomalous Current-Induced Spin Torques in Ferrimagnets near Compensation, Phys. Rev. Lett. 118, 167201 (2017). 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/75246678ebfe0d34b176cd36ac3d9891.png)

Tunable spin orbit torques in Ferrimagnet/Ferromagnetic heterostructures

Pure spin currents produced by the spin Hall effect(SHE) in a nonmagnetic heavy metal has the potential to drive ultra-fast, energy-efficient spintronic devices such as spin Hall nano-oscillators(SHNOs)[1-3], spin-orbit torque based magnetic random access memory(SOT-MRAM), and spin logic devices[4,5]. One key challenge in such emerging devices remains to reduce their high current densities(Jc) and energy consumption to drive magnetization switching and auto oscillations in the adjacent ferromagnets. Efforts have been made to increase the SOT efficiency(ζSOT) by choosing suitable material combinations and/or use oxygen incorporation[6]. However, the increased ζSOT is observed at the expense of an equivalent increase in the longitudinal resistivity(ρ), which essentially cancels out any reduction in the power consumption. A more successful direction is instead to alloy elements, which are in themselves already good SOT material[7]. Here, we demonstrate such alloying of W with Ta to obtain a simultaneous increase of both the ζSOT and the spin Hall conductivity(σSH), which in turn substantially reduces the Jc in W100-xTax(5nm)/CoFeB(tCFB=1.4 to 2 nm)/MgO(2nm) SHNOs. First, we performed STFMR measurements on microbars (6 ×18 µm2, 6 ×12 µm2) to determine their magnetodynamical properties such as current induced ζSOT and σSH. The ζSOT varies from maximum ~ -0.57(x=10%) to ~-0.18(x = 50%) as compared to pure β-W, ~-0.36(x=0). Thanks to a reduced resistivity of W100-xTax, the increased ζSOT is accompanied with a significant increase of the σSH of about ~140% as compared to pure β-W. Finally, we investigated the alloying effect on the auto-oscillation threshold current(Ith) densities by fabricating SHNOs of different widths varying from 10nm to 150nm on a 4 × 14 µm2 mesa[1] and observed about 40% reduction in Ith (at x=12%) as compared to pure β-W measured. The trade-off between ζSOT, ρ, and σSH demonstrates the promising aspects of a W-Ta alloying approach for energy-efficient operation of emerging spintronic devices

![](https://assets.underline.io/uploads/markdown_image/1/image/5e590e6a057e20864ec79b7e0c117dad.jpg)

Fig.1: (a) Variation of ρ(blue) and σSH(red) vs. W100-xTax alloy composition. (b-c) Power spectral densities showing a 40% reduction in the SHNO Ith for two SHNOs with identical constriction width of 50nm.

Alloying effect on auto-oscillation properties of W100-xTax/CoFeB/MgO spin Hall nano-oscillators

EOF: Spin-Orbitronics II

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GOH - Spin Dynamics

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Nilamani Behera

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Tunable spin orbit torques in Ferrimagnet/Ferromagnetic heterostructures

Alloying effect on auto-oscillation properties of W100-xTax/CoFeB/MgO spin Hall nano-oscillators

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