Enhanced spin-orbit torque efficiency in Pt100−xNix alloy based magnetic bilayer∗

Congli He(何聪丽), Qingqiang Chen(陈庆强), Shipeng Shen(申世鹏), Jinwu Wei(魏晋武), Hongjun Xu(许洪军),Yunchi Zhao(赵云驰), Guoqiang Yu(于国强), and Shouguo Wang(王守国),‡

1Institute of Advanced Materials,Beijing Normal University,Beijing 100875,China

2Beijing National Laboratory for Condensed Matter Physics,Institute of Physics,Chinese Academy of Sciences,Beijing 100190,China

Keywords: spin-orbit torque,magnetic doping,spin-torque ferromagnetic resonance

Spin-orbit torque (SOT) has been widely employed for driving magnetization switching,[1-8]magnetization oscillation,[9-12]domain wall motion,[13-15]and skyrmion motion,[16-19]exhibiting promising potentials in spintronic device applications.A typical SOT sample has a bilayer structure consisting of heavy metal(HM)/ferromagnetic metal(FM).In general, the HMs are chosen from 5d elements, such as Ta,W, and Pt.[1-7,9-21]When passing an electrical current in the HM/FM,it generates an out-of-plane spin current due to spin Hall effect and/or Rashba effect.The spin current diffuses into the adjacent FM layer and exerts torques, i.e., the SOTs, acting on the magnetization of FM.To reduce the electrical current density for magnetization manipulation and hence lower the power consumption,it is essential to enhance the SOT efficiency (ξDL), which has become one important goal in the study and application of SOT.

In this work,the magnetic properties and SOT efficiency in Pt100−xNix/ Ni78Fe22(NiFe) bilayers were studied by spintorque ferromagnetic resonance (ST-FMR) technique.[37-40]The effective anisotropy field and effective damping constant(αeff)were obtained via analyzing the ST-FMR spectra.The effective spin-mixing conductance and the interfacial spin transparency were further extracted. The results show that the SOT efficiency can be enhanced by doping the magnetic Ni into Pt,and it reaches the maximum value when x=18.These results may be useful for designing and developing SOT-driven spintronic devices.

Two series of multilayer stacks were prepared:(I) Pt100−xNix(6)/NiFe(5)/MgO(2)/Pt(2) (thicknesses in nm) with x = 0, 6, 12, 18, and 30, respectively;(II)Pt70Ni30(6)/NiFe(t)/MgO(2)/Pt(2)with t =2, 3.5, 5, 6.5,and 8,respectively.All the stacks were deposited on thermally oxidized Si substrates by magnetron sputtering tool in a vacuum of less than 5×10−8Torr and at Ar pressure of 3 mTorr at room temperature. The composition of Pt100−xNixwas tuned by co-sputtering Pt and Ni under different powers. The multilayers were then patterned into Hall bars and rectangular strips by UV lithography and Ar ion milling for typical fourpoint resistance measurements and ST-FMR measurements,respectively. The contact electrodes with Cr/Au were used for electrical measurements.

The schematic of the devices for ST-FMR measurements and the coordinates are illustrated in the inset of Fig.1(a). The details for ST-FMR measurement can be found in our previous work.[37,38,40]The power of the microwave used for this measurement is 13 dBm. Figure 1(a) shows the ST-FMR spectra for the Pt100−xNix(6)/NiFe(5) sample with x=18 under the frequency of 9.0 GHz. The results can be well fitted by a sum of symmetric and antisymmetric Lorentzian functions

where ΔH is the linewidth,H0is the resonant magnetic field,S is the symmetric Lorentzian coefficient,and A is the antisymmetric Lorentzian coefficient. Figure 1(b) presents the resonance frequency f as a function of the resonant fieldµ0H0for different samples. The results for different samples are quite identical, and they can be well fitted by the Kittel equation f =(γ/2π)[µ0H0(µ0H0+4πMeff)]1/2(γ is the gyromagnetic ratio and µ0is the vacuum permeability). The effective magnetization fields (4πMeff) for different samples are extracted from the fitting,[38]as shown in Fig.1(d), which exhibit a weak dependence on the Ni concentration. Figure 1(c)shows the frequency dependence of resonance linewidth ΔH for the samples with various Ni concentrations. The results can be linearly fitted by ΔH =ΔH0+(2παeff/γ)f, based on which the effective damping constants are thus obtained, as shown in Fig.1(d). Similar to the effective magnetization field, the effective damping constant also exhibits a weak dependence on the Ni concentration,indicating that the Ni doping does not have an obvious influence on the magnetic properties of the adjacent NiFe layer.

Fig.1. (a)ST-FMR spectrum for Pt100−xNix(6 nm)/NiFe(5 nm)/MgO(2 nm)/Pt(2 nm)with x=18 under the frequency of 9.0 GHz,which is fitted to a Lorentzian function consisting of a symmetric and an antisymmetric Lorentzian component(the solid curves). Inset: schematic of a Pt100−xNix/NiFe device for ST-FMR measurements. (b)Resonance frequency f versus the resonant fieldµ0H0 for Pt100−xNix/NiFe devices,which can be well fitted to the Kittel equation.(c)The linewidth as a function of the resonance frequency f,which can be linearly fitted.(d)The effective damping constant αeff determined by the linear fitting,and the effective magnetization fields 4πMeff for Pt100−xNix/NiFe devices with different Ni contents.

The Pt70Ni30/NiFe(t) samples with different thicknesses of NiFe layer were also measured for investigating the interfacial spin transmission. Figure 2(a) shows the resonance frequency f as a function of the resonant field µ0H0. These results can also be well fitted by the Kittel equation. Figure 2(b) shows the effective magnetization fields 4πMefffor the Pt70Ni30/NiFe(t) devices, which decrease from 0.910±0.023 T to 0.349±0.004 T as the thickness of the NiFe layer decreases from 8.0 nm to 2.0 nm, reflecting the gradual increasing contribution of the interfacial anisotropy with decreasing thickness.Figure 2(c)shows the linewidth ΔH versus the resonance frequency f for different samples,which allows us to extract the αeffvalues for different samples. The αeffas a function of tNiFeis given in the inset of Fig.2(d),which increases from 0.017 to 0.104 as the NiFe thickness decreases from 8.0 nm to 2.0 nm.

Based on the previous studies,[40,41]the damping can be approximately given by

Here, tHM, λHM, and ρHMare the thickness, spin-diffusion length, and resistivity of the HM layer (here is the Pt70Ni30layer), respectively. We note that the spin diffusion length of pure Pt (λPt= 1.9 nm) is used for estimating the interfacial spin transparency.[25]Tin≈0.59 was obtained for the Pt70Ni30/NiFe interface.

Fig.2. (a)Resonance frequency f as a function of the resonant fieldµ0H0 for Pt70Ni30/NiFe devices with different thicknesses of NiFe layer.(b) The effective magnetization fields 4πMeff for Pt70Ni30/NiFe devices plotted as a function of the NiFe thickness. (c) The linewidth ΔH versus the resonance frequency f for Pt70Ni30/NiFe devices with different thicknesses of NiFe layer. (d) The effect damping constant αeff plotted as a function of and fitted by Eq.(2)(the solid curve). Inset: effective damping constant as a function of tNiFe.

Fig.3. (a)Angular dependence of symmetric and antisymmetric components for Pt100−xNix/NiFe devices with x=18,which are mainly dependent on angle relation of cosφ sin2φ. (b) The values of SOT efficiency ξSOT and resistivity ρHM as a function of Ni content.

Next, we turn to analyze and discuss the SOT efficiency of the samples. The SOT efficiency can be quantitatively determined from the ratio between the symmetric(Vs)and antisymmetric(Va)components of the resonance curve,which are expressed as[40]

Figure 3(a) shows the angular dependence of the Vsand Vacomponents for the Pt100−xNix/NiFe devices with x = 18,which can be fitted well to cosϕ sin2ϕ. A quantity ξSOTcan be estimated from the Vs/Varatio,[44]

In summary,the magnetic properties and SOT efficiency of Pt100−xNix/NiFe bilayers were studied by ST-FMR. The effective anisotropy field and effective damping constant exhibit a weak dependence on the Ni concentration. The effective spin-mixing conductance and the interfacial spin transparency were extracted for the Pt70Ni30/NiFe. The SOT efficiency can be beneficially enhanced by adding scattering centers to increase the electrical resistivity of the HM,which increases with the Ni concentration and reaches the maximum value when x =18. Our results indicate that the SOT efficiency can be tuned by doping Ni. Considering the low cost of Ni compared to Pt and the enhanced SOT efficiency in Pt100−xNix/NiFe,this study may help to reduce the cost of the SOT devices.