Anisotropic Absorption of Pure Spin Currents

Spin transfer in magnetic multilayers offers the possibility of ultrafast, low-power device operation. We report a study of spin pumping in spin valves, demonstrating that a strong anisotropy of spin pumping from the source layer can be induced by an angular dependence of the total Gilbert damping parameter, $\alpha$, in the spin sink layer. Using lab- and synchrotron-based ferromagnetic resonance, we show that an in-plane variation of damping in a crystalline ${\mathrm{Co}}_{50}{\mathrm{Fe}}_{50}$ layer leads to an anisotropic $\alpha$ in a polycrystalline ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ layer. This anisotropy is suppressed above the spin diffusion length in Cr, which is found to be 8 nm, and is independent of static exchange coupling in the spin valve. These results offer a valuable insight into the transmission and absorption of spin currents, and a mechanism by which enhanced spin torques and angular control may be realized for next-generation spintronic devices.

Figure 1

SQUID-VSM and XMCD hysteresis measurements of spin valves with $t_{\mathrm{Cr}}=1\mathrm{nm}$ (a),(b), 1.5 nm (c),(d), and 2 nm (e),(f). As the thickness of the Cr layer increases from 1 to 1.5 nm the coupling changes from FM to AFM before vanishing at $t_{\mathrm{Cr}}=2\mathrm{nm}$. XMCD reveals the element-specific steps in the switching, particularly the AFM coupling of the Ni at low fields for $t_{\mathrm{Cr}}=1.5\mathrm{nm}$.

Figure 2

(Left panels) Field vs frequency-transmission maps, showing Kittel curves of two resonances of the $t_{\mathrm{Cr}}=1\mathrm{nm}$ sample with the bias field aligned along the easy (a) and hard (b) axis of the CoFe. (c) Angular dependence of the two resonances at driving frequency 14 GHz. Solid lines are fits to the data using Eq. (1). (d) Extracted interlayer exchange coupling, $A_{\mathrm{ex}}$, as a function of Cr interlayer thickness, showing FM and AFM coupling before being suppressed for a large $t_{\mathrm{Cr}}$. Error bars are comparable to the point size.

Figure 3

Extracted total Gilbert damping for NiFe (a) and CoFe (b) layers, measured along the easy and hard axes of the CoFe layer. The flat line indicates the damping of the bare ferromagnets, while dashed lines are fits to the data that yield a spin diffusion length of 8 nm. The observed anisotropy of damping is shown in more detail in (c), plotting the Gilbert damping of the NiFe layer as a function of bias field angle with respect to the hard axis of the CoFe layer.

Figure 4

Phase of precession for Co (black squares) and Ni (red circles) moments as measured by XFMR for the $t_{\mathrm{Cr}}=1\mathrm{nm}$ sample with the bias field along the easy (a), intermediate (22° away) (b), and hard (c) axes of the CoFe layer. Solid lines are model calculations using parameters extracted by the fitting of Kittel curves (see Fig. 2) and the damping (see Fig. 3). Dashed lines show the effects of dynamic coupling only (green) and static coupling only (blue). The large error bars in (c) are due to a canting of magnetization reducing the projection along the beam, in turn reducing the measured XMCD signal. (d) Differing effects of static and dynamic exchange on induced precession. Note that the couplings can also interfere with each other, e.g., the increased linewidth from the dynamic interaction leads to a larger phase shift from the static interaction. The dashed lines corresponding to static only are for a spin valve with no spin pumping, and therefore they are the same for all magnetization orientations. In the case of the dynamic-only dashed lines, turning off the static coupling causes the resonance field to shift by a few milliteslas; this effect has been removed for ease of comparison.