Role of Spin Momentum Current in Magnetic Non-Local Damping of Ultrathin Film Structures
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Role of Spin Momentum Current in Magnetic Non-Local Damping of Ultrathin Film Structures G. Woltersdorf, R. Urban, and B. Heinrich Simon Fraser University, 8888 University Dr., Burnaby, BC, V5A 1S6, Canada Non-local damping was investigated by Ferromagnetic Resonance (FMR) using ultrathin magnetic single and double layer structures prepared by Molecular Beam Epitaxy (MBE). The double layer structures show magnetic damping which is caused by spin transport across a normal metal spacer (N). In double layer structures a thin Fe layer, F1, was separated from a thick Fe layer, F2, by a Au(001) spacer. The interface magnetic anisotropies separated the FMR fields of F1 and F2 by a big margin allowing one to investigate FMR in F1 while F2 had a negligible angle of precession, and vice versa. The Fe films in magnetic double layers acquire non-local interface Gilbert damping. It will be shown that the precessing magnetic moments act as spin pumps and spin sinks. This concept was tested by investigating the FMR linewidth around an accidental crossover of the resonance fields for the layers F1 and F2. There is another possible mechanism for non-local damping which is based on a ”breathing Fermi surface” of the spacer. The temperature dependence of the non-local damping indicates that this mechanism is weak in Au spacers. Surprisingly the Au spacer acts as an additional impedance for the spin pump mechanism. Finally, it will be shown that electron-electron correlations in a Pd spacer can lead to a significant enhancement of the non-local damping.
I.
INTRODUCTION
The small lateral dimensions of spintronics devices and high density memory bits require the use of magnetic metallic ultrathin film structures where the magnetic moments across the film thickness are locked together by the intra layer exchange coupling. Since spintronics and high density magnetic recording employ fast magnetization reversal processes it is important to understand the spin dynamics and magnetic relaxation processes of multilayers in the nano-second time regime. The spin dynamics is described by the Landau Lifshitz Gilbert (L.L.G.) equation of motion 1 ∂M G ∂M = − [M × Hef f ] + 2 2 M × , (1) γ ∂t γ Ms ∂t where γ is the absolute value of the electron gyromagnetic ratio, Ms is the saturation magnetization and G is the Gilbert damping parameter. The effective field Hef f is given by the derivatives of the Gibbs energy, U, with respect to the components (Mx , My , Mz ) of the magnetization vector M(t) [1]. The second term in eq. 1 represents the well known Gilbert damping torque. II.
NON-LOCAL DAMPING: EXPERIMENT
The role of non-local damping was investigated in high quality crystalline Au/Fe/Au/Fe(001) structures grown on GaAs(001) substrates, see details in [2–5]. In-plane Ferromagnetic Resonance (FMR) experiments were carried out using 10, 24, 36 and 72 GHz systems [4, 6]. Single Fe ultrathin films with thicknesses of 8, 11, 16, 21, and 31 monolayers (ML) were grown directly on GaAs(001) and covered by a 20 ML thick Au(001) cap layer for protectio
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