Two-magnon Scattering in Polycrystalline Fe and FeV Alloy Thin Films

Abstract

Identifying the origins of ferromagnetic resonance (FMR) linewidth broadening is essential for the fundamental understanding of magnetic relaxation, often discussed in terms of “damping”. It is not yet well understood how inhomogeneity and structural disorder contribute to magnetic relaxation in polycrystalline thin films, even though they are widely used in device applications. In this presentation, we systematically examine the impact of inhomogeneity and disorder on the FMR linewidth of sputter-grown polycrystalline Fe and Fe80V20 films with thickness 2-25 nm and different seed layers. A highly nonlinear frequency dependence of FMR linewidth is observed for Fe films on Cu/Ti seed layers. As shown in Fig. 1, all linewidth vs frequency data are quantitatively reproduced with a two-magnon scattering model [1, 2, 3] that accounts for a random distribution of the effective anisotropy field among grains in the polycrystalline film. The fit results indicate that a larger grain size is correlated with stronger two-magnon scattering, which – somewhat counterintuitively – may be accompanied by a lower Gilbert damping parameter. Films with the strongest two-magnon scattering contribution (cf. 10-nm-thick Fe/Cu/Ti, Fig. 1) exhibit the highest coercivity, pointing to a connection between magnetic inhomogeneity and the observed non-Gilbert relaxation. In addition, it is found that two-magnon scattering is reduced by changing the seed layer or alloying Fe with V (Fig. 2). We will discuss the possible mechanisms that lead to narrower FMR linewidths in polycrystalline Fe(V) films. The fundamental understanding gained from our study enables a coherent approach to develop low-loss polycrystalline magnetic media for power-efficient spintronic devices.