Damping at normal metal/permalloy interfaces

We have determined the Gilbert damping parameter as a function of layer thickness in six sets of samples with either a normal metal (NM)/Permalloy (NiFe) or NM/NiFe/NM structure by measuring the ferromagnetic resonance (FMR) linewidth for all series and correcting for extrinsic broadening effects. In the NM/NiFe samples, we find that increasing the interface roughness increases the damping quadratically from 0.01 at 3 nm to 0.11 at 47 nm, and by controlling the roughness of the copper layer, we have found there is no evidence of increased damping due to the thickness of the copper layer between 100-2050 nm of copper.

I. Introduction

With the drive for thinner and smaller elements in magnetic recording, the effects of surfaces increasingly influence the behavior of magnetic devices [1]. Since both switching and noise are based upon the magnetodynamic damping of a ferromagnet, surface contributions to the damping modify the behavior of devices operating at gigahertz speeds and faster [2], [3]. Coupling between magnetic and normal metal films raises the possibility of damping by spin transport, i.e., spin pumping [4], [5] and by electron transport, i.e., eddy currents. We use the convention here that damping mechanisms are those that remove energy from the magnetic system. By this convention, we regard two-magnon scattering as an inhomogeneous broadening mechanism since it conserves the magnetic energy.