Two-Magnon Frequency Pulling Effect in Ferromagnetic Resonance

Abstract

Two-magnon scattering (TMS) is a well-known extrinsic scattering process in ferromagnets, which is usually observed through its broadening effect on ferromagnetic resonance (FMR) lineshapes for in-plane magnetic field [1]. A lesser known effect is frequency pulling on FMR as a result of the hybridization between interacting magnon modes. The frequency pulling is not independent of the linewidth broadening effect and can be calculated if the two-magnon linewidth is known. We present broadband ferromagnetic resonance measurements for in-plane (IP) and perpendicular-to-plane (PP) directions of applied field in order to demonstrate the two-magnon frequency pulling effect in Fe0.7Ga0.3 thin films. We first show that a satisfactory fit of the resonance frequencies cannot be obtained when the magnetization is IP, and further, that the fit parameters which are obtained differ greatly from the fit parameters obtained for PP magnetization. We address these problems by first fitting the linewidths (where the two-magnon effect is very pronounced) to a combined two-magnon scattering and Gilbert damping model, and with those fit parameters calculate the corresponding two-magnon resonance line shifts (see figure). The fit to the two-magnon model yields a defect correlation length which we corroborate with structural characterization. We then show that this leads to a much better fit of the IP field-dependent dispersion. In addition, the effective demagnetizing field can be fixed to the value obtained for PP magnetization, and a good fit is still obtained. Thus the inconsistencies between IP and PP dispersions are eliminated by accounting for the frequency pulling effect of two-magnon scattering, and we emphasize the importance of this effect when using FMR to characterize static magnetic properties of thin films. This work was supported by SMART, a center funded by nCORE, a Semiconductor Research Corporation program sponsored by NIST.