Tuning Optical Mode of Ferromagnetic Resonance in Exchange-Coupled CoFe/Ta/(NiFe)1–x Cuₓ Trilayers

Abstract

We have theoretically and experimentally investigated magnetic resonance modes, i.e., optical mode (OM) and acoustic mode (AM), in two coupled ferromagnetic (FM) layers. We calculate the magnetization dynamics of the system by correlating each FM layer via exchange coupling (EC)-induced effective fields. Experimentally, we use CoFe/Ta/NiFe(Cu) trilayers in which the EC is tailored by varying the Ta spacer layer thickness and the saturation magnetization in the (NiFe) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloy layer. The good agreement between theory and experiment clearly demonstrates that: 1) the resonance frequency of the OM can be significantly higher than that of the individual FM layer; 2) the intensity of the OM can be significantly enhanced by increasing the contrast in the saturation magnetization in two FM layers; and 3) the intensity of the OM, unlike the FMR of a single FM layer, increases with increasing applied magnetic field or frequency. By studying the EC as a function of spacer layer thickness and the saturation magnetization, we further verify that the Néel “orange-peel” coupling is a dominating mechanism in these trilayers, giving rise to coupling between FM layers.