Strong impact of the eddy-current shielding on ferromagnetic resonance response of sub-skin-depth-thick conducting magnetic multilayers

TL;DR

This study reveals that eddy-current shielding in conducting multilayers significantly reduces ferromagnetic resonance signals at microwave frequencies, impacting the design of spintronic devices and broadband FMR measurements.

Contribution

It demonstrates experimentally and theoretically that eddy currents in nonmagnetic capping layers strongly shield ferromagnetic layers from microwave magnetic fields, especially at high frequencies.

Findings

Eddy currents diminish FMR amplitude in permalloy layers.

Shielding effect is significant even with sub-5 nm NM layers.

High-frequency (30 GHz) eddy-current shielding is very strong.

Abstract

Exchange-coupled nonmagnetic (NM) and ferromagnetic (FM) conducting multilayers are crucial for microwave spintronic devices of the future. We demonstrate, experimentally and theoretically, that in broadband measurements of ferromagnetic resonance (FMR) 10-70 nm-thick permalloy (Py) layers are shielded from the dynamic magnetic field of the microstrip line by eddy currents circulating in the NM capping layers, which strongly diminishes the amplitude of magnetisation precession in the FM material. Our findings have direct implications for designing broadband FMR and measurements of spin current injection through interfaces realised by placing a conducting multilayer above a microwave microstrip line. We show that the eddy-current shielding is very strong at high microwave frequencies (30 GHz) even when the thickness of the NM capping layer is <5 nm, which is well below the microwave skin depth for Au, Cu, Ta, Pd, Pt and other NM metals technologically important for spintronics.