Ferromagnetic resonance of exchange-coupled perpendicularly magnetized bilayers

TL;DR

This paper investigates how broadband ferromagnetic resonance and magnetometry can be used to measure strong interlayer exchange couplings in perpendicularly magnetized bilayers, providing analytical tools and practical insights.

Contribution

It introduces analytical expressions linking eigenmode frequencies and linewidths to interlayer exchange coupling, aiding experimental measurement of $J$ in bilayer systems.

Findings

Eigenmode frequencies and linewidths are sensitive to exchange coupling.

Switching fields are largely independent of the coupling strength.

Analytical models are demonstrated on a system resembling magnetic tunnel junctions.

Abstract

Strong ferromagnetic interlayer exchange couplings $J$ in perpendicularly magnetized systems are becoming increasingly desirable for applications. We study whether ferromagnetic interlayer exchange couplings can be measured by a combination of broadband ferromagnetic resonance methods and magnetometry hysteresis loops. For this we model the switching and the eigenexcitations in bilayer systems comprising a soft layer coupled to a thicker harder layer that possesses higher perpendicular magnetic anisotropy. For large $J >0$ the switching fields are essentially independent of $J$ but the frequency of the optical eigenmode of the bilayer and the linewidth of the acoustical and optical eigenmode are directly sensitive to the coupling. We derive a corpus of compact analytical expressions to analyze these frequencies, their linewidth and discuss the meaning thereof. We illustrate this corpus on a system mimicking the fixed layers of a magnetic tunnel junction meant for spin torque applications.