# Tunable ferromagnetic resonance in coupled trilayers with crossed in-plane and perpendicular magnetic anisotropies

## TL;DR

This study demonstrates a method to significantly tune the ferromagnetic resonance frequency in coupled magnetic trilayers by controlling interlayer coupling and anisotropies, achieving a 20% frequency tunability at low magnetic fields.

## Contribution

It introduces a novel approach to tune FMR frequencies in coupled trilayers with crossed anisotropies using non-magnetic spacers, revealing large frequency shifts and hysteresis effects.

## Key findings

- FMR frequency increased from 1.3 GHz to 6.6 GHz due to coupling.
- Observed a 20% tunability of resonance frequency at low fields.
- Detected frequency hysteresis in the FMR spectra.

## Abstract

A novel approach to tune the ferromagnetic resonance frequency of a soft magnetic Ni$_{80}$Fe$_{20}$ (Permalloy = Py) film with in-plane magnetic anisotropy (IMA) based on the controlled coupling to a hard magnetic NdCo$_\text{x}$ film with perpendicular magnetic anisotropy (PMA) through a non-magnetic Al spacer is studied. Using transverse magneto-optical Kerr effect (TMOKE), alternating gradient magnetometry (AGM) as well as vector network analyzer ferromagnetic resonance (VNA-FMR) spectroscopy, the influence of both Co concentration and Al spacer thickness on the static and dynamic magnetic properties of the coupled IMA/PMA system is investigated. Compared to a single Py film, two striking effects of the coupling between IMA and PMA layers can be observed in their FMR spectra. First, there is a significant increase in the zero-field resonance frequency from 1.3 GHz up to 6.6 GHz, and second, an additional frequency hysteresis occurs at low magnetic fields applied along the hard axis. The maximum frequency difference between the frequency branches for increasing and decreasing magnetic field is as high as 1 GHz, corresponding to a tunability of about 20% at external fields of typically less than $\pm$70 mT. The origin of the observed features in the FMR spectra is discussed by means of magnetization reversal curves.