Homodyne-detected ferromagnetic resonance of in-plane magnetized nano-contacts: composite spin wave resonances and their excitation mechanism

TL;DR

This study investigates the complex ferromagnetic resonance spectra of a nano-contact spin valve, revealing composite modes, the role of rf Oersted fields, and the influence of nano-contact size on spin wave excitation.

Contribution

It provides a detailed analysis of in-plane FMR spectra in nano-contacts, highlighting the excitation of multiple modes and the mechanisms behind them, supported by both experiments and micromagnetic simulations.

Findings

Spectra are composite and asymmetric, requiring two Lorentzians for fitting.

rf Oersted field dominates the excitation of observed spectra.

Nano-contact diameter influences the wavevector of exchange spin waves.

Abstract

This work provides a detailed investigation of the measured in-plane field-swept homodyne-detected ferromagnetic resonance (FMR) spectra of an extended Co/Cu/NiFe pseudo spin valve stack using a nanocontact (NC) geometry. The magnetodynamics are generated by a pulse-modulated microwave current and the resulting rectified dc mixing voltage, which appears across the NC at resonance, is detected using a lock-in amplifier. Most notably, we find that the measured spectra of the NiFe layer are composite in nature and highly asymmetric, consistent with the broadband excitation of multiple modes. Additionally, the data must be fit with two Lorentzian functions in order to extract a reasonable value for the Gilbert damping of the NiFe. Aided by micromagnetic simulations, we conclude that (i) for in-plane fields the rf Oersted field in the vicinity of the NC plays the dominant role in generating the observed spectra, (ii) in addition to the FMR mode, exchange dominated spin waves are also generated, and (iii) the NC diameter sets the mean wavevector of the exchange dominated spin wave, in good agreement with the dispersion relation.