Anomalous temperature dependence of phonon pumping by ferromagnetic resonance in Co/Pd multilayers with perpendicular anisotropy

TL;DR

This study reveals an unexpected increase in phonon pumping efficiency at low temperatures in Co/Pd multilayers with perpendicular anisotropy, challenging conventional magnetostriction-based predictions.

Contribution

It demonstrates anomalous temperature dependence of phonon pumping by ferromagnetic resonance in multilayers, with experimental validation and theoretical correlation.

Findings

Phonon pumping is enhanced by a factor of 4 at 10 K.

The damping enhancement correlates with a shift in FMR field.

Anomalous temperature dependence contradicts magnetostriction predictions.

Abstract

We demonstrate the pumping of phonons by ferromagnetic resonance in a series of [Co(0.8 nm)/Pd(1.5 nm)]$_n$ multilayers ($n =$ 6, 11, 15, and 20) with large magnetostriction and perpendicular magnetic anisotropy. The effect is shown using broadband ferromagnetic resonance over a range of temperatures (10 to 300 K), where a resonant damping enhancement is observed at frequencies corresponding to standing wave phonons across the multilayer. The strength of this effect is enhanced by approximately a factor of 4 at 10 K compared to room temperature, which is anomalous in the sense that the temperature dependence of the magnetostriction predicts an enhancement that is less than a factor of 2. Lastly, we demonstrate that the damping enhancement is correlated with a shift in the ferromagnetic resonance field as predicted quantitatively from linear response theory.