Tuneable electron-magnon coupling of ferromagnetic surface states in PdCoO$_2$

TL;DR

This study reveals highly tunable electron-magnon coupling at the surface of PdCoO$_2$, demonstrating control over spin wave interactions via surface disorder and doping, with implications for magnonics and spintronics.

Contribution

It uncovers how surface charge and disorder modulate electron-magnon coupling in PdCoO$_2$, introducing a method to control this interaction in itinerant magnetic systems.

Findings

Electron-magnon coupling can be enhanced 7-fold with surface disorder and doping.

Strong coupling induces a polaronic regime with quasiparticle mass enhancement.

Surface charge mediates a Stoner transition to surface ferromagnetism.

Abstract

Controlling spin wave excitations in magnetic materials underpins the burgeoning field of magnonics. Yet, little is known about how magnons interact with the conduction electrons of itinerant magnets, or how this interplay can be controlled. Via a surface-sensitive spectroscopic approach, we demonstrate a strong and highly-tuneable electron-magnon coupling at the Pd-terminated surface of the delafossite oxide PdCoO$_2$, where a polar surface charge mediates a Stoner transition to itinerant surface ferromagnetism. We show how the coupling can be enhanced 7-fold with increasing surface disorder, and concomitant charge carrier doping, becoming sufficiently strong to drive the system into a polaronic regime, accompanied by a significant quasiparticle mass enhancement. Our study thus sheds new light on electron-magnon interactions in solid-state materials, and the ways in which these can be controlled.