Tuneable electron\u2013magnon coupling of ferromagnetic surface states in PdCoO2

F Mazzola,V Sunko,C -M Yim,A P Mackenzie,O J Clark,I Marković,S Khim,T K Kim,P Kushwaha,P D C King,P Wahl,D Chakraborti,M Hoesch,L Bawden

doi:10.1038/s41535-022-00428-8

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 electron–magnon coupling at the Pd-terminated surface of the delafossite oxide PdCoO2, where a polar surface charge mediates a Stoner transition to itinerant surface ferromagnetism. We show how the coupling is enhanced sevenfold 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 light on electron–magnon interactions in solid-state materials, and the ways in which these can be controlled.

Highlights

Its high associated density of states would sit at the Fermi level in the non-magnetic state; this is responsible for triggering a Stoner transition to itinerant surface ferromagnetism,[8,11] pushing this flat band below EF as observed here
From model fits to the electronic self energy from our angle-resolved photoemission (ARPES) measurements, we find that it is the indicative of polaron formation. c Energy distribution curves taken at the momentum value indicated by the green line in (a) show how this replica band feature is independent of photon energy, pointing to a strong intrinsic electron–magnon coupling regime
We note that the magnetism predicted in this surface layer by density-functional theory is only stabilised by a surface relaxation which increases the surface Pd-O bond length[8]

Summary

INTRODUCTION

Low-dimensional systems offer enormous potential for stabilising and controlling magnetic states and textures[1,2]. While magnetic anisotropy can allow long-range order to develop again[4,5], fluctuations can still be expected to play a crucial role This necessitates their fundamental study, and potentially provides exciting opportunities in which to control magnonic excitations for spintronic technologies. For the Pd-termination, the resulting selfdoping is electron-like, which acts to shift a large peak in the unoccupied density of states towards the Fermi level This in turn triggers a Stoner instability, generating a 2D ferromagnetic surface layer, as predicted by density-functional theory[8,11] and confirmed from electronic structure[8] and anomalous Hall measurements[13]. This provides a model environment, accessible to spectroscopic probes, in which to study the influence of magnetic excitations on the electronic structure of a 2D magnet. We use angle-resolved photoemission (ARPES) and scanning-tunnelling microscopy and spectroscopy (STM/S) to investigate this system, finding evidence for a strong and highly tuneable electron–magnon coupling

RESULTS

Mazzola et al 2

DISCUSSION