Increased muon field at surface and substrate interface of a palladium thin film

Palladium is a versatile transition metal, from a fundamental and applied physics perspective. Being a nearly-ferromagnetic metal, it shows magnetic spin glass behavior and giant magnet moments when doped with small amounts of magnetic elements such as iron. Due to its considerable spin-orbit coupling, it has recently become of interest in the field of spin-orbitronics. Applications of Pd nanomaterial include, among others, hydrogen storage and purification, catalysis in the context of metal-organic frameworks, magnetoresistance spin valves and phase coherence superconducting junctions.
Understanding Pd finite-size effects is therefore relevant for many fields of research.
We performed depth-dependent low-energy muon spin spectroscopy (MuSR) studies on three undoped and iron-doped Pd 100 nm thin films. Muons implanted in the surface and substrate interface of the samples probe an increased local magnetic field. The field increase is temperature-independent, stronger for iron-doped samples, accompanied by an increase in local field inhomogeneity and it extends over a few nanometers. We discuss various potential origins for this magnetic surface state and conclude that orbital moments induced by the surface/interface and by localized spins/charges are the most likely explanation.