Increased muon field at surface and substrate interface of palladium thin films

We performed depth-dependent low-energy muon spin spectroscopy ($\mu$SR) studies on three palladium 100 nm thin films, both undoped and doped with 170 ppm of iron. Muons implanted in the surface and substrate interface region probe an increased local magnetic field compared to the inner part of the sample. The field increase extends over a few nanometers, it is temperature-independent (in the range of 3.7 - 100 K), stronger for the iron-doped samples and accompanied by an increase in local field inhomogeneity. We consider various potential origins for this magnetic surface state, such as adsorbents and supressed d-states. Our conclusion is that orbital moments induced at the surface / interface by localized spins and charges are the most likely explanation, potentially accompanied by magnetic moments due to crystal irregularities.