Because of the Pd-catalyzed hydrogen dissociation and absorption, magnetic Pd-alloys provide a model system for the investigation of the critical hydrogenation effect on magnetism. In this study, Co50Pd50 (CoPd) alloy thin films were fabricated by e-beam-heated co-evaporation on Al2O3(0001) substrates. These films exhibited a thickness-dependent spin reorientation transition (SRT) from perpendicular direction to in-plane direction with increase of thickness. For 10-30 nm-thick CoPd alloy films with perpendicular magnetic anisotropy (PMA), hydrogenation triggered a SRT to an in-plane anisotropy. The reversibility of SRT was demonstrated by cyclicly changing the hydrogen gas pressure. Furthermore, hydrogenation-induced SRT randomized the magnetic domain orientation. The surface morphology of the CoPd thin films was composed of nanoclusters, which may play a crucial role in hydrogen dissociation and affect PMA. In comparison with a bare CoPd film, a stronger PMA and a less pronounced hydrogenation-induced SRT were observed in a Pd-capped CoPd film. These observations suggest that the hydrogen content in CoPd alloy films can drastically and reversibly modify PMA, inferring the possible hydrogenation-induced charge transfer and modulation of electronic structure in CoPd.
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