Modulation of magnetic anisotropy through self-assembled surface nanoclusters: Evolution of morphology and magnetism in Co-Pd alloy films

作者：Hsu Chuan Che; Chiu Hsiang Chih^*; Mudinepalli Venkata Ramana; Chen Yu Chuan; Chang Po Chun; Wu Chun Te; Yen Hung Wei^*; Lin Wen Chin^*

来源：Applied Surface Science, 2017, 416: 133-143.

DOI：10.1016/j.apsusc.2017.04.191

• Surface nanostructure
• Hydrogenation
• Magnetism
• Thin film

In this study, the self-assembly of surface nanoclusters on 10-20-nm-thick Co50Pd50 (Co-Pd) alloy thin films deposited on the Al2O3(0001) substrate was systematically investigated. The time-dependent evolution of the nanocluster size and magnetic properties was monitored using an atomic force microscope (AFM) and the magneto-optical Kerr effect. When the Co-Pd alloy films were stored in an ambient environment, small nanodots gradually gathered to form large nanoclusters. Approximately 30 days after growth, a nanocluster array formed with an average lateral size of 100 +/- 20 nm and average height of 10 +/- 3 nm. After 100 days, the average lateral size and average height had increased to 140 +/- 20 and 25 +/- 5 nm, respectively. The AFM phase image exhibited a structured contrast on the nanocluster surface, indicating the nonuniform stiffness distribution of the nanoclusters. A microscopic Auger spectroscopy measurement suggested that in contrast to the Pd-rich signal in the flat area, the nanoclusters were cobalt and oxygen-rich areas. Cross-sectional investigation through transmission electron microscopy coupled with energy dispersive spectroscopy showed that the nanoclusters were mostly composed of Co oxide. A uniform Pd-rich underlayer had been maintained underneath the self-assembled Co-oxide nanoclusters. With the formation of a Co-oxide nanocluster array and Pd-rich underlayer, the magnetic easy axis of the Co-Pd film gradually altered its direction from the pristine perpendicular to in-plane direction. Because of the change in the magnetic easy axis, the hydrogenation-induced spin-reorientation transition was suppressed with the evolution of the surface Co-oxide nanoclusters.

• 出版日期2017-9-15