Hydrogenation Induced Spin-Reorientation Transition and Domain Wall Motion in CoPd Alloy Thin Films
Po-Chun Chang1*, Chak-Ming Liu1, Yu-Chan Chen1, Chuan-Che. Hsu1, Wen-Chin. Lin1
1Department of Physics, National Taiwan Normal University, Taipei, Taiwan
* Presenter:Po-Chun Chang, email:chang155212@gmail.com
The hydrogenation induced spin-reorientation transition and domain wall motion forming in two CoPd alloy thin films systems grown on Al2O3(0001) and SiO2/Si(100) substrates was investigated using magneto-optical Kerr effect (MOKE) with various hydrogen gas pressure. In first system, for 10-30 nm Co50Pd50 alloy thin films, hydrogenation induced a reversible spin reorientation transition (SRT) as the hydrogen gas pressure increased. The magnetic easy axis switched from perpendicular to in-plane direction after hydrogen absorption and recovered after hydrogen desorption. Our data shows the perpendicular MOKE (P-MOKE) hysteresis loops of a 30 nm Co50Pd50 film, measured in various hydrogen gas pressures. In the initial condition of air environment and a vacuum, the P-MOKE revealed square shaped hysteresis loops, indicating the stable magnetization in perpendicular direction. Under the hydrogen gas pressure of 50-500 mbar, the hysteresis loop became tilted, indicating the switching of magnetization toward the in-plane direction. Besides, the reversible changes of magnetic coercivity (Hc) and remanence (Mr/Ms) for CoPd film, was also measured as a function of cyclic changed hydrogen gas pressure. This hydrogenation induced SRT disappeared in a 60 nm Co50Pd50 film, because of its dominant in-plane magnetic anisotropy. Besides, the SRT was accompanied by demagnetization effect. The cyclic change of hydrogen pressure switched the magnetization, which was observed in the initial magnetization curve of MOKE measurement. In second system, the hydrogenation effect on the magnetic domain formation and domain wall motion of 25 nm Co30Pd70 alloy thin films grown on SiO2/Si(100) substrates was investigated using magneto-optical Kerr microscopy. In vacuum, there was no domain wall motion observed but only fine domain structure that was changed to be darker and darker with the increase of magnetic field. Under hydrogen, the nucleation and domain wall motion was formed, and the nucleation size was ranged with hydrogen pressure rose. Not only the domain formation but also the reversal motion was changed. Our data shows the time-dependent magnetization reversal curves with different reversal magnetic field under vacuum and hydrogen. The domain can’t be reversed even the reversal magnetic field large than coercivity in vacuum, but can be driven to reverse all the sample under few bar hydrogen with 0.9 Hc. In addition, if the reversal magnetic field was larger, the speed of domain wall was much fast. Finally, series of time constant τ under various magnetic field and hydrogen pressure were obtained from the curve fitting. This study explored the microscopic origin for the hydrogenation effect on magnetic materials and will be valuable for the application of spintronic devices in hydrogen sensing.

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Keywords: Hydrogenation, spin-reorientation transition, domain wall motion, Kerr microscopy