The Magnetic Relaxation of MgB ₂ Thin Films Grown by Hybrid Physical-Chemical Vapor Deposition

The magnetic relaxation of MgB ₂ thin film deposited on Al ₂O ₃ substrate by hybrid physical-chemical vapor deposition method was investigated. The rate of magnetic relaxation (the logarithmic decay rate) S, which was obtained by the time dependence of magnetic moment m(t) at fixed magnetic field H and temperature T, of the MgB ₂ thin film was investigated. The decay of magnetic moment m(t) was carried out at temperatures between 5 and 37.5 K in the field ranges of 0.1-1.0 T, using PPMS-VSM (Quantum Design). The decrease of magnetic moment m(t) for 7200 s at T = 5 K was about 2%-3% and 0.2%-0.3% when the field was applied parallel to the c-axis and the ab-plane, respectively. The logarithmic decay rate S in the field parallel to the ab-plane was much smaller than that parallel to the c-axis over all temperature ranges. The time-independent behavior of logarithmic decay rate S at low temperatures below T = 10 K implies the presence of quantum creep effect.