Nature of magnetism in Co- and Mn-doped ZnO prepared by sol-gel technique

摘要

Magnetic properties of sol-gel-prepared bulk samples of Co_(0.05)Zn_(0.95)O and Mn_(0.05)Zn_(0.95)O are reported before and after annealing in 5 % H_2/95 % Ar at 573 K for 6 h. The as-prepared samples are paramagnetic with the magnetic susceptibility χ following the Curie-Weiss law: χ = χ_0 + C/(T-θ). The magnitudes of C are consistent with the magnetic moments expected for the Co~(2+) and Mn~(2+) states. After hydrogenation, the magnetism of Mn/ZnO is unchanged but Co/ZnO acquires room-temperature ferromagnetism (RTFM) with a magnetic moment of 0.35μ_B/Co site and hysteresis loop with coercivity H_c approx = 600 Oe, remanence M_r approx = 0.45 emu/ g, and saturation magnetization M_s approx = 1.2emu/g. Electron magnetic-resonance spectroscopy at 9.28 GHz gives signals corresponding to the Co~(2+) and Mn~(2+) states for the paramagnetic states and a broad FM signal for the hydrogenated Co/ZnO. This difference under hydrogenation between Co/ZnO and Mn/ZnO suggests that n-type doping leads to stabilizing of RTFM in Co/ZnO but not in Mn/ZnO, the latter perhaps requiring p-type doping.