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

Magnetic properties of sol-gel-prepd. bulk samples of Co0.05Zn0.95O and Mn0.05Zn0.95O are reported before and after annealing in 5%H2/95%Ar at 573 K for 6 h. The as-prepd. samples are paramagnetic with the magnetic susceptibility c following the Curie-Weiss law: c = c0 + C/(T-q). The magnitudes of C are consistent with the magnetic moments expected for the Co2+ and Mn2+ states. After hydrogenation, the magnetism of Mn/ZnO is unchanged but Co/ZnO acquires room-temp. ferromagnetism (RTFM) with a magnetic moment of 0.35mB/Co site and hysteresis loop with coercivity Hc .simeq. 600 Oe, remanence Mr .simeq. 0.45 emu/g, and satn. magnetization Ms .simeq. 1.2 emu/g. Electron magnetic-resonance spectroscopy at 9.28 GHz gives signals corresponding to the Co2+ and Mn2+ 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.