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

Abstract

Magnetic properties of sol-gel-prepared 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-prepared samples are paramagnetic with the magnetic susceptibility χ following the Curie-Weiss law: χ=χ0+C∕(T−θ). 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-temperature ferromagnetism (RTFM) with a magnetic moment of 0.35μB∕Co site and hysteresis loop with coercivity Hc≃600Oe, remanence Mr≃0.45emu∕g, and saturation magnetization Ms≃1.2emu∕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.