Co valence and possible spin transformation in diluted magnetic semiconductors Zn1-zMgzCo0.15O and Zn1-xCoxO

In this paper, possible spin transformation and Co valence in dilute magnetic semiconductors is studied. Polycrystalline samples of Zn_1-xCo_xO (0.05≤x≤0.17) and Zn_1-zMg_zCo_0.15O are prepared by rapid oxalate decomposition technique. X-ray diffraction is used to determine phase purity of the samples. Co valence state 2+ is determined by X-ray absorption near edge spectroscopy (XANES) using synchrotron irradiation. Magnetic properties measured show that all samples are paramagnetic and magnetization hysteresis measurement indicated that there is no trace of ferromagnetism. From Curie-Weiss fittings at high temperature region, the effective magnetic moment (μ_eff) is 3.87μ_B/Co which corresponds to that of tetrahedral Co²⁺ high spin state. When fitting at T approaches 0 K, μ_eff = 2.82μ_B/Co is observed indicating a possible spin state transition to Co²⁺ low spin state.