Magnetic properties of bulk Zn1−xMnxO and Zn1−xCoxO single crystals

Manganese and cobalt-doped ZnO have been produced using a modified melt-growth technique. X-ray diffraction measurements indicate that the samples are high-quality single crystals with ω−2θ full width at half maximum values of 78 arc sec for the undoped ZnO and 252 arc sec for Zn1−xMnxO (x=0.05). The lattice parameter of the Zn1−xMnxO was observed to increase with Mn concentration. Transmission measurements showed systematic variations dominated by absorption from interatomic Mn2+ and Co2+ transitions. No evidence of diluted magnetic semiconductor mean-field ferromagnetic behavior was observed in any of these nominally noncarrier-doped samples. The magnetic properties instead showed paramagnetic behavior for Zn1−xMnxO dominated by an antiferromagnetic Mn–Mn exchange interaction at low temperatures. Zn1−xCoxO showed hysteresis that was attributed to superparamagnetic Co clusters embedded in a diamagnetic ZnO matrix. It has been shown that in the bulk single-crystal form, intrinsic and noncarrier-doped Zn1−xTMxO is not ferromagnetic; thus creative processing and doping techniques are necessary to achieve practical ferromagnetism in these materials.