(Georgia Institute of Technology, 2004-12)
Kane, Matthew Hartmann; Shalini, K.; Summers, C. J.; Varatharajan, R.; Nause, J.; Vestal, Christy Riann; Zhang, Z. John; Ferguson, Ian T.
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. Zn ₁₋ₓ CoₓO 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.