The theoretical prediction and experimental observation of ferromagnetism above room temperature in oxide diluted magnetic semiconductors has stimulated intense efforts to understand the origins of the magnetic ordering [1-5]. To test the importance of charge carriers, we have used targeted p- and n-type chemical perturbations to activate and control the high-T_C ferromagnetism in polycrystalline and epitaxial Mn²⁺:ZnO and Co²⁺:ZnO films. In the case of Mn²⁺:ZnO, p-type chemical perturbations resulted in robust ferromagnetism (T_C > 350 K, M_S ~ 1.5 μ_B/Mn²⁺), whereas n-type chemical perturbations yielded only paramagnetism at all temperatures. For Co²⁺:ZnO, the exact opposite correlation was observed. These results provide experimental evidence for the importance of bound carriers in the magnetic ordering of these materials.

[1] T. Dietl, H. Ohno, F. Matsukura, J. Cibert and D. Ferrand, Science 287, 1019 (2000).

[2] K. Sato and H. Katayama-Yoshida, Semicond. Sci. Technol. 17, 367 (2002).

[3] J.M.D. Coey, M. Venkatesan and C.B. Fitzgerald, Nature Materials 4, 173 (2005).

[4] D.A. Schwartz and D.R. Gamelin, Adv. Mater. 16, 2115 (2004).

[5] K.R. Kittilstved, N.S. Norberg and D.R. Gamelin, Phys. Rev. Lett. 94, 147209 (2005).

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1. Dopants and Carriers in Oxide Diluted Magnetic Semiconductor Nanocrystals and Thin Films
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