The II-VI semiconductor ZnO with both crystal structure (wurtzite) and wide-gap (3.2 eV) similar to GaAs attracted recently considerable attention because of room-temperature ferromagnetism reported in transition metal substituted compounds. If confirmed, these reports, guided by theoretical predictions, would provide practical material for incorporation of spin into semiconductor industry. We have studied structural and magnetic properties of the Mn, Fe, and Co substituted samples synthesized under a wide range of oxygen pressures. No bulk ferromagnetism was observed for any of the single-phase samples obtained over a range of substituted compositions 0 - 20 %. Single-phase compounds demonstrate paramagnetic Curie-Weiss behavior with antiferromagnetic interactions, similar to other diluted semiconductors. Imperfect synthesis conditions could lead to formation of second phases that are responsible for high-temperature ferromagnetic behavior, such as Co-metal and zinc-ferrospinel, or spin-glass behavior observed for ZnMnO3. Absence of ferromagnetism may be caused by our inability to achieve p-type conduction in pure and substituted ZnO.
Supported by the NSF-DMR-0302617 and by the US Department of Education