Ferromagnetism in colloidal Mn²⁺-doped ZnO nanocrystals

High-temperature hydrolysis of Zn(II) and Mn(II) alkoxides in a high boiling point solvent in the presence of surfactants was used to prepare surfactant-coated Zn_1-xMn_xO nanocrystals with average size of 5.5 nm and x = 0.04 ± 0.03. The magnetic properties of the nanocrystals were measured both for isolated particles diluted in a hydrocarbon matrix and for a nanocrystal powder. Nanocrystals of manganese oxide and ZnO coated with manganese oxide were prepared for comparison to the Zn _1-xMn_xO nanocrystals. We find that the manganese ions primarily substitute zinc ions in the hexagonal ZnO lattice, and part of them are ferromagnetically coupled up to room temperature even in isolated noninteracting nanocrystals. The rest of the ions are magnetically disordered or uncoupled. Surprisingly, these small Zn_1-xMn_xO nanocrystals poses relatively large low-temperature magnetic coercivity and relatively high blocking temperature in the isolated form, which indicate large magnetic anisotropy. In the nanocrystal powder the coercive field decreased significantly. This study highlights the advantages of working with noninteracting single domain particles of these intriguing materials.