High-Temperature Ferromagnetism inNi2+-Doped ZnO Aggregates Prepared from Colloidal Diluted Magnetic Semiconductor Quantum Dots

Ferromagnetism with $T_c>350\mathrm{K}$ is observed in the diluted magnetic semiconductor ${\mathrm{N}\mathrm{i}}^{2+}:\mathrm{Z}\mathrm{n}\mathrm{O}$ synthesized from solution. Whereas colloidal ${\mathrm{N}\mathrm{i}}^{2+}:\mathrm{Z}\mathrm{n}\mathrm{O}$ nanocrystals are paramagnetic, their aggregation gives rise to robust ferromagnetism. The appearance of ferromagnetism is attributed to the increase in domain volumes and the generation of lattice defects upon aggregation. The unusual temperature dependence of the magnetization coercivity is discussed in terms of a temperature-dependent exchange interaction involving paramagnetic ${\mathrm{N}\mathrm{i}}^{2+}$ ions.