High-Temperature Ferromagnetism in ${\mathrm{N}\mathrm{i}}^{2+}$-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.

Figure 1

(a) XRD pattern of $0.93\%$ ${\mathrm{N}\mathrm{i}}^{2+}:\mathrm{Z}\mathrm{n}\mathrm{O}$ aggregates prepared by reaction-limited aggregation of colloidal ${\mathrm{N}\mathrm{i}}^{2+}:\mathrm{Z}\mathrm{n}\mathrm{O}$ nanocrystals. The bulk ZnO diffraction pattern is indicated (▼). (b) TEM image of freestanding nanocrystals from which the aggregates were prepared. (c) TEM image of the aggregates used for (a). (d) Selected area TEM image of the same aggregates.

Figure 2

Absorption spectra of [(a) solid line] colloidal $0.93\%$ ${\mathrm{N}\mathrm{i}}^{2+}:\mathrm{Z}\mathrm{n}\mathrm{O}$ DMS-QDs, [(b) dotted line] reaction-limited aggregates formed from the nanocrystals in (a), and [(c) dashed line] bulk $0.1\%$ ${\mathrm{N}\mathrm{i}}^{2+}:\mathrm{Z}\mathrm{n}\mathrm{O}$ single crystal [14].

Figure 3

350 K $M$ vs $H$ measurements of rapidly (▲) and slowly (●) aggregated $0.93\%$ ${\mathrm{N}\mathrm{i}}^{2+}:\mathrm{Z}\mathrm{n}\mathrm{O}$ nanocrystals. Inset: Temperature dependence of $M_s$ for the latter.

Figure 4

(a) Temperature dependence of $H_c$ for $0.93\%$ ${\mathrm{N}\mathrm{i}}^{2+}:\mathrm{Z}\mathrm{n}\mathrm{O}$ nanocrystalline aggregates. The error bars represent the standard deviation from three measurements. The insets illustrate the change in the barrier to magnetization reversal ($\Delta {\Omega }^{*}$) as a function of temperature. (b) Temperature dependence of ${\mu }_{\mathrm{e}\mathrm{f}\mathrm{f}}$ for bulk $0.1\%$ ${\mathrm{N}\mathrm{i}}^{2+}:\mathrm{Z}\mathrm{n}\mathrm{O}$ [18]. The inset shows the spin-orbit (center) and trigonal (right) splitting pattern of the tetrahedral ${\mathrm{N}\mathrm{i}}^{2+}$ ${}^{3}T_1(F)$ ground state (left).