Annealing-induced enhancement of ferromagnetism and nanoparticle formation in the ferromagnetic semiconductorGeFe

We report the annealing-induced enhancement of ferromagnetism and nanoparticle formation in group-IV-based ferromagnetic semiconductor $\mathrm{GeFe}$. We successfully increase the Curie temperature of the $\mathrm{G}{\mathrm{e}}_{0.895}\mathrm{F}{\mathrm{e}}_{0.105}$ film up to 210 K while keeping a nearly single ferromagnetic phase when the annealing temperature is lower than 600 \ifmmode^\circ\else\textdegree\fi{}C. In contrast, when it is annealed at 600 \ifmmode^\circ\else\textdegree\fi{}C, single-crystal GeFe nanoparticles with stacking faults and twins, which have high Curie temperature up to room temperature, are formed in the film. We show that the inhomogeneity of the Fe concentration plays an essential role in determining the ferromagnetism in both cases. Although all the GeFe films show weak spin-glass-like behavior in the very low-temperature region (lower than $\ensuremath{\sim}26$ K), which is insensitive to the annealing temperature, due to the nonuniform distribution of the Fe atoms, the ferromagnetism is much stronger than the spin glass and it dominates the system.