Annealing-induced enhancement of ferromagnetism and nano-particle formation in ferromagnetic-semiconductor GeFe

Ge-based ferromagnetic semiconductor GeFe is a promising material for future Si-based spintronic devices because of the high-quality single crystallinity and good compatibility with Si. However, its Curie temperature ($T_{C})$ is currently at the highest 170 K. In this study, we investigate the annealing effect on GeFe in order to enhance the ferromagnetism. The Ge$_{0.895}$Fe$_{0.105}$ thin film was epitaxially grown on a Ge(001) substrate by low-temperature molecular beam epitaxy. Then, post-growth annealing was carried out. We have analyzed GeFe films both crystallographically and magnetically by using transmission electron microscopy, transmission electron diffraction, energy-dispersive X-ray spectroscopy, magnetic circular dichroism, and superconducting quantum interference device. We have successfully increased the $T_{C}$ of Ge$_{0.895}$Fe$_{0.105}$ up to $\sim$ 220 K while keeping a single ferromagnetic phase when the annealing temperature was lower than 500$^{\circ}$C. In contrast, when annealed at 600$^{\circ}$C, single-crystal GeFe nano-particles with stacking faults and twins, which have a high $T_{C}$ nearly up to room temperature, were formed in the film. Both types of films have a flat surface (roughness of 2-5 MLs), and thus they are promising for Si-based spin devices.