Room-temperature local ferromagnetism and its nanoscale expansion in the ferromagnetic semiconductor Ge1–xFex

Abstract

We investigate the local electronic structure and magnetic properties of the group-IV-based ferromagnetic semiconductor, Ge1−xFex (GeFe), using soft X-ray magnetic circular dichroism. Our results show that the doped Fe 3d electrons are strongly hybridized with the Ge 4p states, and have a large orbital magnetic moment relative to the spin magnetic moment; i.e., morb/mspin ≈ 0.1. We find that nanoscale local ferromagnetic regions, which are formed through ferromagnetic exchange interactions in the high-Fe-content regions of the GeFe films, exist even at room temperature, well above the Curie temperature of 20–100 K. We observe the intriguing nanoscale expansion of the local ferromagnetic regions with decreasing temperature, followed by a transition of the entire film into a ferromagnetic state at the Curie temperature.

Origin of robust nanoscale ferromagnetism in Fe-doped Ge revealed by angle-resolved photoemission spectroscopy and first-principles calculation

“…A significant difference between Ge:Fe and (Ga,Mn)As is that there is an additional Fe 3d electron in Ge:Fe which occupies the minority-spin 3d(e) states at the Fermi level. This means that Fe is in the Fe 2+ state with 3d 6 (sp) 2 configuration, consistent with the XAS and XMCD measurements [15] and a previous calculation on a 2 × 2 × 2…”

“…The present picture explains the observed increase of T C with Fe concentration [9] and with the inhomogeneity of Fe distribution [11]. The same picture explains the observation of nanoscale ferromagnetic domains formed in Fe-rich regions well above the T C [15].…”

“…Unlike (Ga,Mn)As, the T C does not depend on carrier concentration [13]. Recent x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements [15] have revealed the valence of Fe substituting Ge to be 2+, which indicates that each Fe atoms would provide two holes. It was also found that nanoscale ferromagnetic domains exist even above the T C , the origin of which was attributed to the inhomogeneous distribution of Fe atoms on the nanoscale.…”

“…The T C was estimated to be 100 K from the growth condition. In order to remove the oxidized surface layer, just before loading the sample into the vacuum chamber of the spectrometer, we etched the sample in a hydrofluoric acid (HF) solution (3 mol/L) for 5 seconds and subsequently rinsed it in water, which is known to be an efficient way to clean the surfaces of Ge [24] as well as those of Ge:Fe [15]. changing the photon energy, we could cover the entire Brillouin zone.…”

First-principles calculations on the origin of ferromagnetism in transition-metal doped Ge

“…According to their study, T C of Ge 0.895 Fe 0.105 is improved and reaches 210 K by the annealing below 600C, while the T C increases up to the room temperature when the annealing temperature is 600C; however, stacking fault and twins happen. They also investigated the local magnetic properties of (Ge,Fe) by the X-ray absorption spectroscopy (XAS) and soft X-ray magnetic circular dichroism (XMCD), and suggested that even at room temperature the local ferromagnetic region is still kept, and then the expansion of this region progress with decreasing temperature, resulting in the ferromagnetic phase transition 22 . For both the (Ge,Mn) and (Ge,Fe) cases, the inhomogeneous distributions of the Mn and Fe atoms plays an important role for the magnetic properties.…”