Yoshisuke Ban

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

We report the annealing-induced enhancement of ferromagnetism and nano-particle formation in group-IV-based ferromagnetic-semiconductor GeFe. We successfully increase the Curie temperature of the Ge0.895Fe0.105 film up to ~220 K while keeping a single ferromagnetic phase when the annealing temperature is lower than 500{\deg}C. In contrast, when annealed at 600{\deg}C, single-crystal GeFe nano-particles with stacking faults and twins, which have a high Curie temperature nearly up to room temperature, are formed in the film. Our results show that annealing is quite effective to improve the magnetic properties of GeFe for high-temperature-operating spin-injection devices based on Si or Ge.

Carrier transport properties of the Group-IV ferromagnetic semiconductor Ge1-xFex with and without boron doping

We have investigated the transport and magnetic properties of group-IV ferromagnetic semiconductor Ge1-xFex films (x = 1.0 and 2.3%) with and without boron doping grown by molecular beam epitaxy (MBE). In order to accurately measure the transport properties of 100-nm-thick Ge1-xFex films, (001)-oriented silicon-on-insulator (SOI) wafers with an ultra-thin Si body layer (∼5 nm) were used as substrates. Owing to the low Fe content, the hole concentration and mobility in the Ge1-xFex films were exactly estimated by Hall measurements because the anomalous Hall effect in these films was found to be negligibly small. By boron doping, we increased the hole concentration in Ge1-xFex from ∼1018 cm−3 to ∼1020 cm−3 (x = 1.0%) and to ∼1019 cm−3 (x = 2.3%), but no correlation was observed between the hole concentration and magnetic properties. This result presents a contrast to the hole-induced ferromagnetism in III-V ferromagnetic semiconductors.

Important role of the non-uniform Fe distribution for the ferromagnetism in group-IV-based ferromagnetic semiconductor GeFe

We report the growth temperature dependence of the properties of the group-IV-based ferromagnetic semiconductor Ge1-xFex films (x = 6.5% and 10.5%), including the lattice constant, Curie temperature (TC), and Fe-atom locations. While TC strongly depends on the growth temperature, we find a universal relationship between TC and the lattice constant, which does not depend on the Fe content x. By using the channeling Rutherford backscattering and particle-induced X-ray emission measurements, it is clarified that about 15% of the Fe atoms exist in the tetrahedral interstitial sites in the Ge0.935Fe0.065 lattice and that the substitutional Fe concentration is not correlated with TC. Considering these results, we suggest that the non-uniformity of the Fe concentration plays an important role in determining the ferromagnetic properties of GeFe.We investigate the growth-temperature dependence of the properties of the group-IV-based ferromagnetic semiconductor Ge1−xFex films (x = 6.5% and 10.5%), and reveal the correlation of the magnetic properties with the lattice constant, Curie temperature (TC), non-uniformity of Fe atoms, stacking-fault defects, and Fe-atom locations. While TC strongly depends on the growth temperature, we find a universal relationship between TC and the lattice constant, which does not depend on the Fe content x. By using the spatially resolved transmission-electron diffractions combined with the energy-dispersive X-ray spectroscopy, we find that the density of the stacking-fault defects and the non-uniformity of the Fe concentration are correlated with TC. Meanwhile, by using the channeling Rutherford backscattering and particle-induced X-ray emission measurements, we clarify that about 15% of the Fe atoms exist on the tetrahedral interstitial sites in the Ge0.935Fe0.065 lattice and that the substitutional Fe concentration...

Spin-transfer torque in disordered weak ferromagnets

We study theoretically the spin transfer effect on a domain wall in disordered weak ferromagnets. We have identified the adiabatic condition for the disordered case as

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

\lambda \gg \lambda_{\rm D}\equiv \sqrt{{\hbar D}/{\spol}}

Origin of the large positive magnetoresistance of Ge1−xMnx granular thin films

, where

Tunneling magnetoresistance in trilayer structures composed of group-IV-based ferromagnetic semiconductor Ge1-xFex, MgO, and Fe

D

Impurity band conduction in group-IV ferromagnetic semiconductor Ge1-xFex with nanoscale fluctuations in Fe concentration

and

Increase of tunneling magnetoresistance in trilayer structures composed of group-IV ferromagnetic semiconductor Ge 1- x Fe x , MgO, and Fe

\spol

Observation of tunneling magnetoresistance in trilayer structures composed of group-IV ferromagnetic semiconductor Ge 1- x Fe x , MgO, and Fe

are the diffusion constant and the spin splitting energy due to the