Yusuke Shuto

High temperature ferromagnetism in GaAs-based heterostructures with Mn delta doping.

We show that suitably designed magnetic semiconductor heterostructures consisting of Mn delta (delta)-doped GaAs and p-type AlGaAs layers, in which the locally high concentration of magnetic moments of Mn atoms are controllably overlapped with the two-dimensional hole gas wave function, realized remarkably high ferromagnetic transition temperatures (T(C)). A significant reduction of compensative Mn interstitials by varying the growth sequence of the structures followed by low-temperature annealing led to high T(C) up to 250 K. The heterostructure with high T(C) exhibited peculiar anomalous Hall effect behavior, whose sign depends on temperature.

Magneto-optical properties of group-IV ferromagnetic semiconductor Ge1−xFex grown by low-temperature molecular beam epitaxy

Group-IV ferromagnetic semiconductor Ge1−xFex was grown by low-temperature molecular beam epitaxy without precipitation of ferromagnetic Ge–Fe intermetallic compounds. The ferromagnetism of Ge1−xFex films was investigated by magnetic circular dichroism (MCD). In particular, the influence of the Fe content (x=2.0%–17.5%) and growth temperature (100 and 200°C) on the ferromagnetism was carefully studied. The MCD measurements revealed that the overall spectral features reflecting the band structure of the Ge1−xFex films were identical with those in bulk Ge, and that the large spin splitting of the band structure was induced by the incorporation of Fe atoms into the Ge matrix, indicating the existence of s,p‐d exchange interactions. The Ge1−xFex films showed ferromagnetic behavior and the ferromagnetic transition temperature linearly increased with increasing the Fe concentration. These results indicate that the epitaxially grown Ge1−xFex is an intrinsic ferromagnetic semiconductor.

Structural and magnetic properties of epitaxially grown Ge1−xFex thin films: Fe concentration dependence

Ge1−xFex films (x=2.0%–24.0%) grown by low-temperature molecular beam epitaxy were shown to have a diamond-type crystal structure without any other crystallographic phase of precipitates, although they contain slightly nonuniform Fe distribution and tiny stacking fault defects. The lattice constant decreases linearly with increasing the Fe content x from 0% to 13.0%, and is saturated for x>13.0%. The Curie temperature (TC) increases in proportion to x (⩽13.0%) and is saturated for x>13.0%. The maximum TC value was ∼170K at x>13.0%. The structural and magnetic properties indicate that Ge1−xFex is an “intrinsic” ferromagnetic semiconductor.

Epitaxial germanidation of full-Heusler Co2FeGe alloy thin films formed by rapid thermal annealing

The authors demonstrated that a full-Heusler Co2FeGe (CFG) alloy thin film was epitaxially grown by rapid-thermal-annealing-induced germanidation of an Fe/Co/pseudo-Ge(001)-on-insulator (GOI) multilayer formed on a Si-on-insulator (SOI) substrate. X-ray diffraction (XRD) measurements with out-of-plane and in-plane configurations revealed that the CFG film was epitaxially grown along the [001] direction with the in-plane epitaxial relation of CFG[100]‖GOI[100], although the film slightly contained a non-epitaxial component. The strong (111) and (200) superlattice diffraction intensities indicated that the CFG film had a high degree of order for the L21 structure. Cross-sectional high-resolution transmission electron microscopy images of the film revealed that the film had dominant epitaxial and slight non-epitaxial components, which was consistent with the XRD measurements. The epitaxial component was grown directly on the buried oxide layer of the SOI substrate without formation of any interfacial layer.