Kohei Okamoto

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

The group-IV-based ferromagnetic semiconductor Ge1− x Fe x (GeFe) is one of the most promising materials for efficient spin injectors and detectors for Si and Ge. In this study, we show the first successful observation of the tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) containing a group-IV ferromagnetic semiconductor, that is, in MTJs composed of epitaxially grown Fe/MgO/Ge0.935Fe0.065. We find that the p–d(t2) band in GeFe is mainly responsible for the tunneling transport. Although the obtained TMR ratio is small (~0.3%), the TMR ratio is expected to be enhanced by suppressing leak current through amorphous crystal domains observed in MgO.

Quantum size effect in an Fe quantum well detected by resonant tunneling carriers injected from a p-type Ge semiconductor electrode

We report the clear observation of the quantum size effect in an Fe quantum well (QW) detected by resonant tunneling carriers injected from a p-type Ge semiconductor electrode in fully epitaxial double-barrier magnetic tunnel junctions, which are composed of Co/Fe/MgO/Fe QW/MgO/Ge:B grown on a p+-Ge(001) substrate. A large tunnel magnetoresistance (TMR) ratio up to 137% (237%), which is comparable to that in Fe/MgO/Fe, is obtained at 297 K (3.5 K). The quantum oscillations are clearly observed in the dI/dV–V and d2I/dV2–V curves of our devices, and the resonance voltages are in good agreement with the resonant levels calculated by the phase accumulation model. Following these oscillations, the TMR is modulated by the quantum size effect. Our results are promising for realizing future quantum spintronics devices based on semiconductor/metal hybrid heterostructures with advanced functionalities.

Fe concentration dependence of tunneling magnetoresistance in magnetic tunnel junctions using group-IV ferromagnetic semiconductor GeFe

Group-IV-based ferromagnetic semiconductor Ge1−xFex (GeFe) is one of the most promising materials for spin injection/detection in Si and Ge. In this paper, we demonstrate a systematic study of tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) composed of Fe/MgO/Ge1−xFex with various Fe concentrations (x = 0.065, 0.105, 0.140, and 0.175). With increasing x, the TMR ratio increases up to 1.5% when x≤ 0.105, and it decreases when x> 0.105. This is the first observation of the TMR ratio over 1% in MTJs containing a group-IV ferromagnetic semiconductor. With increasing x, while the Curie temperature of GeFe increases, the MgO surface becomes rougher, which is thought to be the cause of the upper limit of the TMR ratio. The quality of the MgO layer on GeFe is an important factor for further improvement of TMR in Fe/MgO/GeFe MTJs.