The electronic structure of half-metallic ferromagnet/semiconductor junctions

摘要

Half-metallic ferromagnets are promising materials for improving the performance of various spin-electronic devices, such as magnetic tunnel junctions (MTJ), spin light-emitting diodes (LED), and some kinds of spin transistors. According to the theory on transport in diffusive regime, the conductivity mismatch between metallic ferromagnets and semiconductors prevents efficient spin injection into semiconductors, except for the case of half-metallic injector [1]. The spin injection across tunnel barrier overcomes the obstacle of the conductivity mismatch [2]. Indeed, efficient spin injection has been achieved for Fe/AlGaAs(001) Schottky junctions [3] and for CoFe/MgO/AlGaAs(001) tunnel junctions [4]. On the other hand, the observed efficiency of spin injection from a half-metallic Heusler alloy Co_2MnGe into AlGaAs/GaAs(001) LED structure is much lower [5]. The disappointing result can be attributed to the influence of interfacial states which degrade half-metallic characteristics [6], since first-principles calculations confirm the spin polarization of half-metallic Heusler alloys Co_2YZ to be hardly unaffected by the B2 disorder, i.e. random interchange of Y and Z atoms [7, 8]. The highly spin-polarized interface between ferromagnetic electrode and semiconductor or insulating barrier is crucial for further improvement of the spin-injection efficiency. In this talk, the issue is addressed from theoretical point of view, regarding half-metallic Heusler alloys as candidates for spin injector.