Fabrication of epitaxial heterostructures with halrf-metallic ferromagnets and development of spin-controlled devices

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The purpose of the present study was to develop spintronic devices utilizing the half-metallicity of Co-based Heusler alloys (Co_2YZ). Fully epitaxial magnetic tunnel junctions (MTJs) with a Co_2YZ thin film or Co_2YZ thin films and a MgO barrier featuring abrupt and extremely smooth interfaces were fabricated. Furthermore, high tunnel magnetoresistance (TMR) ratios were demonstrated at room temperature (RT) for fabricated Co_2YZ/MgO-based MTJs. The main results are summarized as follows.1) The Heusler alloy-based MTJ device technology was developed. This device technology features the followings: a) all layers in the MTJ trilayer structures are epitaxial and single-crystalline, b) abrupt and atomically flat interfaces between a Co_2YZ (Co_2Cr_<0.6>Fe_<0.4>Al (CCFA), Co_2MnSi (CMS), or Co_2MnGe) thin film and a MgO tunnel barrier, and c) the interface region of CMS thin films underneath a MgO barrier is not oxidized.2) The fabricated CCFA/MgO/Co_<50>Fe_<50> MTJs demonstrated a high TMR ratio of 109 % at RT (317% at 4.2 K).3) Fully epitaxial exchange-biased MTJs with CMS thin films as both lower and upper electrodes and with a MgO barrier were fabricated.The TMR ratios at both RT and 4.2 K increased with increasing the in-situ annealing temperature (T_a) just after the deposition of the upper CMS electrode. Furthermore, a high TMR ratio of 179% at RT (683% at 4.2 K) was demonstrated.In summary, it was demonstrated that epitaxial, single-crystalline heterostructures consisting of Co_2YZ thin films and a MgO barrier are highly promising for spintronic devices that utilize the half-metallicity of Co-based Heusler alloys.