Heusler alloy/semiconductor hybrid structures

Heusler alloys are theoretically predicted to become half-metals at room temperature (RT). The advantages of using these alloys are good lattice matching with III-V semiconductors, high Curie temperature above RT and intermetallic controllability for spin density of states at the Fermi level. These alloys are categorized into half and full Heusler alloys dependent upon the crystalline structures, each of which is discussed both experimentally and theoretically in Sections 2 and 3, respectively. As an example, our recent studies on epitaxial L2(1) Co-2 Cr1-xFexAl(001)/GaAs(001) hybrid structures are presented here. Both structural and magnetic characterizations on an atomic scale are typically carried out in order to prove the half-metallicity at RT as described in Section 4. Atomic ordering in the Heusler films is directly observed by X-ray diffraction and is also indirectly measured by the temperature dependence of electrical resistivity. Element specific magnetic moments and spin polarization of the Heusler alloy films are directly estimated by using X-ray magnetic circular dichroism and Andreev reflection, respectively. By employing these alloy films in a spintronic device, diffusive spin-polarized electron transport may offer highly efficient spin injection across a direct interface between the Heusler alloy film and the semiconductor, while ballistic transport in a magnetic tunnel junction may further improve areal density of a magnetic random access memory with a large magneto-resistance ratio at RT as discussed in Section 5. A brief summary is provided at the end of this review. (c) 2006 Elsevier Ltd. All rights reserved.