Fabrication of Magnetic Tunnel Junctions With Co $_{2}$ FeSi Heusler Alloy and MgO Crystalline Barrier

Tunneling magnetoresistance (TMR) characteristics of magnetic tunnel junctions with a Co₂ FeSi electrode and an MgO crystalline barrier have been investigated. Co₂ FeSi Heusler alloy electrode grown on Cr-buffered MgO(100) substrate starts to have an L2₁ structure when annealed above 420degC. In the cases of CoFeB/MgO/Co₂ FeSi junctions, a high TMR ratio of 158% has been achieved after annealing at 350degC, which is obtained by coherent tunneling between the electrodes and barrier, not by the half-metallic nature of Co₂ FeSi Heusler alloy. However, the Co₂ FeSi Heusler alloy electrode degrades the TMR ratio when compared with an amorphous CoFeB electrode and the bottom Co₂ FeSi electrode makes more degradation of the TMR ratio than the top Co₂ FeSi electrode. The major reason for the low TMR ratio in Co₂ FeSi-based junctions is the broken epitaxial relationship between the bottom Co₂ FeSi electrodes and the MgO crystalline barrier, which is investigated by a cross-sectional transmission electron micrograph.