フルホイスラーCo_2FeAl_<0.5>Si_<0.5>合金を有する強磁性トンネル接合の素子構造とトンネル磁気抵抗効果

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We fabricated fully epitaxial magnetic tunnel junctions (MTJs) with full-Heusler Co2FeAl0.5Si0.5 (CFAS) electrodes using the sputtering method. We changed the MgO barrier thickness, the thickness of an Mg layer inserted between the CFAS and MgO layers, and a post-annealing process for CFAS layers in order to obtain conditions necessary for achieving a high tunnel magnetoresistance (TMR) ratio. We found that both the MgO barrier thickness and the post-annealing temperature directly influenced the TMR ratio while the thin Mg layer insertion did not. Post-annealing at higher temperatures above 873 K suppresses the TMR ratio due to degradation of the structure of top CFAS layer during the post-annealing. In addition, a large effective tunnel barrier height was observed in our MTJs because of the poor quality of MgO barrier layer. These facts indicate that the tunneling spin polarization of CFAS/MgO interface is sensitive not only to the degree of ordering of L21 or B2 in CFAS but also to the quality of the MgO barrier and CFAS/MgO interfaces.
2009-05-01