CoFeB/MgO/CoFeB強磁性トンネル接合におけるMgO障壁層の結晶配向性制御および巨大トンネル磁気抵抗効果の導出

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Crystallographic orientation of the MgO barrier in sputter-deposited CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) and its effect on tunnel magnetoresistance (TMR) were investigated. The degree of MgO(OO1) orientation was estimated with the integral intensity ratio (I_<(200)/I_<(220)) of diffraction lines from MgO(200) and MgO(220) planes obtained in grazing incident x-ray diffraction profiles. The main results are stated as follows. (1)I<(200)>/I_<(220)> 〜 4, meaning the (001) orientation of MgO, is realized when the underlaid CoFeB maintains amorphous structure, meanwhile MgO on bcc(110)-oriented CoFe shows (111) orientation (I_<(200)>/I_<(220)> = 0). (2) The prevention of epitaxial growth on hcp(00.l)-oriented Ru layer is effective to maintain amorphous structure of CoFeB. (3) The achievable TMR ratio after high temperature (280℃ - 450℃) annealing is mainly dominated by the MgO orientation and giant TMR ratio exceeding 200% is only obtained with I_<(200)>/I_<(220)> ≥ 3.4, while the resistance area product is independent of I_<(200)>/I<(220)> (4) Thin Mg layer inserted between CoFeB layer and MgO barrier is effective to obtain bcc(001)-oriented crystallization of CoFeB after high temperature annealing and results in a giant TMR ratio, because of its role to avoid surface oxidization of underlying ferromagnetic electrode during the deposition of MgO barrier.
社団法人日本磁気学会の論文
2007-09-01