Abstract
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(001) 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(00.1)-oriented Ru layer is effective to maintain amorphous structure of CoFeB. (3) The achievable TMR ratio after high temperature (280 °C − 450 °C) 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.
