Giant Tunnel Magnetoresistance Effect Derived by Controlling Crystallographic Orientation of MgO Barrier in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

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 ºC 450 ºC) 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.