Improvement of Thermal Tolerance of CoFeB–MgO Perpendicular-Anisotropy Magnetic Tunnel Junctions by Controlling Boron Composition

We investigated annealing temperature
$T_{a}$
dependence of tunnel magnetoresistance (TMR) ratio and magnetic properties for perpendicular-anisotropy (CoFe)
_100–X
B
_X
/MgO magnetic tunnel junctions (MTJs) with single (CoFe)
_100–X
B
_X
/MgO interface (s-MTJ) and double CoFeB–MgO interface (d-MTJ) structures with various boron compositions X. High TMR ratio over 100% was observed in the s-MTJ with
$X= 35$
at.% after annealing at 360 °C–400 °C, whereas the s-MTJ with
$X = 30$
at.% showed the degradation of TMR ratio with the increase of
$T_{a}$
above 360 °C, resulting from the decrease of perpendicular anisotropy. The d-MTJ with
$X = 25$
at.% maintained high TMR ratio up to
$T_{a} = 400~^{\circ }\text{C}$
owing to its higher perpendicular anisotropy compared with the s-MTJ. The difference of perpendicular anisotropy between the s-MTJ and the d-MTJ can be attributed to higher interfacial anisotropy together with lower saturation magnetization of the d-MTJs. The lower saturation magnetization is attributable to two MgO layers that suppress boron diffusion from CoFeB layers, which was verified by cross-sectional line analysis using electron energy-loss spectroscopy.