Bias voltage dependence of magnetic tunnel junctions comprising double barriers and CoFe/NiFeSiB/CoFe free layer

Abstract

The typical double-barrier magnetic tunnel junction (DMTJ) structure examined in this paper consists of a Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlOx/free layer/AlOx/CoFe 7/IrMn 10/Ru 60 (nm). The free layer consists of an Ni16Fe62Si8B14 7 nm, Co90Fe10 (fcc) 7 nm, or CoFe t(1)/NiFeSiB t(2)/CoFe t(1) layer in which the thicknesses t(1) and t(2) are varied. The DMTJ with an NiFeSiB-free layer had a tunneling magnetoresistance (TMR) of 28%, an area-resistance product (RA) of 86 k Omega mu m(2), a coercivity (H-c) of 11 Oe, and an interlayer coupling field (Hi) of 20 Oe. To improve the TMR ratio and RA, a DMTJ comprising an amorphous NiFeSiB layer that could partially substitute for the CoFe free layer was investigated. This hybrid DMTJ had a TMR of 30%, an RA of 68 k Omega mu m(2), and a H-c of 11 Oe, but an increased Hi of 37 Oe. We confirmed by atomic force microscopy and transmission electron microscopy that H-i increased as the thickness of NiFeSiB decreased. When the amorphous NiFeSiB layer was thick, it was effective in retarding the columnar growth which usually induces a wavy interface. However, if the NiFeSiB layer was thin, the roughness was increased and Hi became large because of the magnetostatic Neel coupling.