Bias Voltage Dependence of Magnetic Tunnel Junctions Comprising Double Barriers and CoFe/NiFeSiB/CoFe Free Layer

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/AlO_x/free layer/AlO_x/CoFe 7/IrMn 10/Ru 60 (nm). The free layer consists of an Ni₁₆Fe₆₂Si₈ B₁₄ 7 nm, Co₉₀Fe₁₀ (fcc) 7 nm, or CoFe t₁/NiFeSiB t₂/CoFe t₁ layer in which the thicknesses t₁ and t₂ 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 mum², a coercivity (H_c) of 11 Oe, and an interlayer coupling field (H_i) 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 Omegamu m², and a H_c of 11 Oe, but an increased H_i 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 H_i became large because of the magnetostatic Neacuteel coupling