Magnetoresistance Variation of Magnetic Tunnel Junctions with NiFeSiB/CoFeB Free Layers Depending on MgO Tunnel Barrier Thickness

We developed NiFeSiB/CoFeB hybrid free-layers for magnetic tunnel junctions (MTJs) with MgO tunnel barrier layers. These junctions show tunneling magnetoresistance (TMR) ratios and resistance-area (RA) values ranging from 118-209% and 36-2380 Omega mum² , respectively, obtained at room temperature. Compared to the CoFeB single free-layer case, the NiFeSiB/CoFeB hybrid free-layer approach has the advantage of lowering saturation magnetization. The low magnetization material would be effective to decrease the switching current in spin transfer torque (STT) switching. The experimental results show that the RA value depends not only on the thickness of the MgO barrier but also on the structure of the free layer used. Tunable in the TMR ratio and RA value using the design of the hybrid free-layer, our hybrid free-layered MTJs demonstrate a desirable lower RA value but a similar TMR ratio in comparison to the CoFeB free-layered ones.