Magnetic Tunnel Junctions (MTJ) with Large Tunneling Magnetoresistance (TMR) and Small Saturation Fields

There is a continuing need for more sensitive magnetic sensors. We report here an approach that leads to MTJ structures at the wafer level that have TMR values in excess of 100{\%} and saturation field (B$_{sat})$ values below 1 Oe. The ratio appears to be the largest ever reported. The approach we have used is to fabricate and anneal MTJs of a rather conventional type, SiO2/5Ta/30Ru/5Ta/2CoFe/15IrMn/2CoFe/0.9Ru/ 3CoFeB/2MgO/ 5CoFeB/5Ta/10Ru (thicknesses in nm), then etch down into the free layer of the MTJ, and deposit a thick (100 nm) and very soft magnetic film to lower B$_{sat}$. We have found that incorporating the soft film in the initial structure lowers the TMR significantly upon annealing. Maintaining a large TMR depends on depositing the soft film after annealing. A static field of 1.8 Oe is applied perpendicular to the sweep field ($i.e$. in the hard axis) to suppress the hysteresis of 1.3 Oe. This technique is described in Ref. 1.$^{ }$ The sweep field is in the easy axis of the free layer and the side field is in the hard axis of the free layer. The low-field loop is linear, non-hysteretic, and extrapolates to saturation at 0.8 Oe. The measured TMR is 118{\%}. 1) X. Liu, C. Ren, and G. Xiao 92, 4722 (2002).