High Sensitivity Spin Valve Sensors With AF Coupled Flux Guides

Giant magnetoresistive (GMR) sensors can have their field sensitivity enhanced by many fold if located in the gap of two magnetically soft flux-guides (FG). In this paper, we present spin valve (SV) sensors, with saturation fields of less than 3 Oe and high linearity characterized by coercive forces of less than 0.5 Oe. FG require magnetically soft thin films with thicknesses in the range of 100-500 nm . In previous work, we prepared single-layer (SL) films of amorphous Co _90.7(Zr-Nb)_9.3 that exhibited stripe domains (SD) when patterned into FG, causing Barkhausen noise and complete lost of linearity in the SV sensors response. In this article, single layer films of an amorphous alloy of Co_88.4Zr_3.3Nb_8.3 , patterned into flux guides, do not exhibit SD but well-behaved closure domains. Nevertheless, these induce hysteresis in the sensors response, characterized by a coercive force of 0.7 Oe. This hypothesis is corroborated by focused beam magneto-optic Kerr effect (MOKE) magnetometry, performed in the poles region the CZN FG. By contrast, FG integrating instead multilayer (ML) thin films consisting of ferromagnetic layers of permalloy weakly anti-ferromagnetically (AF) coupled through Ru interlayers cause a strong reduction of hysteresis in the SV sensors response. The sensors in the gap of AF coupled (NiFe/Ru)xn FG, exhibit saturation fields of about 2 Oe and coercive forces of 0.3 Oe, despite the fact that the isolated sensors exhibit coercive forces of 2 Oe.