Study of synthetic ferrimagnet-synthetic antiferromagnet structures for magnetic sensor application

There has been a growing interest in using both synthetic ferrimagnet (SF) free and synthetic antiferromagnet (SAF) pinned layers for head and memory applications. In particular, for linear sensor applications, these structures lower the magnetostatic fields present at the free layer through the reduction of its effective thickness (t_eff^SF). This allows higher sensitivity but at the expense of an increased offset field H₀(Néel coupling field H_f+interlayer demagnetizing field H_d^SAF). In this work, results on a series of patterned 3×1 and 6×2 μm² top-pinned SF-SAF spin valves are analyzed and compared with a three-dimensional micromagnetic simulation in order to clarify the role of the different ferromagnetic layers in the overall offset field and sensitivity. H₀ varies as 1/t_eff^SF[t_eff^SF=(M_at_a-M_bt_b)/M_eff^SF]. The magnetostatic field acting on the SF coming from the SAF (H_d^SAF) can act as a biasing field, partially counterbalancing the Néel coupling field (H_f) leading to a reduction of H₀. In this work the offset field was reduced from an initial value of 25 Oe in a quasicompensated SAF to a value of -6 Oe, by unbalancing the SAF and consequently increasing its effective moment (t_eff^SF=- 15 Å).