Tunnel magnetoresistance sensors with dual soft-pinned free layers exhibiting highly symmetric resistance-field response curves

Abstract

We report on tunnel magnetoresistance (TMR) sensors with two ferromagnetic free layers (FLs) on both sides of a MgO tunnel barrier. The magnetizations of these FLs are weakly (softly) pinned in an anti-parallel configuration by the exchange bias of IrMn antiferromagnetic layers through ferromagnetic and/or antiferromagnetic interlayer couplings of non-magnetic spacer layers. These dual soft-pinned FL (SPFL) sensors show a symmetric resistance (R) change with respect to the polarity of the external magnetic field (H) applied to the hard axis (HA) of the SPFLs and TMR ratios of over 200%, which is much higher than those of the single-SPFL sensors, where one side of the ferromagnetic layers is an SPFL and the other side is a strongly pinned reference layer. When the applied H exhibits an angular misalignment from the HA, dual-SPFL sensors show much-reduced asymmetry in the R–H curves than single-SPFL sensors. Macrospin simulations of the magnetization behavior and R–H curve for both dual- and single-SPFL sensors revealed that the small asymmetry in the R–H curves of dual-SPFL sensors was owing to their significantly smaller dependence of relative magnetization angle between the SPFLs on the magnetic field misalignment from the HA. These results suggest that dual-SPFL TMR sensors are promising for use in magnetic sensors such as encoder sensors for position and rotation detections.