Domain stability effect on magnetoresistance in ferromagnet/superconductor/ferromagnet trilayers

The magnetoresistance (MR) and magnetization of ferromagnet/superconductor/ferromagnet (F/S/F) trilayers with F = permalloy and S = Nb were studied. The trilayers were patterned in a long bar shape and the easy axis of magnetization was aligned either along the long or short axis. Four different configurations of longitudinal easy-axis, transverse easy-axis, longitudinal hard-axis, and transverse easy-axis MR were measured, respectively, and analyzed. For the longitudinal easy-axis MR, an upward resistance jump was observed when the mutual orientation of the magnetization of the two F layers was changed from the parallel to antiparallel (AP) configuration, disagreeing with the theoretical predictions based on the proximity effect. On the other hand, the transverse easy-axis MR exhibited a suppressed MR in the AP state. For the other two configurations, MR increase near the coercive fields were observed. By comparing our results with other reported works, we find that the stability of the domain states is a very important factor for observability of the proximity effect, and the stability can be enhanced by adding an adjacent antiferromagnetic pinning layer or controlling the shape anisotropy.