Anti-Lenz Supercurrents in Superconducting Spin Valves

Abstract

Here, we present a study on Si(111)/\-Ta($150$Å)/\-IrMn($150$Å)/\-NiFe($50$Å)/\-Nb($x$)/\-NiFe($50$Å)/\-Ta($50$Å) and Si(111)/\-Ta($150$Å)/\-NiFe($50$Å)/\-Nb($x$)/\-NiFe($50$Å)/\-IrMn($150$Å)/\-Ta($50$Å) spin-valves with $x=100$ to $500$Å. For both sample families, above a specific critical thickness of the Nb-layer and below $T_c$, the superconducting Nb-layer contributes strongly to the magnetization. These systems show an anomalous hysteresis loop in the magnetization of the superconducting layer; the hysteresis loop is similar to what is generally expected from hard superconductors and many superconductor/ferromagnet hybrid systems, but the direction of the hysteresis loop is inverted, compared to what is generally observed (paramagnetic for up sweeping fields and diamagnetic for down sweeping fields). This means that the respective samples exhibit a magnetization, which is contrary to what should be expected from the Lenz' rule.