Influence of stray fields and the proximity effect in ferromagnet/superconductor/ferromagnet spin valves

We have investigated two switching mechanisms, the proximity effect and the influence of stray fields, in Permalloy (Py)/Nb/Py spin valves. Samples were grown by using dc magnetron sputtering and were patterned to form a bar shape. A coexistence of the proximity effect and the stray-field effect in Py/Nb/Py was found and could be explained by three resistance states: a low-resistance antiparallel state, an intermediate-resistance parallel state, and a high-resistance domain state with multiple domains in both Py layers. In order to compare the stray-field effect with the proximity effect, we increased the bias current density to enhance the Lorentz force on flux lines in the Nb layer in which flux lines are induced by stray fields from the domain walls of the Py layers. An increase in the magnetoresistance due to stray-field-induced flux flow in the domain state with increasing bias current density was observed as expected; meanwhile, a resistance difference between the parallel and antiparallel states due to the proximity effect was also observed. Between the two switching mechanisms, we found that a switch operating on the stray field effect under a high bias current density on the order of 105 A/cm2 was more advantageous for using a superconducting spin-valve as a resistance switch. Finally, we show that the stray-field-induced flux flow can be stabilized by orienting the magnetization easy axis perpendicular to the bar axis.