Magnetic switching of Nb-Ni multilayers near the superconducting critical temperature

The magnetic properties of [Nb_x/Ni_y]₅ multilayers (ML) with x=23nm and y=2.5,3.5 and 5.0 nm, were studied using a superconducting quantum interference device (SQUID) magnetometer over the temperature range 2.0KC near 5.8K and complex magnetic hysteresis that reflects a strong interplay between Meissner currents, flux trapped in the superconducting Nb layers, and the Ni layer magnetization. The y=5.0nm ML exhibited a smooth ferromagnetic hysteresis loop for T>T_C and H applied in the ML plane. In constrast, sharp magnetization jumps of precisely the same magnitude, but opposite polarity, signal abrupt reversals of the ML ferromagnetic moment at two reproducible symmetric switching fields for T just below T_C and H<1KOe. The ML with y=2.5nm exhibited similar switching fields, but they were obscured by numerous random sharp instabilities in the hysteresis loop just below T_C. Behavior similar to the y=5.0nm sample was observed when the random instabilities disappeared at lower temperatures. These results demonstrate the supercurrent response of superconducting/ferromagnetic ML is a very sensitive probe of magnetic layer stability and switching dynamics.