Magnetic Switching of Nb-Ni Multilayers near the Superconducting Critical Temperature

The magnetic properties of [Nb_x/Ni_y]5 multilayers (ML) with x = 23 nm and y -2.5, 3.5 and 5.0 nm, were studied using a superconducting quantum interference device (SQUID) magnetometer over the temperature range 2.0 K < T < 300 K, and magnetic field range 0 < H < 3 KOe. All samples exhibited a zero-field superconducting transition temperature T_c near 5.8 K 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 = 5.0 nm 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 < 1 KOe. The ML with y = 2.5 nm 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. 0 nm sample was observed when the random instabilities disappeared at lower temperatures. These results demonstrate the supercurrent response of superconducting/ferro-magnetic ML is a very sensitive probe of magnetic layer stability and switching dynamics.