Paramagnetic relaxation and Wohlleben effect in field-cooled Nb thin films

The "Wohlleben effect" (paramagnetic Meissner effect) is found in uniform applied magnetic fields only above a threshold value H(0)approximate to 10(2) Oe for field-cooled Nb films of thickness less than 1000 Angstrom. The initial diamagnetic magnetization observed upon field cooling to fixed temperatures just below T-c gives way to a logarithmic time relaxation toward paramagnetic values comparable to the critical state magnetization over a period of several hours (normalized rate S approximate to 0.1), implying that the development of the paramagnetic state is moderated by fast flux creep processes. Small field gradients approximate to 10-20 mOe/cm are found to influence the occurrence of the effect for applied fields H < H-0.