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

Abstract

The ``Wohlleben effect'' (paramagnetic Meissner effect) is found in uniform applied magnetic fields only above a threshold value ${H}_{0}\ensuremath{\approx}{10}^{2}\mathrm{Oe}$ for field-cooled Nb films of thickness less than 1000 \AA{}. 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 $S\ensuremath{\approx}0.1),$ implying that the development of the paramagnetic state is moderated by fast flux creep processes. Small field gradients \ensuremath{\approx}10--20 mOe/cm are found to influence the occurrence of the effect for applied fields $H<{H}_{0}.$