Magnetic instabilities in thin non-planar ferromagnetic films

Lorentz microscopy images of thin permalloy films reveal the presence of vo rtices stabilised by a normal magnetic field. The vortex structures are als o accompanied by a complex domain wall pattern which is nucleated by the ex ternal field and is believed to be related to the shape anisotropy of the s ystem. The non-planar nature of the film investigated is, in fact, the nece ssary condition for the observation of such phenomena. Through the interpre tation of the micrographs, we analyse the energetics of the system by formu lating a magnetic model which encapsulates the shape anisotropy of the samp le. We show that the external field can induce instabilities in the system and the parameters estimated through our model are consistent with the expe rimental values. (C) 2001 Published by Elsevier Science B.V.