Direct observation of switching processes in permalloy rings with lorentz microscopy

To achieve a deeper understanding of the switching process of magnetic Permalloy rings, Lorentz electron microscopy (LTEM) is used for the first time to image the magnetic configuration of such rings under applied external field conditions. Because of the exceptionally high lateral resolution we find two clearly distiguishable wall configurations present in the so-called "onion state." Furthermore, we show that a bias field present during a remagnetization cycle prevents the rings from transforming into a flux closure state, in which case remagnetization is a pure domain wall motion process.