Magnetic bistability and controllable reversal of asymmetric   ferromagnetic nanorings
  Magnetization reversals through the formation of vortex state and the rotation of onion state are two processes with comparable probabilities for symmetric magnetic nanorings with radius of about 50 nanometers. This magnetic bistability is the manifestation of the competition between the exchange energy and the magnetostatic energy in nanomagnets. The relative probability of the two processes in symmetric nanorings is dictated by the ring geometry and can not be altered. In this work, we report the magnetic behavior of a novel type of nanorings -- asymmetric nanorings. By tuning the asymmetry we can control the fraction of the vortex formation process from about 40% to nearly 100% by utilizing the direction of the external magnetic field. The observed results have been accounted for by the dependence of the domain wall energy on the local cross section area of nanoring for which we have provided theoretical calculations. 
F. Q. Zhu, G. W. Chern, O. Tchernyshyov, X. C. Zhu, J. G. Zhu, and C.   L. Chien