Formation of Stripe Domains in Cobalt Bars Via a Magnetic Soft Mode Instability

We theoretically study the relation between stripe domains at remanence and magnetic normal modes in a single crystal Co bar. We find different stripe patterns depending on field history and in each case the domain structure can be related to a soft mode that triggers a phase transition. The stripe domain structure when the external field is along the long axis of the bar is shown to be generated by a standing wave mode, which has the same spatial structure as the stripes. At all fields this mode has the lowest frequency of all the standing wave modes. This mode goes soft at a second-order phase transition where the stripe domains emerge. For other directions of the field, the symmetry of soft modes is found to be consistent with the change in symmetry of the ground state and that the phase transition can be first order. An analytical model relating phase transitions and soft mode behavior is also briefly discussed