Study of in situ magnetization reversal processes for nanoscale Co rings using off-axis electron holography

We report a study of the magnetic switching behavior of nanoscale Co rings using off-axis electron holography. Arrays of 10 nm thick polycrystalline Co rings with 400 nm outer diameter (OD) and different inner diameter (ID) were fabricated by electron-beam lithography. The switching behavior of the rings was studied for different ODID ratios, and two kinds of reversal mechanism were identified. For ODID of 400 nm250 nm and 400 nm50 nm, the reversal started from the so-called onion (bidomain) state, proceeding to a stable vortex state, and finally to the reversed onion state. For intermediate ODID of 400 nm150 nm, the reversal was instead accomplished via rotation of head-to-head domain walls around the rings to the reversed onion state without formation of a vortex state. The ODID ratio of the rings thus played the most important role in determining the switching process. Irrespective of the reversal mechanism, the coercive field of the rings and the range of the field needed to reverse their magnetization, both increased as the inner ring diameter was increased (i.e., narrower ring). The significance of different contributions to the total energy in causing these differences in switching behavior is briefly discussed.