Magnetic Force Microscopic Study on Domain-Wall Molecules in NiFe Nano Rings

A pair of magnetic domain walls (DWs) in ferromagnetic NiFe rings has been investigated in terms of the magnetic force microscopy (MFM). When the distance between the rings d is greater than a threshold value d(th), MFM signals indicate that a DW in the ring is dragged due to a stray magnetic field from an MFM probe tip. When d < d(th), this drag signals disappears; DWs are bound to each other by the DW-DW interaction. This transition can be argued in terms of the competition between the DW-DW magnetostatic interaction and the DW-drag potential. From the d-dependent MFM data, the DW-drag potential was estimated.