Direct visualization of magnetic domain wall motion in Nd-Fe-B magnets by alternating magnetic force microscopy using Co-GdO superparamagnetic tip.

Probing the dynamic magnetic domain of permanent magnet materials under an external AC magnetic field using magnetic force microscopy is challenging as well as important to reveal their pinning sites and local magnetization processes. In this work, we develop alternating magnetic force microscopy (A-MFM) with a Co-GdO superparamagnetic tip for observing the dynamic magnetic domain wall (DW) motion in a, b plane Nd-Fe-B sintered magnets by stimulating the magnetization of the superparamagnetic tip and Nd-Fe-B sample using an external AC magnetic field. This method enables the simultaneous measurement of DC and AC magnetic fields from the sample surface. The measurement results reveal that the DW motion does not happen until the amplitude of the external AC magnetic field reached 77 Oe (zero to peak). Further increasing the external AC magnetic field strength leads to DW motion with larger movement area and stronger AC magnetic field originating from the DW motion. In addition, A-MFM images shows the pinning sites of the DW motion. The present work provides a direct and effective approach to studying the local dynamic magnetization processes of the magnetic materials on the nanometer scale.