Investigation of local events of magnetization reversal in a Nd-Fe-B magnet by use of a Hall-effect microprobe

Magnetization processes in a sintered Nd-Fe-B permanent magnet (NEOMAX-35) were examined on a small scale using a Hall-effect microprobe with an active area 75 μm on a side. Probes were made by evaporating bismuth through a stencil mask onto glass slides. Experiments were performed by placing a probe onto the polished pole face of a Nd-Fe-B magnet and inserting the probe-magnet assembly into an electromagnet. Barkhausen steps, indicating rapid domain wall motion, were observed (superimposed upon the blank probe signal) in the demagnetization of a fully magnetized magnet. Magnetization traces for a thermally demagnetized Nd-Fe-B magnet did not exhibit measurable Barkhausen steps until a field of approximately 1.2 T was applied. The following observations were made for two thermally demagnetized samples which were cycled through minor hysteresis loops (maximum applied field of approximately 2 T). 1) Virgin magnetization traces did not contain measurable Barkhausen steps; however, all other forward and reverse magnetization traces did. 2) The initial reverse magnetization trace exhibited more and larger Barkhausen steps than subsequent traces. 3) Some Barkhausen steps were repeatable, that is, occurring at approximately the same field on each subsequent forward or reverse trace. Hall voltage signals were on the order of millivolts for probe currents of 10 mA.