AC susceptibility and domain wall pinning in ferromagnets

The relationships between the reversible AC susceptibility, χ _AC, of ferromagnets with a random array of inhomogeneities and the effective bias field H are derived on the basis of the theory of strong and weak domain-wall pinning. When H<H _c, the coercive field, χ_AC is proportional to H^-n, where n=1 for weak pinning and n=3/7 (or 1/2 if the demagnetizing effect is neglected) for strong pinning. These relationships are used to investigate the pinning mechanisms of two iron-rich binary alloys, FeAu and FeCu. Specimens of both alloys in the α-phase solid-solution were thermally aged at 700°C for different durations. In the as-quenched FeAu specimen and the FeCu specimens aged for less than 150 mins, it was found that the exponent n has a value ~1. A weak pinning regime is obviously active. For the FeAu and FeCu specimens which were aged for a prolonged period, the exponents drop to average values of 0.61±0.05 and 0.49±0.07, respectively, so that strong pinning in these specimens is expected