Signature of linear (Bulaevskii–Ginzburg) domain walls near T c of Ba-hexaferrite

The domain wall dynamics of a BaFe12O19 single crystal has been studied by measurements of the linear ac susceptibility parallel to the easy axis between 10 and 106 Hz. In the temperature interval 0.98 Tc≤T≤Tc=742.5 K, both dispersion and absorption reveal pure relaxational behavior, the damping rate ΓW of which exhibits a distinct minimum about T*=0.992 Tc. Based on previous results on low-temperature ferromagnets (Tc≤3 K) and recent theoretical work, the speeding up of ΓW∼(Tc−T)−0.7 toward Tc is related to the relaxation of linear domain walls proposed by Bulaevskii and Ginzburg. For the first time, additional support for this wall type is gained from the magnitude of the wall-reconstructuring temperature T*. The steep rise of ΓW below T* can be related to Bloch-wall damping, however, the shape of the crossover of ΓW about T* remains to be explained.