Spontaneous mechanical rotation of as-cast and annealed FeSiBNbCu wires under high-frequency axial ac field

The novel effect consisting of the spontaneous mechanical rotation when subjected to axial high-frequency ac magnetic field is reported for Fe_73.5Nb₃Cu₁Si_13.5B₉ amorphous as-cast wires and after devitrification. The rotation is observed at a spectrum of frequencies of the exciting applied field and as a consequence of the interaction of a generated magnetoelastic standing wave with an induced circular current. A correlation is found between complex spectra of rotational frequencies and the characteristics of the existing structural phases. Four characteristic frequencies and their harmonics are differentiated in the as-cast state. New rotational frequencies are observed at the very early stages of nanocrystallization, which can be ascribed to initial structural rearrangements. However, the spontaneous rotation vanishes in samples annealed at 550 °C and 570 °C where optimum ultra-soft magnetic behaviour is achieved and magnetostriction is drastically reduced. In this state, rotation is not observed as a consequence of the disappearance of the magnetoelastic wave because of the reduced magnetostriction. The rotation of the wire is observed again when annealed at 650 °C, due to a final increase of magnetostriction upon further crystallisation of the system. The direct correlation between magnetostriction and the field-induced rotation of the wire is thus shown.