High Frequency AC Magnetic Field Research Articles

AC field induced rotation of magnetostrictive wires

The last results concerning the novel effect consisting of the spontaneous mechanical rotation of wires when subjected to axial high frequency AC magnetic field are here reported. Wires rotate at frequencies around tens of Hz when subjected to an alternating AC field with a particular frequency of the order of tens of kHz. This phenomenon has been observed in several types of wires, irrespective of their crystalline structure but exhibiting large enough magnetostriction. The studied materials include FeSiB, CoSiB (amorphous) and FeNbCuSiB (nanocrystalline) alloys and polycrystalline Ni. The rotation appears at certain frequencies of the exciting field corresponding to the fundamental and higher harmonics of a set of frequencies. This reveals the resonant nature of the effect, which is connected to the generation of a magnetoelastic standing wave in the wire. A novel mechanical experiment is here introduced that gives a new insight into the phenomenon: a similar response is obtained when samples (even non-magnetostrictive) are mechanically excited by the vibrations induced by a loudspeaker membrane.

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

The novel effect consisting of the spontaneous mechanical rotationwhen subjected to axial high-frequency ac magnetic field is reported for Fe73.5Nb3Cu1Si13.5B9 amorphous as-castwires and after devitrification. The rotation is observed at a spectrum offrequencies of the exciting applied field and as a consequence of theinteraction of a generated magnetoelastic standing wave with an inducedcircular current. A correlation is found between complex spectra of rotationalfrequencies and the characteristics of the existing structural phases. Fourcharacteristic frequencies and their harmonics are differentiated in theas-cast state. New rotational frequencies are observed at the very earlystages of nanocrystallization, which can be ascribed to initial structuralrearrangements. However, the spontaneous rotation vanishes in samples annealedat 550 °C and 570 °C where optimum ultra-soft magneticbehaviour is achieved and magnetostriction is drastically reduced. In thisstate, rotation is not observed as a consequence of the disappearance of themagnetoelastic wave because of the reduced magnetostriction. The rotation ofthe wire is observed again when annealed at 650 °C, due to a finalincrease of magnetostriction upon further crystallisation of the system. Thedirect correlation between magnetostriction and the field-induced rotation ofthe wire is thus shown.