Spontaneous Ac Field Induced Mechanical Rotation in Magnetostrictive Fesib-Based Wires Subjected to Thermal Treatments
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SPONTANEOUS AC FIELD INDUCED MECHANICAL ROTATION IN MAGNETOSTRICTIVE FeSiB-BASED WIRES SUBJECTED TO THERMAL TREATMENTS V.Raposo1,2, A.Mitra1, 3 and M.Vázquez1 1
Instituto de Ciencia de Materiales de Madrid, CSIC, 28049 Cantoblanco, Madrid, Spain 2 Dpto. Física Aplicada, Universidad de Salamanca, 37008 Salamanca, Spain 3 National Metallurgical Laboratory, Jamshedpur, India
ABSTRACT An astonishing new phenomenon has been recently observed in magnetic wires. It consists of the spontaneous rotation of the wires when submitted to an exciting AC axial field with frequency of the order of kHz and amplitude above some threshold. The rotation is believed to appear due to interaction between generated magnetoelastic standing waves and induced eddy currents. In the present work rotational characteristics of Fe77.5Si7.5B15 and Fe73.5Si13.5B9Cu1Nb3 wires in their as-cast amorphous state and after heat treatments leading to devitrification has been investigated. It is proved that this rotational phenomenon is only observed in samples with large enough magnetostriction irrespective of their structural character. Moreover, changes in rotational characteristics are ascribed to the structural modifications accompanying the devitrification process. INTRODUCTION The mechanical rotation of Fe base amorphous wires with bistable magnetic behavior when subjected to an alternating AC field of several kHz has been recently reported [1-3]. Rotational wire frequencies oscillate around tens of Hz. Wire rotation frequency depends on magnetostriction constant, and it has been observed for both positive and negative values as shown in [2]. The influence of the length of the wire in the excitation frequency that causes a rotation and the appearance of rotation at higher harmonics of some fundamental frequencies reveals the existence of a resonant magnetoelastic standing wave in intimate correlation with the origin of the rotation. This phenomenon has been proved to have potential applications in different fields such as in micro-motors or viscosimetry [4,5]. Although a quantitative theory is not currently available, our results show that this is a general property of materials with high magnetostriction constant, no matter their crystalline structure, since it is present in amorphous but also in polycrystalline wires, as recently reported [6]. Consequently, the rotational behavior is nowadays expected to be determined neither by the alloy composition, nor by the structural nature of the material, nor by the domain structure, but seemingly by the magnetostrictive character of the sample. The aim of this work has been to study the AC field induced rotation of Fe77.5Si7.5B15 and Fe73.5Si13.5B9Cu1Nb3 amorphous wires exhibiting large magnetostriction in their as-cast state and after annealing that finally results in the devitrification of the samples [7]. In fact, in the case of Fe73.5Si13.5B9Cu1Nb3 wires, annealing leads to homogeneous and stable partial crystallization with noticeable reduction of magnetostriction and disappearance of that phenomeno
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