Magnetic Properties and Applications of Soft Magnetic Microwires
Recent trends in the development of magnetic sensors are focused on the miniaturization of their size, improvement of their features and on finding of new operating principles based on fundamental studies of new materials. Among new magnetic materials a family of thin wire with reduced dimensions recently gained considerable attention [1,2].
Particularly, studies of giant magneto-impedance (GMI) effect of thin microwires attracted considerable interest. We present the results on tailoring of soft magnetic properties and GMI effect in thin microwires paying special attention to achievement of low hysteretic high GMI effect in extended frequency range (till 4 GHz). Both GMI ratio and hysteresis loops of Co- rich low magnetostrictive amorphous microwires exhibit strong sensitivity to the internal stresses related with the ratio, of the metallic nucleus diameter to the total microwire diameter. The hysteresis loops exhibit low coercivity (generally below 10 A/m) with well defined magnetic anisotropy field, Hk.
Hk increases when decreases. Field dependence of the off-diagonal voltage response measured in pulsed regime (pulsed GMI) exhibits anti-symmetrical shape. On the other hand the magnetic anisotropy of Co and Fe-rich microwires can be tailored by stress or magnetic field annealing. Particularly stress annealed microwires exhibit stress-sensitive GMI effect and hysteretic properties. Varying the time and the temperature of such stress annealing we are able to tailor both magnetic properties and GMI. We found, that if the surface anisotropy is not circumferential, then the MI curve Z(H) present hysteresis. This hysteresis can be suppressed by application of sufficiently high DC bias current IB that creates a circumferential bias field HB. High frequency GMI and stress impedance (SI) can be applied for engineering artificial dielectrics with tuneable electromagnetic properties, i.e. for designing of the composites with embedded arrays of metallic wires exhibiting strong dispersion of the effective permittivity εef in the microwave range .
Among the other magnetic properties suitable for applications the magnetic bistability and fast Domain Wall (DW) propagation in Fe-rich compositions, considerable magnetoresistace in microwires with granular structure and magnetocaloric effect in Heusler-type microwires should be mentioned. The nanocrystalline structure has been observed in FeSiBCuNb and FeHfBSi microwires. We manipulated the DW dynamics of microwires through the magnetoelastic anisotropy: decreasing the magnetoelastic energy, Kme, DW velocity increases.
 DC Jiles Acta Materialia 51 (2003)5907.
 A. Zhukov and V. Zhukova, "Magnetic properties and applications of ferromagnetic microwires with amorphous and nanocrystalline structure," Nova Science Publishers, Inc. 400 Oser Avenue, Suite 1600 Hauppauge, NY 11788, 162 p. 2009, ISBN: 978-1-60741-770-5.