Field-dependent surface impedance tensor in amorphous wires with two types of magnetic anisotropy: Helical and circumferential

Abstract

This paper concerns the theoretical and experimental investigation of the magneto-impedance (MI) effect in amorphous wires in terms of the surface impedance tensor ςˆ. Physical concepts of MI and problems of significant practical importance are discussed using the results obtained. The theoretical analysis is based on employing the asymptotic-series expansion method of solving the Maxwell equations for a ferromagnetic wire with an ac permeability tensor of a general form associated with magnetisation rotation. The magnetic structure-dependent impedance tensor ςˆ is calculated for any frequency and external magnetic field, and is not restricted to the case when only strong skin-effect is present. This approach allows us to develop a rigorous quantitative analysis of MI characteristics in wires, depending on the type of magnetic anisotropy, the magnitude of dc bias current, and an excitation method. The theoretical model has been tested by comparing the obtained results with experiment. For the sake of an adequate comparison, the full tensor ςî s measured in CoFeSiB and CoSiB amorphous wires having a circumferential and helical anisotropy, respectively, by determining the 21 S parameter. In cases, when the rotational dynamics is responsible for the impedance behaviour, there is a reasonable agreement between the experimental and theoretical results. Such effects as the ac biased asymmetrical MI in wires with a circumferential anisotropy, the transformation in MI behaviour caused by a dc current (from that having a symmetric hysteresis to an asymmetric anhysteretic one) in wires with a helical anisotropy are discussed.

Magnetoimpedance in soft magnetic amorphous and nanostructured wires

“…A change of these parameters would result in a different current distribution in the sample's cross section and thus a change in impedance. This approximation has given a reasonable qualitative agreement with experimental results [23]. The magnetic permeability depends as well on the drive current frequency in a complicated way, and is also affected by the magnetic anisotropy induced by the thermal treatment.…”

“…This tensor establishes the relationship between the tangential electric field (which originates the voltage) and the tangential magnetic field (which determines the current) on the wire surface. The diagonal axis ζ zz and offdiagonal ζ φz components of the impedance tensor can be derived from the voltage V z between the wire ends for the conventional MI, or the voltage V φ of the pick-up coil, respectively [23,25]:…”

“…The off-diagonal MI effect originates in the existence of cross magnetization processes when the sample is axially magnetized and an ac current is flowing through it [22][23][24][25]. The large responses are favoured by the existence of a helical path around the direction of the excitation current [45].…”

“…This effect has shown a great potential of amorphous ferromagnetic wires, microwires and ribbons for microminiature magnetic field sensing applications. Their main advantages are the high response non hysteretic behaviour, low power consumption and very simple sensor scheme, which make them very promising materials for sensor technology [22][23][24][25].…”

Development of Ultra-Thin Glass-Coated Amorphous Microwires for High Frequency Magnetic Sensors Applications

“…results in a monotonic decay of the GMI ratio with the axial magnetic field. Consequently, non-diagonal components of the magnetic permeability tensor and impedance tensor were introduced in [6,7] in order to describe such circumferential anisotropy. Therefore, for the samples with a well-defined maximum on the axial field dependence of GMI ratio, the importance of the non-diagonal components of the permeability tensor should be noted.…”