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A Novel Broadband Measurement Method for the Magnetoimpedance of Ribbons and Thin Films

A novel broad-band measurement method of the MI in thin films and ribbons is presented. It is based on the automated measurement of the reflection coefficient of a cell loaded with the sample. Illustrative results obtained with a permalloy multilayer thin film are presented and discussed.


I. INTRODUCTION
The magnetoimpedance (MI) effect consists of a change in the complex impedance of a ferromagnetic conductor under application of a static magnetic field H dc . This effect has been observed over a wide frequency range from few kHz up to several GHz. MI is related to the effective permeability of the sample, and hence, all mechanisms affecting magnetization processes of the material ought to be considered. Albeit this effect is usually weak, a giant magnetoimpedance effect (GMI) in amorphous ferromagnetic FeCoSiB wires with small magnetic fields (a few Oersteds) and at low frequencies (kHz to MHz) was discovered 1 .
A smaller MI effect has also been observed, later on, in ribbons and thin magnetic films 2 .
Several reasons explain that interest. Firstly, potential applications of GMI in magnetic fields sensors and magnetic recording heads have been studied and tested in different systems (thin films, sandwich structures, amorphous ribbons and microwires,...). Secondly, from the theoretical viewpoint, a better understanding of the mechanisms that drive MI and GMI effects provides an additional tool to investigate intrinsic and extrinsic magnetic properties of soft ferromagnetic materials.
In both cases thin films and ribbons possess several advantages with respect to wires because they allow several orientations of H dc versus ac current direction. Moreover, sputtered or otherwise produced films allow multilayering and size reduction required for integrated devices.
At frequencies above a few tens of MHz, sample length is no longer negligible with respect to the ac signal wavelength and consequently, transmission line theory must be used 3 .
In this paper, an appropriate broadband frequency method for the determination of the MI effect, in thin films and ribbons, is presented. The method is based on the automated measurement, by a network analyser, of the reflection coefficient of a cell loaded by the film under test. The field H dc can be applied either in the plane of the sample or out of it with sets of Helmholtz coils. Some illustrative results obtained using this measurement method with a permalloy multilayer thin film are presented and discussed.
The samples used in the present work are single NiFe magnetic thin films of different thicknesses (from a few hundred to a few thousandÅ), NiFe multilayer and sandwich structures deposited by RF diode sputtering on 20x2mm glass subtrates. These samples present an in-plane magnetization perpendicular or parallel to the sample axis.

2
The measuring device is a matched clip fixture, connected to a HP8753 network analyser yielding the complex reflection scattering coefficient S 11 of the clip fixture loaded by the sample.
The complex impedance Z(H, f ) of the sample, is given by with Z 0 = 50 Ω the device characteristic impedance.
The input RF power is 0 dBm and the dc external magnetic field H, applied by two pairs of Helmholtz coils, can be swept from -200 Oe to 200 Oe along or perpendicular to the sample axis.
The apparatus, briefly described here, allows the exploration of the MI effect over a broad frequency band from 0.3 MHz to 500 MHz. The lower bound is imposed by the analyser capabilities and the upper one by our cell design, nevertheless, the method may be easily extended, to lower or to higher frequencies.
The MI ratio used is given by the expression: where H is the dc applied magnetic field, H max the maximum value of H and f the frequency of the driving ac current.
Numerous samples have been tested using this apparatus. As an example, the results