Analysis of the intrinsic field sensitivity and noise of magnetoimpedance sensors

Luis Melo (1) , David Ménard (1) , Arthur Yelon (1) , Basile Dufay (2, 1) , Olivier Mareschal (2) , Sébastien Saez (2) , Christophe Dolabdjian (2)

1 Département de génie physique [Montréal]
2 Equipe Electronique - Laboratoire GREYC - UMR6072

The giant magnetoimpedance (GMI) effect is observed when the impedance of a magnetically soft conductor is measured as a function of frequency of the drive current and the applied external dc field. 1 Technological applications of the GMI effect such as magnetic, current and stress sensors 1 primarily require high impedance variation, high intrinsic sensitivity to dc and ac fields and low intrinsic magnetic noise. The magnitude of the effect, ΔZ, is commonly given by the change of impedance relative to the impedance at saturation. The sensitivity, S (in units of %/Oe or %/T), is proportional to the derivative of ΔZ with respect to the applied dc field. The magnetic noise, MN (in units of T/√Hz), which is a quantity proportional to the voltage-noise-to-sensitivity ratio, defines the minimum measurable field in 1Hz bandwidth and dictates sensor performance. In this work, the experimental and theoretical behaviour of S and MN are analyzed as a function of frequency and applied fields, including the effect of a dc bias current, for a Co-based magnetic conductor. A procedure to obtain the external parameters yielding the highest S and minimum MN is outlined. We have recently proposed a simple model to evaluate the GMI MN and S in the MHz regime of anisotropic magnetic wires. 2 The model accounts for a biasing dc circumferential field, H bias , which is included in the internal field of a single domain cylindrical magnetic conductor. For given frequency , H bias and easy axis direction, S is evaluated as a function of the external axial dc field, H ext. The field yielding the maximum sensitivity (S max), H ext max , is determined, defining the sensor working point. The MN is then evaluated at H ext max. For helical magnetic structures having the easy mag-netization axis close to circumferential, calculations show that both S max and MN (at H ext max) decrease for increasing H bias. 2 The field H bias essentially increases the internal field, which effectively makes the wire magnetically harder and hence, less sensitive to the driving signal when H ext is applied. Calculated normalized sensitivity, Λ, as a function of H ext and H bias at 10MHz for a 35μm diameter is presented in Fig. 1. The quantity Λ is given by Λ=H k S, where H k is the anisotropy field of the wire and S=dη/dH ext. η=(ξμ t 1/2-1)cos 2 θ, ξ is a function of the wave vectors and material parameters , 2 μ t is the transverse permeability and θ is the angle between the static magnetization direction and the direction of H ext. The permeability depends on frequency, internal field and magnetic damping. The parameter values used in the calculations are typical for Co-rich amorphous wires. Also, we have used H k =1.4Oe and an easy axis angle of 72 o (relative to the axial direction) in the calculations. We observe in Fig. 1 that for this frequency and easy axis angle, S broadens with S max (black points) decreasing when H bias is applied (see also broken lines A and B), as a consequence of the increase of the internal field. This may not be the case for a wire with an axial easy axis. Figure 2 shows experimental results of ΔZ/R dc and S from the measurement of the impedance of an as-cast Co-based wire as a function of H ext , when the wire was submitted to a dc bias current, i bias. The impedance was measured in a HP 8753B Network Analyzer (NA). For the i bias , injected into the device, we used the bias input of the NA. The experimental sensitivity shown in Fig. 2 was then obtained from the numerical derivative of the impedance versus field curve. In Fig. 2a, ΔZ/R dc and S vs. H ext at f=10MHz and 100MHz for i bias =20mA are shown. This value of i bias is equivalent to a H bias~2 Oe at the surface of the wire, which is close to the H k of this sample. Figure 2b shows S vs. H ext at f=100MHz for i bias =40mA, 20mA and 4mA. These results show that there are optimal frequency and dc bias conditions to maximize the sensitivity. The high S max for a non-zero H bias opens the possibility for the observation of very low values of MN predicted by the theory. 2 In this work, the behavior of S and MN are analysed and compared with experiment at frequencies up to 500MHz on Co-based amorphous wires.

Domaines: Electronique

Luis Melo, David Ménard, Arthur Yelon, Basile Dufay, Olivier Mareschal, et al.. Analysis of the intrinsic field sensitivity and noise of magnetoimpedance sensors. International Magnetics Conference, Apr 2008, Madrid, Spain. ⟨hal-01109795⟩