D. Seddaoui

Physical models of magnetoimpedance

We recall the methods for the rigorous calculation of the electromagnetic behavior of magnetic metallic samples and their application to the modeling of ferromagnetic resonance and of giant magnetoimpedance experiments. We explain the effect of various approximations and simplifications, particularly of the neglect of the exchange-conductivity effect, which has been the subject of confusion and of misconceptions in the literature, as have questions of domain wall motion and of nonlinear behavior. We show that the rigorous treatment provides a satisfactory description of experimental results, while the simplifications can only do so under limited circumstances.

Second harmonic of nonlinear magnetoimpedance in amorphous magnetic wires with helical anisotropy

The axial magnetic field dependence of the second harmonic of giant magnetoimpedance in Co-rich amorphous wires with helical anisotropy has been measured to high field resolution in the current amplitude range of 2–14 mArms and frequency range of 200 kHz–3 MHz. We have found that the intensity of the inner peaks of the four-peak structure increases with current amplitude until a threshold value, and then begins to decrease without changing position, whereas the outer peaks decrease monotonically and move to higher field. When frequency is increased from 200 kHz to about 2 MHz, all of the four peaks increase in height and move to higher field. Beyond 2 MHz, all of the peaks move to lower field; the intensity of the inner peaks decreases while the outer peaks continue to increase. At low frequency and current, a third pair of peaks appears between the two inner peaks and disappears when the frequency increases. Using a simple quasistatic model, the four-peak and six-peak structures are explained qualitative...

Measurement and Model of the Tensile Stress Dependence of the Second Harmonic of Nonlinear GMI in Amorphous Wires

The second harmonic component of nonlinear GMI in soft magnetic wires with helical anisotropy and near-zero negative magnetostriction subjected to different tensile stresses is studied for various current amplitudes (3-13 mArms) and frequencies (1-3 MHz). In the absence of tensile stress, the dc field dependence of the second harmonic of the voltage across the wire, V2f, exhibits a symmetric four-peak structure. The application of increasing tensile stress at relatively low current (3-5 mArms) causes the V2f signal to convert to a three-peak structure after a complicated series of changes. The three-peak structure consists of two outer peaks (OP) and one small central peak (CP) situated at very low field. When the current amplitude is increased, the V2f signal reverts to the four-peak structure, reversing the same steps. At relatively high frequencies, the V2f signal increases with current amplitude to more than 160 mV at high stress. Using a simple quasi-static model, we were able to qualitatively reproduce the three and four-peak structures and their dependence on current amplitude and tensile stress. Frequency dependence requires a dynamical model, as does precise determination of sizes and positions of peaks. This is being developed

Low Frequency Excess Noise Source Investigation of Off-Diagonal GMI-Based Magnetometers

The equivalent magnetic noise spectral densities of off-diagonal giant magnetoimpedance (GMI)-based magnetometers exhibit significant low-frequency excess noise, proportional to 1/f noise. As it represents a serious limitation to the ultimate sensing performances of high sensitivity magnetometers, possible sources of this 1/f noise are under investigation. Low-frequency magnetization fluctuations have been proposed as the noise source in the case of classical GMI-based sensors. Here, we apply this model to off-diagonal GMI-based magnetometers. This requires the inclusion of magnetization fluctuation noise sources, in addition to white noise sources from electronic conditioning in the GMI effect equations. A pessimistic scenario is presented, predicting the upper limit of low-frequency excess noise from material characteristics. The equivalent magnetic noise level is then computed from the sensitivity of each term of the sensing element impedance matrix to the magnetization angle at the static working point (for both axial and circumferential static magnetic field) and to conditioning circuitry. Based on this, it appears that magnetization fluctuations similarly affect all modes of operation of the two-port network sensing element, inducing identical impedance fluctuations. It also appears that this noise depends only upon the static equilibrium condition. This condition is governed by the effective anisotropy of the magnetic wire and by both axial and circumferential static components of the working point.

Evaluation of the Imaginary Part of the Magnetic Susceptibility,

The equivalent magnetic noise spectral density of giant magnetoimpedance (GMI)-based sensors exhibits a low-frequency excess noise inversely proportional to the frequency (1/f noise). In order to enhance the performance and reach the ultimate and intrinsic limit of the magnetometer, it is necessary to investigate the origin of this excess noise. Cross correlation measurements have permitted the demonstration that the excess low-frequency noise arises from the sensor, under defined excitation conditions, particularly the amplitude of the excitation current and the dc bias current. Given that the GMI effect is based on an impedance variation governed by the magnetization direction under an applied external magnetic field, the low-frequency magnetization fluctuations within the GMI wire have been considered to be a possible noise source. The fluctuation-dissipation theorem permits evaluation of the spectral power density of these fluctuations, which is proportional to the imaginary part of the susceptibility,

\chi ^{\prime \prime }

\chi ^{\prime \prime }

, and Application to the Estimation of the Low Frequency, 1/

. The latter may be evaluated by two different methods, based on the measurement of the real part of the wire impedance and on the evaluation of the circumferential hysteresis loop area. The results obtained by both methods are compared and discussed. The values of

f

\chi ^{\prime \prime }

, Excess Noise in GMI Sensors

thus obtained are then used in order to predict the equivalent magnetic noise level at 1 Hz, and are compared with that measured at 1 Hz. The comparison is conducted for different amplitudes of the dc bias current, and the results obtained indicate that the proposed methods have the potential of predicting the excess noise in GMI sensors.