Abstract-The equivalent magnetic noise spectral densities of off-diagonal 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.
The lowest measurable magnetic field of a high sensitive magnetometer based on giant magnetoimpedance (GMI) effect will be determined by its noise level. Numerous previous works have showed that the equivalent magnetic noise level of GMI magnetometer presents a 1/f behavior at low frequencies and a white noise. Currently, performances of the magnetometer are still limited by the electronic conditioning as well as the intrinsic equivalent magnetic noise of the GMI sensor. To improve sensor performances, particularly at low frequency, we have investigated two focuses on research: 1) reduce the electronic noise of the conditioning electronic by increasing the voltage sensitivity, in units of V/T and 2) reduce the intrinsic equivalent low frequency sensor noise. As previously reported, the equivalent magnetic noise at 1 Hz can be reduced by increasing the excitation current in addition to a dc bias current. By working on these bias conditions, on the given samples, we almost 1 pT/ √ Hz in white noise region and 15 pT/ √ Hz at 1 Hz.Index Terms-Magnetic noise, magnetometer, off-diagonal giant magnetoimpedance (GMI).
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