GMI Noise performance optimisation

International audience1 INTRODUCTIONThe giant magnetoimpedance (GMI) effect has great potential for the design of highlysensitive magnetometers. It has previously been shown that the performance of GMImagnetometers is currently limited by the noise due to electronic conditioning [1, 2].Therefore, an increase of the intrinsic sensitivity of the sensing element is a key point in thereduction of the impact of electronic noise on global performance. A promising approach toreaching that goal is to use a coupled pick-up coil wound around the GMI element in a twoportconfiguration [3]. Here, we propose a model for the equivalent output magnetic noise of acomplete electronic circuit of a magnetometer using the two-port configuration as the sensingelement. This allows us to clarify the main contributions to sensor noise performance,operating at the optimal sensor excitation working point.------------------------------------------------2 NOISE MODELThe sensing element consists of a very thin two-layer pick-up coil wound on a 100μmdiameter CoFeSiB soft amorphous ferromagnetic wire. The response of the device tomagnetic field variation is fully described by its two-port network impedance matrix. Theintrinsic sensitivity of each matrix component is then deduced from the differentialimpedance variation as a function of applied magnetic field. First, we quantify thecontributions from all noise sources of the electronic chain. Then, we predict the outputsensitivity, in V/T, of each of the matrix components. Finally, the output noise level, inT/√Hz, the ratio of the output electronic noise to the output sensitivity, is calculated. Themodel is in good agreement with noise measurements in which the measured white noiseis at the pT/√Hz level. The two-port configuration, with the pick-up coil, clearly yields animprovement in noise performance, compared with the one-port measurement of theimpedance of the wire.--------------------------------REFERENCES:[1] L.G.C. Melo, D. Ménard, A. Yelon, L. Ding, S. Saez, C. Dolabdjian: Optimisation of themagnetic noise and sensitivity of giant magnetoimpedance sensors. Journal of AppliedPhysics, vol. 103, (2008), p.033903[2] L. Ding, S. Nabily, S. Saez, J. Gieraltowski, C. Dolabdjian: Investigation of giantmagnetoimpedance magnetic noise comparison. Sensors Letters, vol. 5, (2007), 171-175.[3] B. Dufay, S. Saez, C. Dolabdjian, D. Seddaoui, A. Yelon, D. Ménard: Improved GMIsensors using strongly coupled thin pick-up coils. Sensors Letters, vol. 7, (2009), 334-338