Optimised GMR sensors for low and high frequencies applications

Magnetoresistive sensors and in particular GMR and TMR sensors are very sensitive but their use for very accurate measurements is often limited by their behaviour in applied magnetic fields. A first constraint is the building of hysteresis free spin valves which is necessary to give an efficient immunity against large random magnetic fields and insure reproducibility of the measurements. A second constraint is the linearity of the sensor in the working region. Finally, the reduction of noise and in particular the magnetic noise is essential for the use of GMR and TMR as sensors. The hysteresis is only dependent on the composition and the shape of the spin valve. The thickness of the soft layer coupled to cross anisotropies plays an important role. This has been highlighted by polarised neutron reflectometry which has shown the existence of small rotations in the soft layer. The shape anisotropy has also a strong influence on the hysteresis but there is a competition between sensitivity and hysteresis free systems. GMR and TMR sensors are nonlinear and this drawback limits their use as replacement of Hall sensors for a large number of applications where sensitivity is not crucial. We will show how we can overcome this problem by combining the design and feedback electronics. Noise behaviour is also crucial for very sensitive sensors. At low frequencies, 1/f noise dominates the thermal noise; it is related to structural defects and magnetic configuration. By a proper design of the sensor, 1/f magnetic noise can be suppressed. A 1/f structural noise can also be reduced by using double modulation technique on the nonlinearity of the spin valve. (c) 2005 Elsevier B.V. All rights reserved.