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Monolithic spin valve compass for autonomous MEMS navigation in geomagnetic field

: Almeida, M.J.; Ueberschär, O.; Ecke, R.; Schulz, S.E.; Matthes, P.; Müller, M.; Exner, H.

Viehweger, C. ; International Measurement Confederation -IMEKO-, Technical Committee on Environmental Measurements -TC 19-:
5th IMEKO TC19 Symposium on Environmental Instrumentation and Measurements 2014. Proceedings : 23/09/2014 - 24/09/2014, Chemnitz, Germany
Budapest: IMEKO, 2014
ISBN: 978-92-990073-6-5
Symposium on Environmental Instrumentation and Measurements <5, 2014, Chemnitz>
Fraunhofer ENAS ()

The natural geomagnetic field has been used for millions of years by various organisms for navigation. The determination of the local field direction (in terms of magnetic north and inclination) enables, for instance, migratory birds to find their annual routes from one continent to another and back home, or magnetotactic bacteria to move towards soil areas rich in nutrients. In analogy, for microelectromechanical systems (MEMS), the capability of detecting the local direction of the geomagnetic field as a 2D or 3D vector enables a reliable autonomous navigation through environments with a complex or unknown topology while being independent of GPS or any other radio-based navigation system (and thus being operable also in obstructed or shielded environments). Such mobile MEMS applications demand, however, a very low power consumption and a high miniaturizability of the sensor, as well as a very fast sensor response time. In the following, we present an innovative 2D GMR spin valve sensor on the basis of exchange-biased NiFe- CoFe / Cu / CoFe / IrMn nanolayers in monolithic integration that fulfils all these requirements. For a maximum signal-to-noise ratio, we have realized a focused double full-bridge layout with an antiparallel alignment of the pinned layers of neighbouring meanders by means of microscopic laser heating and subsequent in-field cooling. A systematic optimization of geometry and magnetic structure further contributed to a maximum signal level and a minimum sensor hysteresis. On the basis of fabricated prototypes with a size of 1.5 mm times we demonstrate that these sensors are readily employed to detect the geomagnetic field as a 2D vector with temporal ( 1 ms) resolution.