Klaus Ludwig

GMR sensors for contactless position detection

Abstract Magnetic sensors are ideal for all kinds of contactless position registration, e.g. distance, speed, angle, rotational speed and sense of rotation. They work even under adverse and dirty environmental conditions. These basic advantages have resulted in the widespread use of all kinds of magnetic sensors in contactless position detection. The magnetic sensors based on the giant magneto resistance (GMR) effect developed for position detection are now finding their way into industrial and automotive applications. They overcome a weakness in conventional magnetoresistors and Hall sensors, because they are less sensitive to air gap deviations apparent in many applications. This paper presents the basic concepts of GMR sensors for contactless position detection. The advantages of the sensor and relevant examples for industrial and automotive applications are discussed.

Adapting GMR sensors for integrated devices

Abstract Magnetoelectronic devices like current sensors or magnetocouplers which integrate the signal generator and the sensor element on one substrate represent an area of major commercial importance for GMR sensors. Due to their intrinsic properties exchange biased GMR spin valves [Phys. Rev. B 42 (1990) 10583] [1] are particularly well suited for these devices. However, the adaptation of the spin valve sensors to such applications is difficult, since only the low field part of the signal characteristic is utilized, which is sensitive to the composition of the GMR stack as well as domain structure. While a large amount of research has focused on the influence of layer composition on the properties of GMR sensors [Annu. Rev. Mater. Sci. 25 (1995) 357] [2] , little is known about the effect of sensor geometry, which influences the domain structure of the sensing layer. In this paper, we introduce the design of an integrated magnetocoupler device and examine the influence of geometry on sensor properties in a design suitable to magnetic field detection.

MI effect in amorphous CoFe thin film trilayers and demonstrator packaging

The magnetoimpedance (MI) effect found in amorphous soft ferromagnetic materials occurs at high frequencies and consists of large changes in the impedance with an external magnetic field. It is the dependence of the transverse permeability on an external magnetic field. A uniaxial magnetic anisotropy is induced having the easy axis transverse to the current direction. Inductance and skin effect depend on permeability thus causing a change of the impedance of amorphous wires, ribbons and of thin films at very high frequencies. Trilayer structures of two 20, 50 and 100 nm thin amorphous CoFeB layers with a central 40, 100 and 200 nm thin Cu layer respectively were sputtered onto a thermally oxidized Si wafer. 300 mum long strips of 3 -20 mum width are structured by plasma etching and connected by ultrasonic bonding to a printed circuit board. Magnetization curves, parallel to the easy axis and hard axis of uniaxial anisotropy, are measured by the magnetooptical Kerr effect exhibiting anisotropy fields of around 1.6 kA/m and low coercivity in the hard axis direction, depending on the film thickness. The domain structure in the strips was observed by Kerr microscopy, indicating possible magnetostatic coupling of the two magnetic layers. The MI effect was measured, before and after field annealing, by means of a Network Analyzer, using the complex ratio of the incoming wave and the transmitted wave through the sample, automatically calculated using a LabVIEW program. The Network Analyzer was calibrated for linear frequency response at 15 mT. The MI maximum of the 20/40/20 nm structure strips is beyond 500 MHz. The MI effect of the 50/100/50 nm structure strips has been improved after field annealing, exhibiting a maximum of up to 9% in a 4 mum wide strip.