Gotthard Rieger

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.

Nanocrystalline soft magnetic composite-cores with ideal orientation of the powder-flakes

Abstract Amorphous Fe 73,5 Cu 1 Nb 3 Si 15,5 B 7 powders with a particle size up to 1400 μm were consolidated to cores by using a press-additive, which allows an ideal orientation of the flakes. The nanocrystalline cores exhibit permeabilities up to e = 6000 and a coercive field of H c = 8 A/m. The magnetic properties can be varied in a wide range depending on particle size and content of press-additive.

Nd–Fe–B permanent magnets (thick films) produced by a vacuum-plasma-spraying process

Thick, hard-magnetic Nd–Fe–B films (∼1 mm) were deposited on different substrates (Cu, steel) by a low-pressure plasma-spraying process. The properties of the applied Nd–Fe–B powders (e.g., grain size, composition) and the conditions of the spraying process were optimized with respect to the mechanical and magnetic properties of the films. Film thicknesses up to 1.2 mm were achieved with good adhesive properties (bond strength>40 MPa). Cracks at the interface or within the films during the deposition process could be suppressed by adjusting the temperature profile of the substrate and controlling the deposition rate. Depending on the maximum temperature of the substrate and the thickness of the Nd–Fe–B films, either amorphous or microcrystalline structures were obtained. In general, the magnetic properties were improved by a postdeposition annealing treatment. Coercivities HcJ up to 16 kA/cm and isotropic remanences of about 0.6 T were achieved.

High frequency performance of laminated soft magnetic films (NiFe, FeAlN, and amorphous CoFeBSi) in external fields

Thin film multilayers of different compositions (NiFe, FeAlN, and amorphous CoFeSiB) were prepared by sputter deposition. The ultrahigh frequency performance was studied using rf fields perpendicular to the in-plane easy axis. By applying external bias fields either along or perpendicular to the easy axis (perpendicular to the rf field excitation), the cutoff frequency of the different alloys can be shifted significantly to higher frequencies (>1 GHz) according to theory. On the other hand, it is possible from these measurements to identify different phenomenological damping of the material systems due to structural effects. Although FeAlN exhibits the highest saturation magnetization compared to NiFe and CoFeBSi, it shows a higher damping which is explained by nonideal structure.

Micromagnetics of shape anisotropy based permanent magnets

In the search for rare-earth free permanent magnets, various ideas related to shape anisotropy are being pursued. In this work we assess the limits of shape contributions to the reversal stability using micromagnetic simulations. In a first series of tests we altered the aspect ratio of single phase prolate spheroids from 1 to 16. Starting with a sphere of radius 4:3 times the exchange length Lex we kept the total magnetic volume constant as the aspect ratio was modified. For a ferromagnet with zero magnetocrystalline anisotropy the maximum coercive field reached up to 0:5 times the magnetization Ms. Therefore, in materials with moderate uniaxial magnetocrystalline anisotropy, the addition of shape anisotropy could even double the coercive field. Interestingly due to non-uniform magnetization reversal there is no significant increase of the coercive field for an aspect ratio greater than 5. A similar limit of the maximum aspect ratio was observed in cylinders. The coercive field depends on the wire diameter. By decreasing the wire diameter from 8:7Lex to 2:2Lex the coercive field increased by 40%. In the cylinders nucleation of a reversed domain starts at the corners at the end. Smoothing the edges can improve the coercive field by about 10%. In further simulations we compacted soft magnetic cylinders into a bulk-like arrangement. Misalignment and magnetostatic interactions cause a spread of 0:1Ms in the switching fields of the rods. Comparing the volume averaged hysteresis loops computed for isolated rods and the hysteresis loop computed for interacting rods, we conclude that magnetostatic interactions reduce the coercive field by up to 20%.

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.

High speed signal transmission with magneto-couplers

Couplers in general serve for galvanically isolated signal transmission between sensitive equipment. Standard opto-couplers are generally limited to a bandwidth of 25 MHz. However, with ever-increasing data transfer rates the need for couplers with much higher bandwidth is steadily growing. For this purpose magneto-couplers are very promising because their bandwidth is expected to be limited only by the Larmor precession of the magnetic moments with frequencies in the range of 1–10 GHz. In magneto-couplers the signal to be transmitted is converted into a current through a micro-coil. The resulting magnetic field is detected by a giant magnetoresistance(GMR)sensor element separated by an isolation layer. In the present case this element is a Wheatstone bridge consisting of four spin valves. The voltage across the bridge resulting from the magnetization change of the soft layers due to the field pulses is detected with a 1 GHz storage oscilloscope. Present experiments indicate a bandwidth larger than 500 MHz. After proper termination of the signal paths and applying a dc-bias field the rise time of the input signal of less than 1 ns is not increased by the coupler. Furthermore we show that the driving pulse may be completely reproduced in shape by applying external dc-bias fields. It will also be discussed how capacitive and inductive crosstalk between the coil and the spin valves and nonideal termination of the signal paths affect the achievable bandwidth.

45.3: Novel Electrochromic Systems for Flexible Displays

Electrochromic (EC) displays show several unique benefits such as high contrast at high viewing angle and extremely low power consumption combined with a potential low-cost manufacturing process. We have developed novel electrochromic material systems for application in flexible devices. Stability and overall performance of these devices are affected by compositional variations of the material formulation as well as electrochemical interactions between electrode and electrochromic material. Electro-optical properties will be discussed as well as stability issues.

High speed signal transmission with GMR based magnetocouplers

Summary form only given. With ever increasing data transfer rates between sensitive equipment, the need for high speed, galvanically isolated signal transmission is steadily growing. Standard optocouplers are reaching technological limits at a bandwidth of 25 MHz. Magnetocouplers are theoretically capable of transferring data at rates up to 1 GHz or more. They also feature several other advantages over optocouplers. They can be operated at low supply voltages, have a larger temperature range, and may be integrated with standard VLSI technology.