M. Löhndorf

Highly sensitive strain sensors based on magnetic tunneling junctions

Micrometer-sized highly sensitive strain sensors are presented. The sensors are based on magnetic tunneling junctions (MTJs) incorporating magnetostrictive free layers. The influence of mechanical strain upon the free layer is explained by a model taking into account the total free energy of the sensing layer. Those MTJ devices prepared in situ with magnetostrictive Fe50Co50 layers exhibit a tunneling magnetoresistance (TMR) ratio of 48%. The changes in strain Δe on the order of 0.4 parts per thousand (‰) result in resistance changes of 24%, which in turn leads to gauge factors [(ΔR/R)/Δe] on the order of 600, whereas gauge factors of 2–4 are typical for metal based, and 40–180 for piezoresistive semiconductor strain gauges.

Microfabricated high-performance microwave impedance biosensors for detection of aptamer-protein interactions

High-frequency impedance biosensors with nanometer gaps have been prepared for the detection of biomolecular interactions such as protein-antibody and protein-aptamer binding. The sensor principle is based on electrical impedance changes measured at 1.2 GHz due to changes of the effective dielectric constant within the 68 nm gaps between two gold electrodes. As a model system, the specific binding of the blood clotting factor human thrombin with different concentrations to its ribonucleic acid (RNA) α-thrombin aptamer, as well as the immobilization process of the RNA-aptamer, have been detected in real time. By using a similar 68 nm-gap sensor blocked with bovine serum albumin and a reference sensor with 10μm electrode spacing, signal changes due to variations of the bulk dielectric constant due to buffer/analyte solutions, and unspecific binding events have been analyzed.

Novel strain sensors based on magnetostrictive GMR/TMR structures

Summary form only given. While there has been considerable research devoted to the use of GMR (giant magnetoresistance) and TMR (tunnel magnetoresistance) layered structures-these studies have focused mainly on data storage applications (e.g., MRAM, Read-write heads) and magnetic field sensors. Comparatively few studies have been devoted to the exploitation of these materials in broader industrial applications for use such as stress, strain or pressure sensors. This study presents an investigation into various magnetic layer structures suitable for devices sensing mechanical responses such as stress, strain and pressure.

Micro-sensor coupling magnetostriction and magnetoresistive phenomena

Abstract Giant magnetoresistance or tunnel magnetoresistance (TMR) structures with magnetostrictive free layers are attractive approaches for the realization of novel strain sensors. It has been found that the strain response can be enhanced by using magnetostrictive materials such as Fe 50 Co 50 . First experiments have further revealed that the tunnel barrier of TMR structures withstands a strain level of at least 0.1%.

Magnetoelastic thin films for high-frequency applications

High-frequency magnetic thin films with cut-off frequencies well above 2 GHz and good magnetoelastic properties are key materials for the development of new magnetic devices such as remote-interrogated magnetoelastic sensors or micro-inductors. In order to achieve high cut-off frequencies the films must have high magnetization, high anisotropy field, moderate permeability and resistivity. Therefore, multilayered thin films consisting of the transition metal alloy Fe/sub 60/Co/sub 60/ and the amorphous soft magnetic alloy Co/sub 80/B/sub 20/ have been developed which show cut-off frequencies higher than 2 GHz combined with good magnetoelastic properties.

Magnetoelectronical Sensors for Mechanical Measurements

Recently, highly sensitive strain gauges were developed, which are based on TMR (tunnel magnetoresistance) or GMR (giant magnetoresistance) effects combined with the inverse magnetostriction. GMR and TMR structures generally possess a symmetrical characteristic which reflects the switching fields of the soft and hard layers, respectively. This characteristic can be changed by a stress field if the soft layer is replaced by a suitable magnetostrictive layer leading to a stress induced rotation of the magnetostrictive layer with respect to the reference layer.

Switching of magnetostrictive micro-dot arrays by mechanical strain

This paper investigates the switching behavior of magnetostrictive micro-dot arrays by mechanical strain. These amorphous FeCoBSi micro-dot arrays with diameters of 1-5 /spl mu/m and a thickness of 20 nm have been prepared on MEMS fabricated membrane structures. Magnetic force microscopy (MFM) has been used to obtain a spatially resolved switching behavior of these dots at different level of mechanical strain. For 20-nm-thick FeCoBSi dots, we have observed a strain induced switching starting at a level of about 0.04% of strain. In addition, finite-element simulations have been performed in order to correlate the MFM results with the local strain distribution of the membrane structure.

Wireless Tyre Sensors Based on Amorphous Magneto-Elastic Materials

Nowadays customised sensors and actuators are very important in order to optimise and control process parameters of engines, machines and other moving parts. Wireless interrogation of the sensors is necessary in order to gain information from hidden spots within the engine or from rotating shafts, axles or car tyres. Therefore we have exploited the possibility of using magneto-elastic wires as sensor elements to monitor the frictional conditions between the tyre and the roadway in order to enhance the active vehicle safety. In vehicles having wheels with rubber tyres, the deformation and forces that occur in the tyre material provide information about the driving status of the vehicle. Especially, it is possible to monitor current frictional conditions between the tyre and the roadway.