Magnetic sensors for measuring of rotary motion have gained importance in recent years. Sensors consisting of CoFe/Cu multilayers show a big dependence of electric resistance on the change of an outer magnetic field, the so-called “Giant Magnetoresistance Effect” (GMR). Because of this effect they have a high potential for the industrial use in such applications. Proceeding miniaturization and increasing demands on reliability are challenging the control of manufacturing processes and robustness of the device during the use. Both topics were addressed in this work by studying the short-term and long-term stability of the nanoscale metallic multilayers at elevated temperatures. Reasons and mechanisms for breakdown of the GMR-effect have been analyzed by different physical methods. It was predicted that the use of alloyed intermediate layers consisting of Cu, Ag, and Au should increase the stability at elevated temperatures. The multilayered samples investigated in this work exhibit a GMR effect of GMR (alloy) = 20.7 % which is significantly smaller than the effect of the standard system with pure Cu interlayers (GMR (Cu) = 25.2 %). Although the change in resistance is smaller for the alloyed system the sensitivity S for both are equally at S = 1.3 %/mT. The reason for this is the difference in the characteristic hysteresis curve of the alloyed system which shows less hysteresis and a smaller saturation field. For protection against oxidation during the use a passivation coating consisting of SiO2 and Si3N4 has been deposited by the means of plasma CVD. Typical parameters for this process are times of tshort-term = 1 h in the temperature range of 200 °C ≤ Tshort-term ≤ 250 °C. Experiments on the short-term stability showed that both systems will not break down under these conditions. Especially the alloyed structure was characterized by an increased GMR effect of GMR  2 % and a stable sensitivity S = 1.3 %/mT up to Tshort-term = 250°C. Because of this the passivation coating was deposited at TCVD = 250 °C. The suitability for automotive applications was tested by a long-term study with tightened surrounding conditions. The sensitivity Sminor of the minor loop was set as criterion for the time to failure tfailure. This means that the magnetic field was varied in steps of ± 0.5 mT around predefined working points of different field strengths (WP [mT]). By applying an Arrhenius-relation for the breakdown at the working point WP = 12 mT for test sensors with non-alloyed Cu interlayers it was possible to predict a lifetime of tfailure  56 000 h for Tuse = 150 °C. Target lifetime for the heavy use in freight vehicles of tfailure  40 000 h have been fulfilled. For Tuse = 160 °C lifetime is tfailure  21 000 h. Maximum lifetime for the alloyed systems was determined to be tfailure  15 000 h, meaning that it is not suitable for automotive applications. The effective activation energy EA for the breakdown of the tested sensors of the long-term study was laying in the range of 1.4 eV ≤EA ≤ 1.6 eV, indicating for grain boundary diffusion as the dominating mechanism for the structural changes of the multilayered samples. These changes have been verified by applying different techniques like X-ray Reflectivity, Transmission Electron Microscopy, or Moessbaur Spectroscopy to three different combinations of materials: • the CoFe/Cu basic system • the CoFe/CuAgAu alloyed system • the model system consisting of Fe/Cu multilayers The breakdown of the multilayered systems is passing three main stages: • Samples examined directly after the preparation show mixing of different atomic species at the interfaces. This intermixing goes along with lattice disturbances (e.g. vacancies, interstitial atoms, metastable phases), and together they cause an increased basic electric resistance • At moderate temperatures lattice imperfections are healing out due to diffusion processes. This decreases the basic resistance giving the spin dependent scattering a higher contribution to the total resistance and resulting in a measurable increase of the GMR effect. • At high temperatures the diffusion processes are accelerated leading to breaking up of the multilayered structure and grain growth. Magnetic short cuts are generated between neighboring magnetic layers accompanied by direct magnetic coupling. The antiferromagnetic coupling, which is the basis of the GMR effect in the studied systems, is destroyed. TEM proved a directed growth from the buffer layer to the surface of the samples across the interfaces of different layers. It was also possible to visualize the layered structure in nanoscale dimension by working with a strong over focus. The idea for the use of the alloyed interlayers was to stabilize the structure by segregation of Ag and Au in the grain boundaries. In contrast to this predication Ag was found to be precipitating in particles of the size of 5 to 10 nm on the surface of them sample. The stabilizing function of the alloyed layer could not been proven. The test results of the study of short- and long-term stability demonstrate that it is possible to develop a market-ready sensor system based on exchange coupling multilayers for automotive application. Because of the similarity in the layered structure the same methodology of accelerated lifetime testing can be applied to spin valve systems as well. Due to the nanoscale dimensions of the layers diffusion lengths are very short and even little amount of diffusion can cause a breakdown in the electronic function. The parameters of sample preparation and the selection of the materials have a big influence on the thermal stability under operational conditions. For every change in materials combination and in preparation conditions a study based on this concept has to be carried out to determine the lifetime.