TEM IMAGING AND EVALUTION OF MAGNETIC-STRUCTURES IN CO CU MULTILAYERS/

In this paper, a method is described which uses electron microscopic i maging of magnetic domains (''Lorentz microscopy'') to measure the deg ree of ferromagnetic coupling between magnetic layers of a multilayer system. Additionally, the in situ observation of magnetisation reversa l processes during a hysteresis cycle allows insights into the microma gnetic structural changes during the cycle, which are useful to explai n the macroscopic magnetic properties of the material under investigat ion. This application of electron microscopic imaging of magnetic stru ctures is given for a system of multilayered material which exhibits t he giant magnetoresistance effect (GMR effect). This effect shows a de pendency of the electrical conductivity on the strength and direction of an applied external magnetic field, which makes it an interesting e ffect for magnetic sensor applications. For a complete understanding o f the physical causes of this effect it is not sufficient to perform m acroscopic magnetic measurements only, such as measurements of the mac roscopic anisotropies or the macroscopic hysteresis loop. Instead, a m icroscopic investigation is necessary to prevent speculative misinterp retations of the macroscopic magnetic behaviour. These microscopic inv estigations can easily be done with an electron microscope, using tech niques for the imaging of magnetic structures, such as Fresnel and Fou cault imaging, or the differential phase contrast technique (DPC). The investigations presented in this paper deal with the micromagnetic do main structures of a multilayered [Co/Cu](N) system that exhibits the GMR effect, and their structural reasons as well as the behaviour of t he magnetic structures under the influence of an external magnetic fie ld. The observations lead to a clearer understanding of the physical r easons that contribute to the GMR effect. The results include the meas urement of the volume fraction of ferromagnetically or antiferromagnet ically coupled regions in the samples, the structural reasons for an i mperfect coupling of adjacent layers, the observation of local microma gnetic changes through a hysteresis cycle and the formation of 360 deg rees walls in the samples. The combination of these results yield a mo re complete understanding of the factors that contribute to the GMR ef fect.