TEM imaging and evalution of magnetic structures in Co/Cu multilayers

Abstract

In this paper, a method is described which uses electron microscopic imaging of magnetic domains (“Lorentz microscopy”) to measure the degree of ferromagnetic coupling between magnetic layers of a multilayer system. Additionally, the in situ observation of magnetisation reversal processes during a hysteresis cycle allows insights into the micromagnetic structural changes during the cycle, which are useful to explain the macroscopic magnetic properties of the material under investigation. This application of electron microscopic imaging of magnetic structures is given for a system of multilayered material which exhibits the giant magnetoresistance effect (GMR effect). This effect shows a dependency of the electrical conductivity on the strength and direction of an applied external magnetic field, which makes it an interesting effect for magnetic sensor applications. For a complete undestanding of the physical causes of this effect it is not sufficient to perform macroscopic magnetic measurements only, such as measurements of the macroscopic anisotropies or the macroscopic hysteresis loop. Instead, a microscopic investigation is necessary to prevent speculative misinterpretations of the macroscopic magnetic behaviour. These microscopic investigations can easily be done with an electron microscope, using techniques for the imaging of magnetic structures, such as Fresnel and Foucault imaging, or the differential phase contrast technique (DPC). The investigations presented in this paper deal with the micromagnetic domain structures of a multilayered [Co/Cu] N system that exhibits the GMR effect, and their structural reasons as well as the behaviour of the magnetic structures under the influence of an external magnetic field. The observations lead to a clearer understanding of the physical reasons that contribute to the GMR effect. The results include the measurement of the volume fraction of ferromagnetically or antiferromagnetically coupled regions in the samples, the structural reasons for an imperfect coupling of adjacent layers, the observation of local micromagnetic changes through a hysteresis cycle and the formation of 360° walls in the samples. The combination of these results yield a more complete understanding of the factors that contribute to the GMR effect.