Investigation of magnetization structures in ferromagnetic and superconducting samples by magnetic force microscopy

Abstract

Even though the phenomenon of magnetic ordering in solids was already known to the ancient Greek, the microscopic understanding of why certain materials show magnetic order dates from this century. In particular, even though some form of magnetic domains was already expected by Weiss when he formulated his theory of Ferromagnetic ordering in 1907, direct experimental evidence of their existence was only provided in 1931 by measurements of v. Hámod and Thiessen and Bitter. In these experiments, the domain structure was determined by the imaging of small magnetic particles that decorate regions with high magnetic stray fields. This decoration is due to the interaction force between these particles and the sample stray field. Interestingly enough, the reason why the domains were formed was still unclear, and was only clarified in 1935 by Landau and Lifschitz. The phenomenon of Superconductivity on the other hand, was discovered much more recently in 1911 by Kamerlingh Onnes. After its discovery, it took until 1933, when Meissner and Ochsenfeld found that superconductors are ideal diamagnets, repelling the magnetic flux from their inside, even if the field is applied before the superconductor becomes superconductive. Again, the existence of domains was first predicted from the theory published by Landau in 1937, but it took until the fifties before the first magnetic flux structures in superconductors were imaged, again using the decoration technique. Thus, even though the theoretical understanding of the domains in ferromagnets and superconductors evolved almost simultaneously, the first direct observation of the latter took 23 years longer, which was probably due to the experimental diffculties of studying superconductors. Magnetic force microscopy (MFM) is a relatively new technique for imaging these magnetization structures. It combines the properties of the decoration technique (the contrast formation is due the magnetic interaction between the stray field of the sample and a small magnetic particle) with the properties of the scanning force microscopy technique developed by Binnig, Quate and Gerber (measuring the interaction be- tween the particle and the sample as a function of position through the deflection of a cantilever beam). The first MFM measurements were made on ferromagnets. Again, the first observation of magnetization structures in superconductors took somewhat longer, until 1994. Nowadays with time and effort, the experimental diffculties of working with MFM at low temperatures have been diminished by the development of better instruments and improved measurement methods. The measurements presented in this thesis were made with such an instrument, the design of which is discussed in chapter 2. Compared to other types of magnetic imaging,2 the advantages of the MFM technique are a high spatial resolution imaging and relatively low requirements for sample preparation. Another, unique property of the MFM is that it can be used as a tool for determining the response of the sample to a local applied field and for modifying the sample. One of the main disadvantages of the MFM until now has been the diffculty to interpret the measured signal. In recent years, the improvement in the quality of the instrument and the subsequent improvement of the measurement quality has allowed the development of procedures that allow the quantitative interpretation of the measured contrast. The methods developed for quantitative evaluation of the MFM measurements as part of this thesis-work are described in more detail in chapter 3. The application of the MFM method to the analysis of ferromagnetic materials is described in chapter 4. A point of interest in the research of these materials is the influence of the interfaces between ferromagnetic and other materials on the magnetic properties of the sample. Here, this influence was studied using Cu/Ni/Cu/Si(001) sandwich structures, because they show a particularly interesting dependence of the preferred orientation of the magnetization on the thickness of the nickel layer. Finally, the application of the MFM method to the study of superconductors is described in chapter 5. In addition to the imaging of the magnetic structures occurring in the superconductor, the use of the MFM to study the response of the superconductor to a local applied field is discussed