Low Temperature Magnetic Force Microscopy

This thesis project involves the construction of a Low-Temperature Magnetic Force Microscope (LTMFM) which is used for the study of the magnetic structure of materials. In this dissertation I describe the instrumentation of the microscope and the micromagnetic studies which have been performed on single crystal magnetite. The LTMFM operates at temperatures in the range between 77K and 300K. It is sensitive to the force gradients on the probe (tip) due to its interaction with the stray fields from the sample. The micromagnetic structure of the sample may be inferred by mapping out these interactions as the tip is scanned across the sample. In this thesis, I have focused my attention on the micromagnetic structure of single crystal magnetite at 77K. This is below its crystallographic transition which occurs at about 110K. We find that the observed domain structures are consistent with our understanding of the uniaxial anisotropy which is exhibited in magnetite below this transition. Some of the more interesting micromagnetic features observed for this system are the 180^circ domain walls. I have modeled the LTMFM response over a 180^circ Bloch wall with a Neel cap. The calculated response has then been fitted to the experimental data. The wall width and the cap depth are obtained as fitting results. In this way, I find that these calculated parameters are larger than those obtained from classical domain theory. The reasons for this discrepancy are discussed. Future directions for such tip-sample modeling are considered.