Lorentz Microscopy of MFM Tips and Related Structures

The work described in this thesis is focused on the fabrication and understanding of the characteristics of small magnetic thin film elements and ferromagnetic thin film magnetic force microscope (MFM) tips so as to optimise the performance of the MFM used in our collaboration with a group at Manchester Metropolitan University. The small elements which have similar geometry as the magnetic film on the tip are studied in this thesis not only because some properties they exhibit are beneficial for quantitative MFM imaging but also because they enable the domain structure in the MFM tip to be observed directly. The basic concepts of ferromagnetism are given in Chapter 1, which serve as a starting point for further discussions.

Magnetic characterisations are performed using Lorentz microscopy in the modified JEOL 2000FX CTEM/STEM and VG HB5 STEM. The imaging formation of the Fresnel, Foucault and Differential Phase Contrast (DPC) modes of Lorentz microscopies are discussed in Chapter 2. The principle of MFM imaging technique and its basic instrumentation are also discussed there.

Chapter 3 gives a detailed description of the specimen fabrication process and some devices assisting the characterisations. The Si3N4 membrane is discussed along with the photolithography technique. Electron beam lithography as a major technique employed to define small element patterns on Si3N4 membrane is described. The oblique thermal evaporation produces MFM tips suitable for the MFM equipped with an interferometric detecting system. The preparation of such tips for observation in the VG HB5 STEM also forms a part of this chapter.

The main body of results is presented in Chapter 4 - 7. Chapter 4 focuses on the anisotropy induced by oblique thermal evaporation in the rectangular permalloy thin film elements. The nature of this anisotropy is investigated in various magnetic states as well as through in-situ magnetising experiments. In chapter 5, the stray field of the MFM tip is characterised experimentally using MDPC imaging and compared with the computer simulated MDPC images of a uniformly magnetised MFM tip. In chapter 6, magnetic structures of permalloy and cobalt thin film elements fabricated on the Si3N4 membranes to simulated the magnetic films on MFM tips are described. Direct imaging of the domain structure in various states is presented along with in-situ magnetising experiment. The characterisations of NanoprobeTM and needle tips are presented in Chapter 7. The conclusions drawn from the above projects and future work are given in Chapter 8.