The Study of Stray Magnetic Fields from Digital Recording Heads for Tape Applications

The study of stray magnetic fields from magnetic recording heads for tape applications was undertaken using the Differential Phase Contrast (DPC) mode of Lorentz microscopy and Magnetic Force Microscopy (MFM). Electron Beam Tomography (EBT), at Glasgow was derived from the DPC technique and this was carried out in a highly modified Philips CM20 (S)TEM; previous EBT work had been carried out using a modified JEOL 2000 FX. A Digital Instruments MFM 3100 equipped with an Extender(TM) Electronics Module was used to study a production data head and an experimental Emboss head supplied by Onstream(TM). This thesis starts with a brief discussion of the basics of ferromagnetism and the application of magnetic materials in magnetic recording technology. Development trends and some recent advances in magnetic recording head design for tape applications are also discussed. Chapter 2 gives a review of quantitative techniques developed for magnetic stray field measurement. The Differential Phase Contrast (DPC) mode of Lorentz microscopy is presented as is an introduction to Electron Beam Tomography. The second method used to study the Onstream samples namely Magnetic Force Microscopy is discussed with particular attention given to the novel features that Digital Instruments employ in making their instrument an effective tool for the study of magnetic samples. There were several types of tip available and they fell into two categories, magnetically hard or soft; in the final section their characteristics are presented. The results of the electron beam studies on the Emboss head and on the modified data head are discussed in Chapter 3. This chapter starts with a brief description of the samples. The preparations of the samples were carried out by Onstream to adapt them for electron microscopy and is discussed in this chapter. The remaining sections concentrate on the results obtained from the JEOL 2000 FX on which the Emboss head was studied and Electron Beam Tomography carried out on the data head on the Philips CM20. In Chapter 4 the Magnetic Force Microscope method is applied to study the stray field from inductive heads. The microscope was operated exclusively in Tapping Mode(TM) and with Interleave(TM) mode engaged. Three samples were studied, two Emboss heads, one of which was experimental and a production data head. One of the aims of this study was to determine which type of tip would be most suitable for the study of the stray field and we found that a CoPt coated tip had the magnetic characteristics suitable for the study of stray field. Conclusions and suggestions for further work are given in Chapter 5. In an Appendix the topic of noise in the DPC data for tomographic reconstruction is discussed. This was inspired by the major problems encountered with the stability in the JEOL2000FX gun output. The influence on tomographic field reconstruction of increasing noise added to DPC data for a model recording head is analysed