A Characterisation of the Micromagnetic Properties of Thin Film Magnetic Recording Media by Lorentz Electron Microscopy

This thesis describes the electron microscope based analysis of two thin film magnetic recording media: iron cobalt chromium and cobalt nickel chromium. In particular, Lorentz electron microscopy was employed to characterise the micromagnetic domain structures of square wave written tracks recorded on to these two media. From the images obtained from this examination of the medium a computational technique was developed to calculate the transition noise power spectra for written tracks from Lorentz images of their domain structure. These Lorentz derived spectra can then be directly compared to the spectra measured electronically from the readback signal measured from the hard disk. As an introduction chapter one presents the basic energy considerations associated with ferromagnetic materials. The second half of the chapter briefly reviews the relevant aspects of longitudinal magnetic recording. Electron microscopy provides the means through which the media can be both physically and micromagnetically characterised. The appropriate techniques are therefore described in chapter two, with particular emphasis on qualitatively describing the imaging methods of Lorentz electron microscopy. To obtain thin films of a recording medium on which tracks have been written three stages of preparation are required. Chapter three describes these processes in detail. The bulk of the electron microscope analysis is contained in chapters four and five. Chapter four begins by reporting the characteristics of the physical structure of the iron cobalt chromium thin film and proceeds to describe the micromagnetic characterisation of tracks written on to the medium both along and off the easy axis. A breakthrough in the preparation of the isotropic cobalt nickel chromium thin film medium from hard disk came in the latter stages of this Ph.D. study. The success of the technique on what is a new medium for analysis was thought worthy of inclusion; hence chapter five reviews the first images of the physical and micromagnetic structures of this medium. There has been little or no work done to try and quantitatively relate the micromagnetic domain structures observed in Lorentz images of a recording medium with the corresponding noise performance of the medium. Chapter six describes estimation of the noise spectrum from a.c. erased iron cobalt chromium from Lorentz images and how this spectrum compares favourably with that measured electronically for the a.c. erased medium. The successful estimation of the a. c. erased noise enabled the same method of noise estimation to be employed to characterise the transition noise spectra from square wave written tracks. Chapter seven discusses the limited success of analysing sections of track (10 periods in length) and describes the evolution of a second method based on analysing individual magnetisation reversals. Chapter eight contains the conclusions drawn from the results of chapters four through seven along with an indication of possible future work