Micromagnetic and Structural Studies of CoPtCr Longitudinal Recording Media

The work presented in this thesis is a study of the micromagnetic structure of inductive and magnetoresistive playback CoPtCr longitudinal recording media. Through the use of Lorentz electron microscopy the magnetic structure of the recorded bits can be viewed directly.

An introduction to the theory and properties of ferromagnetic materials and their applicability to digital magnetic recording is presented in chapter 1. An overview of magnetic recording and the properties required for a good recording media are discussed.

Electron microscopy and in particular the Lorentz modes used to view the magnetic structure of the media are discussed in chapter 2. Image formation in an electron microscope and the Lorentz modes of Fresnel, Foucault and MDPC are introduced and explained.

An essential part of electron microscopy is the fabrication of electron transparent samples. Chapter 3 describes techniques of fabricating both planar and cross-sectional specimens from hard disks. To allow the verification that the integrity of the composition of the media has been maintained throughout the preparation an EDX analysis method is provided.

In chapter 4 the macroscopic properties of the media types are investigated. The film thickness and the magnetic layer composition are verified using the EDX techniques outlined in chapter 3. In addition to this, the value of the saturation induction in the DC bands in each medium is also verified using the technique of LAD.

The Lorentz microscopy results obtained from both media types are presented in chapter 5. A tilting method of reducing scratch contrast is introduced and discussed. Preliminary magnetic investigations are performed using the Fresnel and Foucault modes and images are presented from both media types. Magnetic features such as the bit transitions and the side write are examined, discussed and comparisons between the media types are drawn. The remainder of the chapter concentrates on the MDPC imaging of the media. A technique is presented which verifies the mapping directions using the alternate DC banding on the media. A set of characteristic MDPC images are established and MDPC images are presented from tracks written over a range of frequencies. In particular the MDPC technique is applied to imaging the bit transitions at high magnifications to determine the magnetic structure in that area. The MDPC images also suggest the existence of a slight anisotropy in these media which is seen when the head writes off axis or is skewed with respect to the write direction. MFM images are also presented to verify some of the result seen in the MDPC images.

Two methods of MDPC image simulation are presented in chapter 6. The results are shown from each technique and are compared with the experimental MDPC images.

Chapter 7 outlines and shows the result from a reconstruction technique which use the curl and divergence of MDPC image sets to produce one possible magnetisation map which could produce such a set of MDPC images.

Finally chapter 8 discusses the main conclusions obtained from the previous chapters and includes data which is the grounding for future work in this area.