Magneto-optical recording media based on Co/Pt multilayers (1991)

Magnetic recording is nowadays the most widely used method for storing large amounts of data. A lot of effort is being spent on research on magnetic recording to increase the storage density. One principle cause, which limits the linear bit density in longitudinal recording systems, is the length of the magnetic transitions in the recording medium, due to demagnetizing forces. An answer to this problem is offered by perpendicular recording. Since the mid seventies there has been a growing interest in this recording mode. Because the magnetization of a perpendicular recording medium is oriented perpendicular to the substrate, the demagnetizing forces are strongly reduced in high-density recording. In part I of this thesis, the introduction, the principles of perpendicular recording are given and it is schematically shown that the transitions in a perpendicular recording medium can have negligible width, due to the favorable orientation of the magnetization. Also some complications are discussed, which may deteriorate the recording performance of the medium. The common core of these complications is formed by the longitudinal demagnetizing forces, which tend to close the magnetic flux within the medium. Certain cobalt-based alloys, having the hcp structure with perpendicular c-axis orientation, are suitable materials for a perpendicular recording medium. In practice the anisotropy axis coincides with the c-axis. The physical properties of these binary alloys, viz. the magnetization and the phase diagram, are discussed in chapter 4. The hysteresis loops of thin magnetic layers with perpendicular anisotropy is extensively treated in chapter 5. Two different models of magnetic microstructures are discussed, the stripe domain structure and colurnnar particle structure. The principal difference between these two structures is the way the short- range exchange forces extend through the medium. In the former these forces are able to extend throughout the medium, in which case the nucleation of a stripe domain pattern is a commonly observed microstructure in this kind of magnetic layers. In the latter the exchange forces are confined within the volume of the parti~les, which thus define the maximum size of magnetic domains. These physically opposite models are both investigated to find out how far they agree with the experimentally observed hysteretic properties of Co-Cr layers, which were used as recording medium in this work, and form a continuous theme throughout this thesis. The investigations on the Co-Cr medium are described in part II. First our investigations on the RF-sputter deposition of Co-Cr layers are reported in chapter 6. The morphology of the Co-Cr medium, as revealed by means of electron microscopy, is described in chapter 7. The medium appears to consist of columnarlike crystallites, which have grown perpendicular to the substrate (see fig.7.7). It is just this morphology, which strongly suggests a particulate structure because of the distinct profile of the crystallites. Chapter 8 contains the analysis of the magnetic anisotropy of the Co-Cr medium by means of torque and VSM measurements. Especially the magnitude of the anisotropy constants and the different causes of anisotropy were investigated. The intriguing question, whether the Co-Cr medium is .continuous. or .particulate., is discussed in chapter 9 on the basis of the perpendicular easy-axis loops. The experimental loops are compared with the theoretical results of chapter 5 and it appears that the continuous model agrees best with the experimental results. The coercivity of the Co-Cr medium is analysed in chapter 10, also on the basis of the continuous and the particulate models. It again appears that the continuous modeloffers the best explanation, in this case for the experimentallyobserved coercivity. An interim discussion is presented after chapter 10 in which a tentative description of the Co-Cr medium is given and the consequences for the recording behaviour are discussed. The contents of part III refer to investigations on the characteristics of perpendicular recording. The structure of magnetic transitions, which are induced by means of a single- pole type head into the Co-Cr medium, is investigated experimentally as well as theoretically and is described in chapter 12. In this analysis an accurate description of the head field is required, which is therefore first given in chapter 11.