Magneto-optical recording media based on Co/Pt multilayers

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

Magnetic properties of Co/Pt-multilayers

The magnetic properties of (111)-textured Co/Pt-multilayers have been studied theoretically by means of spin-polarized LMTO-band-structure calculations. In contrast to the interpretation of earlier experimental investigations, the Pt-layers have been found to be significantly magnetized by the adjacent Co-layers. This finding ensures that the Co-layers are exchange-coupled even for relatively thick Pt-interlayers. To get some information on the consequences of interface mixing we have used the ASA-version of the spin-polarized relativistic KKR-method of band-structure calculation to investigate the electronic and magnetic properties of Pt-(Co)-impurities in the various layers of the Co/Pt-multilayers. Our non-self-consistent results indicate that for most cases a slight increase in the magnetic moments has to be expected. To study experimentally the magnetic properties of the Pt-layers of vapor-deposited samples we have made use of the magnetic dichroism in X-ray absorption. This new technique allows to probe the Pt-layers separately and to estimate the average moment of this subsystem. While the estimated moments are in all cases somewhat higher than our corresponding theoretical results, their dependence on the Pt-layer thickness is in very satisfying agreement with the theoretical predictions

Considerations in designing and testing plasma devices for medical applications

Cold atmospheric plasma has been shown to have great potential for many applications on or in the human body, such as disinfection, wound healing, and cancer treatment. Several medical plasma devices have been developed and have obtained CE certification. However, the route there and into clinical practice is not straightforward. Already during the design stage, many matters need to be taken into account: (1) application and user requirements, (2) medical device regulations, e.g. on electrical safety and electromagnetic compatibility, and (3) production-related issues. Subsequent research should be conducted using a setup that resembles the clinical situation as closely as possible, since seemingly insignificant factors can have a large influence on the plasma and its effects. To ensure that the CE marked device will actually be adopted in clinical practice requires further actions during the research and development process: the demands and concerns of all parties directly and indirectly involved in its use should be identified and at least the crucial parties should be acquainted with plasma medicine and the specific medical device. Some examples will be given from the R&D process of a new flexible volume Dielectric Barrier Discharge (vDBD)

Spin-dependent x-ray absorption in Co/Pt multilayers and Co50Pt50Co_{50}Pt_{50} alloy

The spin dependence of $L_{2,3}$ absorption in 5d atoms oriented in a ferromagnetic matrix contains information on the spin density of the empty d‐projected states of the absorbing atom. Spin‐dependent absorption spectroscopy using circularly polarized synchrotron radiation was applied to study the polarization of the Pt atoms in the binary alloy $Co_{50}Pt_{50}$ and Pt/Co layered structures, which are promising candidates for magneto‐optical recording. The spin‐dependent absorption signals for vapor‐deposited 250(4 Å Co+18 Å Pt) and 250(6 Å Co+18 Å Pt) multilayers indicate a ferromagnetic coupling on Pt and Co atoms with a significant Pt polarization. This is reduced on average by about 60% with respect to the Pt polarization in the $Co_{50}Pt_{50}$ alloy. The experimental results are discussed on the basis of spin‐polarized band‐structure calculations

Distribution of magnetic moments in Co/Pt and Co/Pt/Ir/Pt multilayers detected by magnetic x-ray absorption

Measurements of the spin-dependent absorption have been performed at the L2,3 edges in Pt and Ir in the multilayered structures 4ÅCo+χÅPt (χ = 9, 16, 23, 30, 40) and 4ÅCo+11ÅPt+8ÅIr+11ÅPt. The samples were prepared by sputtering and evaporating, respectively, and show a large perpendicular anisotropy. The measured circular magnetic X-ray dichroism (CMXD) profiles indicate a significant spin polarization of the Pt and the Ir interlayer. The results show that the moments of the Pt interlayers as well as of the innermost Ir layers couple ferromagnetically to the Co layers. The experimental findings are compared with the magnetic moment distribution for sharp interfaces obtained from linear muffin tin orbital (LMTO) band structure calculations

Sputtering pressure effect on microstructure of surface and interface, and on coercivity of Co/Pt multilayers

Thin Co/Pt multilayers were prepared on Si and glass substrates by sputtering with Ar pressures ranging from 2.5 to 15 mTorr. The bilayer structure of the samples was Co(3 Å)/Pt(15 Å)×17, and all samples had the easy axis of magnetization perpendicular to the sample surface as determined with a SQUID magnetometer. All samples retained the layered structure, as revealed by low-angle x-ray diffraction. In addition, diffraction peaks due to the formation of Co-Pt compounds (presumably at the interfaces between Co and Pt) were identified. The coercivity of samples changed from about 400 Oe for films deposited at low Ar sputtering pressure (2.5 mTorr) to as high as 2300 Oe for films deposited at high Ar pressure (15 mTorr). Ellipsometry and atomic force microscopy were used to study surface roughness and microstructure of samples prepared at different sputtering pressures