CoNi/Pt multilayers for magneto-optical recording


Drent, W.P. van (1995) CoNi/Pt multilayers for magneto-optical recording. thesis.

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Abstract:The amount of digital information increases at an astonishing rate. This trend is not likely to change in the coming years, as technology is still well away from the fundamental limits in handling information. Information can be created, processed, relayed, and it can be stored in a physical way in a medium for future retrieval. One of the possibilities to store information is to write (and read) it by an optical beam in a magnetic medium: Magneto-Optical (M.O.) recording. M.O. media have several distinct merits when they are used as removable media, where resistance agains environment conditions is imperative. Several M.O. drives applying 5¼" or 3½" M.O.-disks have meanwhile appeared on the personal computer market. To achieve substantial increase of the information density on such disks, the light spot that is used for reading and writing has to become smaller. As the spot is focussed to the diffraction limit this can only be achieved by applying a shorter wavelength. The performance of the current generation of M.O. disks based on RE-TM materials decreases at short wavelenghts (300-500 nm), and therefore other media are needed. After 1985 research on Co/Pd and Co/Pt multilayers started on a world-wide scale, as these layers had many merits: high magneto-optical output around a wavelength of 300 nm, high corrosion resistance and strong perpendicular orientation of the magnetization due to interface anisotropy. Solely the Curie temperature was too high (350-400°C for a medium with high M.O. output), resulting in degradation of the medium after many read/write cycles. One of the methods to reduce the Curie temperature of a multilayer is to reduce the Curie temperature of the magnetic layers therein by alloying them with another element. The moment of the magnetic layers should preferably remain high, and regarding magnetic phase diagrams we chose the magnetic element Ni to alloy with Co. This thesis deals with the preparation and characterization of CoNi/Pt multilayers with either 50 or 60 at.% Ni in the magnetic layers. In Chapter 2: M.O. multilayer theory, the M.O. Kerr effect is introduced and a model for the M.O. effects of arbitrary stacked structures is presented. The influence of polarized Pt on the large Kerr rotations at short wavelengths is treated. The magnetic properties and the Curie temperature of Co/Pt multilayers are reviewed in order understand the critical parameters and their dependence on the layer structure and deposition process. The chapter concludes with an overview of possible additions to the Co layers. In Chapter 3: Experimental methods, first the sputter process is introduced, then the UHV-sputter system that was set-up to produce the multilayers is described. Further the magnetic and structural measurement techniques that were used are briefly discussed. Besides the sputter system, the magneto-optical measurement set-up was one of the major technological projects performed during this work. Therefore it is treated seperately in Chapter 4: M.O. measurement set-up. Operating principle, construction and merits of the new set-up are reviewed. It features spectroscopic Kerr rotation and ellipticity measurements, and using the build-in sample heater Curie temperatures can be measured. Chapter 5: Tayloring of magnetic and magneto-optic properties summarizes measurement results on CoNi/Pt multilayers. It is shown that the layers have perpendicular magnetic anisotropy in a large layer thickness range, and that the highest anisotropy corresponds to the lowest internal stress. The coercivity could be increased up to 200 kA/m (with an hysteresis loop that was almost rectangular) by applying a high sputter gas pressure, whereas the application of a bias voltage to the substrate table resulted mainly in lower coercivities. The reversal mechanism is domain wall motion, where the domains are hampered by many small pinning sites, which probably origin at boundaries of columns that extend throughout the multilayer. The Curie temperature was lowered compared to Co/Pt multilayers with the same layer thicknesses, but for samples that show a good magneto-optic performance the Curie temperature was always at the high side (around 300°C). Application of a Pt seedlayer under the multilayer increased coercivity, which we attribute to the higher roughness, and the anisotropy, which is attributed to the improved (111) orientation of the multilayer. Spectroscopic M.O. measurements and simulations finalize this chapter. Large Kerr rotations were found when the Pt layers were very thin, typically thinner than 8.8 Å. This correlates to the magnetic moment of the CoNi layers, which also becomes higher with decreasing spacing. This ferromagnetic coupling is needed to achieve high M.O. output. The Pt layers can be made thinner without consequences: due to the lower magnetic moment of CoNi layers, the multilayer remains perpendicular magnetized when the Pt layer thickness is reduced (even at 2.9 Å Pt!). The figure of merit of our best CoNi/Pt sample is comparable to that of Co/Pt multilayers in the wavelength range 375-520 nm By excursions around the parameters of this sample an even better performance is expected to be found.
Item Type:Thesis
Additional information:Imported from SMI Theses
Electrical Engineering, Mathematics and Computer Science (EEMCS)
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