Magneto-optical recording media - CoNi/Pt and Co/Pt multilayers


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Meng, Q. (1996) Magneto-optical recording media - CoNi/Pt and Co/Pt multilayers. thesis.

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Abstract:Concluding Remarks 5.1. General Statement As described in Chapter 1, magneto-optical recording disks have been used in the audio (MiniDisc) and PC as removable disks with high data capacity. Recently, MO disks have been in the competition with the phase-change type of optical disks. Up to now, the materials used in MO disks have been and still are RE-TM amorphous alloy films although Co/Pt multilayer films have been developed and have reached the same level of recording performance as RE-TM materials. It is now waiting for a laser device with a shorter wavelength around 400 nm in order to realise an even higher storage capacity with using Co/Pt multilayer films. CoNi/Pt multilayers have shown an improved recording performance compared with Co/Pt multilayers, for instance, increasing the writing sensitivity and the number of write/erase cycles mainly because of their lower Curie temperatures. Based on this background, Co(Ni)/Pt multilayers were chosen to be studied in this work, not only with a view to their application but also for some basic research interests. The study was mostly concentrated on the structural and magnetic properties according to the available measuring and analysis methods, such as VSM, Kerr spectrometry, XRD and TEM. 5.2. Preparation of the Multilayers The Co(Ni)/Pt multilayers were prepared by sputtering with using the Ar gas. Among the sputtering conditions and the parameters of the deposited films, the thickness of the individual layers in a multilayer is of prime importance because it very sensitively determines the magnetic properties of a multilayer. The methods as well as the effects to determine the layer thickness have been carefully investigated. It was found that XRD measurement, especially low-angle XRD measurement is an accurate method to measure the thickness of a single layer film and the bilayer thickness of a multilayer film. Furthermore, the different Ar sputtering pressure, the different substrate temperature and the different substrates have been applied to modify the magnetic properties of the deposited films. The annealing experiments were also carried out in order to determine the thermal stability of the multilayers and to study the changes in the structural and magnetic properties. 5.3. Multilayer Structures Magnetic properties of sputtered Co(Ni)/Pt multilayers depend strongly on the film microstructure. The multilayer structures were analysed by using XRD, TEM as well as XPS and AES. 5.3.1. Effects of the Ar sputtering pressure In the sputtered multilayers, the interface roughness and microstructure are most influenced by the bombardment on the substrate by argon neutrals reflected from the target. At low Ar pressures the reflected neutrals bombard the substrate with a high energy, leading to a dense film which is under compressive stress. At high Ar pressures the reflected neutrals become thermalised before reaching the substrate, the films tend to tensile stress and have a microstructure containing a high density of microvoids. As shown in the cross-sectional TEM images, the multilayer deposited at a lower Ar pressure (1.6×10-2 mbar) clearly showed that the individual layers are flat and continuous at the bottom of the film. As the film growing, the layers become bent and broken with the formation of columns because of the relaxation of the internal stress. The multilayer deposited at a higher Ar pressure (4.0×10-2 mbar) showed the enhanced columnar structures because of the low energy bombardment. However, the most important feature is that the individual layers are still clearly distinguishable within the columns. This means there are still sharper interfaces within the columns which could contribute to the increased perpendicular anisotropy due to the possibly enhanced interfacial strain anisotropy. As shown in the plane-view TEM images, the Co(Ni)/Pt multilayer deposited at a lower Ar pressure (1.6×10-2 mbar) showed the high dense grain structure without visible voids. However, the multilayer deposited at a higher Ar pressure (4.0×10-2 mbar) showed big voids, which correlated with the enhanced columnar structures as shown in cross-section TEM images. Such big voids contributed to the large coercivity because they acted as hard nucleation centres to resistant the magnetic domain reversal. 5.3.2. Effects of annealing ¾Interdiffusion and alloying It was found that Pt diffused into the Si substrate and Si diffused up to the surface of the films in the annealed multilayers that were deposited on Si substrates. The atomic diffusion of Si through the grain boundaries could contribute to the enhanced coercivity in the annealed films due to the formation of a non-magnetic compound such as CoSix at the grain boundaries besides other non-magnetic compounds such as CoOx. The possible alloying at the interfaces by annealing could contribute to the enhancement in Kerr rotation. 5.4. Magnetic Properties 5.4.1. Individual layer thickness dependence-Curie temperature dependence The magnetic properties of Co(Ni)/Pt multilayers are very sensitive to their individual layer thicknesses, not only the thickness of the Co(Ni) layers but also the Pt layer. One of the importance results is that the perpendicular magnetic anisotropy (PMA) depends on the individual layer thicknesses. The Co(Ni) layer thickness dependence of PMA is mainly attributed to the interface anisotropy while the Pt layer thickness dependence is related to the thickness dependence of the Curie temperature. The effect of the Curies temperature on the magnetic properties has been confirmed by the enhancement of magnetic properties when the samples were measured at low temperature. However, the thickness dependence of the Curie temperature is originally related to the layer structures with regard to the interdiffusion at the interfaces and the magnetic coupling between the Co or CoNi layers. 5.4.2. Dependence on the number of bilayers-Total thickness dependence In addition to the individual layer thickness dependence, the magnetic properties are also dependent on the number of bilayers and the total thicknesses of the film, especially the coercivity and the perpendicular anisotropy decreases with increasing the number of bilayers. This is attributed to the degradation of the interface sharpness and the relaxation of the internal stress which causes the reduction of the interface anisotropy and the volume anisotropy, respectively. However, the Kerr effect is not dependent on the number of bilayers when the total thickness is thicker than the Kerr information depth. 5.4.3. Effects of the Pt seedlayer The Pt seedlayer contributes to the important effects that increase the perpendicular magnetic anisotropy and the perpendicular coercivity of the multilayers because of the improvement of the fcc-<111> texture. Furthermore, a Pt seedlayer (thicker than the Kerr information depth) is required for the measurement of the Kerr spectra in order to avoid any influence from the substrates. However, as for the application of a MO disk, the Pt seedlayer must be as thin as possible in order to allow the light to be transmitted through the seedlayer without a significant decrease in the light power on the multilayers. 5.4.4. Effects of the Ar sputtering pressure The different Ar sputtering pressures cause significant differences in the microstructures and the magnetic properties. With increasing Ar pressure, the coercivity and PMA increased. The increase in coercivity can be explained with regard to the enhanced columnar structures and the pronounced voids in the films which contributed to the domain wall pinning. The increase in PMA is related the increased interfacial magnetoelastic anisotropy because of the enhanced strain at the interfaces. The enhanced column structure could be also one of the origins for the increased PMA. The effect of the Ar pressure on the Kerr effect was not obvious in the thick multilayers but significantly obvious in the thin multilayers. Furthermore, when Ar pressure was too high, the magnetic properties were damaged because of the degradation in the multilayer structures. 5.4.5. Thermal stability-Effects of the annealing The Co(Ni)/Pt multilayers can be easily affected by the annealing temperature. This was also found during the Curie temperature measurements. This implies that the thermal stability of the Co(Ni)/Pt multilayers is not so suitable. The temperature for thermal stability is only about 250°C which is lower than the applied thermal writing temperature, usually that is about 300°C. That could be attributed to the fact that the studied films were very thin (10-80 nm). However, two remarkable results were found in the annealed CoNi/Pt multilayers. One is enhancement of coercivity in the annealed multilayers with thinner CoNi layers. Another one is enhancements of Kerr rotation in the annealed multilayers having thicker CoNi layers. 5.4.6. Enhancement of coercivity Increasing the coercivity in Co(Ni)/Pt multilayers is one of the demands for MO application. Two methods for doing that have been realised in this work. One is to deposit the multilayer at a relatively higher Ar pressure; another one is to anneal the multilayer at 300°C. However, for the annealing, it is better to be carried out in a vacuum although annealing in air results in an even larger coercivity. Such enhancements are attributed to the rough and columnar structures associated with the segregation and voids at the grain boundaries. However, for the MO application, these two methods are not desirable because the resulting layer structures can cause medium noise during the MO reading. Therefore, other methods to increase coercivity are still required, for instance using a smooth underlayer, such as a SiNx underlayer, associated with sputtering etching. 5.4.7. Enhancement of Kerr rotation Besides the effort to increase the coercivity, a increase in the Kerr rotation is also requred. As mentioned perversely, one method is to anneal the multilayers at a higher temperature such as 450°C, which is attributed to alloying at the interface between the CoNi and Pt layers. However, such Kerr enhancement was only found in the annealed multilayers with thicker Co(Ni) layers in which the perpendicular magnetic anisotropy was very low. Besides this methods, multilayers covered by dielectric layers, such as SiOx and SiNx layers also showed the enhanced Kerr rotation. This method has been applied in the current MO disks. 5.4.8. Thermomagnetic writing and written domains With regard to one of the goals of this work, thermomagnetic writing has already been realised in CoNi/Pt multilayers. Meanwhile the properties of the written domains as well as the factors to achieve regularly written domains were investigated. It was found that thermomagnetic writing was more easily performed in CoNi/Pt multilayers than in Co/Pt multilayers. The domains written in the film that deposited at the lower Ar sputtering pressure showed the higher regularity than that in the film deposited at the higher Ar pressure. The domains written in the latter film showed the subdomains because of the poor microstructure which could cause the recording noise. In order to achieve regularly written domains, a large perpendicular coercivity with a square hysteresis loop is required. The optimum thicknesses in Co(Ni)/Pt multilayers have been determined to be in the region about tCo (or tCoNi)=5 Å, tPt=10 Å and N=10. The Ar sputtering pressure of 1.6×10-2 mbar is preferred. The typical magnetic parameters in an optimum CoNi/Pt multilayers are Hc=113 kA/m, qK,310=0.3° and Tc=255°C. 5.4.9. The mechanism of coercivity and magnetisation reversal The mechanism of coercivity and magnetisation reversal was studied together with the observation of the domain structures, which provides the information for understanding magnetisation hysteresis loops. A lower coercivity with a sharp nucleation field and a sharp magnetisation reversal is attributed to a few hard domain nucleation centres and soft domain-wall pinning sites. The domain images showed that reverse-domains nucleated at a few centres and they propagated rapidly throungh the domain wall motion. A large coercivity with broad nucleation fields and a slow magnetisation reversal is attributed to many domain nucleation centre and many hard domain-wall pinning sites. The images showed that the domains nucleated at several centres and the domain wall propagated slowly. The first type of magnetisation reversal was found in the multilayers that were deposited at the lower Ar sputtering pressure. The second one was in the films deposited at the higher Ar pressure and in the annealed films . It has already been mentioned that the microstructures in these two typical deposited films are quite different. The flat, continuous and smooth layers leads to the soft domain wall pinning sites and fewer number of the nucleation centres. The rough, columnar layers as well as the segregation and voids between the grains result in more nucleation centres and hard domain wall pinning sites. 5.4.10. Perpendicular magnetic anisotropy As an initial topic in this work, an attempt to study the origins of the perpendicular magnetic anisotropy (PMA) in Co(Ni)/Pt multilayers was carried out with regarding to the interface and volume contributions of the magnetoelastic anisotropy. The CoNi layer thickness dependence of the average strain and resulting the compressive in-plane stress in CoNi/Pt multilayers was determined by XRD measurements. There is no stress remained in the Pt seedlayer on the Si substrate as determined by XRD measurements. The Co(Ni) layer thickness dependence of PMA is mainly attributed to the interfacial anisotropy including the Néel type and the strain anisotropy. The Néel type interfacial anisotropy can be reduced with increasing the interface roughness while the interfacial strain anisotropy can be increased with increasing the interface strain. Besides the interfacial strain anisotropy, the magnetoelastic anisotropy can also contribute to the volume anisotropy which is dominated by the internal strain in the layers. PMA in CoNi/Pt multilayers is not only dependent on the Co(Ni) layer thickness but also on the Pt layer thickness. The decrease in PMA with increasing Pt layer thickness is attributed to the decrease in the Curie temperature. The increase in PMA with increasing the thickness of the Pt seedlayer is correlated to the enhanced film texture along the growth direction. Originally, this could be attributed to the texture dependence of the magnetocrystalline and magnetoelastic anisotropy in a polycrystalline film. The reduction of PMA with increasing the number of the bilayers could be caused by the degradation of the interfaces and the relaxation of the internal stress. These two effects could also contribute to the reduced PMA in the annealed films and in the films deposited at a high temperature substrate. The increased PMA with increasing the Ar sputtering pressure is explained with regard to the increase in the interfacial strain anisotropy although the Néel type interfacial anisotropy and the volume strain anisotropy could be reduced. 5.4.11. Effects of the substrates and the dielectric coverlayers No significant difference has been found in the films which were deposited on the different substrates, such as Si, glass, a dielectric layer (SiOx or SiNx) on Si substrate except in the Kerr measurements if the films are thinner than the Kerr information depth. However, differences could occur in the annealing effect because of the different diffusion effect between the substrate and the film. In order to avoid the annealing diffusion between the Si substrate and the film, a SiOx or a SiNx layer could be used between the Si substrate and the film. In this case the effect of the dielectric underlayer on the Kerr spectra should be carefully considered. On the other hand, a dielectric layer can be deposited on the top of the film to act as a prevent layer, a Kerr enhancement layer and a thermal absorption enhancement layer. It has been shown that a SiNx window with a thickness of 40 nm or a SiNx coverlayer (80 nm) is very useful for the thermomagnetic writing.
Item Type:Thesis
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Electrical Engineering, Mathematics and Computer Science (EEMCS)
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