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van Kranenburg, H. (1992) Obliquely co-evaporated thin films for magnetic recording. PhD thesis, University of Twente. ISBN 90 900 5476 6
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A systematic research is carried out on obliquely ( co- ) evaporated media for magnetic recording applications. The investigated materials concern Co-alloys, being Co-Cr, Co-Ag and Co- Ta. The re1ations between deposition parameters, morphology , texture and rnagnetic behaviour were swdied. The accent was put on understanding and tm1oring the compositional separation. This in turn provides better understanding of the micromagnetic behaviour of the media. The oblique deposition promotes the growth of a columnar morphology. Growth proceeds according to a competition between the shadowing mechanism and surface diffusion. If one vapour direction dominated during deposition then the columns that developed incline towards that direction. In the Co-Cr films the hcp Co lattice ensures a strong crystalline anisotropy. This is not the case in the, in majority, cubic structures of the Co-Ag and Co- Ta fi1ms. In these films the anisotropy is not strong enough to overcome the film demagnetization and their coercivities are small. The Co-Ag and Co- Ta fi1ms therefore form no suitable magnetic recording media. In the (low temperature) obliquely co-evaporated ftlms, where the two vapour beams of different materials arrived at the deposit surface from opposing directions, a chemical inhomogeneous distribution of the two materials arose. We have called this geometry effect the process-induced compositional separation. Preheating of the substrate is thus not necessary to create chemical inhomogeneities in the 1atera1 fi1m direction. The occurence of process-induced compositional separation is proved by several measurement techniques, both direct and indirect, and by simulations of ftlm growth. The process-induced compositional separation can be exploited to tailor a magnetic particulate behaviour in thin ftlm media, as has been shown in this work for the obliquely co-evaporated Co-Cr samples (stand still substrates during deposition). The opposing vapour directions of the ferromagnetic Co and the non-ferromagnetic Cr induce separated Co- and Cr-rich regions in the films, where the Cr-rich parts are in majority present at one side of the columns. This favours discoupling of adjacent columns. Also the open regions in between the columns, that arise due to the shadowing mechanism, promote this discoupling. An enhanced energy product (MsxHc.J is the result. The existence of a certain degree of discoupling of the columns yields a columnar shape anisotropy that, together with the hcp crystalline anisotropy, contributes to the total out-of-plane anisotropy. This out-of-plane anisotropy could be tailored to be stronger than the film demagnetization. If the substrate was rotated during deposition the media lost their enhanced perpendicular coercivities, whereby they became unsuitable for high density recording. The discontinuities, being the open regions between the columns and the chemical inhomogeneities, i.e. the Cr-rich regions which were formed due to the process-induced compositional separation, further provide energy barriers for domain wall movement. The reversal of the magnetization is most likely to proceed through either rotation or domain-wall movement with largely hindered displacement of domain walls.
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