Co-Cr-(X) thin films for high density recording

Magnetic recording has been the dominant recording technology for information storage since the invention of the computer. The first commercial IBM computer (Ramac) in 1965 consisted of 50 hard disks having a diameter of 24 inches (» 610 mm). The total capacity was 5 Mbytes and an access time of about 1 second. In 1994 there were 3.5 inch (» 89 mm) hard disks on the market having a capacity of more than 1 Gbyte with an access time of 6-11 seconds. Formally the 10.5 inch disk (» 267 mm) had a drive volume of 30 litres and a weight of 36 kg. The present 3.5 inch disks are housed in a drive volume of 0.6 litre and have a weight of 1 kg. The demand from the user is for compacter information carriers. Smaller formats are useful and can be produced more cheaply. For such developments knowledge and expertiese of the many relevant subjects is needed like the mechanics of the recording system, the intelligent electronic system, new knowlegde in the area of heads and, last but not least, a better knowledge of the interface between head and medium. The combined approached of all these types of knowledge have lead to the realisation of the 1 Gbit/inch2 systems. The availability of the thin-film media was essential to reach this goal. The research described in this thesis is focussed on the limited facts determining the maximum bit density in thin-film media based on Co-Cr-(X) alloys. Microstructural aspects like crystal size, texture and chemical composition play an important role and are mainly determined by the deposition process and its parameters. The magnetic properties and behaviour of the thin-film media are strongly related to the microstructure and chemical composition and homogeneity. The compositional separation (CS patterns) play a very important role in the magnetization behaviour of the thin-film media. NMR techniques are used to discover the chemical inhomogeneities (chapter 5). The linear bit length approaches the fundamental dimension of the microstructure (crystal size) as the bit density becomes larger (= smaller bits). Because the medium noise is mainly related to the number of individual crystals participating in one bit, the crystal size (diameter) should become smaller and smaller by increasing density. Intimate knowledge about the relation between microstructure, deposition technology and magnetic behaviour is necessary in order to tailor media to ensure the correct recording properties. One of the macroscopical measurements used for obtaining information about local behaviour is the study of the rotational hysteresis loss obtained from torque experiments (see chapter 4). The questions still remaining: are the magnetic properties, measured macroscopically, relevant for understanding the behaviour at nano level? And what can we learn from the hysteresis loop, measured with a VSM (obtained from about 1010 crystals), about the local magnetic behaviour of a small bit consisting of about 100 crystals? A preliminary start has been made in chapter 6 by studying Co-Cr thin films having perpendicular anisotropy. The use of a Magnetic Force Microscope is extremely important for obtaining information on a bit scale. A more correlated study between this type of experimental research, micromagnetic simulations and recording experiments can bridge the cap between macro-and micro-magnetic behaviour. The thesis has shown that using the right deposition technology, suitable materials and advanced analysing methods can offer some design parameters for developing thin-film media for high density recording.