Patterned and non-patterned media

Most modern hard disk drives have non-patterned media platters. The platter consists of a substrate, usually made of glass (though in the past aluminium and magnesium have been used), and is plated with nickel-phosphorous. On top of this substrate a magnetic cobalt-chromium-platinum-tantalum (CoCrPtTa) film is deposited by evaporation. This results in an irregular structure approximated in figure 2.22; the sizes of the individual grains within this structure are approximately 20nm in size (IBM, 2002), though the grain size in the most recent disks is around 5mn (de Groot, 2005). When pieces of data are written to this film, they are written along tracks shown by the dashed lines in the figure; the magnetisation of the crystals within the area representing that bit is set to a binary state. When the pieces of data are read back, the drive takes a mean of the magnetisation as measured by the head and decides whether that particular segment should be a `zero' or a `one', i.e. a bit of data. Typically, one bit is made up of 20 grains by 50 grains, giving a total physical bit size of 400nm $\times$ 1000nm ( $0.4\mu\mathrm{m} \times 1\mu\mathrm{m}$) (IBM, 2002). Figure 2.1 shows the relationship between bit size and storage capacity.

Advancements in hard disk drive technologies have allowed capacity and performance increases in a similar order of magnitude to that of computer processors -- traditionally doubling approximately every eighteen months but recently every twelve months; for example, the giant magnetoresistance (GMR) effect discovered in 1988 (Baibich et al., 1988) gave manufacturers a head technology capable of reading smaller physical data bits on account of their increased sensitivity. Subsequently, to increase data density, manufacturers can reduce the width of the tracks; at 6 Gbit/in$^2$ this is approximately $1\mu\mathrm{m}$.

At some point it becomes impossible to reduce the track width any further whilst ensuring reliability, and consequently data integrity, as there are too few CoCrPtTa grains within the track width to guarantee a particular overall state (i.e. zero or one); a very narrow track may reduce the strength and therefore the ``clarity'' of the read signal. To increase reliability, manufacturers must reduce the size of the particles which coat the substrate, thereby increasing the relative number of grains representing one state; if these particles fall below a certain critical size, the superparamagnetic effect may reverse the magnetisation in individual grains due to thermal fluctuations.

Patterned magnetic arrays (Ross, 2001) at a nanoscopic level are becoming feasible as hard disk storage media (Chou, 1997, Chou et al., 1996) owing to advancement in fabrication processes (Cowburn et al., 1999a, Cowburn et al., 1999b), including self-assembly techniques (Hoinville et al., 2003, Mayes et al., 2003, Zhukov et al., 2003, Bartlett et al., 2003a, Zhukov et al., 2004a). However, to understand what precisely should be assembled, a study is needed into the relative merits of the entities making up the array; indeed, as nanoscopic length scales are approached, the physical shape of an entity becomes more important (Aharoni, 2000, p115) than other factors, such as magnetocrystalline anisotropy, because it will affect increasingly the characteristics of the hysteresis loop (see section 2.8.1). In chapter 5 we investigate the properties of such nanodots.