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Perpendicular storage breaks disk-drive density barrier
Hard disk memory capacities have grown rapidly over recent years, but new ‘perpendicular' magnetic storage technology developed in the HIDEMAR project could boost densities to Tbits/in2.
 In recent years, the global volume of stored digital data has been more than doubling annually, passing the 2.2 million Tbytes (2.2 x 1015 bytes) mark in 2004. But industry and the public continue to show a voracious appetite for more.
This soaring demand arises from data-intensive computer applications, including graphics, animation, multimedia and desktop publishing, to which can be added a growing market for non-PC consumer devices such as set-top boxes, cameras, laser printers and satellite navigation systems. And, despite recent advances in solid-state memory, disk-based magnetic recording remains the dominant storage mode.
Conventional disks are ‘longitudinal' media, in which data are recorded as individual bits in a continuous film of switchable magnetic domains of ferromagnetic material deposited on a metal, ceramic or glass substrate. This technology has proved remarkably durable – capacity growth advanced from 25-30% annually throughout the 1980s to a current rate exceeding 100%.
This dramatic acceleration resulted principally from the successive introduction of magneto-resistive and giant magneto-resistive read heads, which made it possible to retrieve information from smaller and smaller bits. However, when these become so small that their magnetic energy is on par with ambient thermal energy at the operating temperature, they can become unstable and lose their information content. This phenomenon is known as the superparamagnetic limit – and, notwithstanding the best efforts of engineers and scientists, longitudinal media are fast approaching the end of the road. The latest disks have densities of 100 Gb/in2, but significant progress beyond this point will require a radically different solution.
Groundbreaking research
Now, eight European partners in the project Hidemar have led the world in demonstrating environment-friendly production techniques for a new generation of materials. Their groundbreaking research, introducing concepts that could bring a remarkable tenfold increase in storage density by the end of the decade, resulted in the nomination of the consortium as finalists in the 2005 Descartes Prize for scientific excellence.
The underlying principle is to employ ‘perpendicular' technology, whereby numbers of bits are stacked in multilayers with their magnetic moments oriented perpendicularly to the surface of the disk. Patterned media, consisting of regular arrays of single-domain nanomagnets separated by a non-magnetic matrix, are widely seen to be the most promising way ahead – perhaps culminating in ultra-high-density disks holding up to Tbits/in2.
To address these issues, the Hidemar project brought together a consortium comprising four national research institutes, two universities and two industrial companies, together representing six different EU Member States. Heading the three-year initiative was the Italian Consiglio Nazionale delle Ricerche (CNR).
The collaboration focused on CoPt, FePt and CoPd films and multilayers with high uniaxial anisotropy (the tendency of magnetisation to align along specific low energy spatial directions). Specific goals were to reach a density of 200 Gbits/in2 by patterning disks via two different routes: nanolithographic techniques and environment-friendly chemical self-assembly of nanoparticles.
Apart from making it possible to overcome the superparamagnetic limit, patterned materials would offer improved signal-to-noise ratios and simpler writing, while retaining compatibility with existing rotating disk technology. The challenges were to optimise the materials themselves, and to develop manufacturing processes that will be cost-effective on an industrial scale.
Multidisciplinary effort
“Combining thin film growth by sputtering and pulsed laser deposition with advanced nanolithography and nanopatterning, and a variety of chemical self-assembly routes, enabled us to complete a wide-ranging exploration of the available materials preparation options,” observes coordinator Dr Dino Fiorani. “These were validated using a number of techniques – while modelling and numerical micromagnetic simulations, plus fundamental studies of the underlying magnetisation processes were also undertaken.”
The partners succeeded in depositing large area Co/Pd multilayers with high perpendicular anisotropy onto disks by sputtering under industrial conditions. These were patterned using electron beam lithography (EBL) and focused ion beam (FIB) lithography. Ultimately, regions could be formed by EBL with circular tracks of magnetic dots measuring just 27 nm in diameter and having a periodicity of 60 nm. Corresponding to a target-beating data density of 208 Gb/in2, the lab demonstrator was a world premier.
Substantial progress was also made in the development of magnetic anisotropy nanopatterning (MAN), a new technique for patterning by ion implantation, which is being patented by CSIC, Spain, and Unaxis Balzers, Liechtenstein. The latter commercial organisation is already incorporating the project's findings into its latest equipment for hard disk fabrication.
The chemical self-assembly studies produced microns-square regular patterns of perpendicularly anisotropic Fe/Pt nanoparticles as small as 2-3 nm, at much lower temperatures than had previously been achieved. Although not aligned in the circular tracks required for conventional disk reading, self-assembled nanoparticles could eventually permit storage at much higher data densities than those in immediate prospect.
Massive market
Since the close of the project in April 2005, the consortium has continued to collaborate, now using magnetic force microscopy with a low-moment magnetic tip to observe the bit domains. “We are aiming to present a follow-up project for consideration under FP7,” says Dr Fiorani. “Our intention is to explore new methods for self-assembly, together with fast, affordable alternatives to EBL for industrial production. This would give Europe a lead in what will surely be a massive market. In conjunction with heat-assisted magnetic recording devices, mini-media holding tens of terabits per square inch could be just a few years away.” |
