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Nanostructures to boost hard disks' capacities

Imagine that you could instantly boost your computer's hard disk capacity by a factor of 260... A scientist's dream, as the inflowing stream of data grows everyday. Well, this dream may come true in a few years' time, thanks to a team of researchers at the brand new Quantic Materials and Phenomenons laboratory.1

 Created last January, the lab is dedicated to scrutinizing matter at the smallest scales, ranging from molecular to quantum.

 In particular, nanomaterials are designed and their properties thoroughly studied. Vincent Repain and his colleagues focus on ultra high-density metallic lattices. They have grown a single gold crystal, cut it along a specific direction to reveal regular atom-scale “stairs,” and, through evaporation, deposited on it cobalt atoms with magnetic properties.2 Finding the optimal temperature for this process was crucial.
 The result is a “superlattice” with a very homogeneous distribution of cobalt atoms, which self-assemble on the gold matrix, forming billions of “nanostructures” that measure 3.5 by 7 nanometers in size. Its magnetic properties were measured using magneto-optical techniques, involving a laser facility at the Ecole Polytechnique de Lausanne and an X-ray synchrotron beam at the European Synchrotron Radiation Facility (ESRF) in Grenoble. The sample showed a remarkable magnetic homogeneity. “We have beaten a world record of homogeneity for these lattices,” beams Repain. It also appeared that the nanostructures have the same out-of-plane magnetization axis, and therefore do not interact with each other.

 Such a “bottom-up” approach yields interesting properties. Consider current hard disks. They are made of a glass matrix on which a mixture of chromium, platinum and cobalt nanograins is deposited through a well-known industrial process. A head then passes over the nanograins to orient their magnetic field, leading to a '0' or '1' encoding. But careful examination under the microscope shows that the distribution of nanograins is not homogeneous. Furthermore, they are randomly oriented regarding their magnetic field.  Therefore, a “track” of at least 100 grains is required to reliably code a single bit of data. Average PCs have hard drives storing 10 gigabits per square inch, and the most advanced, fresh out of the labs, can go up to 100 gigabits per square inch. In contrast, the superlattice could code up to 26 terabits (1012 bits) per square inch, a boost in storage capacity by a factor of 260! What's more, the single magnetic orientation of the grains significantly lowers the signal-to-noise ratio.

 “It is fundamental research,” warns Repain. “What we have determined are the properties a surface must have for high-density storage. But we cannot yet build a material suitable for industrial purposes.” Indeed, two major difficulties have to be overcome. First, grain distribution must be perfectly homogeneous in order to code one bit per nanostructure.  Second, such minute structures are prone to a phenomenon called “superparamagnetism.” Under 50K, the magnetic field is stable, but above 50K, it varies, hence storage is not reliable. This is a big obstacle, for information must last at least ten years on conventional hard disks. Further research on more stable alloys (cobalt-platinum, cobalt samarium, iron-platinum) could help solve this problem. “So far, the capacity of conventional hard disks has been increasing according to Moore's law (i.e., almost doubling every year), but it will reach a ceiling within a few years,” Repain notes. No doubt that by then superlattices will be a credible alternative.

Jean-François Haït

Notes :

1. Laboratoire Matériaux et Phénomènes Quantiques. Joint lab CNRS / University Paris-vii.
2. N. Weiss, T. Cren, M. Epple, S. Rusponi, G. Baudot, S. Rohart, A. Tejeda, V. Repain, S. Rousset, P. Ohresser, F. Scheurer, P. Bencok, and H. Brune, “Uniform Magnetic Properties for an Ultrahigh-Density Lattice of Noninteracting Co Nanostructures,” Phys.Rev. Lett 95 (15): 157204. 2005.

Contacts :

Vincent Repain
Laboratoire Matériaux et Phénomènes Quantiques, Paris
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