DEVELOPMENT of the next generation of high-capacity hard disc drives is being threatened by a row over whether the sensors required to read data on the discs could ever work.
Hard drives read digital information by detecting changes in the magnetic properties of a disc. One of the major factors determining the capacity of the disc is the size and sensitivity of the 鈥渉ead鈥� that does this work. The current generation of heads relies on a phenomenon known as giant magnetoresistance (GMR), in which the resistance of a material changes dramatically in a magnetic field. GMR is the reason today鈥檚 hard disc drives can store up to 300 gigabytes in a device no bigger than a paperback book.
In the last 18 months, however, scientists have measured a new effect at least 100 times stronger than GMR known as ballistic magnetoresistance (BMR). The discovery generated excitement because hard disc drives relying on BMR materials would have 100 times the capacity of today鈥檚 discs. But last week, one of the scientists involved in the research withdrew his results, saying the work was flawed and throwing the entire field of study into question.
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When electrons travel along a wire, they are normally scattered by imperfections in the wire鈥檚 structure and the vibrations of atoms in their way. But when a wire is thin enough, scientists predict that electrons can only pass through them by travelling 鈥渂allistically鈥�, in straight lines. When that happens, the presence of even the tiniest magnetic field would bend the path of the electrons into nearby atoms, scattering them and dramatically changing the wire鈥檚 resistance.
In July last year, Harsh Deep Chopra, a material scientist, and Susan Hua a physicist, both at the State University of New York, Buffalo, reported startlingly large BMR effects. They found that the resistance of nickel whiskers only a few nanometres wide and long could change by 100,000 per cent in the presence of tiny magnetic fields. But since then, others have not been able to consistently reproduce their dramatic results.
In December last year, the work got tentative backing when William Egelhoff, Chopra鈥檚 postdoctoral advisor and a research chemist at the National Institute for Standards and Technology in Gaithersburg Maryland, was part of a team that reported a smaller 400 per cent change in resistance.
But last week, Egelhoff backtracked by releasing the results of a follow-up study that he says proves that the change in resistance was not what it seemed. 鈥淣o theory predicts such large BMR effects,鈥� Egelhoff told an American Vacuum Society meeting in Baltimore.
He says that in experiments, the magnetic field exerts a force not only on electrons but also on the wires themselves, changing their shape. It is this bending of the wires that he believes changes their resistance not BMR. Future hard disc drives could not exploit such an effect because repeated flexing would soon break the nanowires.
Egelhoff鈥檚 U-turn came after he repeated his experiments with nanoconductors that do not change shape in a magnetic field and found that the high changes in resistance simply disappear. 鈥淚f a real BMR effect is ever found, it is not clear it will be a big one,鈥� says Egelhoff.
However, Chopra disagrees with his former advisor. This week, he plans to present evidence explaining how his huge BMR effect occurs. He says the material is not a single nanowire but hundreds of them packed together. Chopra believes that BMR occurs in all the conductors simultaneously, and that the combined effect leads to the large effect he measures. He adds that he has been careful to prevent the magnetic distortions Egelhoff describes.
But Egelhoff remains sceptical: 鈥淭here鈥檚 an old principle in science 鈥� the most likely explanation is the simplest one.鈥�
Hitachi, one of the world鈥檚 largest makers of hard disc drives declined to comment on the details, but added that such discussions were part of the normal back and forth process of research.
