NIST Measurements of Nanosized Magnetic Sensors Question the Size of BMR Effect

Measurement results presented today by scientists at the Commerce Department’s National Institute of Standards and Technology (NIST) do not corroborate reports of dramatic changes in electrical resistance attributed to ultra-small magnetic sensors that exploit a quantum phenomenon regarded, by some, as the next big thing in magnetic-data-storage technology.

The findings were presented at the American Vacuum Society’s 50th annual international symposium and exhibition in Baltimore, Md.

Reports since mid-2002 have credited the so-called ballistic magnetoresistance (BMR) effect with increases in electrical resistance ranging from a few hundred percent to a million percent, fueling preliminary prospects for vastly improved read heads in hard-disk drives.

NIST research chemist William Egelhoff Jr. said such “impressive” changes in electrical resistance described in articles and at conferences appear to stem from “artifacts,” or factors unrelated to the quantum effect. For example, when two nickel wires in a T-shaped arrangement were exposed to a magnetic field, they contracted. Shortening of the wires can stretch and severely distort the cluster of atoms that formed the nanometer-scale contact between them, resulting in large increases in resistance.

In contrast, Egelhoff said, measurements on nanocontacts between wires that do not shorten in the presence of a magnetic field, thereby eliminating this artifact, failed to show sizable resistance increases.

In fact, one set of studies demonstrated that repeatedly exposing a tiny contact, about 10 nanometers in diameter, to a magnetic field results in the equivalent of molecular-scale tug of war. As the two nickel wires shortened or lengthened by a calculated 35 nanometers, the nanocontact between them would break and then reform. If unrecognized, the cycle of breaking and rebuilding the narrow bridge between the wires would yield values that could be mistaken for an infinite BMR effect, Egelhoff said.

The NIST team has demonstrated several other types of artifacts that can mimic electrical-resistance swings corresponding closely to BMR-effect values reported in scientific journals and at conferences.

To prevent unwanted influences that can mask true experimental results, Egelhoff and his colleagues developed a set of recommended procedures for “performing measurements in an artifact-free manner.”

The NIST work is continuing, with the aim of achieving a more complete understanding of BMR. The phenomenon is predicted to occur when electrons, the carriers of electric current, traverse through channels so narrow that they must proceed in a straight line, all with an unchanged spin, or magnetic orientation. However, theoretical physics does not offer an explanation for why large changes in electrical resistance would result.

NIST results to date might account for why some researchers have not been able to reproduce the high BMR-effect values reported by others.

“The BMR effect is predicted by theory,” Egelhoff explains, “but the ‘impressive’ results are much too large. At this point, it’s inconclusive as to whether a real BMR effect will be found and, if so, whether it will prove large enough to be of much interest to the magnetic-sensor community.”

Magnetic sensors serve as read heads, a key technology of the estimated $50 billion hard-disk-drive industry. As read heads decrease in size and increase in sensitivity to magnetic signals, more data can be squeezed onto smaller spaces on disks. Since the late 1990s, data-storage capacity on magnetic disks has been doubling almost annually.

Reports that nanometer-scale metal sensors were tremendously more sensitive to magnetic fields—due, seemingly, to the BMR effect—sparked speculation that storage capacity might be increased by a factor of a thousand or more.

Egelhoff and his team have collaborated with several laboratories that reported these initial results. The NIST team evaluated the experiments and identified several potential factors unrelated to BMR that could lead to results attributed to the quantum phenomenon.

Last December, Egelhoff co-authored a paper, based on work carried out by collaborators at two other institutions, that attributed a 400 percent change in electrical resistance to the BMR effect. On the basis of follow-up studies, he now concludes that the reported change was the consequence of contact deformation and other artifacts that were not reckoned with during the initial experiment.

Posted 3rd November2003

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