Measuring Absolute Magnetic Moments

As long ago as Lord Kelvin's time, a century ago, scientists recognized that a change in magnetic environment could alter a material's electrical resistance. Back then this was a 5 percent effect. By the 1990's, however, if the material consisted of a sandwich of alternating magnetic and nonmagnetic films the effect could amount to as much as 60 percent.

This "giant magnetoresistance" is now the operative mechanism behind much of hard-drive and tape data storage. By forcing the tiny induced electric currents to jump across insulating layers as it passes from one magnetic layer after another, the change in resistance can be as high as 400 percent. This translates into data storage densities as high as 300 billion bits per square inch.

Fabio da Silva of the University of Colorado at Denver and Health Science Center (fcss@boulder.nist.gov) and his colleagues at the National Institute of Standards and Technology (NIST) in Boulder, Colorado now employ the magnetoresistance effect not to store data but to study the very process by which the tiny domains in the storage medium are magnetized in the first place. Using anisotropic magnetoresistance (AMR), in which the resistance of the sample changes when the magnetic material is remagnetized in a direction different from that of the flowing current, the researchers could measure the fundamental magnetism of the tiny (4-micron) domain itself.

The team hopes these results can serve as a reference for calibrating conventional magnetometers, devices that measure magnetic fields in objects ranging from consumer electronics devices to geophysical specimens. These results are being reported next week at the AVS (vacuum science) symposium in San Francisco.

AVS meeting paper: Wednesday, November 15 8:40 am
Contact Fabio da Silva
University of Colorado at Denver and Health Science Center
fcss@boulder.nist.gov
Abstract at the AVS Symposium site