Research finds long range effects on thin-film magnetization

Research finds long range effects on thin-film magnetization

A tiny grid pattern has led materials scientists at the National Institute of Standards and Technology (NIST) and the Institute of Solid State Physics in Russia to an unexpected finding about certain electromagnetic nanostructures used in data storage.

The team was studying the behavior of nanoscale structures that sandwich thin layers of materials with differing magnetic properties. Such structures have been researched for a few decades because they can have unusual and valuable magnetic properties. The data read heads on modern HDDs usually exploit a version of the giant magnetoresistance (GMR) effect, which uses such layered structures.

NIST metallurgist Robert Shull looked at covering a thin layer of a ferromagnetic material, in which the magnetic direction of electrons, or “spins,” tend to order themselves in the same direction, with an antiferromagnetic layer in which the spins tend to orient in opposite directions. The layers bias the ferromagnet in one preferred direction, essentially pinning its field in that orientation. In a magnetoresistance read head this pinned layer serves as a reference direction that the sensor uses in detecting changing field directions on the disk that it is “reading.” Researchers have long understood this pinning effect to be a short-range phenomenon, felt only a few tens of nanometers down into the ferromagnetic layer vertically.

Looking at a sideways orientation, the NIST/ISSP team started with a thin ferromagnetic film covering a silicon wafer and then added on top a grid of antiferromagnetic strips about 10 nm thick and 10 µm wide, separated by gaps of about 100 µm. As expected, the ferromagnetic material directly under the grid lines showed the pinning effect, but, quite unexpectedly, so did the uncovered material in regions between the grid lines, far removed from the antiferromagnetic material. The pinning effect extends for ten to fifty nanometers down into the ferromagnet right underneath at least 1,000 times further than was previously known to be possible. This may change how closely such structures can be packed without interfering with each other. See http://www.nist.gov for more information.

Jim Harrison