Magnetic devices show unexpected effects

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 - the surprisingly strong and long-range effects of certain electromagnetic nanostructures used in data storage.

Their recently reported findings* may add new scientific challenges to the design and manufacture of future ultra-high-density data storage devices.

The team was studying the behaviour of nanoscale structures that sandwich thin layers of materials with differing magnetic properties. In the past few decades, such structures have been the subjects of intense research because they can have unusual and valuable magnetic properties.

The data read heads on modern high-density disk drives usually exploit a version of the giant magnetoresistance effect that uses such layered structures for extremely sensitive magnetic field detectors.

Arrays of nanoscale sandwiches of a similar type might be used in future data storage devices that would outdo even today's capacious microdrives because in principle the structures could be made even smaller than the minimum practical size for the magnetic islands that record data on hard disk drives, according to NIST metallurgist Robert Shull.

The key trick is to cover 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.

By itself, the ferromagnetic layer will tend to magnetise in the direction of an externally imposed magnetic field - and just as easily magnetise in the opposite direction if the external field is reversed.

For reasons that are still being debated, the presence of the antiferromagnetic layer changes this. It biases the ferromagnet in one preferred direction, essentially pinning its field in that orientation.

In a magnetoresistance read head, for example, 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. The influence of the antiferromagnetic layer is felt only a few tens of nanometres down into the ferromagnetic layer - verticallly. But what about sideways?

To find out, 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.

Using an instrument that provided real-time images of the magnetisation within the grid structure, the team watched the grid structure as they increased and decreased the magnetic field surrounding it.

What they found surprised them.

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.

"This pinning effect extends for maybe tens of nanometres down into the ferromagnet underneath," explains Shull, "so you might expect that there could be some residual effect maybe tens of nanometres away from it to the sides.

"You wouldn't expect it to extend 10 µm away - that's 10 thousand nm." In fact, the effect extends to regions 50 µm away from the closest antiferromagnetic strip, at least 1000 times further than was previously known to be possible.

The ramifications, says Shull, are that engineers planning to build dense arrays of these structures onto a chip for high-performance memory or sensor devices will find interesting new scientific issues for investigation in optimising how closely they can be packed without interfering with each other.