New probe zooms in on midgets of magnetism

Every year, computer-disk makers manage to cram about twice as much information onto their products as the year before. At that furious pace of shrinkage, the already tiny regions of magnetic material representing bits of digital information will soon span only tens of atoms on an edge. One problem: Researchers don’t know if such small domains are suited for the data-storage job.

In novel magnetic images of an ultrathin iron band, differing field directions appear in red and white. Under an applied field that grows stronger from left to right, even minuscule regions (a and b in 2, arrow in 3) flip directions independently. Pietzsch et al./Science

In a study that may help fill that knowledge gap, a team of German researchers has now demonstrated a technique for making images of ultrasmall magnetic regions. The new tool may enable scientists to test materials and minuscule structures for magnetic stability and other traits crucial to data storage. Moreover, its inventors say, the technique itself may give rise to a new and better method for reading digital information, the researchers propose.

In a magnetic-storage material, each atom acts as a minuscule bar magnet thanks to the quantum mechanical property of spin. Regions of storage media known as magnetic domains, in which almost all atoms have the same one of two possible spin orientations, serve as the ones and zeros of digital information.

In today’s most advanced magnetic disks, domains are comparable in size to large viruses, a few hundred nanometers by tens of nanometers in area. As domains shrink, higher-capacity disks become possible, says Roland Wiesendanger of the University of Hamburg in Germany. However, theorists have predicted that domains smaller than 10 nm on a side will be unable to maintain their spin orientations–that is, their ones and zeros–unless chilled well below room temperature.

“From the technological point of view, it’s extremely important to understand how magnetism works at this scale,” says Oswald Pietzsch, a member of the Hamburg team.

With that goal in mind, Wiesendanger and his colleagues have been tapping the spin of another type of particle, the electron. In the past decade, these researchers have developed a specialized type of scanning tunneling microscope (STM) (SN: 10/24/98, p. 268) that injects electrons of a particular spin direction into a surface. By measuring the current that the surface accepts and the voltage between the microscope tip and the surface, the researchers can tell which spin orientations the atoms beneath that tip possess.

The technique also gauges the strength of the magnetic field within the surface caused by the atoms’ spins. In contrast, conventional data-reading heads detect only the much weaker fields that protrude up from the surface. Because the STM technique tunes in to the stronger in-surface magnetic signal, it may someday serve as the basis for a head that will read smaller domains in future generations of disk drives, Wiesendanger says.

The Hamburg team has already used the microscope’s remarkable resolution of atomic spins to distinguish the orientations of domains less than 10 nm wide. The researchers observed the tiny domains in ultrathin films of iron deposited on a tungsten backing and cooled to a frigid 16 kelvins, a temperature at which thermal effects are negligible.

The team reports that it now can also track the changes that take place in those iron domains as they are subjected to magnetic fields comparable to those used for writing data in commercial computer disks. The scientists were able to discern in nanometer-scale domains magnetic effects that were previously detectable only at much larger scales.

Other forms of magnetic microscopy can’t function in the presence of such fields, Pietzsch says. The scientists report their work in the June 15 Science.

“It’s quite elegant to actually see domains on this scale,” comments Daniel T. Pierce of the National Institute of Standards and Technology in Gaithersburg, Md.

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