Magnetic interface photographedBy Kimberly Patch, Technology Research News
The magnetic media that things like floppy and hard disks are made out of is more complicated than a simple explanation may lead you to believe.
It's generally true that magnetic bits store information as the ones and zeros of computer information depending on how their magnetic poles point. But when you get down to the microscopic level, there are a lot of parameters that effect the efficiency of storage media.
A new imaging technique developed at Lawrence Berkeley National Laboratory is giving researcher's their first view of this tiny magnetic realm.
Magnetic media are made up of layers of ferromagnets, whose bits, or magnetic domains, are lined up parallel to one another, and antiferromagnets, where each magnetic domain points in the opposite direction of its neighbor. Antiferromagnets are insensitive to applied magnetic fields, while ferromagnets are affected by them. In media like computer disks the insensitive antiferromagnetic layers are used to pin, or hold steady, every second ferromagnetic layer. These layers, in turn, act as magnetic references for the remaining ferromagnetic layers, which are free to change their bit orientations in response to an applied magnetic field in order to store data.
As the computer age has worn on, scientists have managed to cram more bits onto smaller disks.
But despite the steady advances, not a whole lot is known about how the antiferromagnetic layer pins the ferromagnetic layer, largely because it's occurring between microscopic layers of material. The pinning phenomenon is known as the exchange bias. Although there are several theories about how this exchange bias works, none of the theories is able to describe all observed effects, said Frithjof Nolting, a visiting researcher at the Department of Energy's Lawrence Berkeley National Laboratory. "What is lacking is a microscopic understanding of the [magnetic] spin arrangement at the interface" of magnetic media, he said.
Nolting and his colleagues have taken a large step toward a better understanding with their pictures of the microscopic phenomenon. The researchers used a Photoelectron Emission Microscope (PEEM) to image the magnetic structure at the interface of a ferromagnetic layer of cobalt and an antiferromagnetic layer of lanthanum iron oxide. The PEEM uses x-rays to stimulate photoelectrons. Researchers tuned the energy of the x-ray to stimulate photoelectrons of the two different elements, and polarized the beem to show magnetic domains. The images magnified the layers by 10,000 times at a resolution of 20 nanometers. (See photo.)
The photos showed something unexpected. When magnetic media is manufactured, a strong magnetic field is used to set a magnetic bias. Nolting's photographs were of layers that had not been biased, but they showed natural local magnetic biases within each individual domain.
This is just one example of what is not known about exchange bias in an industry where devices are largely produced by trial and error, Nolting said, adding that the process of searching for a better exchange bias includes many parameters.
"You can try whether a rough or smooth surface is better, single or polycrystal, more oxidation or less, higher temperature, annealing... and this is in combination with many [magnetic materials.] But if you know how this effect works, then you know which buttons you have to turn in order to get the best system," said Nolting.
The work could eventually make a difference in the many devices that use magnetic domains, said David Laughlin, professor of materials science and engineering at Carnegie Mellon University. "The practical aspects [of magnetic storage] have progressed at a rate faster than our understanding. This work should help us catch up a bit," Laughlin said.
With a greater understanding of the phenomenon, and greater control of it, there's a lot of potential for improvements, Laughlin added. "Perhaps it will be possible to pattern the underlayer in such a way that instead of the current two states (say right or left magnetization) there could be four states (right or left and back or front) This would immediately increase the density of recorded bits on a disk. [This is] just speculation but this is within the realm of possibilities," he said.
Nolting and his colleagues published their findings in the June 15 issue of Nature magazine. They are continuing to image exchange bias areas and are readying a paper on oxidation and exchange bias.
According to Nolting, the imaging will allow for a better understanding of the exchange bias within a year, and that knowledge could quickly be translated into optimized exchange bias devices.
Nolting's colleagues in this study were Andrea Scholl Simone Anders and Howard A. Padmore of the ALS, project leader Joachim Stohr of Stanford University, Jin Won Seo of the University of Neuchatel and IBM's Zürich Research Laboratory, Jean Fompeyrine, Heinz Siegwart and Jean-Pierre Locquet of the Zürich Research Laboratory, Jan Luning of the Stanford Synchrotron Radiation Laboratory, Eric E. Fullerton and Michael F. Tony of IBM's Almaden Research Center and Michael R. Scheinfeld of Arizona State University.
Timeline: <2 years
TRN Categories: Semiconductors and Materials; Data Storage Technology
Story Type: News
Related Elements: Technical paper "Direct Observation of the Alignment of Ferromagnetic Spins by Antiferromagnetic Spins," Nature, June 15, 2000
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