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UD-led researchers may have found a way to create ultrahigh-density magnetic media for computers

3:07 p.m., Sept. 15, 2003--University of Delaware scientists are leading efforts to beat the superparamagnetic limit, a fundamental physical constraint that restricts the density of information that can be stored reliably on magnetic computer disk files.

The members of UD’s superparamagnetics research team are seated (from left) Stoyan Stoyanof and Vassil Skumryev and standing (from left) Yong Zhang and George Hadjipanayis

The UD-led research team, which includes Vassil Skumryev, Stoyan Stoyanov and Yong Zhang, researchers in the Department of Physics and Astronomy, and George Hadjipanayis, department chair, reported its findings in a letter to the journal Nature published on June 19.

“The density at which information can be stored in magnetic disk files has doubled every two to three years since the early 1960s, with a present growth rate of about 60 percent,” Hadjipanayis said. “Higher density means the ability to record certain information in a smaller area and thus more powerful and more compact computers. However, past performance is no guarantee of further advances.”

The Nature letters notes that interest in the use of magnetic nanoparticles in high-density recording has increased in the last few years and that most applications rely on the magnetic order of the nanoparticles remaining stable over time.

The UD team said those magnetic recording materials face a fundamental limit related to the minimum particle size in which the magnetic moment—and hence the information—is stable against thermal fluctuations and over time.

“With decreasing particle size, the magnetic anisotropy energy per particle responsible for holding the magnetic moment along a certain direction becomes comparable to the thermal energy. When this happens, the thermal fluctuations induce random flipping of the magnetic moment with time, and the nanoparticles lose their stable magnetic order and become superparamagnetic,” Skumryev explained. “Thus, the demand for further miniaturization comes into conflict with the superparamagnetism caused by the reduction of the anisotropy energy per particle. This constitutes the so-called superparamagnetic limit in recording media.”

The UD team has found that magnetic exchange coupling induced at the interface between ferromagnetic nanoparticles and an antiferromagnetic matrix can provide an extra source of anisotropy, leading to a marked improvement of the thermal stability of the moments of the ferromagnetic nanoparticles. This mechanism provides a way to beat the superparamagnetic limit in isolated particles, the authors said.

“Our approach is different from the recently proposed magnetization stabilization because of the antiferromagnetic coupling of ferromagnetic layers via a non-magnetic spacer,” Stoyanov said.

The authors claim that the UD finding is noteworthy because the thermal energy not only affects the recording media but could also affect any magnetic device that is sufficiently reduced in size. “Our approach, or modifications to it, could be applied to circumvent thermal effects in other types of devices,” Skumryev said.

Hadjipanayis said the team has been working with Dieter Weller at Seagate Technology to study the viability of this new approach.

In their finding, the UD team has collaborated with Dominique Givord of the Laboratoire Louis Neel in France and Josep Nogues of the Universitat Autonoma de Barcelona in Spain.

Article by Neil Thomas
Photo by Duane Perry

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