New insights into the stable magnetism of phase-change semiconductors
could enable the development of ultra-high-speed data storage
Phase-change semiconductors have
the ability to switch back and forth between amorphous (non-crystalline solid)
and crystalline phases upon heating. As such, they are used widely in data
storage and computer memory applications, for the reason that information can
be written in binary form using the two distinct states.
One particular phase-change alloy
currently used in rewritable disc technology is that of germanium, antimony and
tellunium, or Ge2Sb2Te5 (GST). Researchers believe that this material may prove
useful for the field of spintronics, generating a way of storing data which
takes advantage of the inherent angular momentum, or spin, of electrons present
in the material.
Recent research indicates that
the atoms in GST could naturally create a stable bond with certain metals,
thereby generating a permanent and stable ferromagnetic state potentially
useful for high-speed read/write storage. However, to date, researchers have
been unsure exactly how GST is able to form a stable ferromagnetic state.
Now, Kewu Bai at the A*STAR
Institute for High Performance Computing, together with co-workers from
A*STAR’s Data Storage Institute and the Singapore University of Technology and
Design, have completed an in-depth analysis of GST and its ability to maintain
stable ferromagnetism when doped with iron1.
“Alloying magnetic elements such
as iron with semiconductors provides the materials necessary for future
spintronics applications,” explains Bai. “We know very little about the
processes behind ferromagnetism from doping phase-change materials with metals,
because the commonly used experimental techniques, such as X-ray diffraction,
transmission microscopy and X-ray absorption, are not sufficient to
characterize material microstructures.”
The research team instead used
first-principle calculations to determine the validity of the experiments they
carried out. First-principle calculations use the inherent laws of nature — for
example, bonding laws between atoms and laws for electron movements — to build
up an exact picture of the chemical structures at work, rather than relying on
best-fit parameters in computer models.
“We used first-principle
calculations to locate the site in GST at which iron molecules preferred to
bond,” explains Bai. “The mechanism that led to the observed ferromagnetism was
then uncovered.”
The researchers discovered that
the iron molecules preferred to bond with the antimony molecules in GST. Along
certain orientations within the crystalline phase, the iron–antimony bonding
becomes dominant, leading to a stable ferromagnetism in the material.
“We are still in close
collaboration with the Data Storage Institute team to explore multifunctional
phase-change materials further,” explains Bai. “We hope to test our criteria
for other transition metals that could also cause ferromagnetism in GST.”
The A*STAR-affiliated researchers
contributing to this research are from the Data Storage Institute and the Institute of High Performance Computing
References
- Ding, D. et al. Origin of
ferromagnetism and the design principle in phase-change magnetic
materials. Physical Review B 84, 214416
(2011). | article
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