A magnet will take a standard metal paper clip as a hold. These
iron-containing substances are referred to as ferromagnets by scientists.
Albert Einstein and Wander de Haas, two scientists, described a startling
occurrence using a ferromagnet a little more than a century ago. If you just
switch the magnetic field's direction while an iron cylinder is suspended
from a wire and exposed to it, it will begin to rotate.
According to Haidan Wen, a physicist of the Materials Science and X-ray
Science divisions of the U.S. Department of Energy's (DOE) Argonne National
Laboratory, "Einstein and de Haas's experiment is almost like a magic show."
"You don't need to ever touch a cylinder to make it rotate,"
A group of scientists from Argonne and other American national laboratories
and universities have just published a study in Nature describing an
identical but distinct behavior in a "anti"-ferromagnet. This may be useful
for machines that need extremely quick and accurate motion control.
High-speed nanomotors are one example that may be used in biomedical
applications, such as in nanorobots for minimally invasive surgery and
diagnostics.
The distinction between ferromagnets and antiferromagnets is based on an
attribute known as electron spin. This rotation has a purpose. Scientists
use an arrow to depict the direction, which might point up, down, or in any
other direction in between. The arrows connected to each iron atom's
electrons can all point in the same direction, say up, in the previously
stated magnetized ferromagnet. The direction of the electron spins is
reversed when the magnetic field is turned around. Thus, every arrow is
heading downward. The rotation of the cylinder is caused by this
reversal.
"In this experiment, an electron's spin, a microscopic property, is
exploited to elicit a mechanical response in a cylinder, a macroscopic
object," explained Alfred Zong, a Miller Research Fellow at the University
of California, Berkeley.
In antiferromagnets, for instance, the electron spins alternate from up to
down between neighboring electrons rather of always pointing up.
Antiferromagnets do not react to changes in a magnetic field as ferromagnets
do because their opposing spins cancel each other out.
"The question we asked ourselves is, can electron spin elicit in an
antiferromagnet a response that is different but similar in spirit to that
from the cylinder rotation in the Einstein-de Hass experiment?" said
Wen.
The researchers created a sample of the antiferromagnet iron phosphorus
trisulfide (FePS3) to provide an answer to that query. Each layer of the
sample, which was made up of many FePS3 layers, was only a few atoms
thick.
According to Xiaodong Xu, professor of physics and materials science at the
University of Washington, "FePS3 is special because it is formed in a
layered structure, in which the interaction between the layers is extremely
weak."
Wen said, "We constructed a set of corroborative experiments in which we
irradiated this layered material with ultrafast laser pulses and assessed
the resulting changes in material characteristics using optical, X-ray, and
electron pulses.
The researchers discovered that the pulses alter the material's magnetic
properties by jumbling the ordered orientation of electron spins. Instead of
alternating between up and down in a systematic way, the arrows representing
electron spin are now disorganized.
The entire sample experiences a mechanical response as a result of the
electron spin jumbling. The Massachusetts Institute of Technology's (MIT)
Nuh Gedik, a professor of physics, noted that one layer of the sample is
able to slide back and forth with regard to a neighboring layer because the
connection between layers is minimal.
The oscillation time for this motion is extremely short—10 to 100
picoseconds. The definition of a picosecond is one trillionth of a second.
Light only moves a third of a millimeter in one picosecond because of how
quickly this happens.
World-class scientific facilities are needed to conduct measurements on
samples with atomic-scale spatial resolution and picosecond-scale temporal
resolution. The scientists used state-of-the-art ultrafast probes that
analyze atomic structures using electron and X-ray beams to achieve
this.
The early experiments used the mega-electronvolt ultrafast electron
diffraction equipment at SLAC National Accelerator Laboratory and were
inspired by optical observations at the University of Washington. At MIT's
ultrafast electron diffraction setup, more research was done. Work at the
11-BM and 7-ID beamlines at the Advanced Photon Source (APS) and the
ultrafast electron microscopy facility in the Center for Nanoscale Materials
(CNM) added to these findings. At Argonne, CNM and APS are both DOE Office
of Science user facilities.
A multilayer antiferromagnet also experiences effects from the electron
spin at durations longer than picoseconds. Members of the team found that
fluctuating movements of the layers significantly slowed down around the
point when the electron spins switched from disordered to ordered behavior
in a previous work employing APS and CNM equipment.
The key finding in the current study, according to Zong, was the connection
between electron spin and atomic motion that is unique to the layered
structure of this antiferromagnet. The capacity to regulate this motion by
altering the magnetic field or, alternatively, by applying a little strain
will have enormous consequences for nanoscale devices since this connection
emerges at such rapid time scales and miniscule length scales.
Provided by
Argonne National Laboratory
