Scientists discover unusual ultrafast motion in layered magnetic materials

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.