2D magnetism takes a new step

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image: Bulk (a) and monolayer (b) NiPS3 reveal different signature in Raman spectra. The large peak at about 550 cm-1 in the sample one atom thick is a sign that the magnetic order is lost.
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Credit: IBS

Researchers at the Center for Correlated Electron Systems, at the Institute for Basic Science (IBS) in South Korea, in collaboration with Sogang University and Seoul National University, reported the first experimental observation of a XY type antiferromagnetic material, the magnetic order of which becomes unstable. when reduced to an atomic thickness. Posted in Nature Communication, these results are consistent with theoretical predictions dating back to the 1970s.

Dimensionality in physics is an important concept that determines the nature of matter. The discovery of graphene opened the doors to the 2D world: a place where a thickness of one or two atoms makes all the difference. Since then, several scientists have become interested in experimenting with 2D materials, including magnetic materials.

Magnetic materials are characterized by their spin behavior. The spins can be aligned parallel or antiparallel to each other, resulting in ferro-magnets or antiferromagnetics, respectively. Beyond that, all classes of materials can, in principle, belong to three different models according to a fundamental understanding of physics: Ising, XY or Heisenberg. The XY model explains the behavior of materials whose spins move only on a plane made up of the x and y axes.

The behavior of the spins can change dramatically when cutting the magnet to its finest level, as 2D materials are more sensitive to temperature fluctuations, which can destroy the pattern of well-aligned spins. Almost 50 years ago, John M. Kosterlitz and David J. Thouless, and Vadim Berezinskii independently, theoretically described that 2D XY models do not undergo a normal magnetic phase transition at low temperature, but a very unusual shape, later called BKT transition. They realized that the quantum fluctuations of individual spins are much more disruptive in the 2D world than in the 3D world, which can lead to spins taking a vortex pattern. Kosterlitz and Thouless received the Nobel Prize in Physics in 2016.

Over the years, ferromagnetic materials have been widely analyzed, but research into antiferromagnetic materials has not progressed at the same speed. The reason being that the latter require different experimental techniques. “Despite the interest and the theoretical foundations, no one has ever experienced it. The main reason is that it is very difficult to measure the magnetic properties of such a thin antiferromagnetic material in detail,” says PARK Je-Geun. , main author. of the publication.

The researchers involved in this study focused on a class of transition metals suitable for the study of the antiferromagnetic order in 2D. Among them, nickel phosphorus trisulfide (NiPS3) corresponds to type XY and is antiferromagnetic at low temperature. It is also a van der Waals material, characterized by strong intra-layer bonds and easily breakable inter-layer connections. As a result, NiPS3 can be prepared in multiple layers, with a technique called chemical vapor deposition, and then exfoliated as a single layer, allowing the correlation between magnetic order and the number of layers to be examined.

The team analyzed and compared bulk and monolayer NiPS3 with Raman spectroscopy, a technique that helps determine the number of layers and physical properties. They noticed that their magnetism changes according to the thickness: the order of the spins is suppressed at the level of the monolayer.

“The interesting thing is the drastic change between bilayer and monolayer. At first glance, there might not be a big difference between the two, but the effect of switching from two dimensions to three dimensions suddenly switches their physical properties, ”Park explains.

This is another example of thickness dependent magnetic materials. Among them, chromium triiodide (CrI3) is monolayer ferromagnetic, bilayer antiferromagnetic and trilayer ferromagnetic. And unlike iron trithiohypophosphate (FePS3), for which IBS scientists in Professor Park’s group discovered in 2016 that it keeps its antiferromagnetic order intact down to the monolayer.

The group is also studying the Heisenberg model and new phenomena resulting from the combination of antiferromagnetic materials with others.

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