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First Experimental Proof of Strange XY-Type Magnetic-Behavior Transition in Magnetic Materials

The first-ever experimental observation of an XY-type antiferromagnetic material, the magnetic order of which turns unstable upon being reduced to a thickness of single atom, has been reported by scientists from the Center for Correlated Electron Systems, of the Institute for Basic Science (IBS) in South Korea, in collaboration with Sogang University and Seoul National University.

Spins at play. Electrons’ spins in different materials might be constrained in certain orientations. The Ising model deals with spins which are pointing either up or down. The XY model explains the behavior of materials where the spins are free to move only on the x-axis and y-axis, and the Heisenberg model is about spins that point in any direction, much like clock hands. (Image credit: Institute for Basic Science)

The outcomes of the study have been reported in Nature Communications and are consistent with theoretical predictions dating back to the 1970s.

In physics, dimensionality is an important concept that governs the nature of matter. The path toward the 2D realm was paved by the discovery of graphene. In the 2D realm, having a thickness of one atom or two atoms makes a huge difference. From that time, a number of researchers became curious to experiment with 2D materials, including magnetic materials.

Magnetic materials exhibit spin behavior. The spins can be aligned either parallel or antiparallel to one another, giving rise to ferromagnets or antiferromagnets, respectively. Apart from that, all categories of materials can, theoretically, fall in three different models based certain basic understanding of physics: XY, Ising, or Heisenberg. In the XY model, the behavior of materials with spins moving only on a plane containing the x-axis and y-axis is explained.

When the magnet is sliced down to its thinnest level, there can be a drastic change in the spin behavior since 2D materials are highly sensitive to fluctuations in temperature, which can ruin the pattern of well-aligned spins. Nearly five decades ago, John M. Kosterlitz and David J. Thouless, and Vadim Berezinskii independently, theoretically proposed that at low temperatures, 2D XY models do not experience a normal magnetic phase transition but a very strange form, which was later termed the BKT transition. They understood that quantum fluctuations of individual spins are far more turbulent in the 2D realm compared to that in the 3D one, which can result in spins assuming a vortex pattern. In 2016, the Nobel Prize for Physics was awarded to Kosterlitz and Thouless for their study.

Although ferromagnetic materials have been extensively studied over the years, studies on antiferromagnetic materials did not advance at the same rate. The reason is that the latter requires distinct experimental methods.

Despite the interest and theoretical foundations, no one has ever experimented with it. The main reason for this is that it is very difficult to measure in detail the magnetic properties of such a thin antiferromagnetic material.

Park Je-Geun, Study Lead Author, Seoul National University.

The focus of the scientists involved in this research was on a category of transition metals that are apt for the analysis of antiferromagnetic ordering in 2D. Of all, nickel phosphorus trisulfide (NiPS3) corresponds to the XY-type and at low temperatures, it behaves as an antiferromagnet. Since it is characterized by easily breakable inter-layer connections and strong intra-layer bonds, it is also a van der Waals material. Consequently, a method known as chemical vapor deposition can be used to develop NiPS3 in multiple layers and then exfoliate down to monolayer, enabling the analysis of the correlation between the number of layers and magnetic ordering.

The researchers used Raman spectroscopy to analyze and compare NiPS3 in bulk and as a monolayer. This technique enables the number of layers and physical properties to be determined. They observed that their magnetism varied based on the thickness: at the monolayer level, the ordering of the spins is suppressed.

The interesting thing is the drastic change between the bilayer and the monolayer. At first glance, there may not be a big difference between the two, but the effect of moving from two dimensions to three dimensions causes their physical properties to flip abruptly.

Park Je-Geun, Study Lead Author, Seoul National University.

This is another example of magnetic materials that are thickness-dependent. Of all such materials, chromium triiodide (CrI3) behaves ferromagnetic as a monolayer, anti-ferromagnetic as a bilayer, and back to ferromagnetic as a trilayer. Moreover, this is contrary to iron trithiohypophosphate (FePS3), in which case IBS researchers belonging to Prof. Park’s group discovered in 2016 that it retains its antiferromagnetic ordering intact all the way down to monolayer.

The research team is also analyzing the Heisenberg model as well as new phenomena exhibited when antiferromagnetic materials are combined with others.

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