Determine 1: Spins at play. Electrons’ spins in several supplies may be constrained in sure orientations. The Ising mannequin offers with spins that are pointing both up or down. The XY mannequin explains the habits of supplies the place the spins are free to maneuver solely on the x and y axis, and the Heisenberg mannequin is about spins that time in any path, very similar to clock fingers. Credit score: Institute for Fundamental Science

Researchers on the Middle for Correlated Electron Techniques, inside the Institute for Fundamental Science (IBS) in South Korea, in collaboration with Sogang College and Seoul Nationwide College, reported the primary experimental remark of a XY-type antiferromagnetic materials, whose magnetic order turns into unstable when it’s diminished to one-atom thickness. Printed in Nature Communications, these findings are according to theoretical predictions relationship again to the 1970s.

Dimensionality in physics is a vital idea that determines the character of matter. The invention of graphene opened the doorways of the 2-D world: a spot the place being one-atom or two-atom thick makes a distinction. Since then, a number of scientists grew to become thinking about experimenting with 2-D supplies, together with magnetic supplies.

Magnetic supplies are characterised by their spin habits. Spins will be aligned parallel or antiparallel to one another, leading to ferromagnets or antiferromagnets, respectively. Past that, all class of supplies can, in precept, belong to 3 completely different fashions in response to some basic understanding of physics: Ising, XY or Heisenberg. The XY mannequin explains the habits of supplies whose spins transfer solely on a airplane consisting of the x and y axis.

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Spin habits can dramatically change upon slicing down the magnet to its thinnest degree, as 2-D supplies are extra delicate to temperature fluctuations, which might destroy the sample of well-aligned spins. Virtually 50 years in the past, John M. Kosterlitz and David J. Thouless, and Vadim Berezinskii independently, described theoretically that 2-D XY fashions don’t endure a traditional magnetic section transition at low temperatures, however a really uncommon type, later known as BKT transition. They realized that quantum fluctuations of particular person spins are rather more disruptive within the 2-D world than within the 3-D one, which might result in spins taking a vortex sample. Kosterlitz and Thouless have been awarded the Nobel Prize in Physics in 2016.

Through the years, ferromagnetic supplies have been extensively analysed, however analysis on antiferromagnetic supplies didn’t progress with the identical velocity. The reason is that the latter want completely different experimental strategies. “Regardless of the curiosity and theoretical foundations, nobody has ever experimented with it. The principle motive for that is that it is rather troublesome to measure intimately the magnetic properties of such a skinny antiferromagnetic materials,” says PARK Je-Geun, main writer of the publication.

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Determine 2: The comparability between bulk and monolayer NiPS3 at completely different temperatures. Bulk (a) and monolayer (b) NiPS3 reveal a special signature within the Raman spectra. The massive peak at round 550 cm-1 within the one-atom thick pattern is an indication that the magnetic ordering is misplaced. Credit score: Institute for Fundamental Science

The researchers concerned on this examine centered on a category of transition metals which are appropriate for finding out antiferromagnetic ordering in 2-D. Amongst them, nickel phosphorus trisulfide (NiPS3) corresponds to the of XY-type and is antiferromagnetic at low temperatures. Additionally it is a van der Waals materials, characterised by sturdy intra-layer bonds, and easily-breakable inter-layer connections. In consequence, NiPS3 will be ready in a number of layers, with a method known as chemical vapor deposition, after which exfoliated all the way down to monolayer, permitting one to look at the correlation between magnetic ordering and variety of layers.

The workforce analysed and in contrast NiPS3 in bulk and as monolayer with Raman spectroscopy, a method that enables to find out variety of layers and bodily properties. They observed that their magnetism modified in response to the thickness: the spins’ ordering is suppressed on the monolayer degree.

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“The fascinating factor is the drastic change between the bilayer and the monolayer. At first look, there might not be a giant distinction between the 2, however the impact of transferring from two dimensions to 3 dimensions causes their bodily properties to flip abruptly,” explains Park.

That is one other instance of thickness-dependent magnetic supplies. Amongst them, chromium triiodide (CrI3) is ferromagnetic as monolayer, anti-ferromagnetic as bilayer, and again to ferromagnetic as trilayer. And in distinction with iron trithiohypophosphate (FePS3), for which IBS scientists of Prof. Park’s group present in 2016 that it retains its antiferromagnetic ordering intact all the best way all the way down to monolayer.

The group can be investigating the Heisenberg mannequin, and new phenomena arising from the mixture of antiferromagnetic supplies with others.

Discover additional:
Flexy, flat and purposeful magnets

Extra info:
Kangwon Kim et al. Suppression of magnetic ordering in XXZ-type antiferromagnetic monolayer NiPS3, Nature Communications (2019). DOI: 10.1038/s41467-018-08284-6

Journal reference:
Nature Communications

Supplied by:
Institute for Fundamental Science


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