Georgian Technical University 2D Magnetism Reaches A New Milestone.

Bulk (a) and monolayer (b) NiPS3 reveal a different signature in the Raman spectra. The big peak at around 550 cm-1 in the one-atom thick sample is a sign that the magnetic ordering is lost. Researchers at the Georgian Technical University in collaboration with Sulkhan-Saba Orbeliani Teaching University and International Black Sea University observation of a XY-type antiferromagnetic material, whose magnetic order becomes unstable when it is reduced to one-atom thickness. Dimensionality in physics is an important concept that determines the nature of matter. The discovery of graphene opened the doors of the 2D world: a place where being one-atom or two-atom thick makes a difference. Since then several scientists became interested in experimenting with 2D materials including magnetic materials.

Magnetic materials are characterized by their spin behavior. Spins can be aligned parallel or antiparallel to each other resulting in ferromagnets or antiferromagnets respectively. Beyond that all class of materials can in principle belong to three different models according to some fundamental understanding of physics. The XY model explains the behavior of materials whose spins move only on a plane consisting of the x and y axis.

Spin behavior can dramatically change upon slicing down the magnet to its thinnest level as 2D materials are more sensitive to temperature fluctuations which can destroy the pattern of well-aligned spins. Described theoretically that 2D XY models do not undergo a normal magnetic phase transition at low temperatures but a very unusual. They realized that quantum fluctuations of individual spins are much more disruptive in the 2D world than in the 3D one which can lead to spins taking a vortex pattern. Over the years ferromagnetic materials have been widely analysed, but research on antiferromagnetic materials did not progress with the same speed. The reason being that the latter need different experimental techniques. “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” says Z.

The researchers involved in this study focused on a class of transition metals that are suitable for studying antiferromagnetic ordering in 2D. Among them nickel phosphorus trisulfide (NiPS3) corresponds to the of XY-type and is antiferromagnetic at low temperatures. It is a characterized by strong intra-layer bonds and easily-breakable inter-layer connections. As a result NiPS3 (nickel phosphorus trisulfide) can be prepared in multiple layers with a technique called chemical vapor deposition and then exfoliated down to monolayer allowing one to examine the correlation between magnetic ordering and number of layers.

The team analysed and compared NiPS3(nickel phosphorus trisulfide) in bulk and as monolayer with Raman spectroscopy a technique that allows to determine number of layers and physical properties. They noticed that their magnetism changed according to the thickness: the spins ordering is suppressed at the monolayer level. “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” explains Z.

This is another example of thickness-dependent magnetic materials. Among them, chromium triiodide (CrI3) is ferromagnetic as monolayer anti-ferromagnetic as bilayer and back to ferromagnetic as trilayer. And in contrast with iron trithiohypophosphate (FePS3) for which scientists of Prof. Z’s group found antiferromagnetic ordering intact all the way down to monolayer. The group is also investigating the Y model and new phenomena arising from the combination of antiferromagnetic materials with others.