Ising on the 2D cake: Raman in transition

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  • Published: Jan 6, 2017
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Ising on the 2D cake: Raman in transition

Magnetic confusion

Raman spectroscopy on bulk and 2D FePS3 was used to calculate changes in vibration and indirectly magnetization.

Scientists in South Korea have used Raman spectroscopy to obtain the first experimental proof of a physics theory proposed more than 70 years ago.

Je-Geun Park, Associate Director of the Center for Correlated Electron Systems, within the Institute for Basic Science (IBS), working in collaboration with Hyeonsik Cheon of Sogang University and Cheol-Hwan Park of Seoul National University have demonstrated the magnetic behaviour of a special class of two-dimensional materials. They describe their findings in the journal Nano Letters.

2D materials, which includes the monolayer carbon material, graphene, have some intriguing physical and chemical properties which have made them the focus of much research over the last few years not only because they are of fundamental scientific interest but also because of their potential in optical, electronic, catalytic and other applications. The study of 2D materials as opposed to their 3D bulk counterparts beggars many questions regarding magnetic properties, for instance, and the transitions the materials might undergo in changing from one particular set of characteristics to another, paramagnetic to antiferromagnetic, for example.

In transition

Indeed, at extremely low temperatures some materials have a particular arrangement or alignment of electronic spins, but above a threshold the total energy is different and the type of magnetism in the material changes or the alignment is randomized altogether and magnetism lost. In 3D magnetic materials this is a common phenomenon seen above the material's critical temperature. However, science has only recently had the materials and the tools to investigate what happens at the critical temperature in a 2D sheet, or indeed, a 1D chain at low temperatures. Can these phase transitions occur in a 2D lattice at all?

It was approximately a century ago, that physicist Wilhelm Lenz asked his student Ernst Ising to solve this problem for 1D systems. Ising explained it in 1925 and concluded that 1D materials cannot undergo phase transitions. However, he was unable to make the same assertion for a particular type of 2D material. It was not until the work of Lars Onsager in 1943 that it was suggested with some certainty that materials that follow the Ising spin model do have a phase transition. Nevertheless, this was mere theory, no experimental proof was forthcoming at the time.

Spin cycle

"The physics of 2D systems is unique and exciting. The Onsager solution is taught on every advanced statistical mechanics course. That's where I learned this problem. However, when I discovered much later that it has not been tested experimentally with a magnetic material, I thought it was a shame for experimentalists like me, so it was natural for me to look for a real material to test it," explains IBS's Park.

In order to prove the Onsager model, the researchers made crystals of iron trithiohypophosphate using the chemical vapour transport technique. The resulting crystals contain layers bound together by relatively weak van der Waals interactions and so can be peeled apart like layers of wallpaper on the walls of an old house. Using sticky tape, the scientists peeled off successive layers until they were left with just monolayer of the material, the 2D material.  "They are magnetic van der Waals materials or we can call it magnetic graphene as they form a true monolayer of Fe forming a honeycomb lattice just like carbon atoms in graphene. They are very rare, and their physics is still unexplored," he adds.

There are countless analytical tools for measuring the magnetic properties of bulk 3D materials but only Raman spectroscopy would fit the requirements for measuring the properties of a monolayer material. Raman spectroscopy can determine vibrational energies and as such these were measured and used as a proxy of magnetism, the greater the vibration, the less the magnetization.

Initially, the team obtained the Raman spectra of bulk 3D FePS3 at different temperatures and then experimented with a 2D monolayer of the material. "The test with the bulk sample showed us that the Raman signals can be used as a kind of the fingerprint of phase transition at temperatures around 118 Kelvin," Park says. "With this confirmation we then measured the monolayer sample and found the same patterns," he adds. "We conclude that 3D and 2D FePS3 have the same signature of the phase transition visible in the Raman spectrum."

The team has thus showed that in the bulk sample as with the monolayer spins are ordered, making the material antiferromagnetic, at very low temperatures but accumulate disorder and so become paramagnetic above this temperature. "Showing a magnetic phase transition with this tour-de-force experiment is a beautiful test for the Onsager solution," Park says. Next, the team will investigate whether or not 2D equivalents of various transition metal materials will also go beyond the 2D Ising spin model.

Related Links

Nano Lett, 2017, online: "Ising-Type Magnetic Ordering in Atomically Thin FePS3"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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