Probing the properties of magnetic quasi-particles

July 28, 2020

Researchers have for the first time measured a fundamental property of magnets called magnon polarisation -- and in the process, are making progress towards building low-energy devices.

The existence of magnon polarisation has been a theoretical idea in physics for almost 100 years but no one has proved its existence.

Scientists at the University of Leeds and the Tohoku University in Japan set out to try and show it exists by measuring it. Their findings have just been published in the journal Physical Review Letters (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.027201).

Magnons are quasi-particles inside of magnetic materials which are in a continuous process of creation and destruction. They are polarised which allows then to be distinguished as clockwise or anticlockwise (circular polarisation), up or down - and left or right (linear polarisation).

There is intense interest in the polarisation properties of magnons because physicists believe it could be exploited for transporting information in low-energy electrical devices, a field of study called spintronics.

The scientists aimed to measure magnon polarisation in one of the most frequently used magnets in spintronics research, the compound yttrium iron garnet. In many magnets, only anticlockwise magnons exist. But in yttrium iron garnet, both anticlockwise and clockwise polarised magnons were predicted, making it a particularly exciting material to measure.

The team set out to make this measurement using polarised neutron scattering. This involves preparing neutrons in a specific quantum spin state ("up" or "down") and firing them at a magnet in a focused beam.

In the experiment, most neutrons passed straight through the magnet, not interacting at all - making measurements particularly difficult. But, a small number of the neutrons collided with magnons and scattered out of the magnet in all directions. A detector measured the neutrons as they flew out of the sample. By analysing the location, energy and final spin state of the neutrons, the magnon properties were revealed.

Crucially in this work, by comparing the spin state of the neutrons before and after the scattering, the clockwise or anti-clockwise polarisation of the magnons was determined.

Dr Joseph Barker, from the School of Physics and Astronom at Leeds, said: "In Physics, theories remain as predictions until experimental measurements confirm if they are correct or not. A famous example is the search for the Higgs Boson, but there are many untested theories across the sciences.

"Magnon polarisation has recently become an important topic in spintronics so it was the perfect time to try and measure it and verify that it exists."

Dr Barker added: "The experiments and analysis were difficult and complex. In fact, it took two attempts, once in the United States and then in France, to perfect the experimental method.

"We also had to create a precise computer model to ensure we understood what we were seeing correctly because the neutron scattering measurements come from a series of physical processes which cannot be untangled into individual parts."

Researchers can now focus their studies on how to exploit the polarisation of magnons for making new types of spintronic devices for low energy technology.

The research was funded by The Royal Society, Japan Society for the Promotion of Science Grant-in-Aid for Scientific Research, JST ERATO, Tohoku University GP-Spin Program, US Department of Energy and the US-Japan Cooperative Program on neutron scattering.
-end-
Notes to editors

For further information, please contact David Lewis in the University of Leeds press office: d.lewis@leeds.ac.uk

University of Leeds

The University of Leeds is one of the largest higher education institutions in the UK, with more than 38,000 students from more than 150 different countries, and a member of the Russell Group of research-intensive universities. The University plays a significant role in the Turing, Rosalind Franklin and Royce Institutes.

We are a top ten university for research and impact power in the UK, according to the 2014 Research Excellence Framework, and are in the top 100 of the QS World University Rankings 2020. Additionally, the University was awarded a Gold rating by the Government's Teaching Excellence Framework in 2017, recognising its 'consistently outstanding' teaching and learning provision. Twenty-six of our academics have been awarded National Teaching Fellowships - more than any other institution in England, Northern Ireland and Wales - reflecting the excellence of our teaching.

Over a third of our academics are involved in applied research or as consultants to industry, and over the last ten years, the University of Leeds has produced more than 100 'spin-out' companies.

http://www.leeds.ac.uk

University of Leeds

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