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Hoffmann: Heat reveals hidden magnetic secrets

1/7/2021

A new paper by Professor Axel Hoffmann and researchers at the Argonne National Laboratory reveals insights into atomic movement in antiferromagnets, special magnetic materials that may provide a novel way for storing and manipulating information.

“Antiferromagnets are much harder to understand than your well-known fridge-type magnet, the ferromagnet,” said Axel Hoffmann, Founder Professor of Materials Science and Engineering.

The magnetic properties of typical ferromagnets are determined by small magnetic moments associated with individual atoms that when added up have a magnetic direction. In antiferromagnets, on the other hand, these magnetic moments pair up so that two adjacent moments cancel each other out.  

“This results in the absence of an overall net magnetization and therefore makes it hard to gain direct insights into the magnetic states. In fact, when antiferromagnets were theoretically predicted by Louis Néel about a century ago, it took two decades before they were experimentally verified,” he said. 

Revealing the hidden order of antiferromagnets still remains a major challenge, but it turns out that one solution is very similar to exposing secret messages written with lemon juice.  As many children know, if you write a message with lemon juice it becomes invisible after the juice dries but can be made visible again by heating the text up, which causes the acid in the lemon juice to degrade and become visible again. 

Schematic of a heat current via magnetic excitations within the antiferromagnet Cr2O3 sandwiched between two platinum (Pt) layers.  As the magnetic field is rotated from being parallel to the heat current and antiparallel there are different voltages developing on both ends of the antiferromagnet, which are characteristic for the behavior of either the blue or red magnetic moments, respectively.
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Schematic of a heat current via magnetic excitations within the antiferromagnet Cr2O3 sandwiched between two platinum (Pt) layers.  As the magnetic field is rotated from being parallel to the heat current and antiparallel there are different voltages developing on both ends of the antiferromagnet, which are characteristic for the behavior of either the blue or red magnetic moments, respectively.

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In order to uncover the secret magnetic order, the researchers turned up the heat. They used a single crystalline film of chromia (an oxide of chromium) together with additional platinum layers at its top and bottom surfaces. By heating this layered stack from one side, they generated a heat current across each of the interfaces, which would then drive a magnetic spin current as well, resulting in characteristic electric voltages known as the spin Seebeck effect. 

Surprisingly, the dependence of the electric voltages on applied magnetic fields was different for the top and bottom surfaces. By comparing this behavior to numerical simulations, it became clear that the voltages are individually indicative of each component of the magnetic moment pairs. 

“We found that, even though the heat current is carried by magnetic excitations throughout the whole thickness of the antiferromagnetic layer, only the very last magnetic moments at the surface determine the electric voltage response,” said Hoffmann.

The finding is good news for other researchers focused on antiferromagnets. Getting information about individual magnetic spins can require large scale facilities, such as for neutron scattering, or very specialized equipment, such as spin polarized scanning tunneling microscopy. The technique used here can be done at a small scale.

“Here we present a method that can be adopted by almost anybody who works on antiferromagnetic or other magnetic materials.” said Hoffmann. “Generalizing these ideas to other antiferromagnetic materials may reveal many other exciting secrets that they could possibly harbor!”

Professor Axel Hoffmann is also affiliated with Materials Research Laboratory and the Illinois Materials Research Science and Engineering Center (i-MRSEC). He is currently an Associate Editor of the Journal of Applied Physics.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.

The full article is available here:  https://journals.aps.org/prb/abstract/10.1103/PhysRevB.103.L020401