Unusual Crystal Surprises Physicists with Cooling Effect

Natural crystals have long captivated us with their vivid colors, flawless geometry, and striking symmetry. But for scientists, these beautiful formations offer more than just visual delight. Hidden within their structures are often rare and powerful properties like unusual magnetism, the journal Physical Review Letters reported.

One such crystal is atacamite, a brilliant emerald-green mineral first discovered in Chile’s Atacama Desert. While its striking color comes from its copper content, what truly excites researchers is its magnetocaloric effect. This means that when atacamite is exposed to a magnetic field, its temperature changes, a rare and valuable behavior that could help shape the future of cooling technologies.

Now, a research team led by TU Braunschweig and Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has taken a closer look at this intriguing phenomenon. Their goal? To better understand how materials like atacamite respond to magnetism — and to one day use that knowledge to create energy-efficient magnetic refrigeration systems.

At the heart of atacamite’s magnetic behavior are its copper ions. Each one carries an unpaired electron, which acts like a tiny magnetic needle, or compass. Together, these spins interact with magnetic fields in a way that makes the entire crystal behave in fascinating and potentially useful ways.

“The distinct feature of atacamite is the arrangement of the copper ions,” explains Dr. Leonie Heinze of Jülich Centre for Neutron Science (JCNS).

“They form long chains of small, linked triangles known as sawtooth chains.” This geometric structure has consequences: although the copper ions’ spins always want to align themselves antiparallel to one another, the triangular arrangement makes this geometrically impossible to achieve completely. “We refer to this as magnetic frustration,” continues Heinze. As result of this frustration, the spins in atacamite only arrange themselves at very low temperatures – under 9 Kelvin (−264°C) – in a static alternating structure.

When the researchers examined atacamite under the extremely high magnetic fields at HZDR’s High Magnetic Field Laboratory (HLD), something surprising emerged: the material exhibited a noticeable cooling in the pulsed magnetic fields – and not just a slight one, but a drop to almost half of the original temperature. This unusually strong cooling effect particularly fascinated the researchers, as the behavior of magnetically frustrated materials in this context has scarcely been studied. However, magnetocaloric materials are considered a promising alternative to conventional cooling technologies, for example for energy-efficient cooling or the liquefaction of gases. This is because, instead of compressing and expanding a coolant – a process that takes place in every refrigerator – they can be used to change the temperature by applying a magnetic field in an environmentally friendly and potentially low-loss approach.

Additional studies at various labs of the European Magnetic Field Laboratory (EMFL) provided more in-depth insights. “By using magnetic resonance spectroscopy, we were clearly able to demonstrate that the magnetic order of atacamite is destroyed when a magnetic field is applied,” explains Dr. Tommy Kotte, a scientist at HLD. “This is unusual as the magnetic fields in many magnetically frustrated materials usually counteract the frustration and even encourage ordered magnetic states.”

The team found the explanation for the mineral’s unexpected behavior in complex numerical simulations of its magnetic structure: while the magnetic field aligns the copper ions’ magnetic moments on the tips of the sawtooth chains along the field and thus reduces the frustration as expected, it is precisely these magnetic moments that mediate a weak coupling to neighboring chains. When this is removed, a long-range magnetic order can no longer exist. This also provided the team with an explanation for the particularly strong magnetocaloric effect: it always occurs when a magnetic field influences the disorder – or more precisely, the magnetic entropy – of a system. In order to compensate for this rapid change in entropy, the material has to adjust its temperature accordingly. This is the very mechanism the researchers have now managed to demonstrate in atacamite.

“Of course, we do not expect atacamite to be extensively mined in the future for use in new cooling systems,” says Dr. Tommy Kotte, “but the physical mechanism we have investigated is fundamentally new and the magnetocaloric effect we observed is surprisingly strong.” The team hopes their work will inspire further research, especially a targeted search for innovative magnetocaloric materials within the extensive class of magnetically frustrated systems.

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