Natural crystals fascinate with their vibrant colors, their nearly flawless appearance and their manifold symmetrical forms. But researchers are interested in them for quite different reasons: among the countless minerals already known, they always discover some materials with unusual magnetic properties. One of these is atacamite, which exhibits magnetocaloric behavior at low temperatures – that is, the material's temperature changes significantly when it is subjected to a magnetic field. A team headed by TU Braunschweig and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has now investigated this rare property (DOI: 10.1103/PhysRevLett.134.216701). In the long term, the results could help to develop new materials for energy-efficient magnetic cooling.
The emerald-green mineral atacamite, named for the place it was first found, the Atacama Desert in Chile, gets its characteristic coloring from the copper ions it contains. These ions also determine the material's magnetic properties: they each have an unpaired electron whose spin gives the ion a magnetic moment – comparable to a tiny needle on a compass. "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.
What is the origin of this strong magnetocaloric effect?
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.
Publication:
L. Heinze, T. Kotte, R. Rausch, A. Demuer, S. Luther, R. Feyerherm, E. L. Q. N. Ammerlaan, U. Zeitler, D. I. Gorbunov, M. Uhlarz, K. C. Rule, A. U. B. Wolter, H. Kühne, J. Wosnitza, C. Karrasch, S. Süllow, Atacamite Cu₂Cl(OH)₃ in High Magnetic Fields: Quantum Criticality and Dimensional Reduction of a Sawtooth-Chain Compound, in Physical Review Letters, 2025 (DOI: 10.1103/PhysRevLett.134.216701)
Additional information:
Dr. Leonie Heinze
Institute of Condensed Matter Physics
TU Braunschweig
and
Jülich Centre for Neutron Science (JCNS) at the Heinz Maier-Leibnitz Zentrum (MLZ)
Forschungszentrum Jülich GmbH
