Frustrated, more frustrated, more complicated International research team discovers unusually strong competition between magnetic states in the mineral clinoatacamite

What is a fascinating mineral for collectors becomes a subject of study under extreme conditions for researchers: an international team, with Professor Stefan Süllow at the head of it, is investigating mineral samples at temperatures close to absolute zero and in strong magnetic fields. In doing so, the researchers are exploring how novel magnetic states arise and compete with one another. A recently published study now shows how an unusual competitive effect in the mineral clinoatacamite causes particularly strong magnetic disorder. Research into magnetically frustrated materials such as clinoatacamite and such unusual spin states could be of great significance in the future for quantum technology applications, particularly in the field of quantum computing. The study, in which Jochen Litterst, former president of TU Braunschweig, was also actively involved, has now been published by the researchers in the highly prestigious journal Physical Review Letters.

The dark-green mineral clinoatacamite (Cu₂Cl(OH)₃) has a major problem from a magnetic point of view: it is frustrated. The exchange couplings between the spins of the magnetic ions – a kind of angular momentum – do not match their geometric arrangement in triangular structures. “In clinoatacamite, the magnetic copper ions are arranged in what is known as a Kagome lattice,” explains Dr. Leonie Stödter, winner of the Heinrich Büssing Prize, who is currently conducting postdoctoral research at the Jülich Centre for Neutron Science at the research centre Jülich in Garching. “They are subject to an alternating spin arrangement, which they are, however, unable to realise within the triangles.” In Kagome lattices – named after a Japanese woven basket pattern – the competition between the spins can be so severe that no magnetic order emerges, even at the very lowest temperatures. This disordered state is referred to as a ‘quantum spin liquid’.

However, the arrangement in the Kagome lattice is not the only reason why the mineral is magnetically frustrated and why magnetic order can only emerge at very low temperatures of eighteen kelvin, i.e. –255 degrees Celsius. This has been demonstrated by calculations carried out by the research team. “The Kagome lattice in clinoatacamite is slightly distorted. The antiferromagnetic couplings are therefore of varying strengths, meaning that certain arrangements of the spins are energetically more favourable and are preferred,” explains Carolin Kastner, who is completing her PhD at the Institute of Condensed Matter Physics at TU Braunschweig.

In addition to the competition within Kagome lattices, the authors identified an additional competitive effect: different local environments around the copper sites mean that several different magnetic arrangements can be energetically similar, causing the spin arrangements to compete with one another. What is more, this can lead to unusual magnetic states in which different spin arrangements coexist or alternate.

Based on experimental investigations, in particular using large-scale research facilities that provide experimental measurement techniques such as neutron scattering and muon spin rotation, the research team demonstrated that such coexistence of different spin arrangements occurs at temperatures below six kelvin, i.e. –267 degrees Celsius. “Clinoatacamite is an example of magnetic frustration at a higher, non-local level,” summarises Dr. Stödter.

“The study highlights a possible way in which spin-liquid states can be stabilised relative to conventional long-range magnetic states.” – The aim of the research is to achieve controlled access to a spin liquid. If this is successful, spin liquids can be used as the basis for constructing novel, improved qubits (units of information for quantum computers).