Researchers Demonstrate Complex Magnetic Phases in Clinoatacamite below 18.1 K

Scientists are unraveling the magnetic mysteries within clinoatacamite Cu Cl(OH), a compound closely related to the intriguing spin liquid candidate herbertsmithite and a long-standing challenge for researchers studying frustrated magnetism. Led by L. Stödter, C. Kastner, and H. O. Jeschke, with contributions from M. Reehuis, K. Beauvois, and B. Ouladdiaf, this new research demonstrates that clinoatacamite is a distorted kagome antiferromagnet embedded within a complex crystal structure. Their detailed measurements, utilising muon spin rotation/relaxation and neutron diffraction, reveal a sequence of magnetic phases emerging below 18.1 K, suggesting a competition between different antiferromagnetic ordering modes and potentially a unique metamagnetic texture even without an applied field. This work is significant because it provides crucial insight into how structural distortions impact magnetic behaviour in kagome materials, potentially guiding the search for novel quantum magnetic states.

This work is significant because it provides crucial insight into how structural distortions impact magnetic behaviour in kagome materials, potentially guiding the search for novel quantum magnetic states.

Clinoatacamite reveals complex antiferromagnetic phases and order

Scientists have unveiled a complex magnetic order within clinoatacamite, a distorted kagome material and long-standing enigma in frustrated quantum magnetism. The work builds upon decades of research into antiferromagnetic quantum kagome materials, driven by the pursuit of the elusive Quantum spin liquid ground state. While herbertsmithite has emerged as a promising candidate, exhibiting a lack of magnetic order down to Millikelvin temperatures, most similar materials adopt static order instead. This research shifts focus towards distorted kagome materials, demonstrating that non-uniform exchange couplings can yield exotic quantum behaviour, such as patterned valence-bond solids and unusual magnetic properties. Experiments show that clinoatacamite undergoes a magnetic transition at 18.1 K, and further anomalies in thermodynamic quantities are observed at 6.4 K, 6.2 K, and 4.6 K. The team’s detailed analysis of the crystal structure and exchange interactions, derived from DFT calculations, establishes a clear distinction between clinoatacamite and other atacamite family members, highlighting a weak network of kagome planes and interlayer sites.

Clinoatacamite Structure, Crystal Growth and Magnetic Characterisation

This analysis demonstrated weak ferromagnetic couplings to the interlayer copper sites, distinguishing it from other atacamite materials and creating a network of kagome planes and interlayer sites. Subsequently, researchers grew single crystals of clinoatacamite to facilitate detailed measurements. These techniques were applied to the single crystals to probe the magnetic structure and transitions. Further analysis of the data below 6.4 K identified an antiferromagnetic state with a wavevector qm = (0, 0, 0) and weak spin canting. The team determined the strength of nearest-neighbour copper-copper exchange interactions, labelled J1 through J6, using DFT with the GGA+U method, plotting these values as a function of on-site Coulomb repulsion U. At U = 7.8 K, J2 = −14.5 K, J3 = −30.3 K, J4 = 120.1 K, J5 = 211.6 K and J6 = 367.1 K via integration of cmag/T, finding it to be approximately 1/3 of 2 R ln(2), the expected entropy for two spin-1/2 per formula unit. A remarkably sharp and intense anomaly was recorded at 6.2 K, closely resembling a first-order phase transition. Magnetometry performed in weak dc magnetic fields aligned parallel to the b axis revealed distinct behaviour below 6.2 K for both field-cooled and zero-field-cooled samples, extending up to approximately 0.1 T, consistent with earlier polycrystal results. At 4.7 K, slightly offset from the temperature of the weak anomaly in the specific heat, the M(T) data exhibited a kink and maximum at 0.01 T for both field-cooled and zero-field-cooled samples.

The magnetization anomaly at 18.1 K was observed to be weak, visible only on a reduced scale. Ac magnetic susceptibility χac measurements at all frequencies studied showed an intense anomaly at 6.4 K with zero dc magnetic field and a 0.5 mT ac field. Frequency-dependent behaviour of |χac| began at the 6.4 K anomaly and persisted down to approximately 4.5 K, where a weak, kink-like anomaly was present. Researchers observed no resolvable anomaly at 18.1 K in ac susceptibility measurements. Single-crystal muon spin rotation/relaxation experiments confirmed long-range magnetic order at temperatures below the 18.1 K transition. Exemplary zero-field asymmetry spectra taken at 1.73 K, 5.3 K, and 10.8 K displayed characteristics typical of the temperature ranges below 4.6 K, above 6.4 K, and between these transitions, indicating different types of long-range magnetic order. Neutron diffraction experiments revealed magnetic reflections below 6. The transition around 6.4 K appears to be initiated when a second ordering mode becomes thermodynamically stable, reorganising the high-temperature textured state into a long-range ordered phase, potentially accounting for the observed narrow intermediate states. Clinoatacamite therefore exemplifies a geometrically frustrated antiferromagnet where competing modes can coexist, creating textured magnetic states influenced by the parity of the ordering modes. The authors acknowledge that the low symmetry of the crystal structure and the resulting anisotropic coupling terms significantly influence the magnetic phase diagram. Future research could explore the behaviour of this material under applied magnetic fields to further characterise the metamagnetic texture. This work contributes to a better understanding of geometrically frustrated antiferromagnets and the mechanisms by which complex magnetic states can emerge from competing interactions, offering insights into the broader field of quantum magnetism and potentially informing the search for novel quantum spin liquid materials.