Magnetic frustration, a phenomenon where magnetic moments cannot simultaneously satisfy their preferred alignments, complicates magnetic ordering in materials with atoms arranged in triangular networks. Clinoatacamite, Cu2Cl(OH)3, features a distorted kagome lattice, similar to herbertsmithite, a candidate for a quantum spin liquid state. Unlike herbertsmithite, clinoatacamite develops long-range magnetic order below 18.1 K, but the exact nature of this state has remained enigmatic for decades.
Complementary measurements and single-crystal neutron diffraction, including on instruments D10 and D10+ at the ILL, have uncovered a sequence of distinct magnetic states. These results show that lattice distortion does not simply remove magnetic frustration. Instead, different spin arrangements remain in competition and can even coexist over parts of the temperature range.
The magnetic behaviour of a material originates from tiny magnetic moments on atoms, linked to electron spin. In antiferromagnets, neighbouring spins prefer to point in opposite directions. On a triangle, this preference creates a problem for the third spin. In a kagome lattice, this problem repeats, leading to many spin arrangements with similar energies, making it difficult for the system to select a single state.
While herbertsmithite remains magnetically disordered at low temperatures, clinoatacamite develops long-range magnetic order below 18.1 K. Earlier muon experiments had detected this long-range order, whereas neutron diffraction on powder samples only saw it at lower temperatures (6-7 K). The new study, using single crystals and multiple techniques, has resolved this disagreement.
Temperature-dependent measurements on D10+ at the ILL revealed magnetic reflections characteristic of one type of order between 18.1 and 6.4 K. Below approximately 6.4 K, a second set of magnetic reflections appears. Crucially, the two signatures coexist between roughly 6.4 and 4.6 K before the higher-temperature one disappears. This confirms that long-range magnetic order exists below 18.1 K, but its form changes with temperature.
Detailed analysis of the magnetic diffraction pattern at 1.7 K, performed on D10, revealed that even the low-temperature ordered state combines two distinct magnetic ordering modes with different symmetries. The researchers propose that competition persists at another level: different collective ways of organising spins remain closely matched and can mix with one another. This situation, where several possible ordered states are close to becoming stable, is described as multicritical. The distortion of the kagome lattice has therefore not eliminated frustration, but transformed it into a competition between different forms of global magnetic order.
