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The key ingredient for unconventional magnetic states such as quantum spin liquids and magnetization plateaus is magnetic frustration – often encountered in systems where the local geometries and the magnetic interactions are incompatible. A unique and fascinating member among such frustrated quantum magnets is the natural mineral atacamite, Cu₂Cl(OH)₃ [1,2]. This compound has recently been established as material realization of the quantum sawtooth chain – a seminal model in frustrated quantum magnetism. In this system, corner-sharing spin triangles are arranged in a chain with two exchange interactions: J along the chain spine, J’ within the sawtooth triangles. In atacamite, the sawtooth chains were found to be anisotropic with J’/J ~ 1/3 [1].

Remarkably, the high-field magnetic phase diagram of atacamite has been found to host an unusual state with a wide plateau-like magnetization at M ~ M_sat/2, directly above the antiferromagnetic ground state phase (T_N = 8.4 K) [1]. While for the bare quantum sawtooth chain, 1/2-magnetization plateaus are predicted by theory for certain coupling ratios J’/J, the case seems different for atacamite. The flattening of the magnetization is unrelated to a quantum plateau. Rather, this behavior is likely to stem from the residual weak 3D connectivity of the sawtooth chains [1]. Here, we present our investigation of the magnetic phase diagram of atacamite with special emphasis on the unusual, plateau-like state in high magnetic fields. In particular, in order to resolve its nature, we probe this high-field state by a combined study using thermodynamic as well as microscopic measurement techniques [3] and discuss the results in context of the proposed high-field state of weakly coupled sawtooth chains.

[1] L. Heinze et al., Phys. Rev. Lett. 126, 207201 (2021).

[2] L. Heinze et al., Physica B 536, 377 (2018).

[3] T. Reimann, L. Heinze et al., in preparation.