Abstract
Magnetic and crystallographic properties of the mineral langite Cu4(OH)6SOH2O are reported. Thermodynamic measurements combined with a microscopic analysis, based on density-functional bandstructure calculations, identify a quasi-two-dimensional (2D), partially frustrated spin-1/2 lattice resulting in the low Néel temperature of K. This spin lattice splits into two parts with predominant ferro- and antiferromagnetic (AFM) exchange couplings, respectively. The former, ferromagnetic (FM) part is prone to the long-range magnetic order and saturates around 12 T, where the magnetization reaches 0.5 /Cu. The latter, AFM part features a spin-ladder geometry and should evade long-range magnetic order. This representation is corroborated by the peculiar temperature dependence of the specific heat in the magnetically ordered state. We argue that this separation into ferro- and antiferromagnetic sublattices is generic for quantum magnets in Cu2+ oxides that combine different flavors of structural chains built of CuO4 units. To start from reliable structural data, the crystal structure of langite in the 100–280 K temperature range has been determined by single-crystal x-ray diffraction, and the hydrogen positions were refined computationally.
Highlights
Low-dimensional magnets show unique diversity of crystal structures and associated spin lattices, where a plethora of quantum phenomena can be observed [1,2,3]
To start from reliable structural data, the crystal structure of langite in the 100–280 K temperature range has been determined by single-crystal x-ray diffraction, and the hydrogen positions were refined computationally
The physics of quantum spin chains has been actively explored in Cu2+ compounds featuring chains of corner- or edge-sharing CuO4 plaquette units
Summary
Low-dimensional magnets show unique diversity of crystal structures and associated spin lattices, where a plethora of quantum phenomena can be observed [1,2,3]. It gives rise to competing nearest-neighbor and next-nearest-neighbor (NNN) couplings, where the former (J1) is typically ferromagnetic (FM), while the latter (J2) is antiferromagnetic (AFM). Such J1 - J2 frustrated spin chains develop incommensurate spin correlations and helical magnetic order [8, 9], few instances of FM intrachain spin order are known as well [10, 11]. The helical spin arrangement observed in simple binary compounds CuCl2 [12] and CuBr2 [13] and in more complex materials like linarite PbCu(OH)2SO4 [14], all being frustrated J1 - J2 spin chains, may trigger electric polarization induced by the magnetic order, leading to multiferroic behavior [15,16,17,18]. The complex interplay of frustration and anisotropy needs further investigations on different systems as, e.g., LiCuVO4 [20,21,22,23]
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