Abstract
We determined the spin exchanges between the Cu2+ ions in the kagomé layers of volborthite, Cu3V2O7(OH)2·2H2O, by performing the energy-mapping analysis based on DFT+U calculations, to find that the kagomé layers of Cu2+ ions are hardly spin-frustrated, and the magnetic properties of volborthite below ~75 K should be described by very weakly interacting antiferromagnetic uniform chains made up of effective S = 1/2 pseudospin units. This conclusion was verified by synthesizing single crystals of not only Cu3V2O7(OH)2·2H2O but also its deuterated analogue Cu3V2O7(OD)2·2D2O and then by investigating their magnetic susceptibilities and specific heats. Each kagomé layer consists of intertwined two-leg spin ladders with rungs of linear spin trimers. With the latter acting as S = 1/2 pseudospin units, each two-leg spin ladder behaves as a chain of S = 1/2 pseudospins. Adjacent two-leg spin ladders in each kagomé layer interact very weakly, so it is required that all nearest-neighbor spin exchange paths of every two-leg spin ladder remain antiferromagnetically coupled in all spin ladder arrangements of a kagomé layer. This constraint imposes three sets of entropy spectra with which each kagomé layer can exchange energy with the surrounding on lowering the temperature below ~1.5 K and on raising the external magnetic field B. We discovered that the specific heat anomalies of volborthite observed below ~1.5 K at B = 0 are suppressed by raising the magnetic field B to ~4.2 T, that a new specific heat anomaly occurs when B is increased above ~5.5 T, and that the imposed three sets of entropy spectra are responsible for the field-dependence of the specific heat anomalies.
Highlights
We discovered that the specific heat anomalies of volborthite observed below ~1.5 K at B = 0 are suppressed by raising the magnetic field B to ~4.2 T, that a new specific heat anomaly occurs when B
We verified this conclusion by acquiring new magnetic susceptibility data and re-analyzing reported magnetization data of volborthite to show that the kagomé layer of Cu2+ ions is hardly spin-frustrated, and the low-temperature magnetic properties of volborthite should be described by an antiferromagnetic uniform Heisenberg (AUH) chain composed of S = 1/2 pseudospin units
The spin exchanges of volborthite show that each kagomé layer of Cu2+ ions is hardly spin-frustrated, but rather consists of very weakly interacting two-leg spin ladders with linear trimers as rungs
Summary
To explore the magnetic order below 1.5 K, especially its dependence on the magnetic field, as well as the origin of the apparent finite susceptibility as T →0 K, we re-analyzed the spin lattice of volborthite by performing an energy-mapping analysis based on DFT+U calculations [3,4] to find that the magnetic properties of volborthite below ~75 K should be described by two-leg spin ladders with rungs of S = 1/2 pseudospin units rather than by a trigonal lattice of S = 1/2 pseudospin units, as proposed by Janson et al [11] We verified this conclusion by acquiring new magnetic susceptibility data and re-analyzing reported magnetization data of volborthite to show that the kagomé layer of Cu2+ ions is hardly spin-frustrated, and the low-temperature magnetic properties of volborthite should be described by an AUH chain composed of S = 1/2 pseudospin units. B > ~5.5 T, and that these field-dependent behaviors of the magnetic ordering originate from the three sets of magnetic entropy spectra of each kagomé layer of Cu2+ ions, created by constrained interactions between adjacent two-leg spin ladders
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