Absence of Spin Frustration in the Kagomé Layers of Cu2+ Ions in Volborthite Cu3V2O7(OH)2·2H2O and Observation of the Suppression and Re-Entrance of Specific Heat Anomalies in Volborthite under an External Magnetic Field

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.

A cascade of magnetic phase transitions and a 1/3-magnetization plateau in selenite–selenate Co3(SeO3)(SeO4)(OH)2 with kagomé-like Co2+ ion layer arrangements: the importance of identifying a correct spin lattice

“…However, it should be recalled that a kagomé-like arrangement of magnetic ions does not necessarily lead to a kagomé spin lattice. 6 For example, volborthite Cu 3 V 2 O 7 (OH) 2 •2H 2 O, which has a kagomé-like arrangement of Cu 2+ ions, had been regarded for years as a representative example of a kagomé spin lattice, [7][8][9][10][11][12] but its correct spin lattice was found to be a spin-half antiferromagnetic uniform Heisenberg (SHAUH) chain. 6 There are three factors responsible for such a striking observation.…”

“…6 For example, volborthite Cu 3 V 2 O 7 (OH) 2 •2H 2 O, which has a kagomé-like arrangement of Cu 2+ ions, had been regarded for years as a representative example of a kagomé spin lattice, [7][8][9][10][11][12] but its correct spin lattice was found to be a spin-half antiferromagnetic uniform Heisenberg (SHAUH) chain. 6 There are three factors responsible for such a striking observation. [13][14][15][16] (1) The spin exchanges between magnetic ions, not the patterns of their geometrical arrangements, are what govern the spin lattice.…”

“…This accounts for why volborthite exhibits SHAUH chain behavior. 6 It is undesirable to end up interpreting the experimental properties of a magnet using an incorrect spin lattice. A situation like the SHAUH chain behavior of volborthite can also occur in a magnet with a kagomé-like arrangement of S > 1/2 magnetic ions, especially when its crystal structure consists of several different sites for the magnetic ions.…”