High-field Phase Diagram and Spin Structure of Volborthite Cu3V2O7(OH)2/2H2O

TL;DR

This study extends NMR investigations of volborthite to high magnetic fields, revealing a new high-field phase with heterogeneous spin states and a second magnetization step, advancing understanding of frustrated quantum magnetism.

Contribution

It reports the discovery of a new high-field phase (phase III) in volborthite with complex spin heterogeneity, extending the magnetic phase diagram up to 31 T.

Findings

Identification of a new high-field phase above 25 T.

Observation of heterogeneous spin states with two types of V sites.

Detection of a second magnetization step at 26 T.

Abstract

We report results of 51V NMR experiments on a high-quality powder sample of volborthite Cu3V2O7(OH)2/2H2O, a spin-1/2 Heisenberg antiferromagnet on a distorted kagome lattice. Following the previous experiments in magnetic fields $B$ below 12 T, the NMR measurements have been extended to higher fields up to 31 T. In addition to the two already known ordered phases (phases I and II), we found a new high-field phase (phase III) above 25 T, at which a second magnetization step has been observed. The transition from the paramagnetic phase to the antiferromagnetic phase III occurs at 26 K, which is much higher than the transition temperatures from the paramagnetic to the lower field phases I (B < 4.5 T) and II (4.5 < B < 25 T). At low temperatures, two types of the V sites are observed with different relaxation rates and line shapes in phase III as well as in phase II. Our results indicate that both phases II and III exhibit a heterogeneous spin state consisting of two spatially alternating Cu spin systems, one of which exhibits anomalous spin fluctuations contrasting with the other showing a conventional static order. The magnetization of the latter system exhibits a sudden increase upon entering into phase III, resulting in the second magnetization step at 26 T.We discuss the possible spin structure in phase III.