High-Field Multi-Frequency ESR in the Quasi-1D S = 1/2 Ising-Like Antiferromagnet BaCoVO in a Transverse Field.

We have performed high-field multi-frequency electron spin resonance (ESR) measurements of a single crystal of the S = 1/2 quasi one-dimensional (1D) Ising-like antiferromagnet BaCoVO at 1.5 K in magnetic fields of up to 14 T applied along [110] and [100] directions. Two main ESR branches, which are ascending and descending with increasing magnetic fields, are observed from 400 GHz at zero magnetic field in both cases. The low ESR mode shows a softening near 10 T for H || [100], while no softening was observed below 14 T for H || [110]. This difference in the magnetic excitations may explain the distinct phase diagrams for these two directions. [ABSTRACT FROM AUTHOR]