High-field multi-frequency ESR in the S=2 heisenberg antiferromagnetic chain compound MnCl₃(bpy)

We report the results of high-field multi-frequency electron spin resonance (ESR) experiments on a powder sample of MnCl₃ (bpy) (bpy=2, 2′-bipyridine). This compound is one of the rare examples of the spin (S) 2 quasi one-dimensional (1D) Heisenberg antiferromagnets (HAFs), and magnetic properties of tiny single crystal samples were reported previously (G. E. Granroth et al., Phys. Rev. Lett. 77, 1616 (1996).). In our previous paper on this compound (M. Hagiwara et al., J. Phys.: Conf. Ser. 400, 032014 (2012).), we reported, for the S=2 1D HAF, good agreement between experiment and calculation for the temperature dependence of magnetic susceptibility and the high-field magnetization process at 1.3 K. In this work, we have observed some high-frequency resonance modes with zero-field gaps of about 800 GHz that may correspond to antiferromagnetic resonance modes with orthorhombic anisotropy in a long-range-ordered phase that emerged at low temperatures above the critical field at which the energy gap (Haldane gap) closes. The single-ion anisotropy constant D evaluated from the analysis of these ESR modes is larger than that reported previously.