Fractional chain-end effects in quasi-one-dimensional S = 1 material NTNB: electron spin resonance studies

We report on X-band electron spin resonance studies (ESR) of the quasi-one-dimensional anisotropic S = 1 Heisenberg antiferromagnet compound Ni(tn)₂NO₂BF₄ (or NTNB for short). The presence of interacting S = 1/2 chain-end states is clearly observed in the ESR spectra even in nominally pure samples, visible as a g ~ 2 main peak with several satellites. The data are analyzed in the frame of the theory proposed by Batista et al. [Phys. Rev. B 60, R12533 (1999)]. The ESR angular dependence revealed a pronounced anisotropy, whose origin can be explained assuming the presence of the field-induced staggered magnetization.