Electron-spin-resonance study of the one-dimensional magnet (NH4)2MnF5

Electron-spin-resonance (ESR) experiments have been performed on the quasi-one-dimensional magnet (NH4)2MnF5. The ESR spectrum consists of two components, an extremely broad signal and a weak but sharp resonance on top of the broad signal. The resonance field of the dominating broad signal is most probably shifted by strong non-diagonal terms of the dynamic susceptibility to g-values well below g=2 and becomes undetectable upon the onset of strong one-dimensional correlations below T≤60 K. We attribute these two signals to Mn3+ ions of the chains (broad resonance, intrinsic response of the sample) and to defects (small resonance), respectively. The ESR spectra reflect the evolution from the paramagnetic state (T>60 K) to one-dimensional magnetic behaviour (T≤60 K). The results are discussed with respect to the local anisotropy of the Mn3+ ions and comparison is made with the behaviour of other one-dimensional magnetic compounds. (author)