Magnetic excitations in an ionic spin-chain system with a nonmagnetic ferroelectric instability

Abstract

Cross-correlation between magnetism and dielectric is expected to offer distinct emergent phenomena. Here, magnetic excitations in the organic donor-acceptor spin-chain system, tetrathiafulvalene-bromanil (TTF-BA), with a ferroelectric ground state is investigated by ^{1}H-NMR spectroscopy. A nonmagnetic transition with a ferroelectric order is marked by sharp drops in the NMR shift and the nuclear spin relaxation rate T_{1}^{−1} at 53 K. Remarkably, the analyses of the NMR shift and T_{1}^{−1} dictate that the paramagnetic spin susceptibility in TTF-BA is substantially suppressed from that expected for the one-dimensional Heisenberg spins. We propose that the spin-lattice coupling and the ferroelectric instability cooperate to promote precursory polar singlet formation in the ionic spin system with a nonmagnetic ferroelectric instability.