Utilizing magnetic susceptibility, specific heat, thermal expansion and IR spectroscopy we provide experimental evidence that the two subsequent antiferromagnetic transitions in ZnCr_2S_4 at T_N1 = 15 K and T_N2= 8 K are accompanied by significant thermal and phonon anomalies. The anomaly at T_N2 reveals a strong temperature hysteresis typical for a first-order transformation. Due to strong spin-phonon coupling both magnetic phase transitions induce a splitting of phonon modes, where at T_N1 the high-frequency and at T_N2 the low-frequency modes split. The anomalies and phonon splitting observed at T_N2 are strongly suppressed by magnetic field. Regarding the small positive Curie-Weiss temperature Theta= 8 K, we argue that this scenario of two different magnetic phases with concomitant different magneto-elastic couplings results from the strong competition of ferromagnetic and antiferromagnetic exchange of equal strength.
@article{arxiv.cond-mat/0603627,
title = {Spin-driven Phonon Splitting in Bond-frustrated ZnCr2S4},
author = {J. Hemberger and T. Rudolf and H. -A. Krug von Nidda and F. Mayr and A. Pimenov and V. Tsurkan and A. Loidl},
journal= {arXiv preprint arXiv:cond-mat/0603627},
year = {2009}
}