Spin-driven phonon splitting in bond-frustrated ZnCr2S4

Magnetic susceptibility, specific heat, thermal expansion, and IR spectroscopy provide experimental evidence that the two subsequent antiferromagnetic transitions in ZnCr2S4 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 temperature hysteresis typical for a first-order transformation. Because of strong spin-phonon coupling, both magnetic transitions at T-N1 and T-N2 induce a splitting of phonon modes. The anomalies and phonon splitting observed at T-N2 are suppressed by strong magnetic field. Regarding the small positive Curie-Weiss temperature Theta similar or equal to 8 K, we argue that this scenario of two different magnetic phases with different magnetoelastic couplings results from the strong competition of ferromagnetic and antiferromagnetic exchange.