Magnetic susceptibility, specific heat, thermal expansion, and IR spectroscopy provide experimental evidence that the two subsequent antiferromagnetic transitions in ${\mathrm{ZnCr}}_2{\mathrm{S}}_4$ at $T_{N1}=15\mathrm{K}$ and $T_{N2}=8\mathrm{K}$ 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 $\Theta \simeq 8\mathrm{K}$, we argue that this scenario of two different magnetic phases with different magnetoelastic couplings results from the strong competition of ferromagnetic and antiferromagnetic exchange.

• Received 17 February 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.087204