Spin-driven Phonon Splitting in Bond-frustrated ZnCr2S4
Abstract
Utilizing magnetic susceptibility, specific heat, thermal expansion and IR spectroscopy we provide experimental evidence that the two subsequent antiferromagnetic transitions in ZnCr2S4 at TN1 = 15 K and TN2= 8 K are accompanied by significant thermal and phonon anomalies. The anomaly at TN2 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 TN1 the high-frequency and at TN2 the low-frequency modes split. The anomalies and phonon splitting observed at TN2 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.
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