Spin-driven Phonon Splitting in Bond-frustrated ZnCr2S4

TL;DR

This study provides experimental evidence of spin-driven phonon splitting in ZnCr2S4, revealing how magnetic transitions induce phonon anomalies and mode splitting due to strong spin-phonon coupling, influenced by magnetic field and exchange interactions.

Contribution

It demonstrates the direct link between magnetic phase transitions and phonon mode splitting in ZnCr2S4, highlighting the role of spin-phonon coupling in bond-frustrated systems.

Findings

Magnetic transitions cause phonon mode splitting.

Thermal and phonon anomalies are observed at transition temperatures.

Magnetic field suppresses phonon anomalies.

Abstract

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.