Polar phonons and spin-phonon coupling in HgCr2S4 and CdCr2S4

Abstract

Polar phonons of HgCr2S4 and CdCr2S4 are studied by far-infrared spectroscopy as a function of temperature and external magnetic field. Eigenfrequencies, damping constants, effective plasma frequencies and Lyddane-Sachs-Teller relations, and effective charges are determined. Ferromagnetic CdCr2S4 and antiferromagnetic HgCr2S4 behave rather similar. Both compounds are dominated by ferromagnetic exchange and although HgCr2S4 is an antiferromagnet, no phonon splitting can be observed at the magnetic phase transition. Temperature and magnetic field dependence of the eigenfrequencies show no anomalies indicating displacive polar soft mode behavior. However, significant effects are detected in the temperature dependence of the plasma frequencies indicating changes in the nature of the bonds and significant charge transfer. In HgCr2S4 we provide experimental evidence that the magnetic field dependence of specific polar modes reveal shifts exactly correlated with the magnetization showing significant magneto-dielectric effects even at infrared frequencies.

8 pages, 8 figures

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• Dynamical magneto-electric coupling in helical magnets
• Colossal magnetocapacitance and colossal magnetoresistance in HgCr2S4
• Spin-phonon coupling in antiferromagnetic chromium spinels
• Coupling of phonons and electromagnons in GdMnO_3
• Spin-driven Phonon Splitting in Bond-frustrated ZnCr2S4
• Charge-stripe order in the electronic ferroelectric LuFe2O4
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Cited by in corpus (4)

• Broadband dielectric response of CaCu3Ti4O12: From dc to the electronic transition regime
• Magnetic-order induced phonon splitting in MnO from far-infrared spectroscopy
• Colossal magnetocapacitive effect in differently synthesized and doped CdCr2S4
• Optical properties of ZnCr2Se4 - Spin-phonon coupling and electronic d-d-like excitations