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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• Far-infrared optical excitations in multiferroic TbMnO_3
• Colossal magnetocapacitive effect in differently synthesized and doped CdCr2S4
• Optical properties of ZnCr2Se4 - Spin-phonon coupling and electronic d-d-like excitations
• Infrared-active phonon modes in monoclinic multiferroic MnWO4
• Magentically-Induced Lattice Distortions and Ferroelectricity in Magnetoelectric GdMnO3
• Transverse acoustic phonon anomalies at intermediate wavevectors in MgV${}_2$O${}_4$
• On the complexity of spinels: Magnetic, electronic, and polar ground states