Abstract
In this paper, we present a detailed Raman study of the non-multiferroic compounds PrMnO3 and NdMnO3 and the multiferroic compounds TbMnO3 and DyMnO3 as a function of temperature and magnetic field. All studied systems show anomalous phonon shifts close to the Néel transition TN. In PrMnO3 and NdMnO3, the frequency softenings are partly attributed to an orbital-spin-phonon coupling whereas in TbMnO3 and DyMnO3, the relatively weak frequency shifts are rather attributed to an expansion of the Mn−O bond lengths. On the other hand, the frequencies of TbMnO3 phonons are shifted as a function of magnetic field, while those of PrMnO3 remain unaffected. These frequency shifts are interpreted in terms of local oxygen rearrangements under magnetic field that could play an important role in the multiferroicity of TbMnO3 and DyMnO3.
Highlights
In the last decade, the study of the perovskite manganites RMnO3 (R = lanthanides, Y and Bi) have attracted a considerable interest because of their fascinating properties such as colossal magnetoelectric and magnetocaloric effects[1,2]
We investigate the Raman-active phonons in PrMnO3, NdMnO3, TbMnO3 and DyMnO3 compounds as a function of temperature and magnetic field
We focus on the microscopic mechanisms and the theoretical models required to explain the phonon frequency shifts and how they differ in the multiferroic TbMnO3 and DyMnO3 compounds as compared to the non-multiferroic compounds PrMnO3 and NdMnO3
Summary
The study of the perovskite manganites RMnO3 (R = lanthanides, Y and Bi) have attracted a considerable interest because of their fascinating properties such as colossal magnetoelectric and magnetocaloric effects[1,2] Their physical properties are determined by a delicate interplay of charge, spin, orbital, and lattice degrees of freedom[3,4,5]. The orbital ordering temperature in RMnO3 is significantly enhanced by increasing the GdFeO3-type lattice distortion with decreasing the ionic radius of R (rR)[3,11]. The Raman spectra of RMnO3 orthorhombic compounds have been previous studied[21,22,26] They have explained the frequency softening of the 490 cm−1 and 600 cm−1 modes below TN in terms of spin-phonon coupling caused by the phonon modulation of the superexchange integral[21,22,26]. The objectives are (i) to study and compare the Raman response of the non-multiferroic systems PrMnO3 and NdMnO3 to those of the multiferroic compounds TbMnO3 and DyMnO3, (ii) to study the effect of a magnetic field on the Raman-active phonons in the representative compounds PrMnO3 and TbMnO3 at low temperature
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