Study of crystal-field excitations and infrared active phonons in TbMnO3.

The Tb3+ (4f 8) crystal-field (CF) excitations and the infrared phonons in TbMnO3 are studied as a function of temperature and under an applied magnetic field. The phonon energy shifts reflect local displacement of the oxygen ions that contribute to the CF energy level shifts below 120 K and under magnetic field. The CF polarized transmission spectra provide interesting information about the debated nature of the excitations at 41, 65, 130 cm-1. We also evaluate the contribution of the charge transfer mechanism to the magnetoelectric process in TbMnO3 under magnetic field.

Raman and infrared study of 4f electron-phonon coupling in HoVO3.

First-order Raman scattering and multiphonons are studied in RVO3 (R = Ho and Y) as a function of temperature in the orthorhombic and monoclinic phases. Raman spectra of HoVO3 and YVO3 unveil similar features since both compounds have nearly identical R-radii. However, the most important difference lies in the transition temperature involving the V(3+) orbitals, the V(3+) magnetic moments as well as the crystallographic structure. Particularly, the magnetic and orbital reorientations occur at T N2 = 40 K for HoVO3 instead T N2 =77 K in the case of YVO3. For both systems, anomalous phonon shifts which are related to spin-phonon coupling are observed below the V(3+) magnetic ordering temperature (T N1 ≈ 110 K) while additional phonon anomalies are exclusively observed in HoVO3 around T (*) ≈ 15 K. On the other hand, infrared (IR) transmittance measurements as a function of temperature reveal Ho(3+5)I8 → (5)I7 excitations and additional excitations assigned as vibronics. These latter combined with drastic changes in Ho(3+5)I8 → (5)I7 excitations at T N2, are indicative of a strong coupling between the Ho(3+) ions and the ligand field. This could explain the large magnetocaloric capacity shown by HoVO3.

Micro-Raman and infrared studies of multiferroic TbMn2O5.

We have studied the Raman and infrared spectral response of TbMn2O5 under an applied magnetic field parallel to the easy magnetic a-axis at 4.2 K. Strong spin-lattice coupling in TbMn2O5 is evidenced by a frequency shift of Raman and infrared phonons as a function of magnetic field compared to the phonon response of BiMn2O5 that remains unaffected. The magnetic field behavior of the highest frequency phonons retraces the polarization switching in TbMn2O5 and shows an important frequency softening below 3 T that is modulated by the J 3 and J 4 exchange parameters. The role of the Tb(3+) spin alignment with H is interpreted in terms of a local lattice striction and the contribution of the charge transfer mechanism to the magnetoelectric process is evaluated.

Study of crystal-field excitations and infrared active phonons in the multiferroic hexagonal DyMnO3.

In hexagonal DyMnO3, Dy(3+) crystal-field excitations are studied as a function of temperature and applied magnetic field. They are complemented with the measurements of infrared active phonon frequency shifts under applied magnetic field at T = 4.2 K. Between TN = 68 K and T = 10 K, the absence of Dy(3+) Kramers doublet splittings at either the C3 or the C3v site symmetries indicates that the Mn(3+) magnetic order effective exchange field has no component parallel to the c-axis at either site. Below T = 10 K, the ground state Kramers doublet splits under the Dy(3+) internal effective field as well as the applied magnetic field. Also, relatively strong infrared active phonon energy shifts are observed in magneto-infrared reflectance measurements at T = 4.2 K, allowing the calculation of the induced electric polarization changes as a function of the applied magnetic field. Such changes are associated with a large magnetoelectric effect in DyMnO3, arising from a charge transfer between Dy(3+) and apical oxygen ions.

Magnetic and micro-Raman studies of hexagonal-DyMnO3.

Dc-susceptibility measurements and Raman active phonon frequencies of hexagonal DyMnO(3) retrace the Mn(3+) ions antiferromagnetic transition at T(N) ~ 70 K and their spin reorientation at T(SR) ~ 48 K. The temperature evolution of Raman active mode frequencies and their over-hardening are associated with Dy(3+) and Mn(3+) ion displacements below T(N) and with a spin-phonon coupling that involves apical oxygen.

Investigation of phonon behavior in Pr(2)NiMnO(6) by micro-Raman spectroscopy.

The temperature dependence of phonon excitations and the presence of spin-phonon coupling in polycrystalline Pr(2)NiMnO(6) samples were studied using micro-Raman spectroscopy and magnetometry. Magnetic properties show a single ferromagnetic-to-paramagnetic transition at 228 K and a saturation magnetization close to 4.95 µ(B)/f.u. Three distinct Raman modes at 657, 642, and 511 cm( - 1) are observed. The phonon excitations show a clear hardening due to anharmonicity from 300 down to 10 K. Further, the temperature dependence of the 657  cm( - 1) mode shows only a small softening. This reflects the presence of a relatively weak spin-phonon coupling in Pr(2)NiMnO(6) contrary to the case for other double perovskites previously studied.

Raman study of the antiferromagnetic phase transitions in hexagonal YMnO3 and LuMnO3.

The A(1), E(1) and E(2) Raman active modes in hexagonal YMnO(3) and LuMnO(3) single crystals are studied as a function of temperature and compared with previous measurements. In addition to anharmonicity, some phonon frequencies show below T(N) anomalous temperature dependences that reflect the atomic displacements while some other phonon frequencies are more sensitive to the spin-phonon coupling.

A micro-Raman study of a Pr(0.5)Ca(0.5)MnO(3) single crystal and thin films.

We present a micro-Raman study of a high quality Pr(0.5)Ca(0.5)MnO(3) single crystal and thin films on SrTiO(3) and LaAlO(3) substrates. Ten A(g) and seven B(2g) Raman-active modes (simplified symmetry: Pmma space group) have been observed and correlated to charge ordering around T(co) = 240 K and antiferromagnetic spin-orbital ordering at T(N)∼ 170 K. Our data reveal that coherent Jahn-Teller MnO(6) distortions prevail at T