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Phase competition in transition metal oxides has attracted remarkable interest for fundamental aspects and technological applications. Here, we report a concurrent study of the phase transitions in undoped and Cr-doped VO$$_2$$ 2 thin films. The structural, morphological and electrical properties of our films are examined and the microstructural effect on the metal–insulator transition (MIT) are highlighted. We further present a distinctive approach for analyzing the Raman data of undoped and Cr-doped VO$$_2$$ 2 thin films as a function of temperature, which are quantitatively correlated to the electrical measurements of VO$$_2$$ 2 films to give an insight into the coupling between the structural phase transition (SPT) and the MIT. These data are also combined with reported EXAFS measurements and a connection between the Raman intensities and the mean Debye–Waller factors $$\sigma ^2$$ σ 2 is established. We found that the temperature dependence of the $$\sigma _{R}^{2}(V-V)$$ σ R 2 ( V - V ) as calculated from the Raman intensity retraces the temperature profile of the $$\sigma _{EXAFS}^{2}(V-V)$$ σ EXAFS 2 ( V - V ) as obtained from the EXAFS data analysis. Our findings provide an evidence on the critical role of the thermal vibrational disorder in the VO$$_2$$ 2 phase transitions. Our study demonstrates that correlating Raman data with EXAFS analysis, the lattice and electronic structural dynamics can be probed.

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The multiferroic and the rotating magnetocaloric properties of Nd 0.8 Tb 0.2 Mn 2 O 5 are investigated by microscopic optical probes and macroscopic magnetic measurements. Raman-active phonons as a function of temperature, and Nd 3+ and Tb 3+ infrared active crystal-field (CF) excitations as a function of temperature and under magnetic fields up to 11 T have been studied in Nd 0.8 Tb 0.2 Mn 2 O 5 . The obtained results are compared to those of NdMn 2 O 5 and TbMn 2 O 5 reference compounds. The observation of one set of Raman-active phonons and CF excitations rule out possible twinning while their energy positions and thermal evolutions indicate noticeable changes of Mn1-O3-Mn1 and TbO 8 structural units. This would explain the nature of separated magnetic phases in Nd 0.8 Tb 0.2 Mn 2 O 5 . The degeneracy of the ground-state Kramers doublet is lifted ( 0 ∼ 9 cm -1 ), indicating that the Nd 3+ -Mn 3+ interaction impacts the magnetic and ferroelectric properties of Nd 0.8 Tb 0.2 Mn 2 O 5 . The Zeeman splitting of excited crystal-field levels of the Nd 3+ ions at low temperatures shows that the g z factor is weak compared to that in NdMn 2 O 5 . This indicates that the R 3+ spins in Nd 0.8 Tb 0.2 Mn 2 O 5 are mostly aligned within the ab-plane. The nature of magnetocrystalline anisotropy in Nd 0.8 Tb 0.2 Mn 2 O 5 as well as in all RMn 2 O 5 compounds is quantitatively investigated by studying the anisotropy of paramagnetic Curie temperatures along (θ || ) and perpendicular (θ ⊥ ) to the c axis, (θ || − θ ⊥ ), as a function of the rare-earth atomic number. It is particularly found that the magnetocrystalline anisotropy is mainly determined by the quadrupolar charge distribution of 4 f shells. The rotating magnetocaloric effect in Nd 0.8 Tb 0.2 Mn 2 O 5 is also evaluated and compared to that in NdMn 2 O 5 and TbMn 2 O 5 . Our findings show that Nd-and Tb-separated magnetic phases independently contribute to the magnetocaloric effect of Nd 0.8 Tb 0.2 Mn 2 O 5 .