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The magnetic properties and spin–phonon coupling of Pr2CoMnO6 particles

Pr2CoMnO6 particles with different sizes (45 nm–1 μm) have been synthesized by a citrate-gel method with different annealing temperatures and investigated by x-ray diffraction, Raman spectroscopy, and magnetometry. It is found that at a lower annealing temperature related to sample synthesis, the samples with particle size D ≤ 200 nm show a higher ferromagnetic (FM) transition temperature Tc1, which can be attributed to the FM superexchange interaction of the ordered Co2+–O–Mn4+ pairs. As the annealing temperature increasing, a new FM transition originating from FM vibronic superexchange interaction of Co3+–O– Mn3+ pairs in the disordered phase appears at temperature Tc2 lower than Tc1. Raman spectra at room temperature show that both the antisymmetric stretching and symmetric stretching modes at around 490 and 637 cm−1, respectively, show a decrease in the linewidth and an increase in the intensity with the annealing temperature decreasing. All the results indicate that the ratio of the disordered phase to the ordered phase in the sample decreases as the annealing temperature decreasing. It is suggested that the annealing temperature dependent oxygen vacancies play a key role in the magnetic properties of the Pr2CoMnO6 particles. Furthermore, the temperature-dependent Raman spectrum for Pr2CoMnO6 particles reveals obviously softening of the phonon modes involving stretching vibrations of the (Co/Mn)O6 octahedra at Curie temperature, indicating a close correlation between magnetism and lattice in Pr2CoMnO6 particles. The Pr2CoMnO6 compound with smaller particle size retains a higher FM transition temperature and obvious spin–phonon coupling, which are of special interest for the application.