Spin-phonon coupling driven ferroelectricity and magneto-dielectric changes in multiferroic YMn2O5

Abstract

The correlations between the magnetic, dielectric, and lattice degrees of freedom have been investigated utilizing Raman spectroscopy, magnetization, specific heat, and dielectric measurements under magnetic field in order to understand magnetoelectric and magneto-dielectric behaviour in YMn₂O₅. It is reported before that the RMn₂O₅ family of compounds show colossal magneto-dielectric behaviour which is attributed to the unique commensurate-incommensurate magnetic transition shown by the system. In the present study the magnetization and specific heat studies revealed three magnetic transitions at T_N = 44 K, 41 K and 20 K along with a field-induced transition at 35 K under 12 T. Dielectric studies under magnetic field showed ∼1 % magneto-dielectric effects (MD) in the incommensurate spin order state below 20 K, and the MD becomes smaller in the commensurate magnetic state displayed in 20–41 K region. To elucidate the coupling between lattice dynamics and spin ordering, we performed temperature-dependent Raman spectroscopy, revealing clear signatures of spin-phonon coupling and lattice modulation at the three magnetic transitions. Density functional theory calculations are employed to estimate phonon frequencies and associated eigen displacements, demonstrating that only phonon modes with atomic displacements in the ab plane are significantly influenced by magnetic ordering and so, ferroelectricity. This finding highlights that Mn displacement associated with ferroelectric polarization is intrinsically linked to magnetic ordering through spin-phonon coupling, thereby driving magnetoelectric behaviour in YMn₂O₅.