Optical and magnetic investigation of multiferroic and magnetocaloric properties of Nd0.8Tb0.2Mn2O5

Abstract

The multiferroic and the rotating magnetocaloric properties of ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ are investigated by microscopic optical probes and macroscopic magnetic measurements. Raman-active phonons as a function of temperature, and ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Tb}}^{3+}$ infrared active crystal-field (CF) excitations as a function of temperature and under magnetic fields up to 11 T have been studied in ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$. The obtained results are compared to those of ${\mathrm{NdMn}}_{2}{\mathrm{O}}_{5}$ and ${\mathrm{TbMn}}_{2}{\mathrm{O}}_{5}$ 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 ${\mathrm{TbO}}_{8}$ structural units. This would explain the nature of separated magnetic phases in ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$. The degeneracy of the ground-state Kramers doublet is lifted $({\mathrm{\ensuremath{\Delta}}}_{0}\ensuremath{\sim}9\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}1})$, indicating that the ${\mathrm{Nd}}^{3+}\text{--}{\mathrm{Mn}}^{3+}$ interaction impacts the magnetic and ferroelectric properties of ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$. The Zeeman splitting of excited crystal-field levels of the ${\mathrm{Nd}}^{3+}$ ions at low temperatures shows that $\mathrm{the}\phantom{\rule{4pt}{0ex}}{g}_{z}$ factor is weak compared to that in ${\mathrm{NdMn}}_{2}{\mathrm{O}}_{5}$. This indicates that the ${R}^{3+}$ spins in ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ are mostly aligned within the $ab$-plane. The nature of magnetocrystalline anisotropy in ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ as well as in all $R{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ compounds is quantitatively investigated by studying the anisotropy of paramagnetic Curie temperatures along (${\ensuremath{\theta}}_{||}$) and perpendicular (${\ensuremath{\theta}}_{\ensuremath{\perp}}$) to the $c$ axis, $({\ensuremath{\theta}}_{||}\ensuremath{-}{\ensuremath{\theta}}_{\ensuremath{\perp}})$, 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 $4f$ shells. The rotating magnetocaloric effect in ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ is also evaluated and compared to that in ${\mathrm{NdMn}}_{2}{\mathrm{O}}_{5}$ and ${\mathrm{TbMn}}_{2}{\mathrm{O}}_{5}$. Our findings show that Nd- and Tb- separated magnetic phases independently contribute to the magnetocaloric effect of ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$.