Optical and magnetic investigation of multiferroic and magnetocaloric properties of Nd(0.8)Tb(0.2)Mn(2)O(5)
Abstrakt
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 (delta(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 (theta(||)) and perpendicular (theta(|)) to the c axis, (theta(||) - theta(|)), 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 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).