Study of multiferrocity in h-YMnO3

Materials that possess coexistence of magnetic and electric ordering are known as “Multiferroics” and are gaining much attention not only due to the promising multifunctional device applications but also due to rich & fascinating physics. Hexagonal YMnO3 is a member of special class of multiferroic family i.e. “Geometrically frustrated multiferroics” and one of the most intensively studied h-RMnO3 having ferroelectric transition at TC ~900K and antiferromagnetic ordering at TN~70K. To study the effect on multiferroicity, perturbative (i.e. low-level) Fe substitution in YMnO3; the YMn1-xFexO3     (x = 0.0, 0.05, 0.08, and 0.10) have been done. We have identified the relevant structure-property correlations, along with the exact fixation of the charge states of Mn and Fe as Mn+3/Mn+4 and of Fe as Fe+4. XANES results also confirm that Fe replaces Mn-sites only. Furthermore, in order to study the effect of magnetic doping at Mn site, on the magnetic structure of multiferroic hexagonal manganite YMnO3 has been studied by detailed temperature and magnetic field dependent neutron diffraction measurements. The neutron diffraction study revealed that the magnetic structure of pure YMnO3 has 1 irreducible representation. Upon doping Fe at Mn-site the magnetic structure gradually changes to 1+2 for YMn0.95Fe0.05O3 and then to 2 irreducible representations for YMn0.90Fe0.10O3. This change of magnetic structure is a composition analog of spin-reorientation transition (TSR), to be observed first time by the perturbed Fe-doping. Moreover, in order to study dominance of magnetoelasticity, non-magnetic A-site (Y) is replaced by another non-magnetic ion (Eu) in YMnO3. SXRD results provided rather unexpected micro-structural changes concerning the magnetic properties (e.g., planar bond -lengths and -angles). Magnetization M(T) shows antiferromagnetic ordering which gets suppressed with Eu-doping. This is evident by specific heat measurements also. The change in magnetic entropy has also been calculated by doing Debye analysis. Dielectric ε′(T) results showing highly doping-dependent anomaly at TN indicating linear magneto-electric coupling. These results are understood in terms of modification in magneto-elastic coupling triggered by non magnetic Eu+3 doping at Y plane of YMnO3.