Evidence for magneto-electric and spin-lattice coupling in PbFe0.5Nb0.5O3 through structural and magneto-electric studies

Neutron diffraction (ND) studies were carried out on polycrystalline single-phase multiferroic Pb(Fe0.5Nb0.5)O-3 (PFN) in the temperature range of 290-2 K to understand the structural and magnetic properties as a function of temperature. ND data were refined using the Rietveld refinement method for both crystallographic and magnetic structures. The structure at room temperature was found to be monoclinic, in Cm space group. No structural transition was observed till 2 K. At low temperatures (i.e., from T < T (N); T (N) = 155 K), an additional peak appears at scattering vector, Q = 1.35 (-1), indicating the onset of antiferromagnetic ordering. The magnetic structure was found to be commensurate with the crystallographic structure and could be refined using the propagation vector, k = [0.125, 0.5, and 0.5]. Magnetization, ferroelectric P-E loops, and dielectric measurements on PFN reveal a strong anomaly at the antiferromagnetic transition temperature (T (N)) indicating the magneto-electric coupling. The refined temperature-dependent structural parameters such as unit cell volume and monoclinic distortion angle (beta) reveal pronounced anomalies at the magnetic ordering temperature (T (N)), which indicates strong spin-lattice coupling. An anomaly in lattice volume was observed with a small negative thermal expansion below and a large thermal expansion above the T (N), respectively. It shows the occurrence of isostructural phase transition accompanying the magnetic ordering below T (N) 155 K, leading to significant change in ionic polarization, octahedral tilt angle, and lattice strain around T (N). We have used refined atomic positional coordinates from the nuclear and magnetic structures, to obtain ionic polarization. These detailed studies confirm the magneto-electric and spin-lattice coupling in PFN across T-N.