Observation of relaxor ferroelectricity and multiferroic behaviour in nanoparticles of the ferromagnetic semiconductor La2NiMnO6

Abstract

We report a diffuse phase transition (extending over a finite temperature range of ∼50 K) in sol–gel derived nanoparticles (∼25 nm) of the ferromagnetic double perovskite La2NiMnO6. The macroscopic polarization (P–E hysteresis loop), validity of the Vogel–Fulcher relation and high dielectric permittivity (∼9 × 102) confirm relaxor ferroelectric phenomena in these magnetic nanoparticles. Compared to the corresponding bulk sample, appreciably large enhancement of the magnetocapacitive effect (MC ∼ 30%) is observed even under low magnetic field (0.5 T) around the broad relaxor dielectric peak temperature (∼220 K), which is close to the ferromagnetic transition temperature (θf ∼ 196 K). All of these features establish the multiferroic character of the La2NiMnO6 nanoparticles. The inhomogeneities arising from chemical and valence mixing in the present La2NiMnO6 nanoparticles and the inter-site, Ni/Mn-site disorder along with surface disorder of the individual nanoparticles resulting in local polar regions are attributed to the observed dielectric behaviour of the nanoparticles. The wave vector dependent spin-pair correlation is considered to be the plausible cause of the colossal magnetocapacitive response near the transition temperature. High permittivity and large magnetocapacitive properties make these ferromagnetic La2NiMnO6 nanoparticles technologically important.