Observation of Enhanced Dielectric Coupling and Room- Temperature Ferromagnetism in Chemically Synthesized BiFeO 3 @ SiO 2 Core-Shell Particles Mandar M. Shirolkar, †,§ Raja Das, ‡ Tuhin Maity, ‡ Pankaj Poddar, ‡ and Sulabha K. Kulkarni* ,§ † Department of Physics, University of Pune, Pune-411007, India ‡ Physical and Materials Chemistry Division, National Chemical Laboratory, Pune-411008, India § DST unit on Nanoscience, Indian Institute of Science Education and Research, Pune-411021, India * S Supporting Information ABSTRACT: We report the effect of SiO 2 coating on the structural, magnetic, and dielectric properties of chemically synthesized high-purity BiFeO 3 particles. The as- synthesized BiFeO 3 particles demonstrate properties comparable with those reported for bulk BiFeO 3 . On the other hand, the structural measurement on BiFeO 3 @SiO 2 shows that the SiO 2 coating has anisotropically compressed the lattice of BiFeO 3 particles and stimulates the variation in the electron density. This affects magnetic and dielectric behavior of material. Frequency-dependent dielectric constant study at low temperature (20-325 K) revealed slight reduction (8-10%) in the dielectric constant of BiFeO 3 @SiO 2 particles compared to uncoated BiFeO 3 particles. The study reveals five anomalies at 234, 206, 146, 84, and 25 K located in close proximity to the linear magnetodielectric coupling and spin reorientation transitions. The loss tangent (≈10 -3 ) and alternating current (ac) conductivity (≈10 -8 Ω -1 cm -1 ) of BiFeO 3 @SiO 2 particles are orders of magnitude lower than those observed for the BiFeO 3 particles. The magnetic measurement shows the existence of room-temperature ferromagnetism in BiFeO 3 @SiO 2 particles with average value of magnetic moment per Fe atom ≈0.030 μ B and appreciable coercivity as high as 120 Oe. The canted spin structure in the surface shell of BiFeO 3 @SiO 2 particles show an enhanced magnetic property and shifted hysteresis loop. The magnetic measurement in close proximity to dielectric transitions revealed enhanced magnetization, suggesting the presence of anisotropies. It has been observed that SiO 2 coating alters the properties of BiFeO 3 particles. Our dielectric and magnetic measurements show enhanced coupling among the electric and magnetic ordered parameters in BiFeO 3 @SiO 2 core-shell particles compared to uncoated BiFeO 3 particles. The magnetic and dielectric properties of SiO 2 - coated BiFeO 3 are similar to nanoparticles of BiFeO 3 where interface plays a significant role. 1. INTRODUCTION Among the known magnetoelectric multiferroics, currently bismuth ferrite (BiFeO 3 , or BFO) has been a scientifically and technologically important material because of its vast potential applications. 1-7 BFO exhibits antiferromagnetic Ne ́ el temper- ature (T N ≈ 643 K) and ferroelectric Curie temperature (T c ≈ 1100 K). 8,9 The coexistence of such mutually exclusive smart ferroic properties above room temperature makes BFO one of the main potential contenders for room-temperature-based magnetoelectric devices, data storage applications, spintronics, and so on. 1,9,10 In spite of its excellent properties, the defects and nonstoichiometry in the material gives rise to low resistivity. However, its long incommensurate spiral spin structure, which suppresses net microscopic magnetization and magnetoelectric effect, hinders its potential use for the device applications. 1 Even though extensive investigations have been reported on the multiferroic properties of BFO, the nature of its multiferroic properties such as the existence of weak ferromagnetism and weak coupling between magnetic and electric components are still controversial. It has been observed that simultaneous enhancement in magnetic and electric properties is rather unfeasible, except for the controversial result observed on thin films, which demonstrates that epitaxially strained thin films show enhancement in the multiferroic properties. 11 However, consequent theoretical and experimental studies have shown conflicting reports, suggesting that epitaxial strain did not enhance the multiferroic properties. 12,13 Hence, there is an urgent need to perform experiments in order to improve the properties of BFO from the point of view of device applications. Many approaches have been reported to develop high-quality BFO in different forms. 14-16 Different strategies were also applied to further improve the properties of BFO. 17-19 In recent time, several groups have explored core-shell technique on different materials and reported novel properties Received: March 16, 2012 Revised: August 16, 2012 Published: August 17, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 19503 dx.doi.org/10.1021/jp3025683 | J. Phys. Chem. C 2012, 116, 19503-19511