IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 20 (2008) 355003 (4pp) doi:10.1088/0953-8984/20/35/355003 Magnetodielectric response in epitaxial thin films of multiferroic Bi 2 NiMnO 6 P Padhan 1 , P LeClair 1 , A Gupta 1,3 and G Srinivasan 2 1 Center for Materials for Information Technology, University of Alabama, Tuscaloosa, AL 35487, USA 2 Physics Department, Oakland University, Rochester, MI 48309, USA E-mail: agupta@mint.ua.edu Received 25 April 2008, in final form 9 July 2008 Published 1 August 2008 Online at stacks.iop.org/JPhysCM/20/355003 Abstract Thin films of multiferroic Bi 2 NiMnO 6 (BNMO) have been epitaxially stabilized on various lattice-matched substrates using the pulsed laser deposition technique. BNMO films deposited on conducting Nb doped SrTiO 3 substrates exhibit a magnetodielectric effect, in which the effective dielectric constant is dependent on the applied magnetic field and measurement frequency. A rapid change in the effective dielectric constant with a corresponding maximum in the magnetodielectric effect is observed near the ferromagnetic transition temperature of BNMO. The temperature-dependent magnetodielectric effect is attributed to the coupling between electric dipole ordering and fluctuations and magnetic ordering and fluctuations. 1. Introduction The perovskites and double perovskites exhibit a wide variety of electrical and magnetic ordering behavior that can be tuned by compositional changes, making them a natural choice to search for ‘multiferroic’ materials, in which magnetic and dielectric processes coexist. For example, the local distortion of the transition metal octahedra in TiO 6 -based compounds induces polar electric domains [1], and in Bi-cation-containing perovskites, this leads to ferroelectric order [2, 3]. On the other hand, ferromagnetic order in double perovskites arises from a 180 ◦ superexchange interaction between two transition metal cations (Goodenough–Kanamori rules) [4]. Bismuth-based perovskite ferroelectrics are being seri- ously considered as materials for nonvolatile ferroelectric ran- dom access memory devices to replace the more toxic Pb-based compounds, and also because of their fast switching charac- teristics [5]. Generally, the dielectric properties of ferroelec- tric thin films are strongly influenced by the finite screening length of the electrode material, soft-mode hardening of sur- face dipoles, and the thickness-dependent strain relaxation [6]. Ferroelectric ordering at ∼485 K has recently been observed in bulk Bi-based double perovskite Bi 2 NiMnO 6 (BNMO) samples synthesized at high pressures [7]. Inter- estingly, these samples also exhibit magnetic ordering below ∼140 K, which is significantly lower than the ferroelectric 3 Author to whom any correspondence should be addressed. ordering temperature. BNMO has a heavily distorted double perovskite structure with Ni 2+ and Mn 4+ ions ordered in a rock-salt configuration similar to La 2 NiMnO 6 (LNMO). The presence of the 6s 2 lone pairs of Bi 3+ ions and the covalent Bi– O bonds results in ferroelectric properties, while the –Ni 2+ –O– Mn 4+ –O–Ni 2+ – magnetic path leads to ferromagnetism [7]. Thin films of BNMO have been grown on lattice-matched SrTiO 3 substrates via epitaxial stabilization that exhibit ferro- magnetic and ferroelectric ordering [8]. The lower value of the saturation magnetization ( M S ∼ 4.1–4.5 μ B at 5 K) of BNMO compared to the theoretical prediction of M S ∼ 5 μ B , obtained in both bulk and thin film samples [7, 8], has been explained based on partial B -site disorder of the Ni 2+ and Mn 4+ [9]. We have successfully stabilized thin films of BNMO on various substrates which provide different degrees of lattice mismatch. The interesting feature observed in these films is the remarkable change of the effective dielectric constant at the ferromagnetic transition temperature of about 130 K, compared to a very small anomaly at T C observed in [8]. Our BNMO films also exhibit a ‘magnetodielectric effect’ in which the effective dielectric constant is dependent on the applied magnetic field. We find a maximum magnetodielectric effect around the ferromagnetic transition temperature. 2. Experimental details Pulsed laser deposition was used to grow epitaxial thin films of Bi 2 NiMnO 6 (BNMO) on (001)-oriented 1.0 wt% Nb doped 0953-8984/08/355003+04$30.00 © 2008 IOP Publishing Ltd Printed in the UK 1