Multiferroic oxide composites: synthesis, characterisation and applications G. Sreenivasulu 1 , H. Qu 2 and G. Srinivasan* 1 The nature of mechanical strain mediated electromagnetic coupling in multiferroic composites has been studied extensively in recent years. This review is on composites with ferromagnetic or ferrimagnetic oxides and ferroelectrics. Systems studied so far include samples with spinel ferrites, hexagonal ferrites or lanthanum manganites for the ferromagnetic phase and barium titanate, lead zirconate titanate (PZT), lead magnesium niobate–lead titanate (PMN-PT) or lead zinc niobate–lead titanate (PZN-PT) for the ferroelectric phase. Bilayer and multilayer heterostructures, bulk composites, core shell nanoparticles and core shell nanotubes and nanowires were investigated for their response to magnetic fields, termed direct magnetoelectric effect (DME). Several systems show a giant low frequency DME and resonance enhancement at bending and electromechanical resonance. The response of the composites to an electric field, called converse ME effect, is found to be strong in several ferrite–ferroelectric composites. The potential for use of the composites for pico-Tesla magnetic sensors and high frequency electric field tunable ferrite signal processing devices are also addressed in this review. This paper is part of a special issue on Smart Materials Introduction A multiferroic is a material that exhibits two or more of primary ferroic properties such as ferromagnetism, ferroelectricity or ferroelasticity. A composite made of ferromagnetic and ferroelectric phases allows for cou- pling between the electric and magnetic order parameters and, thereby, represents an engineered multiferroic. 1–8 In such a composite with a ferrite and a piezoelectric as in Fig. 1, the mechanical strain mediates the magneto- electric (ME) effect that is defined as the polarisation P of the composite in an applied magnetic field H [direct ME effect (DME)] or an induced magnetisation M in an electric field E [converse ME effect (CME)]. 1 The polarisation P is related to H by the expression, P5aH, where a is the second rank ME susceptibility tensor. Studies on DME involve measurements of P versus E in H, dependence of the permittivity e on H, or voltage response dV of the sample to an ac magnetic field dH. The ME voltage coefficient (MEVC) a E 5dV/(tdH) and is related to a by a5e o e r a E , where t is the composite thickness and e r is the relative permittivity. 1 The CME is studied through techniques including M versus H under E or magnetic anisotropy field induced by an electric field through ferromagnetic resonance (FMR). 1–8 This review is on multiferroic composites with specific focus on bulk, layered and nanocomposites with ferromagnetic or ferrimagnetic oxides such as spinel ferrites, hexagonal ferrites or lanthanum manganites for the magnetic phase and lead zirconate titanate (PZT), barium titanate (BTO) or lead magnesium niobate–lead titanate (PMN-PT) for the ferroelectric phase. 1,5,7 Recent efforts on composites with piezoelectric AlN, lanthanum–gallium–tantalate (langatate) and quartz are also addressed. 9–11 A brief discussion is provided on the synthesis of the composites, characterisation in terms of DME and CME, and applications for useful technolo- gies such as magnetic sensors and E tunable ferrite devices for use over 1–110 GHz. 12–15 Early works on ME composites dealt with bulk composites prepared by mixing and sintering ferrite and BTO powders, whereas recent works have mainly focused on layered composites and nanostructures. 16–43 Bulk composites of nickel ferrite NiFe 2 O 4 (NFO) or cobalt ferrite CoFe 2 O 4 (CFO) with BTO showed poor ME coupling attributed to large leakage currents due to low resistivity for ferrites. 1 The strongest ME coupling is expected in a layered structure due to (i) low leakage currents and (ii) ease of poling to align the electric dipoles and strengthen the piezoelectric effect. 5,7 Layered samples are synthesised by laminating and sintering thick films of the two phases made by tape casting or by bonding single crystal or polycrystalline platelets. 16–32 Efforts to date on nanocomposites include nanobilayers by deposition tech- niques including pulsed laser deposition (PLD) and chemical vapour deposition (CVD), nanopillars of ferrites in ferroelectrics by PLD, core shell particulate composites by self-assembly, core shell nanowires and tubes by electrospinning, and template assisted synthesis. 33–43 Several techniques were employed for studies on DME and CME coupling. 1 A giant low frequency 1 Physics Department, Oakland University, Rochester, MI 48309–4401, USA 2 Electrical and Computer Engineering Department, Oakland University, Rochester, MI 48309–4401, USA *Corresponding author, email srinivas@oakland.edu ß 2014 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 22 October 2013; accepted 7 March 2014 DOI 10.1179/1743284714Y.0000000537 Materials Science and Technology 2014 VOL 30 NO 13a 1625