Nanoindentation behaviour of nano BiFeO 3 Pintu Sen a , Arjun Dey b,1 , Anoop K. Mukhopadhyay b, * , S.K. Bandyopadhyay a , A.K. Himanshu a a Variable Energy Cyclotron Centre, 1/AF, Bidhan Nagar, Kolkata 700064, India b Mechanical Property Evaluation Section, CSIR-Central Glass and Ceramic Research Institute, Jadavpur, Kolkata 700064, India Received 30 July 2011; received in revised form 3 September 2011; accepted 3 September 2011 Available online 10 September 2011 Abstract Bismuth ferrite (BiFeO 3 ) is a unique magnetoelectric multiferroic that exhibits the coexistence of ferroelectricity and antiferromagnetism at room temperature. This unique combination of properties has pumped a huge surge in current research on BiFeO 3 as a future material for very important technological applications such as magnetic detectors and as an active layer in magnetoelectric memories. For such applications involving miniaturized components and devices, it is essentially important to have an idea of the mechanical integrity of the system at the scale of the microstructure. In spite of the wealth of the literature, however, the attempt to evaluate the mechanical integrity of nano BiFeO 3 at a scale comparable with the local microstructural length scale was almost non-existent. Here we report, possibly for the first time the nanoindentation behaviour of a sol–gel process derived nano BiFeO 3 having particle size of 5–25 nm. The nanoindentation studies were conducted at 100–1000 mN loads on a green pellet annealed at a low temperature of only 300 8C to avoid particle coarsening. The results showed interesting dependence of nanohardness and Young’s modulus on the nanoindentation load which could be explained in terms of elastic recovery and plastic deformation energy concepts. # 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Nano BiFeO 3 ; Multiferroic; Nanoparticle; Nanoindentation 1. Introduction Bismuth ferrite (BiFeO 3 ) is a well known magnetoelectric multiferroic (MM) material [1,2]. MM materials are being coined as an ‘accidental beauty of nature’. The beauty of these materials is that in them the ferroelectricity and magnetism coexist in a coupled fashion. Currently there is a huge research interest in these MM materials in general and in bismuth ferrite in particular. The possible applications of these materials include both magnetic detectors and multi-state memory devices. The fundamental research interest in such materials also stem from the fact that the complex yet interesting physics behind the fundamental mechanisms for magnetoelectric coupling is still far from well understood. Bismuth ferrite (BFO) stands out in the ‘genre’ of MM materials which are being most actively researched upon these days simply because of the fact that it is the only known room temperature MM material in which both ferroelectricity and antiferromagnetism coexist [1,2]. It exhibits huge remnant polarization (100 mC cm 2 ) in both single crystal [3,4] and thin film form [5,6]. It also displays very attractive feature of the coupling between the directions of the ferroelectric polarization and the sublattice magnetization [7]. These unique features have inspired the consideration of BFO as the active layer in magnetoelectric memories based on exchange bias and also as a possible active material in the next generation of lead-free ferroelectric materials. BFO has a superimposed incommensurate G-type cycloid spin structure with a periodicity of 62 nm along the [1 1 0] h direction in its rhombohedral structure [1,2,6]. At room temperature it possesses a rhombohedrically distorted perovskite structure (space group R3c). The unique beauty is that this structure is both ferroelectric (with polarization along the <1 1 1> directions and antiferromagnetic [8]). It has been reported that at a temperature of 827 8C BFO shows a transition from rhombohedral to orthorhombic [6] or monoclinic [7] www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 38 (2012) 1347–1352 * Corresponding author. Present address: Central Glass and Ceramic Re- search Institute, 196, Raja S.C. Mullick Road, Kolkata 32, India. Tel.: +91 33 2473 3469/76/77/96; fax: +91 33 2473 0957. E-mail address: anoopmukherjee@cgcri.res.in (A.K. Mukhopadhyay). 1 Presently the author is associated with the Thermal System Group, ISRO Satellite Centre (ISAC), Indian Space Research Organisation, Dept. of Space, Govt. of India, Bangalore, India. 0272-8842/$36.00 # 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2011.09.011