Letter to the Editor Change in periodicity of the incommensurate magnetic order towards commensurate order in bismuth ferrite lead titanate Tim Stevenson a,n , Tim P. Comyn a , Aziz Daoud-Aladine b , Andrew J. Bell a a Institute for Materials Research, University of Leeds, Leeds LS2 9JT, UK b ISIS, STFC, Rutherford Appleton Laboratory, Harwell Science & Innovation Campus, Didcot OX110QX, UK article info Article history: Received 7 May 2010 Received in revised form 7 June 2010 Available online 3 August 2010 Keywords: Bismuth ferrite Multiferroic Neutron diffraction Antiferromagnetic Ferroelectric abstract xBiFeO 3 –(1 x)PbTiO 3 has been synthesised in bulk polycrystalline ceramic form and then self-disintegrated to form powder (P4mm) (x ¼0.7) or crushed (R3c) (x ¼0.75, 0.9) to relieve stress. High-resolution neutron powder diffraction has been employed to observe incommensurate antiferromagnetic ordering within the materials, and its dependence on phase at 7.5 K. It is shown that in the rhombohedral phase the period of the magnetic structure depends on PbTiO 3 addition, and increases from 790 ˚ A (x ¼0.9) to 840 ˚ A (x ¼0.75). The tetragonal phase restores the collinear antiferromagnetic order. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Bismuth ferrite (BiFeO 3 ) has been the focus of a considerable number of publications during the last decade, particularly pertaining to its nature as a room temperature magnetoelectric material [1–7]. The BiFeO 3 crystal structure is rhombohedral perovskite (space group R3c) that exhibits both ferroelectricity (FE) and antiferromagnetism (AFM), with Curie and Ne ´el temperatures of T c ¼ 1103 K [8,9] and T N ¼ 643 K [10,11], respectively. The magnetism of iron-perovskites can be described at first approximation as the collinear and commensurate G-type anti- ferromagnetic order [12] of the Fe 3+ ions [13]. In BiFeO 3 this order is reported as incommensurate (IC) and cycloidal [14], and the Fe 3+ moments rotate with a longer range period [15] of 620 ˚ A [16–20], which is present near T N [21]. A change in the periodicity of the IC structure is observed when doping on the A or B site with metal ions such as lanthanum or manganese, respectively [15]. The addition of Mn shows an increase of the periodicity until BiMn x Fe 1 x O restoration of the collinear structure occurs for x 40.2 [15,16]. Bismuth ferrite forms a solid solution with tetragonal lead titanate (PbTiO 3 , space group P4mm) to form xBiFeO 3 –(1 x)PbTiO 3 (or BFPT), which as a ceramic exhibits a morphotropic phase boundary (MPB) at ca. x ¼ 0.7, and a broad region of mixed R3c and P4mm phases between 0.4ox o0.7. At x ¼ 0.7, ferroelectricity is retained up to T c ¼ 908 K [22] and antiferromagnetic ordering remains in the rhombohedral and tetragonal phase up to T N ¼ 560 K [23] and T N ¼ 220 K [24], respectively. As is observed in lead zirconate titanate (PZT), the morpho- tropic phase boundary in BFPT at x ¼ 0.7 displays an increase in electrical permittivity and piezoelectric activity [25]. In unclamped particles, the spontaneous strain of the tetragonal phase maximises at ca  1.18 near the MPB (  1.06 in PbTiO 3 [22]); hence sintered ceramics of these compositions are subject to large internal stress. Under certain process conditions this is partially relieved by a fraction of the material transforming into the rhombohedral phase (hence the wide mixed phase region). In other cases the stress is relieved by the self-disintegration of the sample to grain size particulates [26,27]. The internal stress has a profound influence on the properties of the ceramics, from polarisation and hardness to simple synthesis. The focus of this work is to determine the magnetic structure in BFPT, observe changes in the ordering with varying composi- tions due to lead titanate addition, and to examine the phase dependence in sustaining the IC order. The use of high-resolution neutron diffraction was employed, as neutrons have a unique ability to provide structural and magnetic data simultaneously. High-resolution techniques are essential for resolving the antiferromagnetic peak that splits into satellites when the commensurate G-type order becomes IC. 2. High-resolution neutron experiment Polycrystalline samples of x ¼ 0.7, 0.75 and 0.9, xBiFeO 3 – (1  x)PbTiO 3 were prepared using conventional mixed oxide Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.07.043 n Corresponding author. Tel.: +44 113 343 2362; fax: +44 113 343 2384. E-mail address: pretjs@leeds.ac.uk (T. Stevenson). Journal of Magnetism and Magnetic Materials 322 (2010) L64–L67