Raman and X-ray diffraction study of (Ba,Sr)TiO 3 /(Bi,Nd)FeO 3 multilayer heterostructures A.S. Anokhin a, ⁎, O.A. Bunina b , Yu I. Golovko a , V.M. Mukhortov a , Yu I. Yuzyuk b , P. Simon c a Southern Scientific Centre, Russian Academy of Sciences, 41, Chekhov str., Rostov-on-Don 344006, Russia b Faculty of Physics and Research Institute of Physics, Southern Federal University, 5, Zorge str., Rostov-on-Don 344090, Russia c CNRS UPR 3079 CEMHTI, 1D, av. de la Recherche Scientifique, F-45071 Orléans, France abstract article info Article history: Received 21 December 2012 Received in revised form 8 August 2013 Accepted 9 August 2013 Available online 19 August 2013 Keywords: Multiferroics Raman spectroscopy X-ray diffraction superlattices Thin films Sputtering We report synthesis, X-ray diffraction (XRD) and Raman scattering characterisation of epitaxial heterostructures containing alternating (Bi 0.98 Nd 0.02 )FeO 3 (BNFO) and (Ba 0.8 Sr 0.2 TiO 3 ) (BST) layers deposited on (100) MgO substrates. A significant shift of the BST soft mode and partial depolarisation in the Raman spectra of multilayer heterostructures caused by epitaxial strains were observed. Satellite peaks typical for superlattices were observed in the XRD patterns of multilayer heterostructures with layer thicknesses below 30 nm. Raman spectra of the BNFO/BST superlattice with a modulation period of 10 nm revealed hardening of the soft mode and a dominating symmetric-stretching mode at 705 cm −1 due to distortion in FeO 6 octahedra enforced by the epitaxial strain in the superlattice. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Currently, multiferroics are the subject of intensive investigations because they are very promising candidates for device applications in spintronics and multiple-state memories that can be addressed both electrically and magnetically [1–3]. Bismuth ferrite BiFeO 3 (BFO) exhibits ferroelectric (T C = 1083 K) and antiferromagnetic (T N = 643 K) phase transitions. It belongs to the multiferroic class of materials and it has attracted significant interest since the pioneering work of Wang et al. [4]. They reported large spontaneous polarisation in constrained epitaxial thin films of BFO grown on SrRuO 3 (SRO)-coated SrTiO 3 (STO) substrates. These films also showed a significant enhancement of magnetisation compared with the bulk, a rather strong piezoelectric response compa- rable with that of widely used Pb(Ti,Zr)O 3 and a large magnetoelectric coefficient of 3.77 V/A. Recently, Zhang et al. [5] reported that tetragonal-like BFO films on a (001) LaAlO 3 substrate exhibit a large spontaneous polarisation of ~130 ± 5 μC/cm 2 , demonstrating the potential of this material for lead-free piezo- or ferroelectric application. The significant magnetoelectric effect in BFO thin films indicates that cycloidal spin modulation, experimentally observed in bulk BFO by neutron scattering [6], is suppressed by epitaxial constraints and related lattice distortions. Epitaxial multiferroics exhibit strong sensitivity of magnetic and/or ferroelectric properties towards lattice strain. Thus, the rich structural diversity of BFO thin films depending on the substrate, substrate orientation, buffer layer and deposition process has been reported [7–16]. Substrate and film thickness are usually varied to modify epitaxial strain effects in thin films in order to achieve desirable properties. Alternatively, multilayer films and superlattices (SLs) offer a very efficient way of strain engineering to design materials with enhanced magnetoelectric responses. In recent years, a num- ber of multilayer heterostructures containing alternating ferroelec- tric and multiferroic layers, such as Pb(Zr,Ti)O 3 /CoFe 2 O 4 [17–19], (Pr 0.88 Ca 0.15 MnO 3 )/(Ba 0.6 Sr 0.4 TiO 3 ) [20], CoFe 2 O 4 /BaTiO 3 [21], BiFeO 3 / BaTiO 3 [22], (Bi,Nd)FeO 3 /(Ba,Sr)TiO 3 [23], BiFeO 3 and BiMnO 3 [24] have been investigated. Also, SrTiO 3 and (Ba,Sr)TiO 3 buffer layers have been used to grow BFO and Nd-doped BFO epitaxial films on (001) MgO substrates [25,26]. It is worth noting that Nd-doped BFO films are of particular interest because the substitution of Bi for Nd improves both the ferroelectric and the magnetic properties [27]. Investigation of BiFeO 3 /BaTiO 3 multilayers [22] revealed larger magnetisation for samples with smaller modulation periods owing to the increasing number of inter- faces present in BiFeO 3 /BaTiO 3 SLs. A recent study of (Bi,Nd)FeO 3 /(Ba,Sr) TiO 3 multilayers with 3 and 6 nm layer thicknesses [23] revealed satura- tion magnetisation of about 5 · 10 4 –6 · 10 4 A/m, which is an order of magnitude higher than that reported for BiFeO 3 /BaTiO 3 SLs [22]. In the present paper, we report on X-ray diffraction (XRD) and Raman investigation of multilayer heterostructures containing (Bi 0.98 Nd 0.02 )FeO 3 (BNFO) and (Ba 0.8 Sr 0.2 TiO 3 ) (BST) alternating layers deposited on (100) MgO substrates, to clarify lattice distortions within multilayer heterostructures and the microscopic origin of enhanced magnetisation for samples with smaller modulation periods. Thin Solid Films 545 (2013) 267–271 ⁎ Corresponding author. Tel.: +7 9604646971. E-mail addresses: anokhin.andrey@gmail.com (A.S. Anokhin), yuzyuk@rambler.ru (Y.I. Yuzyuk). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.08.057 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf