Local ferroelectric switching properties in BiFeO 3 microstructures and their piezomagnetic response
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Local ferroelectric switching properties in BiFeO3 microstructures and their piezomagnetic response C. Harnagea, C.V. Cojocaru, A. Pignolet INRS-Énergie, Matériaux et Télécommunications / Université du Québec, Varennes, Canada ABSTRACT We report here the successful fabrication of BiFeO3 (BFO) isolated micron-sized structures by pulsed laser deposition. The islands have a relatively constant aspect ratio (height/lateral size) of 0.1-0.3. We present their local ferroelectric characterization, using piezoresponse force microscopy (PFM), showing that the micron-sized BFO islands exhibit a strong piezoresponse and have ferroelectric domains with lateral sizes down to the 100 nm range. We also present here the first results of Magnetostriction Force Microscopy experiments performed on these structures. On ferromagnetic samples this method reveals a piezomagnetic or magnetostriction contrast, associated with magnetic domains. In our case, we show that the contrast can be associated to the magnetoelectric effect. INTRODUCTION In its bulk form, BFO was reported as a perovskite showing ferroelectricity with a high Curie temperature (1103 K) and antiferromagnetic properties below Neel temperature (643 K) [1]. Structural analysis of BFO indicates that it possesses a rhombohedrally distorted perovskite structure with R3c symmetry (a = b = c = 5.63 A°, a = b = c = 59.4°) at room temperature [2]. Recently, thin films of BFO have attracted attention because their ferroelectric properties were found to be over an order of magnitude better than that in a bulk single crystal. This enhanced ferroelectricity, together with the fact that BFO is antiferromagnetic with a weak ferromagnetism at room temperature, due to a canted spin structure, have opened perspectives for novel applications [3,4]. However, prior to any application and device fabrication, a good understanding of the basic ferroelectric and electromechanical properties of this class of multifunctional materials, as well as their coupling with the magnetic order parameter is necessary. EXPERIMENTS Sample preparation The BiFeO3 micron-sized islands were obtained by Pulsed Laser Deposition at room temperature. A KrF excimer laser (λ = 248 nm, pulse duration = 14 ns) was employed for ablation with a laser fluence set at 2 J/cm2. We used stoichiometric BFO ceramic target from Praxair Surface Technologies. The as deposited films with a thickness 100 nm were amorphous and a post-deposition thermal treatment at 900º C for 1 h in O2 flow was performed in order to crystallize them into the ferroelectric phase. The annealing process favored the minimization of the free energy of the film-substrate system, and the continuous films broke up into distinct islands which crystallized into the perovskite phase [5]. The average size of the structures was 1.5 µm (see the inset in Fig 1a), their density ~ 3.8*104/mm2 and the aspect ratio of (average height/diameter) ~0.1. The substrates used were SrTiO3 (100)-oriented, Nb-doped in order to
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