Comment on "Epitaxial BiFeO3 multiferroic thin film heterostructures".

Wang et al. (1) recently reported multiferroic behavior, with ferromagnetic and ferroelectric polarizations that are both large at room temperature, in thin strained films of BiFeO 3 (BFO). Although at room temperature, bulk BFO is ferroelectric (2) and antiferromagnetic (3–5), Wang et al. (1) reported that a 70-nm film shows both an enhanced ferroelectric polarization (90 mC cm) and a substantial magnetization (1 m B /Fe). This remains the only report of a robust roomtemperature multiferroic and suggests the potential for novel devices that exploit the anticipated strain-mediated magnetoelectric coupling between the two ordered ground states. In this Comment, we argue that epitaxial strain does not enhance the magnetization and polarization in BiFeO 3 . Like Wang et al. (1), we grew BFO films on 50-nm underlayers of SrRuO 3 (SRO) on SrTiO 3 (001) substrates (STO). In addition, we used plain STO and conducting 0.2% atomic Nb-doped SrTiO 3 substrates (Nb-STO). Both BFO and SRO films were grown by pulsed laser deposition with a KrF excimer laser (248 nm, 1 Hz, target-substrate distance 0 8 cm). BFO films were grown (670-C, 8 Pa O 2 , 1.6 J cm) using a Bi-rich target of Bi 1.2 FeO 3 , because Bi is volatile (6). SRO films were grown (650-C, 9 Pa O 2 , 1.7 J cm) using a stoichiometric target. The growth rate for both BFO and SRO was 10 )/min. After deposition, films were cooled at 5-C/min to 400-C in 40 kPa oxygen, annealed for 1 hour, and then cooled to room temperature at 8-C/min. The crystalline quality of our films was investigated with high-resolution x-ray diffraction (Fig. 1). The reciprocal space maps show that the BFO and SRO in-plane lattice parameters are equal to the STO lattice parameter of 3.905 ), consistent with coherently strained films. It should be noted that Wang et al. (1) performed their calculations of saturation polarization using the bulk SRO lattice parameter of 3.935 ) for the BFO inplane lattice parameter. In our work, BFO films as thin as 40 nm gave equivalent x-ray results, but 300-nm BFO films were found to be relaxed. All coherently strained films showed a root-mean-square surface roughness of 2 nm, as determined by atomic force microscopy. BFO film stoichiometry was determined from quantitative energy dispersive x-ray spectroscopy in a thick (È400 nm) film grown at a laser repetition rate of 2 Hz. The Fe:Bi ratio was found to be unity, within the error of the technique of a few percent. The Fe oxidation state was investigated for BFO/STO samples grown at 1 Hz and 2 Hz with x-ray photoelectron spectroscopy. All samples behaved similarly; a representative scan of the Fe 2p line is shown in Fig. 2. The position of this line is expected to be 711 eV for Fe3þ and 709.5 eV for Fe2þ, and the position of the satellite is expected at 719 eV for Fe3þ and 716 eV for Fe2þ (7). From Fig. 2, we deduce that the oxidation state of Fe in our BFO/STO films is Fe3þ and that there is no evidence for Fe2þ within a resolution of a few atomic percent. To verify that our BFO films are insulating and ferroelectric, we performed piezoresponse microscopy (8) on BFO/Nb-STO and TECHNICAL COMMENT