Article pubs.acs.org/JPCC
Magnetic, Ferroelectric, and Magnetocapacitive Properties of Sonochemically Synthesized Sc-Doped BiFeO3 Nanoparticles Dimple P. Dutta,*,† Balaji P. Mandal,† Ratna Naik,‡ Gavin Lawes,‡ and Avesh K. Tyagi*,† †
Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201, United States
‡
ABSTRACT: We report the synthesis of undoped and Sc3+doped BiFeO3 nanoparticles using the sonochemical technique. X-ray diffraction reveals that all samples are single phase with no impurities detected. EDX analysis was done to confirm the extent of Sc3+ doping in the samples. The size and morphology of the nanoparticles have been analyzed using transmission electron microscopy (TEM). XPS studies were done to check the presence of Fe2+ ions in the samples. The BiFeO3 nanoparticles show a weak ferromagnetic behavior at room temperature, which is quite different from the linear M− H relationship reported for bulk BiFeO3. The substitution of Sc ions for Bi enhances the ferromagnetic as well as ferroelectric properties of this system, which is mainly attributed to the antiferromagnetic core and ferromagnetic surface of the nanoparticles, together with the mild structural distortion. Temperature and field dependence of magnetization curves reveal the frustrated magnetic behavior of this system. The leakage current is considerably reduced, and electric polarization increases significantly in the case of BiFe0.95Sc0.05O3 nanoparticles. Magnetoelectric coupling was observed in the BiFe0.95Sc0.05O3 sample. Thus, it can be inferred that Sc3+-doped BiFeO3 nanoparticles show promise as good multiferroic materials.
1. INTRODUCTION Multiferroic materials have coupled electric, magnetic, and structural order parameters that result in simultaneous ferroelectricity, antiferro/ferromagnetism, and ferroelasticity.1,2 They are rare in nature since for being simultaneously ferroelectric and ferromagnetic, the material requires empty and partially filled transition metal d-orbitals, respectively, which are mutually exclusive properties.3,4 Interest in these materials is due to their potential applications in memory devices, spintronics, and sensors.5−8 BiFeO3 (BFO) is a natural ferroelectromagnet, which exhibit both ferroelectricity and ferromagnetism (i.e., multiferroism, TC ∼ 1100 K, TN ∼ 643 K). However, its spontaneous polarization and saturation magnetization are disappointingly low when compared to many standard ferroelectrics and ferromagnets. This is due to the superimposition of a spiral spin structure on BFO’s antiferromagnetic order. In bulk BFO the Dzyaloshinsky− Moriya (DM) interaction results in a canted AFM ordering of Fe3+ spins in the system, which induce a lattice strain increasing the free energy of the crystal. To minimize the free energy, a spiral spin structure is superimposed on the AFM ordering resulting in rotation of the antiferromagnetic axis through the crystal with an incommensurate long wavelength period of 62 nm. This cancels the macroscopic magnetization and inhibits the observation of the linear magnetoelectric effect in bulk BFO.9,10 Apart from this, in bulk BFO, measurement of ferroelectric and transport properties are also hindered by leakage problems, which arise as a result of defects, nonstoichiometry, and low resistivity. Thus, for practical © 2013 American Chemical Society
applications of this material, it is essential to improve its multiferroic properties. This can be achieved to some extent by decreasing the size of BFO to less than 62 nm and also by introducing suitable dopant ions in the material. The decrease in particle size below the periodicity of helical ordering gives rise to the suppression of modulated spin structure which improves the magnetization in nanoscale particles.11−15 A or B site substitution in perovskite BFO can lead to reduction in leakage current and increase in resistivity by eliminating secondary impurities and oxygen vacancies, thereby improving its ferroelectric properties.16−19 Hence, there is a lot of interest in synthesis of suitably doped phase pure BiFeO3 nanoparticles. Synthesis of phase pure BFO has been a challenging task since the products are usually contaminated with the presence of Bi2Fe4O9. Recently, we had reported the sonochemical synthesis of phase pure BFO nanorods where codoping with various metal ions (Ba2+ and Mn2+) or (Ca2+ and Cr3+) enhanced its magnetic and ferroelectric properties.20 Particularly, the Cr3+-doped samples exhibited saturation of electrical polarization with less leaky characteristics. With Cr 3+ substitution, the acceptor doping of Fe3+ by Fe2+ was reduced since Cr3+ is very stable electronically, and this led to the decreased conductivity. Here we report the sonochemical synthesis of phase pure undoped and Sc3+-doped BFO nanocubes. The choice of the dopant ion was based on the Received: October 30, 2012 Revised: January 3, 2013 Published: January 27, 2013 2382
dx.doi.org/10.1021/jp310710p | J. Phys. Chem. C 2013, 117, 2382−2389
The Journal of Physical Chemistry C
Article
fact that Sc3+ has a stable electronic configuration, which is expected to improve ferroelectric properties and minimize the leakage current in BFO. Sc3+-doped BFO thin films deposited on Pt/Ti/SiO2/Si substrates reported only a slight improvement in magnetic and ferroelectric properties.21 The origin of magnetism in the film was due to coexistence of Fe2+/Fe3+ ions and doping of nonmagnetic Sc3+ ions into the iron sites of BFO reduced the leakage current and hence improved its ferroelectric properties by arresting the proportion of Fe2+/Fe3+ double-exchange interactions. The role of Sc3+ doping in the BFO nanoparticles has yet not been explored. There are also not any reports on the extent of magnetoelectric coupling in this type of substituted BFO material. In this study, we have prepared undoped BiFeO3 and BiFe(1−x)ScxO3 (where x = 0.02, 0.05, 0.1) nanocubes using the sonochemical method. The materials have been characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) techniques. The magnetic properties, such as magnetization dependence on temperature (M−T) and on field (M−H), have been investigated. Five important results are found: (i) sonochemically synthesized BiFeO3 nanocubes are able to incorporate Sc3+ doping up to 5 mol % with no effect on the basic crystal structure, (ii) XPS data show the absence of Fe2+ ions in the BiFeO3 samples, (iii) an enhancement in magnetization is seen on increase of Sc3+ doping up to 5 mol % in BiFeO3, which cannot be explained on the basis of the coexistence of Fe2+/ Fe3+ and their double-exchange interactions as was in the case of BiFeO3:Sc3+ thin films, (iv) leakage current is considerably reduced and electric polarization increases in case of BiFe0.95Sc0.05O3 nanoparticles, and (v) magnetoelectric coupling was observed in the doped sample.
in the scattering angular range (2θ) of 15°−80°. Silicon was used as an external standard for correction due to instrumental broadening. The average crystallite size was calculated from the diffraction line width based on Scherrer’s relation: d = 0.9λ/B cos θ, where λ denotes the wavelength of X-rays and B is the corrected full width at half-maxima (fwhm). EDX analyses was were carried out using an Inca Energy 250 instrument coupled to Vega MV2300t/40 scanning electron microscope. Conventional TEM micrographs were recorded on JEOL 2000FX. The particulates obtained were dispersed in methanol solution and then deposited on the carbon-coated copper grids for TEM/ SAED studies. Raman spectra were recorded using an indigenously developed confocal micro-Raman setup configured around a HORIBA Jobin Yvon spectrograph. 532 nm of the frequency-doubled diode pumped solid state Nd:YAG laser source was used for excitation. X-ray photoelectron spectra (XPS) were recorded using Mg Kα radiation (1253.6 eV) on a VG Microtech FISON instrument. The base pressure of the experimental chamber was
