Effect of rare earth doping on BiFeO 3 magnetic and structural properties (La, Gd) This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2010 J. Phys.: Conf. Ser. 200 012134 (http://iopscience.iop.org/1742-6596/200/1/012134) Download details: IP Address: 128.230.234.162 The article was downloaded on 02/08/2013 at 09:14 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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Effect of rare earth doping on BiFeO3 magnetic and structural properties (La, Gd)
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
Effect of Rare Earth doping on BiFeO3 Magnetic and
Structural Properties (La, Gd)
Julian Andres Munevar Cagigas, Dalber Sanchez Candela and ElisaBaggio-SaitovitchCentro Brasileiro de Pesquisas Fısicas. Rua dr. Xavier Sigaud 150, CEP 22290-180, Rio deJaneiro, Brazil
Abstract. In order to remove the incommensurate cycloid spin coupling in BiFeO3 thatinhibits this multiferroic compound of showing magnetoelectric effect, we doped this perovskiteon Bi site with rare-earth ions (La, Gd), showing that the antiferromagnetic ordering withspace modulated coupling of spins is broken. A correlation between the structure and themagnetic properties is found through Rietveld Refinement of X-Ray diffraction patterns,magnetic hysteresis curves and the hyperfine parameters obtained by Mossbauer Spectroscopy atroom temperature, justified by the doping ion size that induces a magnetic structure distortionreflected in an increase of the coercive field and the remanent magnetization as well as inthe hyperfine parameters behavior, linked with the structural phase transitions caused by theremoval of the Bi 6s2 lone pair that induces structural distortions. It is also shown that thesymmetries used are polar, that in principle can let the compounds keep a ferroelectric behavior.
1. IntroductionThe research on multiferroics has been improved in the recent years because of the discoverof single phase compounds where coexistence of ferroelectricity and ferromagnetism occurs, aswell as potential applications arises [1]. BiFeO3 perovskite-like compound is a well known roomtemperature multiferroic (TC ∼ 1100 K, TN ∼ 650 K), whose crystal symmetry is trigonalR3c related to the FeO6 octahedra tilting and the stereochemically inactive Bi 6s2 lone pair,responsible for the ferroelectricity observed in single crystals [2]. BiFeO3 also shows G-type anti-ferromagnetism with a long range cycloidal ordering incommensurable with the lattice (λ = 620nm) [3] and thus no linear magnetoelectric coupling is possible. One promising route to removethis coupling is making substitutions on the Bi site with alkali metals as well as rare-earth ions[4, 5, 6, 7], that removes the cycloid coupling leading to a nonzero mangetic response, and insome cases is increased the ferroelectric response being evidence of a magnetoelectric effect [7].It was also found [4] a strong dependence of the magnetic response with the size of the dopantion responsible of the cycloidal coupling of spins suppression in BiFeO3. But, surprisingly toour knowledge, there are no systematic studies on the structural consequences of this cycloidsuppression or the magnetic structure changes induced by doping. Thus, we made multiferroicBi1−xRxFeO3 compounds with R being La and Gd with concentrations up to 50%, and roomtemperature powder X-Ray diffraction and Rietveld Refinement, 57Fe Mossbauer Spectroscopyand magnetic hysteresis curves carried out in a SQUID magnetometer.
International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics: Conference Series 200 (2010) 012134 doi:10.1088/1742-6596/200/1/012134
2. Experimental ProcedureSynthesis of polycrystalline Bi1−xRxFeO3 perovskites were carried out through solid state reac-tion route as proposed elsewhere [2]. The powder X-Ray diffraction patterns were obtained witha Panalytical X’pert Pro diffractometer with Cu Kα radiation (λ = 1.5416 A) and 2θi = 20,2θf = 80, ∆θ = 0.02 and t = 2 s. The obtained data were analyzed with the GSAS software [8].The magnetic hysteresis curves were obtained using a SQUID magnetometer at room tempera-ture, with a maximum applied field of 3 T. We also analyzed Mossbauer spectra taken at roomtemperature with a spectrometer in transmission configuration with 57Co radioactive source,the resulting spectra were analyzed with the NORMOS software. Doping with enriched 57Fewas not necessary because of the high absorption of 14.4 keV radiation by the Bi atom, butobtaining Mossbauer spectra with low absorption (< 2%).
3. Results and DiscussionThe X-Ray diffraction patterns obtained for BLFO (Bi1−xLaxFeO3) and BGFO (Bi1−xGdxFeO3)are shown in the figure 1a, showing single phase compounds. As in some previous works [6, 9],for BLFO we use a R3c symmetry for x = 0, P1 for x = 0.1 and x = 0.2, C2 (BiMnO3-like) forx = 0.3 and x = 0.4, and Pbn21 for x = 0.5. For BGFO series the crossover is narrow and wefound a continuous phase transition from R3c to Pbn21 between x = 0.1 and x = 0.2, as has beenfound recently [6]; the refinements [8] were obtained reaching reliable results (R2
Bragg < 10%).Among all the results obtained from the refinements, it’s remarkable the behavior of the per-ovskite cell volume or V
Z (figure 1b), where an abrupt decrease is found for BGFO at the phasetransition crossover, while for BLFO this abrupt decrease seems to be smoothed for higher x.This fact should be related with a change of coordination number of the A site in the perovskitestructure as a consequence of the removal of the Bi ion, in such way we should remove thestructural distortion induced by the Bi electronic configuration, being easier the removal of thedistortion in the BGFO (rGd = 1.053 A) rather than BLFO (rLa = 1.16 A), where the structurekeeps some kind of distortion. The symmetries whose best results were achieved are polar, al-lowing a possible spontaneous polarization.The magnetic hysteresis curves for BLFO and BGFO are shown in the figures 1c and 1d, where
is clearly shown the cycloid spin structure distortion, since for no doping (BiFeO3) we cannotobserve any hysteretic behavior, and for further doping we observe a nonzero magnetic response.Were extracted 2MR and 2HC as a function of doping, finding that in both cases we notice an in-crease and decrease of 2MR and 2HC , possibly related to a progressive destruction of the cycloidcoupling of spins. But even if the behavior is similar for BLFO and BGFO, we notice differentmaximum values of 2MR as well as different transition crossover widths, suggesting that the sup-pression of the cycloid spin structure is directly related to the characteristics of the doping ion.We can notice a more pronounced decrease of 2HC for BGFO if compared with BLFO. Besides,as shown in figures 1c and 1d, we find a correlation between the symmetry transitions and therelease of a remanent magnetization, possibly justified by a structure modification affecting theantiferromagnetic exchange interaction between Fe ions and distorting the cycloid coupling ofspins in the magnetic structure. For BGFO we can’t resolve clearly the transition crossover andthus we reach an orthorhombic symmetry for a relatively low doping, contrary to BLFO wherewe get low symmetries in the crossover directly related with the distortion and suppression ofthe cycloid spin structure.The Mossbauer spectra obtained (not shown) for all the samples exhibited six absorption peaks,
characteristic of a magnetic ordering. We supposed two magnetic components for the same crys-tallographic site fitted with NORMOS/SITE, regarding that we can take one site related withthe iron spin oriented 0 with respect to c crystallographic axis and the other magnetic site withthe iron spin oriented π
2 [2, 3, 10] (figure 2). For BLFO we notice an increase of the magnetic
International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics: Conference Series 200 (2010) 012134 doi:10.1088/1742-6596/200/1/012134
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Figure 1. a) Diffraction patterns of BLFO and BGFO powder compounds. b) Perovskitevolume cell as a function of doping concentration for BLFO and BGFO. Are also shown themagnetic hysteresis curves and 2MR and 2HC for c) BLFO and d) BGFO compounds.
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Figure 2. Magnetic hyperfine fields, quadrupole moments, isomer shifts and areas percentage ofthe a) BLFO and b) BGFO samples obtained supposing two magnetic sites through Mossbauerspectroscopy.
International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics: Conference Series 200 (2010) 012134 doi:10.1088/1742-6596/200/1/012134
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hyperfine field for both sites indicating an increase of the iron magnetic moment. Following theBhf behavior with the areas for each site in the Mossbauer spectra we can notice a distortion ofthe cycloid spin structure, giving us an idea on how is the cycloid coupling breaking mechanism.We also observe through the quadrupole moment for the site 2 that the asymmetry induced onthe Fe surrounding charge distribution is lost, and for the site 1 shows a slight decrease afterx = 0.3 is noticed confirming a magnetic structure distortion. The isomer shift values showsa conservation of the Fe environment throughout the doping. Regarding BGFO results (figure2b), between x = 0.1 and x = 0.2 the magnetic hyperfine fields “switches” as the difference be-tween the sites increases; the quadrupole moment shows a change from asymmetry to symmetryafter x = 0.2, indicating us a cycloid spin structure suppression after x = 0.2 as well as a widercrossover between a cycloid spin structure and an antiferromagnetic ordering for BLFO com-pared to BGFO. The magnetic hyperfine field for the site 2 tends to reach the parent compoundvalue (Bhf = 50.3 T for GdFeO3) and the the hyperfine fields switching can be justified by thesuppression of the cycloid spin structure. The isomer shift for both sites indicates a conserva-tion of the Fe environment, but in the figure 2b we notice a small variation of the isomer shiftmaybe related with a distortion effect of the magnetic structure or the lattice. Since in BiFeO3
coexists ferroelectricity and magnetism, the lack of time and space symmetry yields a cycloidspin coupling correlated with a lattice distortion [11, 10], thus one way to remove the cycloidspin ordering is through a lattice distortion, likely stoichiometric modifications [5, 4, 7, 9]. Thiscrystal symmetry correlated magnetic structure modification found is justified by a substitutioninduced crossover between the cycloid coupling of spins and a G-type antiferromagnetic order-ing, leading to a distorted antiferromagnetic ordering, as seen through the hyperfine parametersfor Fe ion. This crossover is highly influenced by the doping ion, the bigger the ionic radius thebroad the magnetic structure crossover, with a strong influence on the magnetic properties ofthe R doped BiFeO3 compounds.
4. ConclusionsWe prepared R doped BiFeO3 compounds and their further characterization gave us as amain result the distortion of the magnetic structure through a lattice distortion. This is toour knowledge the first work oriented on how the cycloid spin structure is modified by thecation substitutions, and we expect that this work can give a light in the understanding of themultiferroic phenomena. Research supported by the CAPES and CNPq Brazilian agencies.
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International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics: Conference Series 200 (2010) 012134 doi:10.1088/1742-6596/200/1/012134
