1 Department of Mechanical Engineering, University of Puerto Rico at Mayagüez P.O. Box. 9045, Mayagüez, Puerto Rico. 00681-9045 USA. 2Department of ...
Mater. Res. Soc. Symp. Proc. Vol. 1368 © 2011 Materials Research Society DOI: 10.1557/opl.2011.1023
Tuning of Magnetic Properties in Cobalt-Doped Nanocrystalline Bismuth Ferrite Gina Montes Albino1, Oscar Perales-Pérez2, Boris Renteria2, Marco Galvez3 and Maxime J-F Guinel4 1
Department of Mechanical Engineering, University of Puerto Rico at Mayagüez P.O. Box 9045, Mayagüez, Puerto Rico. 00681-9045 USA. 2 Department of Engineering Science and Materials, University of Puerto Rico at Mayagüez, Mayagüez, Puerto Rico 00680- 9044, USA. 3 Department of Physics, University of Puerto Rico at Mayagüez, Puerto Rico 00980, USA. 4 Department of Physics, University of Puerto Rico at Rio Piedras, PO Box 70377, San Juan, Puerto Rico 00936-8377 USA. ABSTRACT This study reports on the structural and magnetic characterizations of free-standing bismuth ferrite, BiFeO3, nanoparticles synthesized in polyol medium. Fine tuning of the ferrite magnetic properties was achieved by adding an excess bismuth species or doping with cobalt ions, coupled with thermal annealing. Crystalline Bi1-yCoyFeO3 powders (where ‘y’ ranges from 0.00 to 0.10) were produced after annealing the precursors for one hour at 700οC. The average crystallite size was calculated to be approximately 22 nm. We found that the synthesis under stoichiometric excess of Bi species (up to 10 at.%) promoted a more complete crystallization of the material, i.e., no precursor phases remained. Furthermore, both the saturation magnetization and the coercivity of the synthesized powders were strongly influenced by the concentration of Co. They increased from 0.13 emu/g and 19 Oe to 3.5 emu/g and 1183 Oe for pure BiFeO3 and 10 at.% Co-doped BiFeO3, respectively. INTRODUCTION Bismuth ferrite BiFeO3 (BFO) is a multiferroic material that can exhibit multifunctional behavior at the nanoscale. Control of the synthesis parameters can enhance its magnetic, electrical and structural properties. It can find applications in electronics devices such as actuators, switches, magnetic field sensors and novel electronic memories [1, 2]. BFO has a symmetric rhombohedral perovskite structure with space group R3c and a = 5.6 Å and c = 13.9 Å [3] and is ferroelectric or antiferromagnetic at temperatures below TC ~ 1,103 K and TN ~ 643 K, respectively. It can also exhibit weak magnetism at room temperature due to residual magnetic moments from a canted spin structure. On the other hand, ferroelectricity in BFO is due to the electrostatic force between Bi3+ and oxygen anions in the octahedral sub-lattice FeO6 (perovskite structure). This electrostatic force induces a large displacement of the Bi ions relative to the FeO6 octahedrons creating a ferroelectric state [4]. In turn, the magnetic behavior observed in BFO is induced by the two iron magnetic moments rotating in the (111) plane [5]. Since BFO has both electric and magnetic polarizations, it is possible to fine tune its properties by appropriate cationic substitutions and by the presence of an excess of Bi3+. The main challenge in BFO synthesis is to avoid the formation of impurity phases such as Bi2Fe4O9. Even though it can be removed using dilute nitric acid, it lacks of reproducibility. Synthesis under Bismuth-stoichiometric excess conditions was suggested to inhibit the formation of impurity phases, promote the incorporation of substituting species and retain magnetic behavior over a range of temperature
[6,7]. On this basis, the present study investigated the synthesis of BFO under stoichiometric excess of Bi ions and co-existence of cobalt species as an attempt to achieve tunability of the magnetic properties of this material. EXPERIMENTAL Materials synthesis Pure and Co-doped BFO powders were synthesized using a polyol medium [9]. 36 mL of ethylene glycol (99.9%, boiling point 1950C) was used as the solvent for Bi(NO3)3.5H2O and Fe(NO3)3.9H2O (99.9 % purity). Co(II)-acetate was used to obtain the desired Co concentrations according to Bi1-yCoyFeO3 stoichiometry. Excess Bi(NO3)3.5H2O was employed to go beyond stoichiometric needs (5, 7 and 10 at.% Bi). The solutions were heated up to 2000C for 90 minutes to remove the ethylene glycol. The resulting solid precursors were annealed for one hour in the 6500C-7000C range. Characterization techniques The phases present in the materials synthesized were determined using x-ray diffractometry (Siemens D 500 diffractometer operating with a Cu-Kα radiation). The average crystallite sizes were calculated using Scherrer’s equation. The magnetic properties were measured at room temperature using a Lake Shore 7410 vibrating sample magnetometer (VSM). RESULT AND DISCUSSION A. X-ray diffractometry The XRD patterns for pure BFO powders synthesized with an excess of Bi (‘x’=5, 7 and 10 at.%) are shown in Fig. 1. All four samples were annealed 1 hour in air at 7000C. As seen, BiFeO3 and Bi-secondary phases (Bi2Fe4O9 or Bi25FeO39) were detected. In order to observe in more detail the information from the secondary phases, Fig. 1b shows the magnification of XRD patterns in the 250-370 2θ range. The presence of these oxides become more evident for x ≥ 7 at.%, which can be attributed to their incomplete conversion into the BFO phase. The average crystallite size of the BFO phase was calculated to be approximately 22 nm and was found to be independent of the bismuth excess.
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Fig.1.- (a) XRD patterns of BFO powders synthesized with an excess of Bi (x = 5, 7 and 10 at.%). They were annealed 1 hour in air at 7000C. (b) XRD data from the same samples in the 250-370 2θ range.
Figure 2 shows the XRD patterns for the Co-doped BiFeO3 powders where the Co concentration, ‘y’, was varied (0, 5, 7 and 10 at.% Co) with a fixed 7 at.% of bismuth excess. Figure 2 (a) shows the patterns obtained from samples annealed at 6500C, and Figure 2 (b) for samples annealed at 7000C. Also here, the corresponding XRD information in the 250-370 2θ range was magnified and presented in Figures (c) and (d). The presence of Co in the solutions tends to inhibit the formation of impurity phases. The annealing at the higher temperature (7000C) may also have promoted the more complete conversion of the Bi-intermediate oxides [11]. Note the formation of cobalt ferrite for the largest Co doping. The average crystallite size of the BFO phase was calculated to be approximately 23 nm for all eight samples.
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Fig. 2.- XRD patterns recorded from the Co-doped BiFeO3 powders (the Co concentration, ‘y’, was varied in the 0-10 at.% Co range) containing a fixed 7 at.% of bismuth excess for samples annealed at 6500C, (a), and samples annealed at 7000C, (b). The XRD data in the 250-370 2θ range for the powders annealed at 650oC and 700oC are shown, respectively, in figures (c) and (d). The effects of the annealing temperature on the formation of the BFO host were also investigated. Figure 3 shows the XRD patterns obtained from powders synthesized with 7 at.% of bismuth excess with two Co doping (a) 5 at.%, and (b) 10 at.%, while varying the annealing
temperature (600oC, 650oC and 700oC). The amounts of bismuth oxides impurities (Bi2Fe4O9) decreased with increasing temperature suggesting its progressive conversion into BFO.
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Fig. 3.- XRD patterns obtained from BFO powders synthesized with a fixed 7 at.% bismuth excess at different temperatures (600oC, 650oC and 700oC). The Co-doping was 5 at.%, (a) and 10 at.%, (b). B. Magnetic Measurements The coercivity values were found to be not affected by the excess of bismuth. The coercivity remained at 203 Oe and 28 Oe for the BFO powders annealed at 6500C and 7000C, respectively. The corresponding room temperature M-H loops for the samples synthesized with a 7 at.% bismuth excess and different concentrations of Co are shown in figure 4. Both the saturation magnetization and the coercivity were found to increase with increasing Co concentration, ‘y’. Magnetic Co ions may be responsible for the rise in magnetization and coercivity but the contribution of isolated ferromagnetic cobalt ferrite should also be taken into account. The maximum magnetization and coercivity values are summarized in table 1.
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Fig. 4.- M-H loops recorded from BFO powders synthesized with a fixed Bi excess of 7 at.% and varying Co concentration (‘y’= 0, 5, 7 and 10 at. %). Results in (a) are for samples annealed at 6500C, and in (b) for samples annealed at 7000C
Table 1. Maximum magnetization, Mmax, and coercivity, Hci, for BFO samples synthesized with a Bi excess of 7 at.% and varying cobalt concentrations at two annealing temperatures (650oC and 700oC). Co at.%
Mmax(emu/g) Hci(Oe) 650oC 0.2 86 1.6 1164 2.5 1200 3.5 982
0% 5% 7% 10%
Mmax (emu/g) Hci(Oe) 700oC 0.1 19 1.3 1164 2.3 1214 3.5 1183
The formation of BFO is strongly dependent on the annealing temperature; therefore, the annealing temperature may also play an important role on the magnetic properties of the bismuth ferrite powders. Figure 5 shows the M-H loops for the powders synthesized at different annealing temperatures and Co-doping levels (5 at. % and 10 at.%). The Bi excess was kept constant at 7 at.%. Table 2 summarizes the magnetic properties measured. The magnetization and coercivity of the powders synthesized at 5 at.% and 10 at.% exhibited opposite trends, i.e. higher annealing temperatures caused the drop in magnetization but a drastic increase in coercivity from 712 up to 1,164 Oe. The enhanced crystallinity and purity of BFO crystals with higher annealing temperatures can explain this increase in coercivity. The magnetization was drastically enhanced with increased temperature when the powders were doped with 10 at.% Co. The rise in magnetization and coercivity can be attributed not only to the incorporation of Co ions into BFO but also to the probable presence of isolated cobalt ferrite, which is known to exhibit high magnetization and moderate to high coercivity values at the nanoscale.
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Fig. 5.- M-H loops for BFO powders synthesized with a Bi excess of 7 at.% and different annealing temperatures (600oC, 650oC and 700oC). The Co concentration were 5 at.% (a), and 10 at.% (b).
Table 2 Maximum magnetization, Mmax, and coercivity, Hci, for samples synthesized with a fixed Bi excess of 7 at.% and different annealing temperatures (600oC, 650oC and 700oC). The Co concentration was 5 at.% and 10 at.%. Annealing Temperature 6000C 6500C 7000C
Mmax(emu/g) 2.7 1.6 1.3
Hci(Oe) Co 5at% 712 1164 1164
Mmax(emu/g) Hci(Oe) Co 10at% 2.2 1101 3.5 982 3.5 1183
The co-existence of cobalt ferrite and BFO structures has been reported in related works [12,13]. The corresponding room-temperature maximum magnetization and coercivity measured at 30kOe were around 1.8emu/g and 800Oe, respectively. The more suitable conditions for atomic blending of ionic species in the precursors formed in polyol medium and their subsequent crystallization at the proper annealing temperature explain the higher coercivity and magnetization levels achieved in our work. Accordingly, our results suggest the possibility of producing pure BiFeO3 and multifunctional CoFe2O4-BiFeO3 nanocomposites with tunable magnetization and coercivity at room temperature. CONCLUSIONS Nanocrystalline powder suspensions of BiFeO3 were produced from ethylene glycol solutions after annealing in air. However, BiFeO3 co-exists with minor amounts of intermediate Bi2Fe4O9. An excess of bismuth ions in the reacting solutions promotes the development of the BFO phase while inhibiting the formation of secondary bismuth phases. Magnetic properties were also affected by the cobalt concentration and the annealing temperature for a fixed excess of Bi ions. Our results suggest the formation of CoFe2O4-BiFeO3 nanocomposite powders with tunable magnetization and coercivity at room temperature. Acknowledgments This material is based upon work supported by the DOE-Grant No FG02-08ER46526. References [1] Gina Montes, et. al, Materials Research Society Symp. Proc. 1256, (2010) 1256-N06-14. [2] M. B. Holcomb, et. al, Physical Review B. 81, (2010), 134406. [3] Kumar M M and Palkar V R , Applied Physical Letters 76,(2000) 2764. [4] H.X. Lu, et.al, Physica B 406 (2011), 305-308. [5] D. Lebeugle, et. al, Physical Review B. 81 (2010), 134411. [6] S.S. Kim, et.al, Ferroelectric Letters, 34 (2007), 84-94. [7] Xiaobo He and Lian Gao, Ceramics International, 35 (2009), 975-978. [8] F. Azough et al., J. European Ceramic Society 30, (2010), 727–736. [9] Tae-Jin Park, Nano Letters 7, 766-772 (2007) [10] Matjaz Valant, Anna-Karin Axelsson and Neil Alford, Chemistry of Materials 19, (2007), 5431-5436. [11] I.V. Linevskgga and A.V. Petrova, Inorganic Materials, Vol. 45 N0 8 (2009), 930-934. [12] F. Azough et al., Journal of the European Ceramic Society, 30 (2010) 727–736 [13] W.-S. Kim et al., Journal of Magnetism and Magnetic Materials, 321 (2009) 3262–3265
