Preparation and Characteristics of BiFeO 3 Ceramics Doped by MnO 2 and Co 2 O 3

Transcription

1 Key Engineering Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Preparation and Characteristics of BiFeO 3 Ceramics Doped by MnO 2 and Co 2 O 3 Yu Xiaohua 1, a, Gu Hongxing 1, Wu Bolin 2, Wang Jian 1, Shao Gangqin 1, b, Li Xibao 1 and Ouyang Shixi 3 1 State Key Laboratory of Advanced Technology for Materials Synthesis & Processing, Wuhan University of Technology, Wuhan , China 2 Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guilin University of Technology, Guilin , China 3 China Building Materials Academy, Beijing , China a yuxiaohua@whut.edu.cn, b shaogangqin@yahoo.com.cn Keywords: Multiferroics; Ceramic; BiFeO 3 ; Magnetization Abstract. BiFeO 3 -based single-phase multiferroics have been widely studied in both ceramics and films. However, the macroscopic magnetic properties of BiFeO 3 ceramics were now most very weak. In this work, MnO 2 and Co 2 O 3 were used as B-site substitutes in BiFeO 3 ceramics in order to the enhancement of magnetic properties. The change of magnetization was analyzed based on the phase composition and the microstructure of ceramics. Introduction Single-phase multiferroics materials have attracted widely interests due to their potential applications in novel magnetoelectricity devices, and theoretic contributions for the coupling mechanism between electronic and magnetic order parameters. BiFeO 3 is now the most extensively studied material because its T C and T N are up to 1043 K and 655 K respectively [1], while other single-phase multiferroics materials behave under room temperature. BiFeO 3 has a rhombohedrally distorted perovskite structure with the space group of R3c at room temperature. The Fe magnetic moments in BiFeO 3 are coupled ferromagnetically within the pseudo- cubic (111) planes and anti-ferromagnetically between adjacent planes, showing G-type anti- ferromagnetic order. Since the spin order of BiFeO 3 ceramic is spatially modulated, there is no macroscopic magnetization at room temperature in it. Thus the weak magnetization has limited its applications. Most researches were focused on the improvement of ferroelectric properties through A-site doping by using Sr, La, Nd, Sm, Pb and so on. Several studies had enhanced the magnetic properties of BiFeO 3 through B-site substitution by using Mn, Nb, Cr and Co [2-6]. The ionic radius and outermost electronic distribution of magnetic Mn and Co were similar with Fe. In this work, the magnetic properties of BiFeO 3 ceramics doped by Co, Mn and Co-Mn were examined. Experimental The polycrystalline BiFeO 3 ceramics modified by Co and Mn, i.e., BiCo x Mn 0.1-x Fe 0.9 O 3 samples (with x = 0, 0.02, 0.05, 0.08, 0.1), were prepared from high-purity oxides ( 99.9%) by solid-state reaction technique (calcined at 750 C for 1 h and sintered at 850 C for 1 h). Phase analysis was carried by X-ray diffraction (XRD, Cu K α, Rigaku D/Max-RB, Japan). Fourier transform infrared (FT-IR) spectra of the samples were determined by a FT-IR spectrometer (Thermo Nicolet Corporation, NEXUS-870, USA). The magnetic hysteresis loops (M-H) were measured by using a vibrating-sample-magnetometer (VSM) supplied by ADE (Model 4), USA. Results and Discussion All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-09/05/16,15:48:36)

2 332 High-Performance Ceramics VI Fig. 1 XRD patterns of the mixed-powder samples which Bi 2 O 3 : Fe 2 O 3 = 1 : 1 (mol) calcinated at different temperatures Fig. 2 XRD patterns of BiFeO 3 and BiCo x Mn 0.1-x Fe 0.9 O 3 bulk samples (x = 0, 0.02, 0.05, 0.08, 0.1) sintered at 850 C Fig. 3 The room-temperature FT-IR spectra of BiCo x Mn 0.1-x Fe 0.9 O 3 bulk samples (x = 0, 0.05, 0.1) sintered at 850 C Fig. 4 The M-H curves measured at room temperature for the BiFeO 3 and BiCo x Mn 0.1-x Fe 0.9 O 3 ceramics (x = 0, 0.02, 0.05, 0.08, 0.1) sintered at 850 C Phase composition. BiFeO 3 can exist stably at narrow temperature range and the secondary phase can form easily during the solid-state reaction. Fig. 1 showed XRD patterns of the mixed-powder samples which Bi 2 O 3 : Fe 2 O 3 = 1: 1 (mol) calcinated at different temperatures. At 750 C, BiFeO 3 became the main phase, which was up to. According to the DTA result of Bi 2 O 3 :Fe 2 O 3 =1:1 composite powders in Ref. [1], the calcinated temperature was determined as 750 C. Fig. 2 showed was the XRD patterns of BiFeO 3 and result of bulk samples BiCo x Mn 0.1-x Fe 0.9 O 3 bulk samples (x = 0, 0.02, 0.05, 0.08, 0.1) sintered at 850 C. Results showed that the Bi 2 Fe 4 O 9 phase presented when x = 0 and there were no Bi 2 Fe 4 O 9 phases when x = 0.02, 0.05, 0.08, and 0.1. That meant the existence of Co was helpful to inhibit the formation of the Bi 2 Fe 4 O 9. FT-IR spectra. The room-temperature FT-IR spectra of BiCo x Mn 0.1-x Fe 0.9 O 3 bulk samples (x = 0, 0.05, 0.1) sintered at 850 C were shown in Fig. 3. The strong absorption peaks near 546cm -1 and 437 cm -1 were assigned to Fe-O stretching and bending vibration, respectively. They were characteristic peaks of the octahedral FeO 6 group in perovskite compounds. The BiFeO 3 ceramics doped by Co, Mn and Co-Mn still maintained the rhombohedrally-distorted perovskite structure because there were no evident changes in the absorption range around cm -1.

3 Key Engineering Materials Vols Table 1 The Hc and Ms of BiFeO 3 and BiCo x Mn 0.1-x Fe 0.9 O 3 ceramics (x = 0, 0.02, 0.05, 0.08, 0.1) sintered at 850 C BiFeO 3 x = 0 x = 0.02 x = 0.05 x = 0.08 x = 0.1 Hc (Oe) Ms (emu/g) Fig. 5 SEM micrographs of the fracture surface of BiFeO 3 and BiCo x Mn 0.1-x Fe 0.9 O 3 ceramics (x = 0, 0.02, 0.05, 0.08, 0.1) sintered at 850 C Magnetic properties. The magnetic hysteresis cycle (M - H) at room temperature for BiFeO 3 and BiCo x Mn 0.1-x Fe 0.9 O 3 ceramics (x = 0, 0.02, 0.05, 0.08, 0.1) sintered at 850 C were shown in Fig. 4. The related coercive fields (Hc) and saturation magnetization (Ms) were shown in Tab. 1. The M - H of BiFeO 3 ceramics was nearly a straight-line and Ms was the lowest. The Co and Mn actually improved the magnetization of BiFeO 3. Ms increased. When x = 0, the BiFeO 3 ceramics doped only by Mn maintained very weak magnetization with Ms = emu/g. When x = 0.1, the BiFeO 3 ceramics doped only by Co got the highest Ms = emu/g and the lowest Hc = Oe. The Hc reached the maximum when x = Usually the largest crystalline size of samples decreased the coercive field. Fig. 5 showed SEM micrographs of the fracture surface of BiFeO 3 and BiCo x Mn 0.1-x Fe 0.9 O 3 ceramics (x = 0, 0.02, 0.05, 0.08, 0.1) sintered at 850 C. It was found in Fig. 5 that the crystalline size of sample with x = 0.02 was not the smallest. But when x = 0.02 there were many pores in the sintered sample in Fig. 5. Hc was affected by the existence of pores too. So the high density of sintered samples would have lower Hc. The Ms increased with the increase of Co content. Enhanced magnetization in small particles was recently reported [7] and it has been attributed to the surface-induced magnetization and ferromagnetism caused by apparent oxygen deficiency. When Co or Mn was solely doped, it inhibited the crystaline growth. When Co and Mn co-existed, the crystaline size turned larger. The samples with x = 0.1 had the highest Ms and lower Hc just because the crystalline size was smallest and there was no Bi 2 Fe 4 O 9 in it. In a word, the existence of Co was helpful to the enhancement of magnetization through inhibiting the formation of Bi 2 Fe 4 O 9 and the grain growth. Conclusion The Co, Mn and Co-Mn all improved the magnetization of BiFeO 3. But they had different effect on the phase composition and crystalline size. Co was helpful to inhibit the formation of Bi 2 Fe 4 O 9 and maintain small crystalline size. Mn could not inhibit the formation of Bi 2 Fe 4 O 9. So BiFeO 3 ceramics doped by Co had the highest Ms and lower Hc. So Co 2 O 3 improved the magnetization of BiFeO 3

4 334 High-Performance Ceramics VI ceramics more than MnO 2 and Co 2 O 3 - MnO 2. Acknowledgments This work was supported by State Key Laboratory of Advanced Technology for Materials Synthesis & Processing, Wuhan University of Technology, Wuhan, China (2008-ZD-1). References [1] V. Fruth, L. Mitoseriu, D. Berger, et al.: Progress in Solid State Chemistry, Vol. 35 (2007), p.193. [2] V.A. Khomchenko, D. A. Kiselev, M. Kopcewicz, et al.: J. Magn. Magn. Mater. Vol. 321 (2009), p [3] L.Y. Wang, D.H. Wang, H. B. Huang, et al.: J. Alloys Comp. Vol. 469 (2009), p.1. [4] Y.G. Wang, G. Xu, L. L. Yang, et al.: Materials Letters, Vol. 62 (2008), p [5] A. K. Eriksson, F. Lindberg, G. Svensson, et al.: J. Solid State Chem. Vol. 181 (2008), p [6] Y.K. Jun and S. H. Hong: Solid State Communications, Vol. 144 (2007), p.329. [7] S. M. Selbach, T. Tybell, M. A. Einarsrud and T. Grande: Chem. Mater. Vol. 19(2007), p

5 High-Performance Ceramics VI / Preparation and Characteristics of BiFeO 3 Ceramics Doped by MnO 2 and Co 2 O / DOI References [2] V.A. Khomchenko, D. A. Kiselev, M. Kopcewicz, et al.: J. Magn. Magn. Mater. Vol. 321 (2009), p /j.jmmm