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DOI: 10.1039/C4RA01807A (Paper) RSC Adv., 2014, 4, 18885-18888

Facile hydrothermal synthesis of core/shell-like composite SrFe12O19/(Ni, Zn)Fe2O4 nanopowders and their magnetic properties

Ailin Xia*a, Suzhen Rena, Conghua Zuoa, Lijiao Zhangb, Mingfu Xiea, Yong Denga, Ruonan Wua, Wei Xua, Chuangui Jina and Xianguo Liua
aAnhui Key Laboratory of Metal Materials and Processing, School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, China. E-mail: alxia@126.com; Fax: +86-0555-2311570; Tel: +86-13665557919
bSchool of Science, Hebei University of Science and Technology, Shijiazhuang 050018, China

Received 1st March 2014 , Accepted 11th April 2014

First published on 11th April 2014

Abstract

SrFe12O19/(Ni, Zn)Fe2O4 nanopowders with exchange coupling were synthesized via a facile hydrothermal method, and their structural and magnetic properties were studied. It is found that only when the mass ratio of SrFe12O19 to (Ni, Zn)Fe2O4 is 2[thin space (1/6-em)]:[thin space (1/6-em)]1, the specimen exhibits a two-phase composite structure. The synthesized composites are quite stable under high temperatures lower than 950 °C. It is also found that the nanocomposites exhibit a core/shell-like structure, and both the saturation magnetization and coercivity are greatly influenced by the content of soft phase and impurity.

1. Introduction

In order to improve the magnetic properties in hard magnetic materials, the exchange coupling between magnetically hard and soft phases is widely studied, especially in rare-earth magnets.1–8 Hexagonal M-type SrFe12O19 (SrM) ferrite is one of the most extensively used hard magnetic materials for its fine chemical stability, magnetic and electrical properties. Recently, the exchange coupling in SrM composite ferrite has attracted more and more interest. Zhang et al.8 studied the magnetic and microwave absorption properties of SrM/CoFe2O4 nanocomposites with a core/shell structure synthesized via a two-step coprecipitation method. Song et al.9 studied the microstructure, magnetic properties and exchange-coupling interactions of one-dimensional SrM/Ni0.5Zn0.5Fe2O4 composite ferrite nanofibers synthesized via the electrospinning and calcination process. Hue et al.10 studied the structure and magnetic properties of SrM/La1−xCaxMnO3 composites synthesized via two stages combined citrate precursor sol–gel and hydrothermal method. Radmanesh et al.11 studied the synthesis and magnetic properties of SrM/Ni0.7Zn0.3Fe2O4 nanocomposites synthesized via a combination of sol–gel self-propagation and glyoxilate precursor methods. Mehdipour et al.12 compared the microwave absorption properties among SrM/NiFe2O4, SrM and NiFe2O4 particles synthesized by the coprecipitation of chloride salts using the sodium hydroxide solution.

As a typical chemical method to synthesize nanopowders, the hydrothermal method is widely used to synthesize oxide powders, including magnetic ferrites.13–16 However, how to synthesize composite ferrite powder with exchange coupling via hydrothermal method is still a challenge, and there is still no relative report. In this study, a novel facile hydrothermal route was used to synthesize SrM/Ni0.4Zn0.6Fe2O4 composite nanopowders with exchange coupling, and their structural and magnetic properties are studied.

2. Experimental

2.1 Synthesis of composite specimens

All the reagents used were analytically pure. First, magnetically hard SrM ferrite nanopowder was prepared via a hydrothermal process. Aqueous solutions of Fe(NO3)3 and Sr(NO3)2 were coprecipitated by NaOH. In order to synthesize single phase SrM powder, the molar ratio of OH−/NO−3 and the atomic ratio of Fe/Sr were set to 1[thin space (1/6-em)]:[thin space (1/6-em)]3 and 1[thin space (1/6-em)]:[thin space (1/6-em)]4, respectively.15 The precipitates and aqueous solution obtained after coprecipitation were hydrothermally reacted in a Teflon liner at 220 °C for 5 h. In order to ensure the purity, as-synthesized SrM powder was washed by diluted hydrochloric acid (22 wt%).16 Then, in order to prepare magnetically soft (Ni0.4Zn0.6)Fe2O4 (NZFO) ferrite, stoichiometric amount of nitrates (Fe(NO3)3, Zn(NO3)2 and Ni(NO3)2) were dissolved in deionized water and coprecipitated by NaOH. The precipitates and aqueous solution obtained were moved to a Teflon liner, and the SrM powder previously synthesized was also added to the Teflon liner with different mass ratio of SrM to NZFO (Rm, 2[thin space (1/6-em)]:[thin space (1/6-em)]1, 1[thin space (1/6-em)]:[thin space (1/6-em)]1, 1[thin space (1/6-em)]:[thin space (1/6-em)]2, 1[thin space (1/6-em)]:[thin space (1/6-em)]3 and 1[thin space (1/6-em)]:[thin space (1/6-em)]4). Finally, the mixture was hydrothermally reacted at 200 °C for 8 h to synthesize SrM/NZFO nanocomposites. The powders obtained after hydrothermal reaction were used as the specimens in this study. The procedure for the hydrothermal synthesis of SrM/NZFO nanocomposites can be schematically illustrated in Fig. 1.
image file: c4ra01807a-f1.tif
Fig. 1 Schematic procedure of the formation of the SrM/NZFO nanocomposites.

2.2 Characterization

The phase identification and microstructural studies were carried out by using X-ray diffractometry (XRD, Rigaku D/max-2550V/PC) with Cu Kα radiation, thermal gravimetric analysis and differential thermal analysis (TGA–DTA, Shimadzu DTG-60(H)), and transmission electron microscopy (TEM/HRTEM, JEOL JEM2010). Magnetic hysteresis loops (MHLs) were measured at room temperature on a vibrating sample magnetometer (VSM, Quantum Design Versalab) with a maximum external field Hm ≈ 2389 kA m−1 (30[thin space (1/6-em)]000 Oe).

3. Results and discussion

Fig. 2(a) shows the XRD pattern of as-synthesized hydrothermal SrM powder. Only characteristic peaks from SrM are found in Fig. 2(a), indicating that the SrM powder used to synthesize composites is single phase. Fig. 3 shows the XRD patterns of hydrothermal SrM/NZFO composites with different Rm. Since the typical characteristic peaks from SrM (“△”) and NZFO (“▲”) ferrites are present in all the patterns, the SrM and NZFO phases do exist in the powder specimens. Moreover, with the decrease of Rm, the relative intensity of peaks from SrM decreases markedly, while that from NZFO increases markedly, indicating an increasing content of NZFO in the composite powders. However, except for Rm = 2[thin space (1/6-em)]:[thin space (1/6-em)]1, also found in the XRD patterns are the characteristic peaks from Fe2O3 (“∇”), especially in Fig. 3(b) (Rm = 1[thin space (1/6-em)]:[thin space (1/6-em)]1), suggesting that only the specimen with Rm = 2[thin space (1/6-em)]:[thin space (1/6-em)]1 is the pure two-phase SrM/NZFO composite. Note that since the relative intensity of peaks from Fe2O3 decreases markedly with the decrease of Rm from 1[thin space (1/6-em)]:[thin space (1/6-em)]1 to 1[thin space (1/6-em)]:[thin space (1/6-em)]4, there is a great amount of Fe2O3 in the specimen with Rm = 1[thin space (1/6-em)]:[thin space (1/6-em)]1, while it continuously decreases to a small amount in the specimen with Rm = 1[thin space (1/6-em)]:[thin space (1/6-em)]4. According to our experience, under the same hydrothermal conditions of synthesizing composites hereinbefore, the as-synthesized NZFO powder, whose XRD pattern is shown in Fig. 2(b), should be single phase. Therefore, the SrM powder added in the Teflon liner influences the phase formation of NZFO during the hydrothermal process. When Rm = 2[thin space (1/6-em)]:[thin space (1/6-em)]1, a great number of SrM nanoparticles in the Teflon liner can act as the nucleation sites for NZFO (Fig. 1), facilitating the phase formation of NZFO. When Rm = 1[thin space (1/6-em)]:[thin space (1/6-em)]1, the number of nucleation sites is greatly reduced, which accounts for the great amount of Fe2O3 in the corresponding specimen. However, the decrease of Rm suggests that more NZFO nanoparticles can be synthesized at the beginning of hydrothermal process. Therefore, more nucleation sites may be supplied by NZFO nanoparticles, which further facilitates the phase formation of NZFO. Consequently, the content of impurity Fe2O3 in the composites is reduced with the decrease of Rm.
image file: c4ra01807a-f2.tif
Fig. 2 XRD patterns of (a) hydrothermal SrM, and (b) NZFO powder under the same hydrothermal conditions of composites.

image file: c4ra01807a-f3.tif
Fig. 3 XRD patterns of hydrothermal SrM/NZFO composites with different Rm. Rm is: (a) 2[thin space (1/6-em)]:[thin space (1/6-em)]1; (b) 1[thin space (1/6-em)]:[thin space (1/6-em)]1; (c) 1[thin space (1/6-em)]:[thin space (1/6-em)]2; (d) 1[thin space (1/6-em)]:[thin space (1/6-em)]3 and (e) 1[thin space (1/6-em)]:[thin space (1/6-em)]4.

Fig. 4 shows the TGA–DTA curves (from room temperature to 950 °C) of the pure two-phase SrM/NZFO composite with Rm = 2[thin space (1/6-em)]:[thin space (1/6-em)]1. The endothermic peak at around 40 °C accompanied with a minor weight loss (about 1.5%) is attributed to the dehydration of absorbing water in the specimen. However, except this peak, there is no other noticeable endothermic peak found in the figure, indicating no phase transition from the room temperature to 950 °C. The endothermic process from about 300 °C to 950 °C could be ascribed to the growth of grains. This result of DTA curve is confirmed by the TGA analysis. Almost no other weight loss is found except for the dehydration of absorbing water, which further implies no phase transition during the heating process. Though not given here, the DTA–TGA curves behave similarly for the specimens with other Rm. Therefore, the synthesized composites are quite stable under high temperatures lower than 950 °C.


image file: c4ra01807a-f4.tif
Fig. 4 TGA–DTA curves of the composite specimen with Rm = 2[thin space (1/6-em)]:[thin space (1/6-em)]1.

Fig. 5(a) gives the typical TEM morphology of the composite with Rm = 2[thin space (1/6-em)]:[thin space (1/6-em)]1. The image reveals that the nanocomposite exhibits a core/shell-like structure in which the NZFO is coated on the surface of SrM nanoparticles. This can be seen more clearly from the inset enlarged figures of selected areas. Note that since the transmittance of electrons in SrM is different from that in NZFO due to the different crystalline structure and magnetism, the SrM phase may absorb more electrons.17 In Fig. 5(a), the core regions present a slightly dark color, while the shell regions present a slightly bright color. Fig. 5(b) and (c) schematically give the HRTEM lattice fringe images of NZFO and SrM phases in the composite ferrite with Rm = 2[thin space (1/6-em)]:[thin space (1/6-em)]1, respectively. Lattice fringes are well matched with the theoretical expected from the P63/mmc and Fd[3 with combining macron]m symmetry for SrM and NZFO, respectively. The interplanar distances observed for (106) and (200) crystallographic planes in SrM are about 0.314 nm and 0.253 nm, respectively, and that for (111) plane in NZFO is about 0.502 nm. This result further confirms the existence of SrM and NZFO phases in the composite.


image file: c4ra01807a-f5.tif
Fig. 5 Typical TEM morphology of (a) composite, and HRTEM lattice fringe images of (b) NZFO and (c) SrM phases in the composite. Rm = 2[thin space (1/6-em)]:[thin space (1/6-em)]1.

Fig. 6 gives the MHLs of SrM/NZFO composites with different Rm, and the corresponding saturation magnetization (Ms) and coercivity (Hc) are listed in Table 1. As is known, for composites with exchange coupling, a wasp-waisted MHL suggests a poor exchange coupling between magnetically hard and soft phases.15 However, all the MHLs in Fig. 6 are very smooth, indicating that though crystallographically composed of two phases, all the specimens magnetically exhibit a fine single phase behavior.18 Therefore, the SrM and NZFO phases in the composites are well exchange coupled. In Table 1, the Ms and Hc of single phase SrM nanopowder are also given. Affected by the finite size effects, the Ms of SrM nanopowder obtained is much smaller than that of bulks.19,20 However, compared with SrM nanopowder, the Ms of composite with Rm = 2[thin space (1/6-em)]:[thin space (1/6-em)]1 is improved slightly from 57.4 to 59.1 emu g−1, which should be ascribed to the exchange coupling between the hard and soft magnetic phases. However, the Ms of specimen with Rm = 1[thin space (1/6-em)]:[thin space (1/6-em)]1 is greatly reduced to 47.1 emu g−1 due to the existence of a lot of impurity Fe2O3. Then, the Ms increases from 47.1 to 52.8 emu g−1 with the decrease of Rm due to both the reduced content of Fe2O3 and the enhanced content of soft NZFO in specimens.


image file: c4ra01807a-f6.tif
Fig. 6 MHLs of SrM/NZFO composites with different Rm.
Table 1 Magnetic properties of single phase SrM nanopowders and SrM/NZFO nanocomposites with different Rm
Specimen Ms (emu g−1) Hc (kA m−1)
SrM 57.4 29.9
Rm = 2[thin space (1/6-em)]:[thin space (1/6-em)]1 59.1 54.7
Rm = 1[thin space (1/6-em)]:[thin space (1/6-em)]1 47.1 23.1
Rm = 1[thin space (1/6-em)]:[thin space (1/6-em)]2 50.6 9.8
Rm = 1[thin space (1/6-em)]:[thin space (1/6-em)]3 52.0 9.9
Rm = 1[thin space (1/6-em)]:[thin space (1/6-em)]4 52.8 4.0

Seen from Table 1, the Hc of SrM nanopowder is 29.9 kA m−1, which is much smaller than the corresponding bulks. As is known, for ultrafine grains, the Hc behaves proportionally to the average grain size (D) as follows:21

 
image file: c4ra01807a-t1.tif(1)
where pc is a dimensionless factor; A is the exchange constant; K1 is magnetocrystalline anisotropy constant. Obviously, the small D results in a great reduction of Hc.22 Compared with SrM nanopowder, the Hc of composite with Rm = 2[thin space (1/6-em)]:[thin space (1/6-em)]1 is improved markedly from 29.9 to 54.7 kA m−1, which confirms that the core/shell structure is helpful to improve the Hc of composites with exchange coupling.7,23 However, the Hc decreases significantly from 54.7 to 4.0 kA m−1 with the decrease of Rm from 2[thin space (1/6-em)]:[thin space (1/6-em)]1 to 1[thin space (1/6-em)]:[thin space (1/6-em)]4. The Hc in a hard/soft exchange coupled system can be approximated by:2,10
 
image file: c4ra01807a-t2.tif(2)
where HcH is the coercivity of the hard phase, fS is the volume fraction of the soft phase, and MH and MS are the magnetizations of the hard and soft phases, respectively. According to the model presented by formula (2), with the increase of fS, the Hc in composites will decrease obviously.

4. Conclusions

In summary, a facile hydrothermal method was used to synthesize the composite SrFe12O19/(Ni, Zn)Fe2O4 nanopowders with different mass ratio (Rm). All the composites contain SrFe12O19 and (Ni, Zn)Fe2O4 phases. However, except for Rm = 2[thin space (1/6-em)]:[thin space (1/6-em)]1, impurity Fe2O3 is also found in all the other specimens. The TGA–DTA study shows that the synthesized composites are quite stable under high temperatures lower than 950 °C. The result of TEM shows that the synthesized nanocomposites exhibit a core/shell-like structure. Seen from the magnetic hysteresis loops, the magnetically hard and soft phases are well exchange coupled, and both the saturation magnetization and coercivity are greatly influenced by the content of impurity and soft phase.

Acknowledgements

This work was supported by the National Natural Science Foundation of China under Grant no. 11204003, the National Training Programs of Innovation and Entrepreneurship for College Students (TPIECS) under Grant no. 201310360013, the Provincial TPIECS under Grant nos. AH201310360013 and AH201310360263, the Student Research Training Program (SRTP) of AHUT under Grant no. 2013017Y and the Scientific Research Foundation for Ph.D. in Hebei University of Science and Technology under Grant no. QD200949.

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