Journal of Magnetism and Magnetic Materials 369 (2014) 23–26
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Solvothermal synthesis and magnetic properties of BaFe12 2x(NiTi)xO19 nanoparticles Min Zhang a, Zhenfa Zi a,b,n, Qiangchun Liu c, Xuebin Zhu a, Changhao Liang a, Yuping Sun a, Jianming Dai a,n a
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Department of Physics and Electronic Engineering, Hefei Normal University, Hefei 230061, China c School of Physics and Electronics Information, Huaibei Normal University, Huaibei 235000, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 16 April 2014 Received in revised form 22 May 2014 Available online 12 June 2014
M-type hexaferrites BaFe12 2x(NiTi)xO19 (x ¼ 0, 0.05, 0.1, 0.3, 0.5, 0.7, and 0.9) were synthesized by the solvothermal method. X-ray diffraction (XRD) analysis reveals that the samples are of pure-phase with the space group of p63/mmc. As the Ni–Ti substitution level increases from x ¼0 to 0.9, the results of field-emission scanning electronic microscopy indicate that the particles are agglomerated and the mean particle size decreases. With the increase in substitution concentration x, the saturation magnetization increases first, and then declines slowly, whereas the coercivity decreases gradually. The variation of magnetic properties can be explained by the effects of Ni–Ti substitution. The results above indicate that our samples have positive effects on high density perpendicular recording. & 2014 Elsevier B.V. All rights reserved.
Keywords: Solvothermal method Magnetic materials Nanoparticles
1. Introduction M-type hexaferrite of BaFe12O19, denoted as BaM ferrite, with the magnetoplumbite structure, due to its high relatively coercivity, large saturation magnetization, high Curie temperature, excellent chemical stability and corrosion resistance, has been extensively used as highdensity perpendicular recording media, permanent magnets, microwave absorbers and so on [1–3]. However, in practical application, how to produce BaM ferrite with high saturation magnetization and coercivity controlled is still a great challenge. In order to obtain M-type hexaferrite materials with suitable saturation magnetization and coercivity, various cations such as Co–Ti [4], Co–Si [5], La–Co [6], Ce [7], Pr [8], Eu [9], etc. have been performed to substitute partially for Fe or Ba ions or both. Among these, Ni–Ti substituted BaM ferrite is rarely studied. It has been reported that Ni2 þ ions have the preferences for the 4f2 sites at lower substitution level but occupy the 12k sites at higher substitution level ( 0.5) [10] and Ti4 þ ions prefer to substitute for Fe3 þ ions in 2a and 4f2 sites with low substitution level (0.2) but in the 2a and 2b sites at higher substitution level [11]. Hence the combination of Ni2 þ and Ti4 þ ions substituted BaM ferrite can result in the enhanced magnetic properties probably. In general, the conventional ceramic route is used to prepare BaM ferrite from iron oxide and barium carbonate at high
temperature (4 1200 1C), producing large multi-domain particles not suitable for recording applications [12], which can result in agglomeration and large size of particles. In order to improve the material properties, some considerable routes such as coprecipitation method [13], sol–gel method [3], glass crystallization [14], electrospinning technique [15] and hydrothermal process [16,17] have been developed to synthesize BaM ferrite. Among these methods, the hydrothermal method requires neither expensive starting materials nor extremely high-temperature sintering. However, the BaM ferrite prepared by the hydrothermal method has so poor magnetic properties that the saturation magnetization is far less than the saturation magnetization of bulk material ( 76 emu/g at room temperature) [17–19]. In the present work, the BaM ferrite prepared by the solvothermal method shows good magnetic properties. To the best of our knowledge, no group reported the magnetic properties of the Ni–Ti substituted barium hexaferrites by the solvothermal method. In a result, single-phase M-type BaFe12 2x(NiTi)xO19 ferrites were successfully prepared. The values Ms and Hc can be tuned by Ni–Ti substitution effectively, which is very favorable for applications in the high-density recording media.
2. Experimental n
Corresponding authors. Tel.: þ 86 551 65595612. E-mail addresses:
[email protected] (M. Zhang),
[email protected] (Z. Zi),
[email protected] (J. Dai). http://dx.doi.org/10.1016/j.jmmm.2014.06.019 0304-8853/& 2014 Elsevier B.V. All rights reserved.
A series of samples with nominal chemical formula BaFe12 2x (NiTi)xO19 (x ¼0, 0.05, 0.1, 0.3, 0.5, 0.7, and 0.9) was prepared by the solvothermal method. All the reagents were of analytical grade
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M. Zhang et al. / Journal of Magnetism and Magnetic Materials 369 (2014) 23–26
and were used without further purification. The molar ration of Ba/Fe was 1/8 to ensure the formation of BaFe12O19. Fe(NO3)3 9H2O (1.64 g), Ba(NO3)2 (0.13 g) and NaOH (1.08 g) were dissolved in 50 ml ethylene glycol (EG), and the mixture was stirred for certain time. Then the mixture was transferred into a stainlesssteel autoclave (70 ml) and maintained at 220 1C for 24 h. After the reaction, the autoclave was cooled to the temperature naturally. The precipitate was filtered, washed with deionized water and ethanol several times, and dried at 80 1C overnight, immediately following the precursor was calcinated at 450 1C for 5 h, finally the as-burned power was calcinated at 900 1C for 6 h. The Ni–Ti substituted BaFe12O19 nanoparticles, with the stoichiometric ratio of Ba/(Fe, Ni–Ti) ¼1/8, were synthesized by following the same preparation process of BaFe12O19. For example, BaFe11(NiTi)0.5O19 nanoparticles were prepared from Ni(NO3)2 6H2O (0.050 g), (C4H9O)4Ti (0.058 g) and Fe(NO3)3 9H2O (1.504 g). Phase analysis of the products was performed by Philips X’pert PRO X-ray diffractometer (XRD) with Cu Kα radiation. Scanning electron microscope (SEM) was used to show the morphology and particle size. Fourier transform infrared (FTIR) spectra were recorded at room temperature for the samples in transmission mode using Nicolet Nexus FTIR spectroscopy. The magnetic properties were measured by using a superconducting quantum interference device magnetometer measurement system (SQUID, MPMS-5T).
Table 1 Crystallite size, lattice parameters a and c, ratio of c/a, saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc) for BaFe12 2x(NiTi)xO19. sample
D (nm)
a (Å)
c (Å)
c/a
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
x ¼0 x ¼0.05 x ¼0.1 x ¼0.3 x ¼0.5 x ¼0.7 x ¼0.9
55 48 52 49 46 44 41
5.896 5.895 5.896 5.894 5.891 5.888 5.889
23.216 23.223 23.223 23.226 23.229 23.236 23.249
3.938 3.939 3.939 3.941 3.943 3.946 3.948
74.8 75.8 74.3 67.8 64.3 59.3 58.9
36.3 36.0 34.6 32.2 30.3 26.2 26.2
5409 5182 4543 4265 3804 3022 2650
3. Results and discussion 3.1. Structure and morphology Fig. 1 shows the XRD patterns of Ni–Ti substituted BaFe12O19 nanoparticles. It can be seen that all the diffraction peaks are in good agreement with the standard pattern (JCPDS 00-043-0002). It is also worth noting that Ni–Ti substitution level in the BaM system results in decrease in intensity of major diffraction peaks, which confirms the incorporation of Ni–Ti into the lattice of BaM without changing the crystal structure of BaM ferrite. Due to the homogeneous of Fe and Ba species in the EG, by solvothermal method, which is beneficial for formation of M-type hexaferrite after heat-treatment. The crystallite sizes (D) evaluated by the Scherrer equation are listed in Table 1. The D decreases with increasing x, indicating the Ni–Ti substitution inhibits the growth of the crystals, which is consistent with the report [20]. The lattice constant values obtained by Rietveld method are also listed in Table 1, and the representative refinement results of x ¼0 and
Fig. 2. Rietveld refinement results of samples for x ¼0 and 0.9.
0.9 are shown in Fig. 2. The lattice constant a and c, the c/a ratio increased with increasing Ni–Ti substitution value, which can mainly be explained on the basis of the ionic radii of the substituted ions. In order to further investigate the structure of our samples, the FTIR spectra for the sintered samples at 900 1C for 6 h are presented in Fig. 3. The characteristic peaks at 591, 544 and 444 cm 1 can be attributed to the metal-oxygen stretching vibrations of tetrahedral and octahedral clusters in the hexagonal lattice [21]. With the Ni and Ti ion content increasing, the characteristic bands become more and more broad and the positions of characteristic band do not change significantly, because the wavenumber is inversely proportional to the atomic weight and here Ni ion is similar atomic weight with Fe ion. These results confirm the formation of hexaferrite, supporting the XRD results indirectly. The SEM micrographs for BaFe12 2x(NiTi)xO19 samples are show in Fig. 4a–d. It is worthy to be noted that most particles are homogeneous relatively with particle size between 100 and 200 nm. The particle sizes decrease with increasing x seeming to be much larger than D from the XRD results, which indicates that our samples are multicrystallite particles. At low substitution level, the morphology is hexagonal structure in platelet form, as seen in Fig. 4a, but the shape becomes very irregular with the increasing substitution level. It is clear that the particles for Ni–Ti substitution with x¼ 0.9 aggregates appreciably. Du et al. [22] prepared BaFe12O19 with ethylene glycol as solvent by solvothermal method, and the sample exhibits very large particle sizes up to several micrometers and serious aggregation. However, the sizes of our sample decrease sharply to nanoscale with increasing x. 3.2. Magnetic properties
Fig. 1. XRD patterns of BaFe12 2x(NiTi)xO19 with different Ni–Ti substitution levels.
The room temperature magnetic hysteresis loops of BaFe12 2x (NiTi)xO19 nanoparticles prepared with different Ni–Ti content are
M. Zhang et al. / Journal of Magnetism and Magnetic Materials 369 (2014) 23–26
shown Fig. 5. The relevant parameters are tabulated in Table 1. All the samples exhibit hysteresis behavior, indicative of ferrimagnetism. The BaFe12O19 exhibits relatively large saturation magnetization (Ms, 74.8 emu/g) and coercivity (Hc, 5409 Oe), which is compared to the theoretically value of 76 emu/g and 6700 Oe. This high Ms and Hc can ascribe to the high crystallinity and small
Fig. 3. FTIR patterns of BaFe12 2x(NiTi)xO19 ferrite samples.
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and homogeneous particle size from the SEM results. The particle sizes of all the samples are far smaller than the critical size (560 nm) of a single-domain particle reported by the reference [23], indicating that we have prepared the nanoparticles of singledomain. The single-domain particles with excellent magnetic
Fig. 5. Field-dependent magnetization curves of BaFe12 2x(NiTi)xO19 with different Ni–Ti substitution levels. The insets show the magnified view of the M–H curves at lower applied field.
Fig. 4. SEM images of BaFe12 2x(NiTi)xO19 for x¼ 0 (a), x ¼0.3 (b), x ¼0.5 (c) and x ¼0.9 (d).
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shape (at the low substitution level) is hexagonal platelet, which has a positive significance to improve the Hc value. Our results imply that substitution of Ni2 þ and Ti4 þ ions for Fe3 þ ions can effectively reduce the Hc without serious deterioration of the magnetization, which is able to achieve suitable Hc for the perpendicular magnetic recording media.
4. Conclusions
Fig. 6. Ms and Hc of BaFe12 2x(NiTi)xO19 with different Ni–Ti substitution levels.
properties are suitable for recording applications. Fig. 5 shows that Ms increases at low substitution, reaching a maximum at x ¼0.05, and then decreases for x Z0.05. Given the Gorter model [13], the 24Fe3 þ ions of M-type hexaferrite are distributed on five different crystallographic sites: three up-spin sites (2a, 12k and 2b) and two down-spin sites (4f1 and 4f2) along the c axis. The Ni–Ti substitution in different Fe3 þ sites makes contribution to the magnetic properties of the barium ferrite. It has been reported that Ni2 þ ions have preference for the 4f2 sites at low substitution level but occupy the 12k sites at high substitution level ( 0.5), while Ti4 þ ions prefer to substitute for Fe3 þ ions in 2a and 4f2 sites with low substitution level ( 0.2) but in the 2a and 2b sites at high substitution level. When Ni2 þ ions substitute for Fe3 þ ions at the 4f1 site with spin down, then the magnetic moments of the anti-parallel decrease and the net magnetization increases, causing the Ms to increase. Liu et al. [24] have reported that the magnetic structure of the M-type is determined by the indirect exchange interaction between the magnetic ions. With increasing x, the Ti4 þ ions substitution for Fe3 þ ions will lead to change in Fe3 þ (high spin) to Fe2 þ (low spin) ions in 2a site to maintain charge neutrality. As Fe2 þ –O2 –Fe3 þ exchange interactions are weaker than Fe3 þ –O2 –Fe3 þ , which can lead to magnetic dilution and hence decrease Ms. In addition, at high substitution level, the Ni2 þ and Ti4 þ substitution for Fe3 þ ions will occur in another sites, due to lower magnetic moment of Ni2 þ (2μB) and nonmagnetic Ti4 þ than that of Fe3 þ (5μB), which results in the decreased Ms. The other possibility for the reduction in Ms and remanent magnetization (Mr) is the spin canting. The Ni–Ti substituted barium hexaferrite can make the electrons spin deviate from collinear to non-collinear arrangement, which will result in the reduction of magnetic moment. This similar result was also found in La3 þ , Al3 þ or La3 þ –Co2 þ substituted barium hexaferrite [24–26]. As shown the inset (a) in Fig. 5, the coercivity decreases drastically with the increasing Ni–Ti substitution level. The variation in Ms and Hc for our samples are shown in Fig. 6. It is wellknown that the decrease in the Hc mainly depends on the reduction of the magnetocrystalline anisotropy of BaM ferrite system. Both 2b and 4f2 sites have been reported to make the greatest contribution to the magnetic anisotropy in the barium hexaferrite [27], while 4f1 tetrahedral site gives the smallest contribution. The Hc drastically decreases as x increases from 0 to 0.9, because Ni2 þ ions substitute Fe3 þ ions in 4f2 sites. Zhang et al. [28] have reported that the nonmagnetic Ti4 þ ions substitute for magnetic Fe3 þ ions also causes the reduced Hc. Additionally, the shape anisotropy can also play an important role. The grain
Ni–Ti substituted M-type barium ferrite nanoparticles have been prepared by the solvothermal method. XRD results show that all the samples are single-phase. It can been seen from FE-SEM results the particle size decreases slightly after Ni–Ti substitution. Ni–Ti combination not only can keep the high Ms value effectively, but also can decrease the Hc drastically. Thus we can modulate the coercivity easily for the perpendicular magnetic recording media.
Acknowledgements This work was financially supported by the National Nature Science Foundation of China (Grant nos. 11274314, 11374304, and U1232210), the National Basic Research Program of China (2014CB931704), and the Postdoctoral Science Foundation of China (Grant no. 2013M541848). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
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