SrFe12O19 Fe2O3纳米复合永磁材料的磁性研究

摘要： 引言

M-type strontium hexaferrite (SrFe12O19) was discovered in the 1950s by Philips laboratories [1]. As one of ferrous magnetic oxide, it has been intensively investigated during the last years due to its appropriate magnetic properties, chemical stability and low cost compared with rare-earth compounds. It has been recognized that it can be used as permanent magnets, high-density magnetic and magneto-optic recording media, and microwave filters [2-3].

In M-type hexaferrites, the iron ions occupy on five different sites: the octahedral sites, crystallographically known as 2a, 12k and 4f2, and the tetrahedral sites 4f1 and 2b. In the magnetically ordered state, the 12k, 2a and 2b sites (eight Fe3+ ions in all) have their spins aligned parallel to each other and to the crystallographic c-axis, whereas spins of 4f1 and 4f2 sites (four Fe3+ ions in all) align in an opposite direction, which leads to the lower saturation magnetization Ms. High-performance permanent magnets for energy-related applications require a large energy product (BH)max. A permanent magnet with a large (BH)max value should exhibit both high remanent magnetization Mr and large coercivity Hc. Both parameters are determined not only by intrinsic properties such as the magnetocrystalline anisotropy Ku and saturation magnetization Ms, but also by structural parameters such as grain sizes and alignment of the granular materials which are sensitive to the preparation conditions. So many research have been made on improving the magnetic properties of SrFe12O19, such as the cationic substitution [4-5], investigation of synthesis method and optimum to the processing conditions, etc. [6-12]. In addition, exchange coupling through the interface between hard and soft magnetic phases was found to drastically modify the magnetic properties of nanocomposite combining the high magnetization of a

soft-magnetic phase with the high anisotropy of a hard one [13].

In this work, crystal structure, magnetic properties and exchange-coupling behavior of M-type strontium hexaferrite prepared by chemical co-precipitation method have been systematically studied by tuning the Fe3+/Sr2+ mole ratios and calcinations temperature. The results show that the lower Fe3+/Sr2+ mole ratio (10:1) greatly reduces the crystallization temperature and benefits to the formation of single phase SrFe12O19. For the single phase SrFe12O19 with calcinations temperature of 1000 ºC, the optimum magnetic parameters are obtained, and the coercivity and saturation magnetization are 4751 Oe and 62.68 emu/g, respectively, while the Fe3+/Sr2+ mole ratio being 11:1. For the samples with Fe3+/Sr2+ mole ratio being 12:1, soft magnetic γ-Fe2O3 and hard magnetic SrFe12O19 phases coexist in the sample and exchange coupling interaction between them results in the improvement of both coercivity and saturation magnetization, which to our knowledge has not been reported before in nanacomposite ferrites, because the previously reported results show that the exchange coupling generally leads to the increase of saturation magnetization but the decrease of coercivity[13-14]. Comprehensively, the exchange coupling between the hard and soft magnetic phases doesn’t realize the distinct enhancement of magnetic parameters as described by R. Skomski and S.D. Bader et al[15-16]. The possible reasons have been suggested and further investigations have been in progress.

实验

Strontium hexaferrite powders were prepared by the chemical co-precipitation method. The analytically pure ferric nitrate (Fe(NO3)3•9H2O), strontium carbonate (SrCO3), nitric acid (HNO3) and sodium hydroxide (NaOH) were used as starting materials. HNO3 was used to dissolve the strontium carbonate and to obtain strontium nitrate solution. First, a series of ferric nitrate (dissolved in distilled water) and strontium carbonate (dissolved in nitric acid) solutions with various Fe3+/Sr2+ molar ratios of 12:1 was mixed by gentle heating and stirring for 1 h using a magnetic stirrer.

Then, sodium hydroxide as precipitant was slowly poured into the compound solution at room temperature at pH = 10. The co-precipitate solution was kept in air for 24 h at room temperature. The co-precipitate was filtered and washed several times using distilled water until the pH value of the solution became neutral, and dried at 90 oC for 24 h. The dried powders were calcined at 500 oC for 5 h, and then sintered at six different temperatures of 600, 700, 800, 900, 1000 and 1100 oC for 2h in air. The obtained samples are listed in Table 1.

The crystalline structural analysis was performed by an X-Ray diffractometer using Cukα radiation source. The morphology of samples was investigated by scanning electron microscopy (SEM). The magnetic properties were measured by Quantum Design superconducting quantum interference device (SQUID) MPMS system (T＝300 K, 0≤H≤2 T).

2 结果与讨论

2.1 样品XRD分析

800C222 1-SrFe12O19 2-Fe2O32Intensity (a.u.)900C21000C2111111121111111111100C20304050degree)607080

图1 样品在不同退火温度下的XRD谱

Fig 1 XRD patterns of different sintering temperatures of the sample

2.2 样品SEM分析

图2 样品在不同退火温度下的SEM图片 (a) 800 oC (b) 900 oC (c) 1000 oC (d) 1100 oC

2.3 样品磁性能分析

表1 A样品不同温度下的磁性能

样品 800 oC 900 oC 1000 oC 1100 oC

磁性能

Hc(Oe) Ms(emu/g) Mr(emu/g) Mr/Ms 4578 4000 2816 1650

62.23 59.19 58.97 58.15

32.27 30.67 24.12 26.19

0.52 0.52 0.41 0.45

0.30.20.10.0m-0.1-0.2-0.3-0.4-0.50.00.51.0 800 C 900 C 1000 C 1100 C1.52.0

ooooH ( T )图3 样品在不同退火温度下的退磁曲线 (a)800 oC (b) 900 oC (c) 1000 oC (d) 1100 oC

The intensity of exchange coupling between hard and soft magnet phases inside materials can be showed by demagnetization curve analyse method (δM) [23-26] , its definition is:

δM (H) = Md (H) — [1－2Mr (H) ]

Md is the remanence ratio of demagnetization curve, Mr is the remanence ratio of initialization curve. If δM > 0, exchange coupling gives priority to crystal grains. The higher the value of δM peak is, the stronger the exchange coupling is. If δM < 0, long distance magnetism action gives priority to crystal g rains. If δM = 0, it shows there is nonentity action between grains.

Fig. 5 shows the δM curves for A samples sintered at 800, 900, 1000 and 1100 °C, respectively. It can be seen from Fig. 4 that when annealed at 800 and 900°C the samples contains SrFe12O19 and Fe2O3 phases, from Fig. 5 we can seen that for annealed at 800 and 900°C, δM shows a positive peak, those indicated that between SrFe12O19 and Fe2O3 line in exchange coupling behavior. When annealed at 800°C its positive peak value higher than annealed at 900°C its positive peak value, it indicated that exchange coupling behavior under at 800°C stronger than at 900°C. That is why

the saturation magnetization of A samples under at 800°C larger than at 900°C. From Fig. 4 we can seen that when annealed at 1000 and 1100°C the single SrFe12O19 phase was appeared and Fe2O3 phases was disappeared. It has to be noted that in Fig. 5, when annealed at 1000 and 1100°C, δM shows a negative peak, those indicating that magnetostatic interactions become dominant. That is why the saturation magnetization of A samples under at 1000°C small than at 900°C. Fig. 5 shows the peak value of δM at 1100°C was the lowest than other temperature, so its saturation magnetization was the minimum.

3 结 语

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