Materials Letters 157 (2015) 277–280

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Microstructure and magnetic properties of W-type hexagonal ferritesBa1�xSrxFe2þ2 Fe3þ16 O27

Farui Lv, Xiansong Liu n, Shuangjiu Feng, Kai Huang, Xiaofei Niu, Xin Huang, Feng Huang,Yuqi Ma, Shuai Jiang, Yingchun WuEngineering Technology Research Center of Magnetic Materials, School of Physics & Materials Science, Anhui University, Hefei 230601, PR China

a r t i c l e i n f o

Article history:Received 17 March 2015Received in revised form29 April 2015Accepted 30 April 2015Available online 28 May 2015

Keywords:W-type ferritesThe ceramic processMicrostructureMagnetic properties

x.doi.org/10.1016/j.matlet.2015.04.1517X/& 2015 Elsevier B.V. All rights reserved.

esponding author. Fax: þ86 551 65107674.ail address: xiansongliu@ahu.edu.cn (X. Liu).

a b s t r a c t

A series of W-type hexagonal ferrites with the composition Ba Sr Fe Fe Ox x1 22

163

27−+ + (0rxr1) were prepared

by the ceramic process in a reducing atmosphere during the whole process of pre-sintering and sintering.The phase composition, micro-morphology, and magnetic properties of the particles were investigatedby XRD, SEM, and VSM. The results of XRD show that the single phase was observed in the W-typeferrites with different Sr content. The micro-morphology of the particles exhibits the uniform planehexagonal structures of W-type ferrites with different Sr content. The coercivity (Hc) of the particlesincreases with the increase of Sr content (x), while the saturation magnetization (Ms) of the particles firstdecreases with x from 0 to 0.2, and then begins to increase when x continues to increase.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

The hexagonal ferrite was first discovered in 1950s [1] and sincethen, people began to become more and more interested in it. Itwas widely used as permanent magnets, microwave devices, mag-neto-optics and magnetic recording media due to their excellentoxidation resistance, high coercivity, remanence, magnetic energyproduct and uniaxial magnetocrystalline anisotropy [2,3]. TheM-type ferrites have become the mainstream of hexagonal ferrites,however, the performance of them is close to its limitation, and it isso difficult to improve their magnetic properties rapidly [4]. Theformula of W-type ferrites is BaMe2Fe16O27 where Me2þ is usually afirst row transition metal or some other divalent cation (e.g. Mg2þ ,Zn2þ , Mn2þ , Fe2þ , Co2þ and Ni2þ), meanwhile, the Ba2þ can bereplaced by one or two metal ions of other groups (e.g. Sr2þ , Pb2þ

and Ca2þ) [5]. In the structure of W-type ferrites, the [0001] axis ofR blocks and [111] axis of S blocks appeared at the same time. Inaddition, the R blocks and S blocks can overlap together along the c-axis in a way of SSRS*S*R*, where the structures of them areBaFe6O11 and Me2Fe4O8 respectively, and the * means a rotation ofthe blocks through 180° around the c-axis [6–8].

W-type ferrites were first reported as the Fe W22+ (BaFe Fe O2

2163

27+ + )

and initially treated only as a combination of the mixed phase of Mand X ferrites [9]. Besides, as a result of the existence of Fe2þ , theconductive performance is far superior to BaM ferrites [10,11].

Nevertheless, the reports of Fe W22+ ferrites prepared by the ceramic

process are relatively less for the reasons that the Fe2þ is easilyoxidized to Fe3þ and makes the experiment so difficult. In thiswork, we prepared Fe W2

2+ ferrites by the ceramic process with themethod of controlling the oxygen partial pressure strictly under theprotection of nitrogen [12].

2. Material and methods

All samples of W-type ferrites Ba Sr Fe Fe Ox x1 22

163

27−+ + (x¼0, 0.2,

0.5, 0.8 and 1.0) were synthesized by the ceramic process. Rawmaterials used in this work were BaCO3 (99% purity), SrCO3 (99%purity), and Fe2O3 (98% purity). The raw materials were weightedaccording to a stoichiometric composition of Ba Sr Fe Fe Ox x1 2

2163

27−+ + ,

where x¼0, 0.2, 0.5, 0.8 and 1.0. Then the raw materials werecharged in a ball mill together with a diameter of 8 mm. Themixtures of the raw materials were milled for 4 h with an angularvelocity of 80 rpm and a ball-to-powder weight ratio of 15:1. Themixed powder was dried at 100 °C for 10 h in an oven, and pre-sintered at 1290 °C in nitrogen for 3 h in a muffle furnace. The pre-sintered samples were crushed to particles by a vibration mill.

The phases of the samples were investigated by X-ray diffrac-tion (XRD) using Cu Kα radiation. And the morphology of thesamples was observed by using a HITACHI S-4800 scanning elec-tron microscopy (SEM). At the same time, the saturation magne-tization (Ms) and coercivity (Hc) were measured by a Riken DenshiBH-55 vibrating sample magnetometer (VSM).

Table 1Effects of Sr-substitution on lattice constants (a and c), c/a, Ms and Hc for the

W-type ferrites Ba Sr Fe Fe Ox x1 22

163

27−+ + samples.

Sample (x) c (Å)70.001 a (Å)70.001 c/a Ratio Ms (emu/g) Hc (Oe)

0 32.810 5.879 5.581 69.95 255.360.2 32.836 5.90 5.566 69.31 257.780.5 32.701 5.88 5.562 69.75 260.980.8 32.667 5.879 5.557 71.01 262.561.0 32.572 5.872 5.547 72.69 263.45

Fig. 2. Crystal axis ratio of c/a with different Sr content (x).

F. Lv et al. / Materials Letters 157 (2015) 277–280278

3. Results and discussion

3.1. Microstructure

Fig. 1 shows the X-ray diffraction patterns ofBa Sr Fe Fe Ox x1 2

2163

27−+ + (x¼0, 0.2, 0.5, 0.8 and 1.0) magnetic powders

pre-sintered in nitrogen at 1290 °C for 3 h. The XRD patterns of allthe samples are totally compatible with the JCPDS data forBaFe18O27 (PDF#75-0406) and do not show any extra peaks com-pared to those observed for the standard W-type ferrites. There-fore, it is convinced that single W-type ferrites are synthesized.The lattice constants a and c for all the samples are calculated fromthe values of the dhkl corresponding to (1010) peaks and (116)peaks by the following relation and the values are listed in Table 1.

⎛⎝⎜

⎞⎠⎟d

h hk ka

lc

43 1

hkl

2 2

2

2

2

1/2

= ⋅ + + +( )

−

Here, dhkl is the interplanar spacing, and h k and l are the Millerindices.

From the table we can see that, the values of a and c for all thesamples first increase and then decrease regularly with the in-creasing of x, which were very different with that of unsubstitutedferrites. Both a and c reach the maximum values (5.8997 Å and32.8364 Å) when x¼0.2, respectively. When x40.2, the latticeconstant a decrease slightly, while the lattice constant c decreasesignificantly. The results are in agreement with that reported byAhmad et al. [5,13]. The variation of the lattice constants a and cwith the increase of Sr content (x) is mainly due to the fact that theionic radius of Sr2þ (1.12 Å) is smaller than that of Ba2þ (1.35 Å).The crystal axis ratios c/a for all samples are also listed in Table 1and shown in Fig. 2. There is considerable variation in constant cthan a because of c-axis is the easy axis in W-type hexaferrites andit is easier to orient the spin directions along c-axis which is ver-tical to the hexagonal based plane [12].

The SEM micrographs for W-type ferrites Ba Sr Fe Fe Ox x1 22

163

27−+ +

(x¼0, 0.2, 0.5, 0.8 and 1.0) are shown in Fig. 3. These micrographsindicated that platelets-shape grains have well defined hexagonalshape with micron scale grain size and the tropism of grains israndom. The shape of the materials plays an important role invarious applications in different field and it has been reported

Fig. 1. X-ray diffraction patterns for the W-type ferrites Ba Sr Fe Fe Ox x1 22

163

27−+ + . (a)

x¼0 (b) x¼0.2 (c) x¼0.5 (d) x¼0.8 (e) x¼1.0 and (f) JCPDS diffraction data.

earlier that the platelet shaped hexaferrites can be used in mi-crowave absorbing coatings [14]. Moreover, the decrease of thegrain size can be attributed to the fact that the ionic radius of Sr2þ

is smaller than Ba2þ with the increase of Sr content.

3.2. Magnetic properties

The magnetic hysteresis loops for W-type ferritesBa Sr Fe Fe Ox x1 2

2163

27−+ + (x¼0, 0.2, 0.5, 0.8 and 1.0) were measured and

the loop for W-type ferrite Ba Sr Fe Fe O0.5 0.5 22

163

27+ + was shown in

Fig. 4(a). The saturation magnetization (Ms) and coercivity (Hc) arealso listed in Table 1, and the influence of Sr content on the sa-turation magnetization (Ms) and coercivity (Hc) of the W-typeferrites is shown in Fig. 4(b). As can be seen from Fig. 4(b), thesaturation magnetization (Ms) is very high, while coercivity (Hc)was low, from which the soft character of magnets was observedand the result is in agreement with that reported by Albanese et al.[15] and Ram et al. [16]. As can be seen from the variation of Ms

and Hc with different x contents, Hc increases slightly with theincrease of x, while Ms decreases first with x from 0 to 0.2, andthen increases from 0.2 to 1.0. Both Hc and Ms reach the maximumvalues (72.79 emu/g and 263.45 Oe) when x¼1, respectively.The high saturation magnetization (Ms) of W-type ferrites iscaused by superexchange interaction across one oxygen atom(Fe3þ–O–Fe3þ) and two oxygen atoms (Fe3þ–O–O–Fe3þ) [17].Due to the substitution of the Ba2þ ions by the smaller Sr2þ ions,the lattice constants first increase and then decrease as Table 1indicates. Correspondingly, the similar changed trend with latticeconstants also appeared over the distance of Fe–O being parallel tothe c-axis, which lead to the superexchange interaction first re-duce and then strengthen, which caused the change of Ms.

The value of coercivity Hc increases with the increases of the Sr

Fig. 3. SEM micrographs for all W-type ferrites Ba Sr Fe Fe Ox x1 22

163

27−+ + samples.

F. Lv et al. / Materials Letters 157 (2015) 277–280 279

content. According to ferromagnetic theory, the coercivity inW-type hexaferrite can be explained by the equation as follows:

HKM

0.642

c1

s=

( )

where K1 is the magnetocrystalline anisotropy constant and Ms isthe saturation magnetization. Since Ms changes slightly, it can beconcluded that reasons for the variation of Hc may be contributedto the difference in the magnetocrystalline anisotropy of the Sr2þ

and Ba2þ ions by substituting Sr2þ (1.12 Å) for Ba2þ (1.35 Å). Theincreased magnetocrystalline anisotropy causes an enhancement

Fig. 4. Magnetic properties of W-type ferrites Ba Sr Fe Fe Ox x1 22

163

27−+ + samples: (a) magn

saturation magnetization (Ms) and coercivity (Hc) with x increase.

in coercivity. In other words, the magnetocrystalline field increaseswhen Ba2þ is replaced by the smaller Sr2þ .

4. Conclusions

W-type hexagonal ferrites Ba Sr Fe Fe Ox x1 22

163

27−+ + (x¼0, 0.2, 0.5,

0.8 and 1.0) have been successfully synthesized by the ceramicprocess in nitrogen. The Sr2þ ions substituted Ba2þ ions in orderto investigate the microstructure and magnetic properties ofhexaferrites. The results of XRD confirmed the appearance of apure phase hexagonal crystal structure. The study of SEM showed

etic hysteresis loop for W-type ferrite Ba Sr Fe Fe O0.5 0.5 22

163

27+ + ; and (b) variation of

F. Lv et al. / Materials Letters 157 (2015) 277–280280

that the ferrites have formed the hexagonal structure and theparticles distributed evenly. The saturation magnetization (Ms) isvery high while the coercivity (Hc) only has a few oersteds, whichshow the soft character of the investigated ferrites.

Acknowledgments

The authors acknowledge the financial support from the Na-tional Natural Science Foundation of China under Grant nos.51272003, 51472004, 11375011 and 21271007, the Natural ScienceFoundation of Anhui Province under Grant no. 1408085MA12, theKey Program of the Education Department of Anhui Province(Grant nos. KJ2013B057 and KJ2012A027), Anhui University “211Project” Academic Innovation Team Projects under Grant no.02303402, and from the Research Fund for the Doctoral Programof Higher Education of China under Project 20123401110008.

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