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SiO2 nanocomposites synthesized by sol–gel method
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 4176–4181 www.elsevier.com/locate/ceramint
The structure, magnetic properties and cation distribution of Co1 xMgxFe2O4/SiO2 nanocomposites synthesized by sol–gel method Li Wang, Ming Lu, Yu Liu, Ji Li, Mei Liu, Haibo Lin Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, China Received 23 October 2014; received in revised form 17 December 2014; accepted 17 December 2014 Available online 24 December 2014
Abstract The structure, magnetic properties and cation distribution of Co1 xMgxFe2O4/SiO2 (x ¼ 0, 0.2, 0.4, 0.6, 0.8) nanocomposites synthesized by the sol–gel method were studied at room temperature by using X-ray diffraction, transmission electron microscopy, vibrating sample magnetometer, and Mössbauer spectroscopy. The lattice constant of Co1 xMgxFe2O4 in the nanocomposite decreased with increasing the Mg2 þ content, and average crystallite size was in the range of 33–39 nm. The saturation magnetization of the samples decreased with increasing composition x, while the coercivtiy increased firstly and then decreased, showing a maximum value of 2604 Oe at x ¼0.6. The Mössbauer spectra were fitted with two Zeeman sextets, indicating that all the samples were completely magnetically ordered. The cation distribution was discussed and the variations of the magnetic moment per formula unit indicated that Co1 xMgxFe2O4 in the nanocomposite was a collinear ferrimagnetic structure. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Magnetic properties; Spinel ferrite; Sol–gel method; Cation distribution; Mössbauer spectroscopy
1. Introduction Spinel ferrites with general formula AB2O4 have been applied for the last 70 years [1–3]. From all ferrites, high magnetocrystalline anisotropy and good chemical stability have made cobalt ferrite (CoFe2O4) a very promising candidate for magnetic recording applications such as audio and video tape and high density digital recording discs [4–6]. Magnetic properties of ferrites can be suitably tailored by varying the composition of cations. This means that it is possible to change the relative strengths of the exchange interactions in spinels by changing the type of the magnetic ions as well as by selective substitution of non-magnetic atoms on the tetrahedral (A) and octahedral (B) sites which lead to interesting spin configurations. Several researches have reported on non-magnetic ions (Al3þ , Y3þ , Zn2þ or Cd2þ ) substituting CoFe2O4 [7–9]. To our knowledge, there are very few detailed studies on the Mg2 þ substituting cobalt ferrite. Magnesium ions with non-magnetic nature are known for achieving control over magnetic parameters n
Corresponding author. Tel.: þ86 434 3290232; fax: þ 86 434 3292233. E-mail address: [email protected] (H. Li).
http://dx.doi.org/10.1016/j.ceramint.2014.12.099 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
in developing technologically important materials and they have strong B sites preference [10]. Thus, the substitution of Mg2 þ ions for Co2þ will markedly alter magnetic properties of cobalt ferrite. The interest in these nanocomposites consisting of nanometric magnetic particles embedded in an insulating matrix such as silica has grown considerably in recent years due to novel properties presented by these kinds of material. Rohilla et al. founded the measured value of the coercivity at room temperature for CoFe2O4/SiO2 was much higher than that of bulk cobalt ferrite [11]. The silica network can also act as a buffer to protect the nanoparticles, and minimize the surface roughness and spin disorder. The absence of spin disorder for NiAl0.2Fe1.8O4 nanoparticles in silica had been observed in our previous work [12]. The sol–gel method had been proved to be an efficient technique to prepare nanocomposites [13,14]. Through this method, a good control of the particle size, dispersion, structure, and chemical composition could be attained by carefully monitoring the preparation parameters. In this work, we prepared Co1 xMgxFe2O4/SiO2 nanocomposites using the sol–gel method. The structure, magnetic properties and cation distribution of Co1 xMgxFe2O4/SiO2 nanocomposites
L. Wang et al. / Ceramics International 41 (2015) 4176–4181
3. Results and discussion Fig. 1 shows the XRD patterns of Co1 xMgxFe2O4/SiO2 nanocomposites. The presence of diffraction peaks (220), (311), (440), (422), (511), and (440) confirms the formation of cubic spinel structure. It is also possible to identify the existence of αFe2O3. The effect of α-Fe2O3 on the magnetic properties of the nanocomposites can be ignored, however, due to the fact that the relative intensity of peak for α-Fe2O3 is weak. There is no obvious change with increasing x (see Table 1, the ratio of I(104)/ I(311) is about a constant 0.180) [15]. In addition, there is no characteristic peak from SiO2, which is due to the low content of SiO2 in the nanocomposites [16]. The crystallite size from the XRD data is most widely estimated by the Scherrer formula [17] in the following form: DXRD ¼
Cλ B1=2 cos θ
ð1Þ
Intendity(a.u)
(440)
a-Fe 2 O 3
(511)
x=0.8
*
(422)
Co1 xMgxFe2O4/SiO2 (x=0, 0.2, 0.4, 0.6, 0.8) nanocomposites where the mass ratio of ferrite and SiO2 being 7:3 were prepared by the sol–gel method. Analytical grade iron nitrate Fe (NO3)3 9H2O, cobalt nitrate Co(NO3)2 6H2O, magnesium nitrate Mg(NO3)2 6H2O, tetraethyl-orthosilicate (TEOS), methoxyethanol, nitric acid and deionized water were used as starting materials. Fe(NO3)3 9H2O, Co(NO3)2 6H2O and Mg (NO3)2 6H2O in a molar ratio of Fe:Co:Mg=2:(1 x):x were dissolved in methoxyethanol to prepare solution A, and TEOS was dissolved in methoxyethanol to prepare solution B. Then, the latter was added dropwise to the former under vigorous magnetic stirring to prepare solution C. Nitric acid and deionized water were also added in the solution C and a gel would be formed by continuously stirring for 5 h at room temperature. After the gel formation, it was evaporated on a water bath at 60 1C for 12 h, and then was dried at 100 1C in a drying oven for 24 h to form the xerogel. At last, the xerogels were annealed at 1100 1C for 2 h under air and cooled slowly in a furnace. The structure of Co1 xMgxFe2O4/SiO2 nanocomposites was characterized by a Rigaku D/max-2500 diffractometer with Cu Kα1 radiation at room temperature. High-purity silicon powders provided by Rigaku Corporation were used as a standard sample to compare the lattice constant, which was determined from the X-ray data using MDI Jade 6.5 software. The morphology and size of the nanoparticles were measured by JEM-2100HR TEM. The magnetic properties of the samples were measured using a Lake Shore 7407 VSM with a maximum applied field of 20 kOe. The 57Fe Mössbauer spectra were collected on a FAST Comtec Mössbauer systems at room temperature, using a 57Co(Pd) source and a constant acceleration mode. All isomer shifts were given relative to that of α-Fe at room temperature. The spectra were fitted with Lorentzian lines via the least squares method.
(111)
2. Experiments
Co 1-x Mg x Fe 2 O 4
(400)
.
(220) (104) (311) (222)
were investigated by using X-ray diffraction (XRD), transmission electron microscopy (TEM), a vibrating sample magnetometer (VSM), and Mössbauer spectroscopy (MS).
4177
x=0.6 x=0.4
.
x=0.2 x=0
20
. .. .
.. .
*
30
40 ο 2θ ( C )
50
60
Fig. 1. XRD patterns of Co1 xMgxFe2O4/SiO2 nanocomposite.
Table 1 The values of lattice constant a of Co1 xMgxFe2O4 in the nanocomposite, the average crystallite size D, and the ratio of I(104)/I(311) in XRD patterns. x
0
0.2
0.4
0.6
0.8
a (nm) D (nm) I(104)/I(311)
0.8392 37 0.178
0.8388 36 0.179
0.8383 33 0.181
0.8374 35 0.178
0.8362 39 0.180
where B1/2 is the full width at half maximum in radian (2θ), θ is the corresponding Bragg angle, and C ¼ 0.9 for spherical particles. Table 1 gives the lattice constant a of Co1 xMgxFe2O4 and its average crystallite size D, which was obtained from the Eq. (1) on (220), (311), (440), (511) and (440) XRD peaks. As x increases from x ¼ 0 to 0.8, a decreases from 0.8392 to 0.8362 nm. The decrease in lattice constant is due to the replacement of the larger ionic crystal radius of Co2 þ (0.072 nm) by the smaller Mg2 þ (0.066 nm) [18]. The average crystallite size of Co1 xMgxFe2O4 was estimated in the range of 33–39 nm. The TEM image of Co0.4Mg0.6Fe2O4/SiO2 nanocomposite is shown in Fig. 2. As can be seen, Co0.4Mg0.6Fe2O4 particles are spherical in shape and homogeneously dispersed in SiO2. There is not obvious agglomerate. It is due to the fact that the silica matrix is considered not only to serve as spatial nucleation sites for Co1 xMgxFe2O4, but also to confine the coarsening of nanoparticles and minimize the degree of crystallite aggregation [19]. The mean particle diameter was estimated to about 37 nm, which is almost consistent with that measured by XRD. The microstructure of uniformly dispersed and small grains is desired for realizing the high coercivity. The hysteresis loops of Co1 xMgxFe2O4/SiO2 nanocomposites are shown in Fig. 3(a). At high field, the magnetization increases almost linearly with the external field and shows a lack of saturation at a field as high as 20 kOe. The variations of the saturation magnetization MS and the coercivity HC of the
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L. Wang et al. / Ceramics International 41 (2015) 4176–4181
On the other hand, as seen in Fig. 4, HC increases and reaches a maximum of 2604 Oe when x is 0.6, thereafter decreases with further increasing the Mg2 þ content. The coercivity of the interaction of particles having the single domain magnetic structure is given by [21] 0:96 K ð1 pÞ μ0 M S
HC ¼
ð2Þ
where p is the volume fraction, which can be described as νp/νs, νs is the total volume of the sample, and νp is the total volume of single domain particles in the sample. K is the anisotropy constant and μ0 is the permeability of the free space. Considering the fact that all the samples were prepared and examined almost under the same conditions, the values of p were roughly assumed to be the same for the samples. From Eq. (2), HC is proportional with K, and inversely proportional with MS. In the present, with increasing x, the decrease in the content of Co2 þ on B sites results in a decrease of K. At the same time, Ms also deceases with increasing x. Thus, the value of HC is dependent on the amplitudes of the decrease in K and MS. In Fig. 4, for the samples with x¼ 0, 0.2, 3000
45
2500
40
2000
35
1500
30
1000
25
500
20
0
0.0
0.2
0.4
0.6
0.8
Ms (emu/g)
Hc (Oe)
samples with x are shown in Fig. 4. The values of MS were obtained by extrapolation to infinite field in an M vs. 1/H2 plot [20] (see Fig. 3(b)). As seen in Fig. 4, as x increases from 0 to 0.8, MS decreases from 41.63 to 18.34 emu/g. The decrease in MS may be attributed to the weakening of exchange interaction due to the substitution of non-magnetic Mg2 þ ions. Moreover, it also can be explained by the cation distribution, since the distribution of cations in A and B sites influences the magnetic properties of ferrites. The obtained cation distribution from MS is listed in Table 3. The substitution of Mg2 þ at B sites for Co2 þ reduces the net magnetic moment at B sites, as the magnetic moment of Mg2 þ (0 μB) is less than that of Co2 þ (3 μB). Thus, the net magnetic moment per molecule is reduced, leading to the decease of MS.
15
x Fig. 4. Variations of the saturation magnetization MS and the coercivity HC of Co1 xMgxFe2O4/SiO2 nanocomposites with the Mg2 þ content.
Fig. 2. TEM image of Co0.4Mg0.6Fe2O4/SiO2 nanocomposite.
45 40
40
x =0
0
M emu/g
M (emu/g)
20
x =0 x =0.2 x =0.4 x =0.6 x =0.8
-20
-40 -20000
-10000
0
H (Oe)
10000
20000
35
x =0.2
30
x =0.4
25 20
x =0.6
15
x =0.8
0.2
0.3
0.4
0.5
0.6 8
0.7 2
0.8
-2
10 / H （Oe
Fig. 3. (a) Hysteresis loops of Co1 xMgxFe2O4/SiO2 nanocomposites. (b) Variations of M with 1/H2.
0.9
1.0
L. Wang et al. / Ceramics International 41 (2015) 4176–4181
0.4 and 0.6, the decrease in MS is obvious, and it may predominate over the decrease in K. So HC increases with increasing x. Meanwhile, for the sample x¼ 0.8, the case is reverse. The decrease in Ms is small, and the decrease in K predominates over the decrease in MS, leading to the decrease of HC. Moreover, the measured values of HC for the present nanocomposite are larger than those of pure Co–Mg ferrite nanoparticles [22], which is due to the interfacial diffusion between Co1 xMgxFe2O4 layer and SiO2 in the nanocomposites. The Mössbauer spectra of Co1 xMgxFe2O4/SiO2 nanocomposites are shown in Fig. 5, and the fitted Mössbauer parameters are listed in Table 2. All spectra for the nanocomposites were fitted with two Zeeman sextets, which indicated that all the samples were the completely magnetically ordered. In Table 2, the IS value varies in the range of 0.260–0.370 mm/s, which is consistent with that of the high spin Fe3 þ state. The components A and B correspond to the Fe3 þ on the A sites and the B sites within the grains, respectively. It could be seen that the value of IS at each site decreases with increasing x, which indicates that the s electron density of Fe3 þ increases because of the lattice contraction induced by the doping of the smaller ionic crystal radius of Mg2 þ . In addition, there is a monotonic decrease in the internal hyperfine filed values with increasing non-magnetic Mg2 þ content, demonstrating the reduction in ferromagnetic behavior and magnetic coupling with increasing x, in agreement with the magnetization results. The relative absorption area of the two Zeeman sextets can reflect the content of Fe3 þ ions on the A and B sites. And it is well known that the preference to occupy B sites of Co2 þ is stronger than that of Mg2 þ . Thus the cation distribution in
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Table 2 Mössbauer parameters of Co1 xMgxFe2O4/SiO2 nanocomposites. x
Component I.S (mm/s) Q.S (mm/s) Hf (kOe) HWHM (mm/s) S (%)
0
A B A B A B A B A B
0.2 0.4 0.6 0.8
0.275 0.372 0.270 0.370 0.260 0.370 0.260 0.369 0.260 0.367
0.033 0.064 0.026 0.053 0.026 0.054 0.025 0.051 0.028 0.050
487.0 514.2 486.7 513.0 486.4 512.4 485.8 511.5 482.1 509.6
0.214 0.294 0.201 0.302 0.199 0.265 0.198 0.250 0.201 0.251
49.6 50.4 48.0 52.0 47.6 52.4 48.9 51.1 47.8 52.2
Note: IS, QS, Hf, HWHM, and S represent the isomer shift, the quadrupole splitting, the hyperfine field, the half width at half maximum, and the relative absorption area, respectively.
Table 3 The values of α, the ratio of Mg2 þ (B)/Mg2 þ (A), and cation distribution of the samples. x
α
0 0.2 0.4 0.6 0.8
0 0.03 0.05 0.02 0.04
Cation distribution
Mg2 þ (B)/Mg2 þ (A)
(Co0.01Fe0.99)A[Co0.99Fe1.01]O4 (Mg0.03Fe0.96)A[Mg0.17Co0.80Fe1.04]O4 (Mg0.05Fe0.95)A[Mg0.35Co0.60Fe1.05]O4 (Mg0.02Fe0.98)A[Mg0.58Co0.40Fe1.02]O4 (Mg0.04Fe0.96)A[Mg0.76Co0.20Fe1.04]O4
0 5.7 7.0 29.0 19.0
Co1 xMgxFe2O4 (xa0) can be written as (MgαFe1 α)A[Mgx α Co1 xFe1 þ α]BO4. Then, the absorption area ratio of A to B sites, SA/SB, based on the above distribution is written as
100 95
x =0.8
90
SA ð1 αÞf A ¼ SB ð1 þ αÞf B
ð3Þ
Relative transmission (%)
100 x =0.6
96 92 100 96
x =0.4
92 88 100 96
x =0.2
92 100 96
x =0
92 88 -12 -10 -8 -6 -4 -2 0 2 4 Velocity (mm/s)
6
8 10 12
Fig. 5. Mössbauer spectra of Co1 xMgxFe2O4/SiO2 nanocomposite.
where fA and fB represent the recoil-free fractions for A and B sites of Fe3 þ , respectively. In the present work, we assume that fA is equal to fB [23]. The 6th column of Table 2 shows the observed values of the absorption area of A and B sites. So α could be calculated. The obtained cation distribution is listed in Table 3. In Table 3, the ratio of Mg2 þ (B)/Mg2 þ (A) increases firstly, and then decreases with increasing x. This indicates that the preference of Mg2 þ to occupy B sites increases and reaches a maximum when x is 0.6. According to Neel's two sublattices model of ferrimagnetism, the calculated magnetic moment per formula unit ηcal B in Bohr magneton is expressed as ηcal B ¼ M B ðxÞ M A ðxÞ
ð4Þ
where MA and MB are the A and B sites magnetic moments. The values of ηcal were calculated by using the cation B distribution in Table 3. The ionic magnetic moments of Fe3 þ , Co2 þ and Mg2 þ are 5 μB, 3 μB, and 0 μB, respectively. The observed magnetic moment per formula unit ηobs B could be
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L. Wang et al. / Ceramics International 41 (2015) 4176–4181
Acknowledgments 4
Neel's model
This work was supported by the National Natural Science Foundation of China (Nos. 21201078, 21371071) and Foundation of Science and Technology of Jilin, China (No. 201205075).
Experiment data from VSM
η (μ ) Β Β
3
References 2
1 0.0
0.2
0.4 x
0.6
0.8
Fig. 6. Variations of ηB with x for the Co1 xMgxFe2O4/SiO2 nanocomposites.
calculated by using the relation [24]. ηobs B ¼
molecular weight M S 5585
ð5Þ
obs Fig. 6 shows the variations of ηcal with Mg2 þ B and ηB content. It can be seen that the values of ηobs B are smaller than those of ηcal B , which may be due to the nano size particles in obs present. It is clearly noted that both ηcal B and ηB decreases with increasing x, which suggests the Co1 xMgxFe2O4 is collinear ferrimagnetic structure.
4. Conclusions In conclusion, Co1 xMgxFe2O4/SiO2 (x¼ 0, 0.2, 0.4, 0.6, 0.8) nanocomposites were obtained by the sol–gel method. When the Mg2 þ content x increased from 0 to 0.8, the lattice constant of Co1 xMgxFe2O4 in the nanocomposite decreased from 0.8392 to 0.8362 nm, the average crystallite size was in the range of 33–39 nm, the saturation magnetization decreased from 41.63 to 18.34 emu/g, while the coercivity had a maximum of 2604 Oe as x was 0.6. The decrease in the saturation magnetization was due to the decrease of net magnetic moment per molecule as magnetic Co2 þ were substituted by non-magnetic Mg2 þ on B sites. The change of the coercivity depended on the amplitudes of the decrease in K and MS with increasing x. The studies of Mössbauer spectra indicated all the samples were the completely magnetically ordered. The results of the cation distribution indicated the preference of Mg2 þ to occupy B sites increased with increasing x and reached a maximum when x was 0.6. The variations of the calculated and observed magneton number with x were almost the same, which suggested that Co1 xMgxFe2O4 in the nanocomposites was collinear ferrimagnetic structure. The results reveal that Mg2 þ doped in CoFe2O4/SiO2, especially with the content of 0.6, is a good candidate for high frequency recording materials because of its high coercivity.
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 4176–4181 www.elsevier.com/locate/ceramint
The structure, magnetic properties and cation distribution of Co1 xMgxFe2O4/SiO2 nanocomposites synthesized by sol–gel method Li Wang, Ming Lu, Yu Liu, Ji Li, Mei Liu, Haibo Lin Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, China Received 23 October 2014; received in revised form 17 December 2014; accepted 17 December 2014 Available online 24 December 2014
Abstract The structure, magnetic properties and cation distribution of Co1 xMgxFe2O4/SiO2 (x ¼ 0, 0.2, 0.4, 0.6, 0.8) nanocomposites synthesized by the sol–gel method were studied at room temperature by using X-ray diffraction, transmission electron microscopy, vibrating sample magnetometer, and Mössbauer spectroscopy. The lattice constant of Co1 xMgxFe2O4 in the nanocomposite decreased with increasing the Mg2 þ content, and average crystallite size was in the range of 33–39 nm. The saturation magnetization of the samples decreased with increasing composition x, while the coercivtiy increased firstly and then decreased, showing a maximum value of 2604 Oe at x ¼0.6. The Mössbauer spectra were fitted with two Zeeman sextets, indicating that all the samples were completely magnetically ordered. The cation distribution was discussed and the variations of the magnetic moment per formula unit indicated that Co1 xMgxFe2O4 in the nanocomposite was a collinear ferrimagnetic structure. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Magnetic properties; Spinel ferrite; Sol–gel method; Cation distribution; Mössbauer spectroscopy
1. Introduction Spinel ferrites with general formula AB2O4 have been applied for the last 70 years [1–3]. From all ferrites, high magnetocrystalline anisotropy and good chemical stability have made cobalt ferrite (CoFe2O4) a very promising candidate for magnetic recording applications such as audio and video tape and high density digital recording discs [4–6]. Magnetic properties of ferrites can be suitably tailored by varying the composition of cations. This means that it is possible to change the relative strengths of the exchange interactions in spinels by changing the type of the magnetic ions as well as by selective substitution of non-magnetic atoms on the tetrahedral (A) and octahedral (B) sites which lead to interesting spin configurations. Several researches have reported on non-magnetic ions (Al3þ , Y3þ , Zn2þ or Cd2þ ) substituting CoFe2O4 [7–9]. To our knowledge, there are very few detailed studies on the Mg2 þ substituting cobalt ferrite. Magnesium ions with non-magnetic nature are known for achieving control over magnetic parameters n
Corresponding author. Tel.: þ86 434 3290232; fax: þ 86 434 3292233. E-mail address: [email protected] (H. Li).
http://dx.doi.org/10.1016/j.ceramint.2014.12.099 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
in developing technologically important materials and they have strong B sites preference [10]. Thus, the substitution of Mg2 þ ions for Co2þ will markedly alter magnetic properties of cobalt ferrite. The interest in these nanocomposites consisting of nanometric magnetic particles embedded in an insulating matrix such as silica has grown considerably in recent years due to novel properties presented by these kinds of material. Rohilla et al. founded the measured value of the coercivity at room temperature for CoFe2O4/SiO2 was much higher than that of bulk cobalt ferrite [11]. The silica network can also act as a buffer to protect the nanoparticles, and minimize the surface roughness and spin disorder. The absence of spin disorder for NiAl0.2Fe1.8O4 nanoparticles in silica had been observed in our previous work [12]. The sol–gel method had been proved to be an efficient technique to prepare nanocomposites [13,14]. Through this method, a good control of the particle size, dispersion, structure, and chemical composition could be attained by carefully monitoring the preparation parameters. In this work, we prepared Co1 xMgxFe2O4/SiO2 nanocomposites using the sol–gel method. The structure, magnetic properties and cation distribution of Co1 xMgxFe2O4/SiO2 nanocomposites
L. Wang et al. / Ceramics International 41 (2015) 4176–4181
3. Results and discussion Fig. 1 shows the XRD patterns of Co1 xMgxFe2O4/SiO2 nanocomposites. The presence of diffraction peaks (220), (311), (440), (422), (511), and (440) confirms the formation of cubic spinel structure. It is also possible to identify the existence of αFe2O3. The effect of α-Fe2O3 on the magnetic properties of the nanocomposites can be ignored, however, due to the fact that the relative intensity of peak for α-Fe2O3 is weak. There is no obvious change with increasing x (see Table 1, the ratio of I(104)/ I(311) is about a constant 0.180) [15]. In addition, there is no characteristic peak from SiO2, which is due to the low content of SiO2 in the nanocomposites [16]. The crystallite size from the XRD data is most widely estimated by the Scherrer formula [17] in the following form: DXRD ¼
Cλ B1=2 cos θ
ð1Þ
Intendity(a.u)
(440)
a-Fe 2 O 3
(511)
x=0.8
*
(422)
Co1 xMgxFe2O4/SiO2 (x=0, 0.2, 0.4, 0.6, 0.8) nanocomposites where the mass ratio of ferrite and SiO2 being 7:3 were prepared by the sol–gel method. Analytical grade iron nitrate Fe (NO3)3 9H2O, cobalt nitrate Co(NO3)2 6H2O, magnesium nitrate Mg(NO3)2 6H2O, tetraethyl-orthosilicate (TEOS), methoxyethanol, nitric acid and deionized water were used as starting materials. Fe(NO3)3 9H2O, Co(NO3)2 6H2O and Mg (NO3)2 6H2O in a molar ratio of Fe:Co:Mg=2:(1 x):x were dissolved in methoxyethanol to prepare solution A, and TEOS was dissolved in methoxyethanol to prepare solution B. Then, the latter was added dropwise to the former under vigorous magnetic stirring to prepare solution C. Nitric acid and deionized water were also added in the solution C and a gel would be formed by continuously stirring for 5 h at room temperature. After the gel formation, it was evaporated on a water bath at 60 1C for 12 h, and then was dried at 100 1C in a drying oven for 24 h to form the xerogel. At last, the xerogels were annealed at 1100 1C for 2 h under air and cooled slowly in a furnace. The structure of Co1 xMgxFe2O4/SiO2 nanocomposites was characterized by a Rigaku D/max-2500 diffractometer with Cu Kα1 radiation at room temperature. High-purity silicon powders provided by Rigaku Corporation were used as a standard sample to compare the lattice constant, which was determined from the X-ray data using MDI Jade 6.5 software. The morphology and size of the nanoparticles were measured by JEM-2100HR TEM. The magnetic properties of the samples were measured using a Lake Shore 7407 VSM with a maximum applied field of 20 kOe. The 57Fe Mössbauer spectra were collected on a FAST Comtec Mössbauer systems at room temperature, using a 57Co(Pd) source and a constant acceleration mode. All isomer shifts were given relative to that of α-Fe at room temperature. The spectra were fitted with Lorentzian lines via the least squares method.
(111)
2. Experiments
Co 1-x Mg x Fe 2 O 4
(400)
.
(220) (104) (311) (222)
were investigated by using X-ray diffraction (XRD), transmission electron microscopy (TEM), a vibrating sample magnetometer (VSM), and Mössbauer spectroscopy (MS).
4177
x=0.6 x=0.4
.
x=0.2 x=0
20
. .. .
.. .
*
30
40 ο 2θ ( C )
50
60
Fig. 1. XRD patterns of Co1 xMgxFe2O4/SiO2 nanocomposite.
Table 1 The values of lattice constant a of Co1 xMgxFe2O4 in the nanocomposite, the average crystallite size D, and the ratio of I(104)/I(311) in XRD patterns. x
0
0.2
0.4
0.6
0.8
a (nm) D (nm) I(104)/I(311)
0.8392 37 0.178
0.8388 36 0.179
0.8383 33 0.181
0.8374 35 0.178
0.8362 39 0.180
where B1/2 is the full width at half maximum in radian (2θ), θ is the corresponding Bragg angle, and C ¼ 0.9 for spherical particles. Table 1 gives the lattice constant a of Co1 xMgxFe2O4 and its average crystallite size D, which was obtained from the Eq. (1) on (220), (311), (440), (511) and (440) XRD peaks. As x increases from x ¼ 0 to 0.8, a decreases from 0.8392 to 0.8362 nm. The decrease in lattice constant is due to the replacement of the larger ionic crystal radius of Co2 þ (0.072 nm) by the smaller Mg2 þ (0.066 nm) [18]. The average crystallite size of Co1 xMgxFe2O4 was estimated in the range of 33–39 nm. The TEM image of Co0.4Mg0.6Fe2O4/SiO2 nanocomposite is shown in Fig. 2. As can be seen, Co0.4Mg0.6Fe2O4 particles are spherical in shape and homogeneously dispersed in SiO2. There is not obvious agglomerate. It is due to the fact that the silica matrix is considered not only to serve as spatial nucleation sites for Co1 xMgxFe2O4, but also to confine the coarsening of nanoparticles and minimize the degree of crystallite aggregation [19]. The mean particle diameter was estimated to about 37 nm, which is almost consistent with that measured by XRD. The microstructure of uniformly dispersed and small grains is desired for realizing the high coercivity. The hysteresis loops of Co1 xMgxFe2O4/SiO2 nanocomposites are shown in Fig. 3(a). At high field, the magnetization increases almost linearly with the external field and shows a lack of saturation at a field as high as 20 kOe. The variations of the saturation magnetization MS and the coercivity HC of the
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On the other hand, as seen in Fig. 4, HC increases and reaches a maximum of 2604 Oe when x is 0.6, thereafter decreases with further increasing the Mg2 þ content. The coercivity of the interaction of particles having the single domain magnetic structure is given by [21] 0:96 K ð1 pÞ μ0 M S
HC ¼
ð2Þ
where p is the volume fraction, which can be described as νp/νs, νs is the total volume of the sample, and νp is the total volume of single domain particles in the sample. K is the anisotropy constant and μ0 is the permeability of the free space. Considering the fact that all the samples were prepared and examined almost under the same conditions, the values of p were roughly assumed to be the same for the samples. From Eq. (2), HC is proportional with K, and inversely proportional with MS. In the present, with increasing x, the decrease in the content of Co2 þ on B sites results in a decrease of K. At the same time, Ms also deceases with increasing x. Thus, the value of HC is dependent on the amplitudes of the decrease in K and MS. In Fig. 4, for the samples with x¼ 0, 0.2, 3000
45
2500
40
2000
35
1500
30
1000
25
500
20
0
0.0
0.2
0.4
0.6
0.8
Ms (emu/g)
Hc (Oe)
samples with x are shown in Fig. 4. The values of MS were obtained by extrapolation to infinite field in an M vs. 1/H2 plot [20] (see Fig. 3(b)). As seen in Fig. 4, as x increases from 0 to 0.8, MS decreases from 41.63 to 18.34 emu/g. The decrease in MS may be attributed to the weakening of exchange interaction due to the substitution of non-magnetic Mg2 þ ions. Moreover, it also can be explained by the cation distribution, since the distribution of cations in A and B sites influences the magnetic properties of ferrites. The obtained cation distribution from MS is listed in Table 3. The substitution of Mg2 þ at B sites for Co2 þ reduces the net magnetic moment at B sites, as the magnetic moment of Mg2 þ (0 μB) is less than that of Co2 þ (3 μB). Thus, the net magnetic moment per molecule is reduced, leading to the decease of MS.
15
x Fig. 4. Variations of the saturation magnetization MS and the coercivity HC of Co1 xMgxFe2O4/SiO2 nanocomposites with the Mg2 þ content.
Fig. 2. TEM image of Co0.4Mg0.6Fe2O4/SiO2 nanocomposite.
45 40
40
x =0
0
M emu/g
M (emu/g)
20
x =0 x =0.2 x =0.4 x =0.6 x =0.8
-20
-40 -20000
-10000
0
H (Oe)
10000
20000
35
x =0.2
30
x =0.4
25 20
x =0.6
15
x =0.8
0.2
0.3
0.4
0.5
0.6 8
0.7 2
0.8
-2
10 / H （Oe
Fig. 3. (a) Hysteresis loops of Co1 xMgxFe2O4/SiO2 nanocomposites. (b) Variations of M with 1/H2.
0.9
1.0
L. Wang et al. / Ceramics International 41 (2015) 4176–4181
0.4 and 0.6, the decrease in MS is obvious, and it may predominate over the decrease in K. So HC increases with increasing x. Meanwhile, for the sample x¼ 0.8, the case is reverse. The decrease in Ms is small, and the decrease in K predominates over the decrease in MS, leading to the decrease of HC. Moreover, the measured values of HC for the present nanocomposite are larger than those of pure Co–Mg ferrite nanoparticles [22], which is due to the interfacial diffusion between Co1 xMgxFe2O4 layer and SiO2 in the nanocomposites. The Mössbauer spectra of Co1 xMgxFe2O4/SiO2 nanocomposites are shown in Fig. 5, and the fitted Mössbauer parameters are listed in Table 2. All spectra for the nanocomposites were fitted with two Zeeman sextets, which indicated that all the samples were the completely magnetically ordered. In Table 2, the IS value varies in the range of 0.260–0.370 mm/s, which is consistent with that of the high spin Fe3 þ state. The components A and B correspond to the Fe3 þ on the A sites and the B sites within the grains, respectively. It could be seen that the value of IS at each site decreases with increasing x, which indicates that the s electron density of Fe3 þ increases because of the lattice contraction induced by the doping of the smaller ionic crystal radius of Mg2 þ . In addition, there is a monotonic decrease in the internal hyperfine filed values with increasing non-magnetic Mg2 þ content, demonstrating the reduction in ferromagnetic behavior and magnetic coupling with increasing x, in agreement with the magnetization results. The relative absorption area of the two Zeeman sextets can reflect the content of Fe3 þ ions on the A and B sites. And it is well known that the preference to occupy B sites of Co2 þ is stronger than that of Mg2 þ . Thus the cation distribution in
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Table 2 Mössbauer parameters of Co1 xMgxFe2O4/SiO2 nanocomposites. x
Component I.S (mm/s) Q.S (mm/s) Hf (kOe) HWHM (mm/s) S (%)
0
A B A B A B A B A B
0.2 0.4 0.6 0.8
0.275 0.372 0.270 0.370 0.260 0.370 0.260 0.369 0.260 0.367
0.033 0.064 0.026 0.053 0.026 0.054 0.025 0.051 0.028 0.050
487.0 514.2 486.7 513.0 486.4 512.4 485.8 511.5 482.1 509.6
0.214 0.294 0.201 0.302 0.199 0.265 0.198 0.250 0.201 0.251
49.6 50.4 48.0 52.0 47.6 52.4 48.9 51.1 47.8 52.2
Note: IS, QS, Hf, HWHM, and S represent the isomer shift, the quadrupole splitting, the hyperfine field, the half width at half maximum, and the relative absorption area, respectively.
Table 3 The values of α, the ratio of Mg2 þ (B)/Mg2 þ (A), and cation distribution of the samples. x
α
0 0.2 0.4 0.6 0.8
0 0.03 0.05 0.02 0.04
Cation distribution
Mg2 þ (B)/Mg2 þ (A)
(Co0.01Fe0.99)A[Co0.99Fe1.01]O4 (Mg0.03Fe0.96)A[Mg0.17Co0.80Fe1.04]O4 (Mg0.05Fe0.95)A[Mg0.35Co0.60Fe1.05]O4 (Mg0.02Fe0.98)A[Mg0.58Co0.40Fe1.02]O4 (Mg0.04Fe0.96)A[Mg0.76Co0.20Fe1.04]O4
0 5.7 7.0 29.0 19.0
Co1 xMgxFe2O4 (xa0) can be written as (MgαFe1 α)A[Mgx α Co1 xFe1 þ α]BO4. Then, the absorption area ratio of A to B sites, SA/SB, based on the above distribution is written as
100 95
x =0.8
90
SA ð1 αÞf A ¼ SB ð1 þ αÞf B
ð3Þ
Relative transmission (%)
100 x =0.6
96 92 100 96
x =0.4
92 88 100 96
x =0.2
92 100 96
x =0
92 88 -12 -10 -8 -6 -4 -2 0 2 4 Velocity (mm/s)
6
8 10 12
Fig. 5. Mössbauer spectra of Co1 xMgxFe2O4/SiO2 nanocomposite.
where fA and fB represent the recoil-free fractions for A and B sites of Fe3 þ , respectively. In the present work, we assume that fA is equal to fB [23]. The 6th column of Table 2 shows the observed values of the absorption area of A and B sites. So α could be calculated. The obtained cation distribution is listed in Table 3. In Table 3, the ratio of Mg2 þ (B)/Mg2 þ (A) increases firstly, and then decreases with increasing x. This indicates that the preference of Mg2 þ to occupy B sites increases and reaches a maximum when x is 0.6. According to Neel's two sublattices model of ferrimagnetism, the calculated magnetic moment per formula unit ηcal B in Bohr magneton is expressed as ηcal B ¼ M B ðxÞ M A ðxÞ
ð4Þ
where MA and MB are the A and B sites magnetic moments. The values of ηcal were calculated by using the cation B distribution in Table 3. The ionic magnetic moments of Fe3 þ , Co2 þ and Mg2 þ are 5 μB, 3 μB, and 0 μB, respectively. The observed magnetic moment per formula unit ηobs B could be
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L. Wang et al. / Ceramics International 41 (2015) 4176–4181
Acknowledgments 4
Neel's model
This work was supported by the National Natural Science Foundation of China (Nos. 21201078, 21371071) and Foundation of Science and Technology of Jilin, China (No. 201205075).
Experiment data from VSM
η (μ ) Β Β
3
References 2
1 0.0
0.2
0.4 x
0.6
0.8
Fig. 6. Variations of ηB with x for the Co1 xMgxFe2O4/SiO2 nanocomposites.
calculated by using the relation [24]. ηobs B ¼
molecular weight M S 5585
ð5Þ
obs Fig. 6 shows the variations of ηcal with Mg2 þ B and ηB content. It can be seen that the values of ηobs B are smaller than those of ηcal B , which may be due to the nano size particles in obs present. It is clearly noted that both ηcal B and ηB decreases with increasing x, which suggests the Co1 xMgxFe2O4 is collinear ferrimagnetic structure.
4. Conclusions In conclusion, Co1 xMgxFe2O4/SiO2 (x¼ 0, 0.2, 0.4, 0.6, 0.8) nanocomposites were obtained by the sol–gel method. When the Mg2 þ content x increased from 0 to 0.8, the lattice constant of Co1 xMgxFe2O4 in the nanocomposite decreased from 0.8392 to 0.8362 nm, the average crystallite size was in the range of 33–39 nm, the saturation magnetization decreased from 41.63 to 18.34 emu/g, while the coercivity had a maximum of 2604 Oe as x was 0.6. The decrease in the saturation magnetization was due to the decrease of net magnetic moment per molecule as magnetic Co2 þ were substituted by non-magnetic Mg2 þ on B sites. The change of the coercivity depended on the amplitudes of the decrease in K and MS with increasing x. The studies of Mössbauer spectra indicated all the samples were the completely magnetically ordered. The results of the cation distribution indicated the preference of Mg2 þ to occupy B sites increased with increasing x and reached a maximum when x was 0.6. The variations of the calculated and observed magneton number with x were almost the same, which suggested that Co1 xMgxFe2O4 in the nanocomposites was collinear ferrimagnetic structure. The results reveal that Mg2 þ doped in CoFe2O4/SiO2, especially with the content of 0.6, is a good candidate for high frequency recording materials because of its high coercivity.
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