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Structural and magnetic properties of Cr-substituted NiZnCo ferrite nanopowders
Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Structural and magnetic properties of Cr-substituted NiZnCo ferrite nanopowders Le-Zhong Li n, Xiao-Qiang Tu, Rui Wang, Long Peng College of Optoelectronic Technology, Chengdu University of Information Technology, Chengdu 610225, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 July 2014 Received in revised form 31 October 2014 Accepted 11 January 2015
Cr-substituted NiZnCo ferrite nanopowders, Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 (0 rx r 0.20), were synthesized by sol–gel auto-combustion method. The effect of Cr substitution on the structural and magnetic properties have been investigated. Differential thermal analysis-thermogravimetry (DTA-TG), X-ray diffraction (XRD), Fourier transform infrared spectro-scopy (FT-IR) and vibrating sample magnetometer (VSM) measurements were used to characterize chemical, structural and magnetic properties. The DTATG results indicate that there are three steps of combustion process. FT-IR spectra confirm the formation of spinel structure during the self-propagation combustion. XRD results indicate that the lattice parameter and the X-ray density decrease, and the average crystallite size increase with increasing Cr substitution. And Fe2O3 secondary impurity phase formed with excess Cr substitution. The saturation magnetization increases with the increase of Cr substitution when x r 0.05, and decreased when x 40.05. Meanwhile, the coercivity monotonically decreases with the increase of Cr substitution. & 2015 Published by Elsevier B.V.
Keywords: NiZnCo ferrite nanopowder Sol–gel auto-combustion process Cr substitution Structure Magnetic property
1. Introduction The unique properties of ferrite nanopowders have generated more interest in the science community because high surface to volume ratio resulting in novel phenomena's of nanomagnetism such as superparamagnetism, magnetic quantum tunneling and spin-glasslike behavior, etc. [1]. It is well known that the ferrites MFe2O4 with the spinel structure are based on a face-center cubic lattice of the oxygen ions. Each spinel unit cell contains eight formula units. In each unit cell, there are 64 tetrahedral sites (A sites) and 32 octahedral sites (B sites). Therefore, the chemical, structural, and magnetic properties of ferrite are strongly influenced by their composition and microstructure, which are sensitive to the preparation methodologies [2]. They show various magnetic properties depending on the cation distribution. Various cations can be placed in the structure of AB2O4 in A site and B site can result in the interesting physical and chemical properties in spinel ferrites [3]. Numerous workers have studied the effect of Cr3 þ substitution in the spinel ferrites, such as Zn ferrite [4], Ni ferrite [5], NiZn ferrite [6,7], NiCuZn ferrite [8], NiMg ferrite [9] and Co ferrite [3,10]. And all the investigations are focus on the effect of Fe3 þ by Cr3 þ substitution on the chemical, structural, and magnetic properties. To the best of our knowledge, no results have been n
Corresponding author. Fax: þ 86 2885966385. E-mail address: [email protected] (L.-Z. Li).
reported about the effects of substitution of Ni2 þ by Cr3 þ on the structural and magnetic properties of NiZnCo ferrite nanopowders. Cr3 þ ions usually occupy the octahedral sites in spinels [10]. The decrease in ionic radius and increase in valence are expected as Cr3 þ with ionic radius of 0.615 Å replaces the lager Ni2 þ (0.69 Å) cation, which can result in the structural variation of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders. At the same time, an increase in total magnetic moment at octahedral sites is expected as Cr3 þ with a magnetic moment of 3μB replaces the weaker Ni2 þ (2 μB) cation. Therefore, the substitution of Ni2 þ by Cr3 þ should influence the structural and magnetic properties of NiZnCo ferrite nanopowders. In the present work, NiZnCo ferrite nanopowders were prepared by sol–gel auto-combustion method because it has advantages of being able to use inexpensive precursors, a simple preparation method and low sintering temperature that results in well dispersed homogenous, nano-sized and highly reactive ferrite powder [9,10]. Accordingly, in the present paper, we report our results on structural and magnetic properties of magnetic Cr3 þ substituted NiZnCo ferrite nanopowders were prepared by sol–gel auto-combustion method.
2. Experimental procedures The samples of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 (x ¼0, 0.05, 0.10, 0.15, 0.20) ferrite nanopowders were synthesized by sol–gel autocombustion method. Stoichiometric quantities of analytical grade
http://dx.doi.org/10.1016/j.jmmm.2015.01.020 0304-8853/& 2015 Published by Elsevier B.V.
Please cite this article as: L.-Z. Li, et al., Journal of Magnetism and Magnetic Materials (2015), http://dx.doi.org/10.1016/j. jmmm.2015.01.020i
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L.-Z. Li et al. / Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Zn(CH3COO)2 2H2O, Ni(CH3COO)2 4H2O, Fe(NO3)3 9H2O, Co(NO3)2 6H2O and Cr(NO3)3 9H2O were first dissolved in deionized water and stirred for 15 min. After the nitrates completely dissolved, the citric acid was added to the resulting solution and the molar ratio of citric acid and metal nitrates was taken as 1:1. Then, the prepared solution was continuously stirred for 3 h at 80 °C to form the gel precursors. To maintain the pH value at 7 an aqueous ammonia solution was added drop-wise after the gel precursors cooled to room temperature. The gel solution was dried at 85 °C until a dry gel precursor was obtained. Finally, the dried gel was ignited in self-propagation combustion to obtain ferrite powders. Simultaneous differential thermal analysis-thrermogravimetry (DTA-TG) measurements were carried out on as-prepared powder, up to 800 °C at a heating rate of 10 °C/min, using Shimadzu DTG60 thermal analyzer. Fourier transform infrared (FT-IR) analysis, with KBr pellet method, was performed at the range of 4000–400 cm 1 using a Shimadzu IRPrestige-21 spectrophotometer. The phase identification of the prepared nanopowders was performed by DX 2700 X-ray diffractometer (XRD) (Cu target, Kα radiation, 35 kV, 25 mA) at room temperature. The hysteresis loops of samples were measured at room temperature by using a TOEI VSM-5S-15 vibrating sample magnetometer (VSM).
3.1. Thermal decomposition of the as-prepared precursor The minimum temperature for the complete decomposition process, which can be considered as the minimum temperature for the calcination process, can be estimated from the thermal behavior measurements of the dried precursors [8]. Fig. 1 shows typical DTA-TG curves in air for the as-prepared gel precursor with Ni0.4Zn0.5Cr0.1Co0.1Fe1.9O4. The DTA and TG results indicate that there are three steps of combustion process. The first step occurred before 200 °C which can be attributed to the evaporation of the residual water. The weight change was approximately 11%. The endothermic DTA peak in the range of 132–167 °C is attributed to the dehydration of the precursor. The second step occurred at about 224 °C which associated with a relatively sharp and intense exothermic peak. The weight change was approximately 69%. This indicated that the decomposition of the gel occurs suddenly in a single step, as observed in other systems [5,8]. The third step occurred in the range 260–400 °C are attributed to the oxidative decomposition of the residual nitrate and organic matter. The
100
10 TG
80
Weight (%)
5
Exo
↑
60
T
↓
0
DTA
40
x=0.20
x=0.10
x=0.00
4000
3500
3000
20
Endo
-5
200
2000
1500
1000
500
-1
Wave number (cm ) Fig. 2. FT-IR spectra of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders.
weight change was approximately 4.5%.The weight of the precursor appears to be constant for temperatures above 400 °C.
Fig. 2 shows the FT-IR spectra of the Cr-substituted NiZnCo ferrite nanopowders at room temperature in the frequency range of 400–4000 cm 1. The band around 415 cm 1 and the band around 580 cm 1 are ascribed to the stretching vibration of octahedral and tetrahedral metal–oxygen bonds, which are characteristics of spinel structure and hence confirms the ferrite formation [11]. The FT-IR spectra also show the absorption bands appearing at 1080, 1350 and 1400 cm 1 confirms the complex formation, since these bands are not characteristic to free ethylene glycol molecule. These bands are ascribed to the symmetric vibration of COH group, symmetric vibration of CO and symmetric vibration of NO3 group [11,12]. An absorption band corresponding to symmetric vibration of COO group is observed at around 1630 cm 1 and one more at around 3500 cm 1 corresponding to the symmetric vibration of O–H group. Owing to the high temperature generated during the combustion process all the carboxyl, hydroxyl and nitrate groups appear with less intensity. 3.3. Structural properties X-ray diffraction patterns of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders are shown in Fig. 3. The patterns match well with the characteristic reflections of cubic spinel structure. From it we can see that there is a Fe2O3 secondary impurity phase when x4 0.10. This mainly due to the valence balance. At present system, the valence of cation is 7.9 þx, which more than 8 when x40.10. Eventually, according to the valence balance, partial Fe3 þ will be combined with oxygen and existed in the phase of Fe2O3.The different structural parameters of all samples are calculated according to XRD data and the results are shown in Table 1. The lattice parameter (a) of the samples is calculated using the relation [13]:
a = dhkl h2 + k 2 + l2
0 0
2500
3.2. FT-IR properties
3. Results and discussion
∇
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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Transmittance (%)
2
400
600
800
Temperature ( °C) Fig. 1. DTA-TG curves in air of as-prepared precursors of Ni0.4Zn0.5Cr0.1Co0.1Fe1.9O4.
(1)
where (h k l) are the Miller indices and dhkl is the inter-planar spacing. The average particle size is calculated according to the Debye–Scherrer equation [13–15]:
Please cite this article as: L.-Z. Li, et al., Journal of Magnetism and Magnetic Materials (2015), http://dx.doi.org/10.1016/j. jmmm.2015.01.020i
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L.-Z. Li et al. / Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
400 (311)
x=0.00 x=0.05 x=0.10 x=0.15 x=0.20
300
(220)
(533) (620)
(422)
(222)
x=0.20
(440)
(511)
(400)
Fe O
x=0.15 x=0.10
200
Magnetization (kA/m)
Intensity (a. u.)
100 0 -100 -200
x=0.05
-300 x=0.00
-400
20
30
40
50
60
-10000
70
-5000
0
Fig. 3. X-ray diffraction patterns of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders.
Table 1 Lattice parameter (a), crystallite size (D) Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders.
and
X-ray
density
5000
10000
H (Oe)
2θ (° )
(dx)
of
Fig. 4. Hysteresis loops of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders.
Ni2 þ ions of higher atomic weight (58.693) with low atomic weight Cr3 þ ions (51.996) result in the slightly decrease of density from 5.3626 to 5.3556 g/cm3.
x
a (Å)
D (nm)
dx (g/cm3)
3.4. Magnetic properties
0.00 0.05 0.10 0.15 0.20
8.3857 8.3821 8.3788 8.3772 8.3736
22.3 23.2 23.4 24.2 25.3
5.3626 5.3620 5.3608 5.3563 5.3556
Fig. 4 shows the hysteresis loops of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders. It is observed the hysteresis loops of NiZnCo ferrite nanopowders without and with Cr substitution is that of a typical ferrimagnet. Furthermore, the saturation magnetization (Ms) and coercivity (Hc) of nanopowders as a function of Cr substitution (x) are shown in Fig. 5. The saturation magnetization (Ms) gradually increases with the increase of Cr substitution when xr 0.05, and decreases when x 40.05. Meanwhile, the coercivity (Hc) monotonically decreases with the increase of Cr substitution. Magnetic properties of ferrites are sensitively dependent on the structure, composition, defects, crystallite size, internal strain and cation distribution. According to Néel's two sublattice model of ferrimagnetisms [20], the cations distribution can be written as follows:
0.9λ β cos θ
(2)
where λ is wavelength of the X-ray radiation, β is the measure of broadening of diffraction due to size effect and θ is Bragg's angle. The X-ray density (dx) is calculated using the following relation [16,17]:
dx =
8MW NA a3
(3)
where MW is the molecular weight; NA is Avogadro's parameter and a is the lattice parameter. From Table 1 it is clear that, the lattice parameter (a) and the X-ray density (dx) decrease, and the average crystallite size calculated by Debye–Scherrer's equation slightly increase with the increase of Cr substitution. The decrease of lattice parameter is attributed to the decrease of ionic radius of Cr3 þ (0.615 Å) as compared to that of Ni2 þ (0.69 Å) ions [18]. The average crystallite size increases with the increase of Cr substitution can be explained in terms of the increased pore mobility due to the creation of excess cation vacancies. Excess vacancy formation with the substitution of Ni2 þ ions of lower valence with higher valence Cr3 þ ions can be argued from the site and charge neutrality as well as the oxidation reduction equilibrium of iron. For a grain boundary with attached pores, the velocity increases with the pore mobility. Another possible reason is that Cr substitution promotes the grain growth of NiZnCo ferrite nanopowders as reaction center by the increase of the bulk diffusion due to the aberration and activation of crystal lattice, which is caused by Cr substitution [19]. This supposition can be supported by the shrinked lattice constant. The decrease in the density mainly depends upon the molecular weight of the samples and their lattice parameters. Consequently, the substitution of
(Zn0.5 Fe 0.5 )[Ni0.5 − x CrxCo0.1Fe1.4 ]O4 . (A site) ⋯⋯ (B site) And the magnetic moment in Bohr magneton of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders is calculated by the following equation: 320
290 MS
285
Hc
300
280 280
Hc (Oe)
D=
Ms (kA/m)
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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
3
275
270 260 265 0.00
0.05
0.10
0.15
0.20
x Fig. 5. Ms and Hc of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders.
Please cite this article as: L.-Z. Li, et al., Journal of Magnetism and Magnetic Materials (2015), http://dx.doi.org/10.1016/j. jmmm.2015.01.020i
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4
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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
M = MB − MA = 5.8 + x .
(4)
where MA and MB are the magnetic moments in the A and B sites, and the magnetic moments of Zn2 þ , Ni2 þ , Cr3 þ , Co2 þ , and Fe3 þ ions are 0, 2, 3, 3, and 5 μB, respectively. Simultaneously, the term of saturation magnetization (Ms) is defined as the vector sum of magnetic moment per unit cell, and is written as the following equation:
Ms =
8M a3
.
(5)
Thus, according to Eqs. (4) and (5), and the variations of lattices parameters (a) with Cr substitution, Ms should be monotonically increases with the increase of Cr substitution. But, Ms gradually decreases with the increase of Cr substitution when x 40.05 as shown in Figs. 4 and 5. The initial increase of Ms with Cr sustitution x is explained on the basis of Néel's two sublattice model theory, but decrease in observed magnetic moment after x40.05 indicates a possibility of non-collinear spin structure in the system which can be explained on the basis of three sub-lattice model suggested by Yafet–Kittel [21]. In addition, the formation of Fe2O3 impurity phase and a large number of point defects present in the spinel structure due to the non-stoichiometry are also the main reasons result in the decrease of Ms. In case of excess Cr substitution, oxygen vacancies should be present. These oxygen vacancies could disrupt the superexchange interaction between the magnetic ions, thus decreasing the saturation magnetization of the nanopowders. The coercivity (Hc) of ferrite thin films as a function of Cr substitution (x) is shown in Fig. 5, which indicates that Hc monotonically decreases with the increase of Cr substitution. The decrease in the Hc values mainly due to the decrease in magnetocrystalline anisotropy. The magnetocrystalline anisotropy constant (K1) is negative for Cr ferrites [3]. The absolute value of K1 is larger for NiZnCo ferrites than that of Cr ferrites. The total anisotropy is equal to the sum of their individual anisotropies. So, K1 decreases with the increase of Cr substitution which results in the decrease of coercivity. Furthermore, the increased average grain size is another reason that results in the decrease of Hc with the increase of Cr substitution.
4. Conclusions Cr-substituted NiZnCo ferrite nanopowders, Ni0.5 xZn0.5Crx Co0.1Fe1.9O4 (0 rx r0.20), have been synthesized by sol–gel autocombustion method. The chemical, structural and static magnetic properties of nanopowders have been investigated and the following results have been obtained: (1) The DTA-TG results indicate that there are three steps of combustion process. And the decomposition of the gel occurs suddenly in a single step at about 224 °C (2) According to FT-IR analysis, there are two peaks at the band around 415 cm 1 and the band around 580 cm 1 which are characteristic bonds of spinel structure and hence confirms the ferrite formation. (3) The XRD results indicate that the lattice parameter and the X-ray density decrease, and the average crystallite size slightly increase with the increase of Cr substitution. Furthermore, there is a Fe2O3 secondary impurity phase when x 40.10. (4) The hysteresis curves of the nanopowders exhibited that the saturation magnetization increases with the increase of Cr substitution when x r0.05, and decreased when x 40.05.
Meanwhile, the coercivity monotonically decreases with the increase of Cr substitution.
Acknowledgments This work is financially supported by the Fund Project of Sichuan Provincial Department of Education and Research Fund for Young Academic Leaders of CUIT under Grant nos. 13ZB0079 and J201306.
References [1] M.S. Khandekar, R.C. Kambale, J.Y. Patil, Y.D. Kolekar, S.S. Suryavanshi, Effect of calcination temperature on the structural and electrical properties of cobalt ferrite synthesized by combustion method, J. Alloy. Compd. 509 (2011) 1861–1865. [2] F. Li, J.J. Liu, D.G. Evans, X. Duan, Stoichiometric synthesis of pure MFe2O4 (M ¼Mg, Co, and Ni) spinel ferrites from tailored layered double hydroxide (hydrotalcite-like) precursors, Chem. Mater. 16 (2004) 1597–1602. [3] S.T. Alone, S.E. Shirsath, R.H. Kadam, K.M. Jadhav, Chemical synthesis, structural and magnetic properties of nano-structured Co–Zn–Fe–Cr ferrite, J. Alloy. Compd. 509 (2011) 5055–5060. [4] A.I. Borhan, T. Slatineanu, A.R. Iordan, M.N. Palamaru, Influence of chromium ion substitution on the structure and properties of zinc ferrite synthesized by the sol–gel auto-combustion method, Polyhedron 56 (2013) 82–89. [5] M.A. Gabal, Y.M.A. Angari, Effect of chromium ion substitution on the electromagnetic properties of nickel ferrite, Mater. Chem. Phys. 118 (2009) 153–160. [6] L. Gama, E.P. Hernandez, D.R. Cornejo, A.A. Costa, S.M. Rezende, R.H.G. A. Kiminami, A.C.F.M. Costa, Magnetic and structural properties of nanosize Ni–Zn–Cr ferrite particles synthesized by combustion reaction, J. Magn. Magn. Mater. 317 (2007) 29–33. [7] A.C.F.M. Costa, V.J. Silva, H.S. Ferreira, A.A. Costa, D.R. Cornejo, R.H.G. A. Kiminami, L. Gama, Structural and magnetic properties of chromium-doped ferrite nanopowders, J. Alloy. Compd. 483 (2009) 655–657. [8] W.A. Bayoumy, M.A. Gabal, Synthesis characterization and magnetic properties of Cr-substituted NiCuZn nanocrystalline ferrite, J. Alloy. Compd. 506 (2010) 205–209. [9] M. Hashim, Alimuddin, S. Kumar, S.E. Shirsath, R.K. Kotnalad, J. Shah, R. Kumar, Influence of Cr3 þ ion on the structural, ac conductivity and magnetic properties of nanocrystalline Ni–Mg ferrite, Ceram. Int. 39 (2013) 1807–1819. [10] M. Han, C.R. Vestal, Z.J. Zhang, Quantum couplings and magnetic properties of CoCrxFe2 xO4 (0 o xo 1) spinel ferrite nanoparticles synthesized with reverse micelle method, J. Phys. Chem. B 108 (2004) 583–587. [11] M.A. Dar, V. Verma, S.P. Gairola, W.A. Siddiqui, R.K. Singh, R.K. Kotnala, Low dielectric loss of Mg doped Ni–Cu–Zn nano-ferrites for power applications, Appl. Surf. Sci. 258 (2012) 5342–5347. [12] A. Ghasemin, M. Mousavinia, Structural and magnetic evaluation of substituted NiZnFe2O4 particles synthesized by conventional sol-gel method, Ceram. Int. 40 (2014) 2825–2834. [13] B.D. Cullity, Elements of X-ray Diffraction, Addison Wesley Publishing, Massachusetts, 1959. [14] M.H. Mahmoud, C.M. Williams, J. Cai, I. Siu, J.C. Walker, Investigation of Mnferrite films produced by pulsed laser deposition, J. Magn. Magn. Mater. 261 (2003) 314–318. [15] M.H. Ibrahim, M.S. Seehra, G. Srinivasan, Observations of magnetization reversal and magnetic clusters in copper ferrite films, J. App. Phys. 75 (1994) 6822–6824. [16] S. Amiri, H. Shokrollahi, Magnetic and structural properties of RE doped Coferrite (RE ¼ Nd, Eu, and Gd) nano-particles synthesized by co-precipitation, J. Magn. Magn. Mater. 345 (2013) 18–23. [17] E.R. Kumar, R. Jayaprakash, S. Kumar, Effect of annealing temperature on structural and magnetic properties of manganese substituted NiFe2O4 nanoparticles, Mater. Sci. Semicond. Process. 17 (2014) 173–177. [18] H.S.C. O’Neill, A. Navrotsky, Simple spinels; crystallographic parameters, cation radii, lattice energies, and cation distribution, Am. Mineral. 68 (1983) 181–194. [19] L. Li, X. Tu, L. Peng, X. Zhu, Structure and static magnetic properties of Zrsubstituted NiZn ferrite thin films synthesized by sol–gel process, J. Alloy. Compd. 545 (2012) 67–69. [20] D.F. Wan, X.L. Ma, The Physics of Magnetism, UESTC Press, Chengdu, 1994. [21] Y. Yafet, C. Kittel, Antiferromagnetic arrangements in ferrites, Phys. Rev. 87 (1952) 290–294.
Please cite this article as: L.-Z. Li, et al., Journal of Magnetism and Magnetic Materials (2015), http://dx.doi.org/10.1016/j. jmmm.2015.01.020i
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Structural and magnetic properties of Cr-substituted NiZnCo ferrite nanopowders Le-Zhong Li n, Xiao-Qiang Tu, Rui Wang, Long Peng College of Optoelectronic Technology, Chengdu University of Information Technology, Chengdu 610225, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 July 2014 Received in revised form 31 October 2014 Accepted 11 January 2015
Cr-substituted NiZnCo ferrite nanopowders, Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 (0 rx r 0.20), were synthesized by sol–gel auto-combustion method. The effect of Cr substitution on the structural and magnetic properties have been investigated. Differential thermal analysis-thermogravimetry (DTA-TG), X-ray diffraction (XRD), Fourier transform infrared spectro-scopy (FT-IR) and vibrating sample magnetometer (VSM) measurements were used to characterize chemical, structural and magnetic properties. The DTATG results indicate that there are three steps of combustion process. FT-IR spectra confirm the formation of spinel structure during the self-propagation combustion. XRD results indicate that the lattice parameter and the X-ray density decrease, and the average crystallite size increase with increasing Cr substitution. And Fe2O3 secondary impurity phase formed with excess Cr substitution. The saturation magnetization increases with the increase of Cr substitution when x r 0.05, and decreased when x 40.05. Meanwhile, the coercivity monotonically decreases with the increase of Cr substitution. & 2015 Published by Elsevier B.V.
Keywords: NiZnCo ferrite nanopowder Sol–gel auto-combustion process Cr substitution Structure Magnetic property
1. Introduction The unique properties of ferrite nanopowders have generated more interest in the science community because high surface to volume ratio resulting in novel phenomena's of nanomagnetism such as superparamagnetism, magnetic quantum tunneling and spin-glasslike behavior, etc. [1]. It is well known that the ferrites MFe2O4 with the spinel structure are based on a face-center cubic lattice of the oxygen ions. Each spinel unit cell contains eight formula units. In each unit cell, there are 64 tetrahedral sites (A sites) and 32 octahedral sites (B sites). Therefore, the chemical, structural, and magnetic properties of ferrite are strongly influenced by their composition and microstructure, which are sensitive to the preparation methodologies [2]. They show various magnetic properties depending on the cation distribution. Various cations can be placed in the structure of AB2O4 in A site and B site can result in the interesting physical and chemical properties in spinel ferrites [3]. Numerous workers have studied the effect of Cr3 þ substitution in the spinel ferrites, such as Zn ferrite [4], Ni ferrite [5], NiZn ferrite [6,7], NiCuZn ferrite [8], NiMg ferrite [9] and Co ferrite [3,10]. And all the investigations are focus on the effect of Fe3 þ by Cr3 þ substitution on the chemical, structural, and magnetic properties. To the best of our knowledge, no results have been n
Corresponding author. Fax: þ 86 2885966385. E-mail address: [email protected] (L.-Z. Li).
reported about the effects of substitution of Ni2 þ by Cr3 þ on the structural and magnetic properties of NiZnCo ferrite nanopowders. Cr3 þ ions usually occupy the octahedral sites in spinels [10]. The decrease in ionic radius and increase in valence are expected as Cr3 þ with ionic radius of 0.615 Å replaces the lager Ni2 þ (0.69 Å) cation, which can result in the structural variation of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders. At the same time, an increase in total magnetic moment at octahedral sites is expected as Cr3 þ with a magnetic moment of 3μB replaces the weaker Ni2 þ (2 μB) cation. Therefore, the substitution of Ni2 þ by Cr3 þ should influence the structural and magnetic properties of NiZnCo ferrite nanopowders. In the present work, NiZnCo ferrite nanopowders were prepared by sol–gel auto-combustion method because it has advantages of being able to use inexpensive precursors, a simple preparation method and low sintering temperature that results in well dispersed homogenous, nano-sized and highly reactive ferrite powder [9,10]. Accordingly, in the present paper, we report our results on structural and magnetic properties of magnetic Cr3 þ substituted NiZnCo ferrite nanopowders were prepared by sol–gel auto-combustion method.
2. Experimental procedures The samples of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 (x ¼0, 0.05, 0.10, 0.15, 0.20) ferrite nanopowders were synthesized by sol–gel autocombustion method. Stoichiometric quantities of analytical grade
http://dx.doi.org/10.1016/j.jmmm.2015.01.020 0304-8853/& 2015 Published by Elsevier B.V.
Please cite this article as: L.-Z. Li, et al., Journal of Magnetism and Magnetic Materials (2015), http://dx.doi.org/10.1016/j. jmmm.2015.01.020i
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Zn(CH3COO)2 2H2O, Ni(CH3COO)2 4H2O, Fe(NO3)3 9H2O, Co(NO3)2 6H2O and Cr(NO3)3 9H2O were first dissolved in deionized water and stirred for 15 min. After the nitrates completely dissolved, the citric acid was added to the resulting solution and the molar ratio of citric acid and metal nitrates was taken as 1:1. Then, the prepared solution was continuously stirred for 3 h at 80 °C to form the gel precursors. To maintain the pH value at 7 an aqueous ammonia solution was added drop-wise after the gel precursors cooled to room temperature. The gel solution was dried at 85 °C until a dry gel precursor was obtained. Finally, the dried gel was ignited in self-propagation combustion to obtain ferrite powders. Simultaneous differential thermal analysis-thrermogravimetry (DTA-TG) measurements were carried out on as-prepared powder, up to 800 °C at a heating rate of 10 °C/min, using Shimadzu DTG60 thermal analyzer. Fourier transform infrared (FT-IR) analysis, with KBr pellet method, was performed at the range of 4000–400 cm 1 using a Shimadzu IRPrestige-21 spectrophotometer. The phase identification of the prepared nanopowders was performed by DX 2700 X-ray diffractometer (XRD) (Cu target, Kα radiation, 35 kV, 25 mA) at room temperature. The hysteresis loops of samples were measured at room temperature by using a TOEI VSM-5S-15 vibrating sample magnetometer (VSM).
3.1. Thermal decomposition of the as-prepared precursor The minimum temperature for the complete decomposition process, which can be considered as the minimum temperature for the calcination process, can be estimated from the thermal behavior measurements of the dried precursors [8]. Fig. 1 shows typical DTA-TG curves in air for the as-prepared gel precursor with Ni0.4Zn0.5Cr0.1Co0.1Fe1.9O4. The DTA and TG results indicate that there are three steps of combustion process. The first step occurred before 200 °C which can be attributed to the evaporation of the residual water. The weight change was approximately 11%. The endothermic DTA peak in the range of 132–167 °C is attributed to the dehydration of the precursor. The second step occurred at about 224 °C which associated with a relatively sharp and intense exothermic peak. The weight change was approximately 69%. This indicated that the decomposition of the gel occurs suddenly in a single step, as observed in other systems [5,8]. The third step occurred in the range 260–400 °C are attributed to the oxidative decomposition of the residual nitrate and organic matter. The
100
10 TG
80
Weight (%)
5
Exo
↑
60
T
↓
0
DTA
40
x=0.20
x=0.10
x=0.00
4000
3500
3000
20
Endo
-5
200
2000
1500
1000
500
-1
Wave number (cm ) Fig. 2. FT-IR spectra of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders.
weight change was approximately 4.5%.The weight of the precursor appears to be constant for temperatures above 400 °C.
Fig. 2 shows the FT-IR spectra of the Cr-substituted NiZnCo ferrite nanopowders at room temperature in the frequency range of 400–4000 cm 1. The band around 415 cm 1 and the band around 580 cm 1 are ascribed to the stretching vibration of octahedral and tetrahedral metal–oxygen bonds, which are characteristics of spinel structure and hence confirms the ferrite formation [11]. The FT-IR spectra also show the absorption bands appearing at 1080, 1350 and 1400 cm 1 confirms the complex formation, since these bands are not characteristic to free ethylene glycol molecule. These bands are ascribed to the symmetric vibration of COH group, symmetric vibration of CO and symmetric vibration of NO3 group [11,12]. An absorption band corresponding to symmetric vibration of COO group is observed at around 1630 cm 1 and one more at around 3500 cm 1 corresponding to the symmetric vibration of O–H group. Owing to the high temperature generated during the combustion process all the carboxyl, hydroxyl and nitrate groups appear with less intensity. 3.3. Structural properties X-ray diffraction patterns of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders are shown in Fig. 3. The patterns match well with the characteristic reflections of cubic spinel structure. From it we can see that there is a Fe2O3 secondary impurity phase when x4 0.10. This mainly due to the valence balance. At present system, the valence of cation is 7.9 þx, which more than 8 when x40.10. Eventually, according to the valence balance, partial Fe3 þ will be combined with oxygen and existed in the phase of Fe2O3.The different structural parameters of all samples are calculated according to XRD data and the results are shown in Table 1. The lattice parameter (a) of the samples is calculated using the relation [13]:
a = dhkl h2 + k 2 + l2
0 0
2500
3.2. FT-IR properties
3. Results and discussion
∇
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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Transmittance (%)
2
400
600
800
Temperature ( °C) Fig. 1. DTA-TG curves in air of as-prepared precursors of Ni0.4Zn0.5Cr0.1Co0.1Fe1.9O4.
(1)
where (h k l) are the Miller indices and dhkl is the inter-planar spacing. The average particle size is calculated according to the Debye–Scherrer equation [13–15]:
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400 (311)
x=0.00 x=0.05 x=0.10 x=0.15 x=0.20
300
(220)
(533) (620)
(422)
(222)
x=0.20
(440)
(511)
(400)
Fe O
x=0.15 x=0.10
200
Magnetization (kA/m)
Intensity (a. u.)
100 0 -100 -200
x=0.05
-300 x=0.00
-400
20
30
40
50
60
-10000
70
-5000
0
Fig. 3. X-ray diffraction patterns of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders.
Table 1 Lattice parameter (a), crystallite size (D) Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders.
and
X-ray
density
5000
10000
H (Oe)
2θ (° )
(dx)
of
Fig. 4. Hysteresis loops of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders.
Ni2 þ ions of higher atomic weight (58.693) with low atomic weight Cr3 þ ions (51.996) result in the slightly decrease of density from 5.3626 to 5.3556 g/cm3.
x
a (Å)
D (nm)
dx (g/cm3)
3.4. Magnetic properties
0.00 0.05 0.10 0.15 0.20
8.3857 8.3821 8.3788 8.3772 8.3736
22.3 23.2 23.4 24.2 25.3
5.3626 5.3620 5.3608 5.3563 5.3556
Fig. 4 shows the hysteresis loops of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders. It is observed the hysteresis loops of NiZnCo ferrite nanopowders without and with Cr substitution is that of a typical ferrimagnet. Furthermore, the saturation magnetization (Ms) and coercivity (Hc) of nanopowders as a function of Cr substitution (x) are shown in Fig. 5. The saturation magnetization (Ms) gradually increases with the increase of Cr substitution when xr 0.05, and decreases when x 40.05. Meanwhile, the coercivity (Hc) monotonically decreases with the increase of Cr substitution. Magnetic properties of ferrites are sensitively dependent on the structure, composition, defects, crystallite size, internal strain and cation distribution. According to Néel's two sublattice model of ferrimagnetisms [20], the cations distribution can be written as follows:
0.9λ β cos θ
(2)
where λ is wavelength of the X-ray radiation, β is the measure of broadening of diffraction due to size effect and θ is Bragg's angle. The X-ray density (dx) is calculated using the following relation [16,17]:
dx =
8MW NA a3
(3)
where MW is the molecular weight; NA is Avogadro's parameter and a is the lattice parameter. From Table 1 it is clear that, the lattice parameter (a) and the X-ray density (dx) decrease, and the average crystallite size calculated by Debye–Scherrer's equation slightly increase with the increase of Cr substitution. The decrease of lattice parameter is attributed to the decrease of ionic radius of Cr3 þ (0.615 Å) as compared to that of Ni2 þ (0.69 Å) ions [18]. The average crystallite size increases with the increase of Cr substitution can be explained in terms of the increased pore mobility due to the creation of excess cation vacancies. Excess vacancy formation with the substitution of Ni2 þ ions of lower valence with higher valence Cr3 þ ions can be argued from the site and charge neutrality as well as the oxidation reduction equilibrium of iron. For a grain boundary with attached pores, the velocity increases with the pore mobility. Another possible reason is that Cr substitution promotes the grain growth of NiZnCo ferrite nanopowders as reaction center by the increase of the bulk diffusion due to the aberration and activation of crystal lattice, which is caused by Cr substitution [19]. This supposition can be supported by the shrinked lattice constant. The decrease in the density mainly depends upon the molecular weight of the samples and their lattice parameters. Consequently, the substitution of
(Zn0.5 Fe 0.5 )[Ni0.5 − x CrxCo0.1Fe1.4 ]O4 . (A site) ⋯⋯ (B site) And the magnetic moment in Bohr magneton of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders is calculated by the following equation: 320
290 MS
285
Hc
300
280 280
Hc (Oe)
D=
Ms (kA/m)
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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
3
275
270 260 265 0.00
0.05
0.10
0.15
0.20
x Fig. 5. Ms and Hc of Ni0.5 xZn0.5CrxCo0.1Fe1.9O4 ferrite nanopowders.
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4
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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
M = MB − MA = 5.8 + x .
(4)
where MA and MB are the magnetic moments in the A and B sites, and the magnetic moments of Zn2 þ , Ni2 þ , Cr3 þ , Co2 þ , and Fe3 þ ions are 0, 2, 3, 3, and 5 μB, respectively. Simultaneously, the term of saturation magnetization (Ms) is defined as the vector sum of magnetic moment per unit cell, and is written as the following equation:
Ms =
8M a3
.
(5)
Thus, according to Eqs. (4) and (5), and the variations of lattices parameters (a) with Cr substitution, Ms should be monotonically increases with the increase of Cr substitution. But, Ms gradually decreases with the increase of Cr substitution when x 40.05 as shown in Figs. 4 and 5. The initial increase of Ms with Cr sustitution x is explained on the basis of Néel's two sublattice model theory, but decrease in observed magnetic moment after x40.05 indicates a possibility of non-collinear spin structure in the system which can be explained on the basis of three sub-lattice model suggested by Yafet–Kittel [21]. In addition, the formation of Fe2O3 impurity phase and a large number of point defects present in the spinel structure due to the non-stoichiometry are also the main reasons result in the decrease of Ms. In case of excess Cr substitution, oxygen vacancies should be present. These oxygen vacancies could disrupt the superexchange interaction between the magnetic ions, thus decreasing the saturation magnetization of the nanopowders. The coercivity (Hc) of ferrite thin films as a function of Cr substitution (x) is shown in Fig. 5, which indicates that Hc monotonically decreases with the increase of Cr substitution. The decrease in the Hc values mainly due to the decrease in magnetocrystalline anisotropy. The magnetocrystalline anisotropy constant (K1) is negative for Cr ferrites [3]. The absolute value of K1 is larger for NiZnCo ferrites than that of Cr ferrites. The total anisotropy is equal to the sum of their individual anisotropies. So, K1 decreases with the increase of Cr substitution which results in the decrease of coercivity. Furthermore, the increased average grain size is another reason that results in the decrease of Hc with the increase of Cr substitution.
4. Conclusions Cr-substituted NiZnCo ferrite nanopowders, Ni0.5 xZn0.5Crx Co0.1Fe1.9O4 (0 rx r0.20), have been synthesized by sol–gel autocombustion method. The chemical, structural and static magnetic properties of nanopowders have been investigated and the following results have been obtained: (1) The DTA-TG results indicate that there are three steps of combustion process. And the decomposition of the gel occurs suddenly in a single step at about 224 °C (2) According to FT-IR analysis, there are two peaks at the band around 415 cm 1 and the band around 580 cm 1 which are characteristic bonds of spinel structure and hence confirms the ferrite formation. (3) The XRD results indicate that the lattice parameter and the X-ray density decrease, and the average crystallite size slightly increase with the increase of Cr substitution. Furthermore, there is a Fe2O3 secondary impurity phase when x 40.10. (4) The hysteresis curves of the nanopowders exhibited that the saturation magnetization increases with the increase of Cr substitution when x r0.05, and decreased when x 40.05.
Meanwhile, the coercivity monotonically decreases with the increase of Cr substitution.
Acknowledgments This work is financially supported by the Fund Project of Sichuan Provincial Department of Education and Research Fund for Young Academic Leaders of CUIT under Grant nos. 13ZB0079 and J201306.
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