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shell nanocomposite prepared by the hydrothermal method
Author’s Accepted Manuscript Structural and magnetic properties of CoFe2O4/NiFe2O4 CORE/shell nanocomposite prepared by Hydrothermal method A.A. Sattar, H.M. EL-Sayed, Ibrahim ALsuqia www.elsevier.com/locate/jmmm
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S0304-8853(15)30345-0 http://dx.doi.org/10.1016/j.jmmm.2015.07.039 MAGMA60404
To appear in: Journal of Magnetism and Magnetic Materials Received date: 15 June 2015 Accepted date: 13 July 2015 Cite this article as: A.A. Sattar, H.M. EL-Sayed and Ibrahim ALsuqia, Structural and magnetic properties of CoFe2O4/NiFe2O4 CORE/shell nanocomposite prepared by Hydrothermal method, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2015.07.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and Magnetic Properties of CoFe2O4/NiFe2O4 Core/Shell Nanocomposite Prepared By Hydrothermal Method A.A.Sattar1, H.M.EL-Sayed1 and Ibrahim.ALsuqia2 1
Department of Physics, Faculty of Science, Ain Shams University, 11566 Abbasia, Cairo, Egypt. 2
Department of Physics, Faculty of Education and Applied Science, Hajjah University, Alshahli, Hajjah, Yemen. E-mail of Corresponding Author : [email protected] Abstract CoFe2O4/NiFe2O4 core/shell magnetic nanocomposite was synthesized by using hydrothermal method.The analysis of XRD indicated the coexistence of CoFe2O4, NiFe2O4as core/shell composite. The core/shell structure of the composite sample has been confirmed by HR–TEM images, EDX and FT–IR measurements. The size of obtained core/shell nanoparticles was 17 nm in core diameter and about 3 nm in shell thickness. The magnetization measurements showed that both the coercive field and the saturation magnetization of the resulting core/shell nanocomposite were slightly decreased compared to those of the CoFe2O4 core but the thermal stability is of the magnetization parameter was enhanced. Furthermore, superparamagnetic phase is established at temperatures higher than the room temperature. The results were discussed in terms of the surface pinning and the magnetic interaction at the interface between the core and shell. Keywords: CoFe2O4/NiFe2O4,Core/Shell Magnetic Hydrothermal Method, Magnetic Properties.
Nanocomposite,
1. Introduction Core/shell magnetic nanocomposites have attracted considerable attention because of their unique physical and chemical properties. They have the advantage of tuning and tailoring their physical properties by changing the chemical composition and size of the core and shell [1].
Recent studies have demonstrated some merits of bimagnetic core/shell nanocrystals in improving the energy product of permanent magnets and in enhancing the thermal stability of magnetic nanocrystals to overcome the “superparamagnetic limitation” in recording media [2]. The exploration of various core−shell combinations of different magnetic materials provided a better fundamental understanding of magnetic interactions and achieved the desirable magnetic characteristics for specific applications [3]. One of the most important ferrites is Cobalt ferrite (CoFe2O4) because of its high magnetic and thermal stability and high anisotropy field. These properties, along with their greatest physical and chemical stability, make CoFe2O4 nanoparticles suitable for high density information recording, ferrofluid technology, storage systems, clinical and biological applications [1], [4-8]. On the other hand,Nickel ferrite (NiFe2O4) is one of the versatile and technologically important soft ferrite (low anisotropy field) materials with spinel structure because of its typical ferromagnetic properties, low electrical conductivity and thus lower eddy current losses, high electrochemical stability, catalytic behavior and abundance in nature [9-12]. The combination of these two ferrites as a core/shell structure may improve their magnetic properties [1]. The purpose of this work is to prepare and characterize high coercive field/low coercive field magnetic core/shell nanocomposites based on cobalt and nickel ferrites, respectively, by the hydrothermal method.
2. Experimental details: 2.1 Materials Analytical grades of Iron (III) nitrate nonahydrate (Fe(NO3)3.9H2O), Cobalt (II) nitrate hexahydrate (Co(NO3)2.6H2O) and sodium hydroxide (NaOH) were used as precursors and deionized water as solvent for preparing the core. Sodium citrate solution for surface modification of the core and Nickel (II) nitrate hexahydrate (Ni(NO3)2.6H2O) for preparing the shell.
2.2 Synthesis of Cobalt Ferrite and Nickel Ferrite by Hydrothermal Method: Cobalt ferrite nanoparticles were prepared by dissolving 1 mole of (Co(NO3) 2.6H2O) and 2 moles of (Fe (NO3) 3.9H2O) in distilled water. Under vigorous stirring, the pH of the precursor solution was adjusted to pH 12 with the drop–wise addition of (NaOH) solution. The cobalt ferrite (CoFe2O4) nanoparticles are prepared according to the following chemical reaction: Co(NO3)2 + 2 Fe(NO3)3 + 8 NaOH →
CoFe2O4 + 8 Na(NO3) + 4H2O
The same method is used for preparing Nickel ferrite nanoparticles by the chemical reaction: Ni(NO3)2 + 2 Fe(NO3)3 + 8 NaOH →
NiFe2O4 + 8 Na(NO3) + 4H2O
After continued stirring for 1 h, the solution transferred into 100 mL Teflon-lined steel autoclave. The autoclave was sealed and treated at 180 ºC for 15 hours. The pressure in the sealed autoclave was lower than the equilibrium vapor tension of water at the same temperature because the solution contained NaOH. After the hydrothermal reaction, the autoclave was taken out and cooled at room temperature. The obtained product of CoFe2O4 and NiFe2O4 were filtered and washed by water several times until the pH decreased to 7.0. The solution of CoFe2O4 was separated into two equal quantities; the first half was dried at 90 ºC for 1 hour and the second half will be used as a core for core/shell composite sample. 2.3 Synthesis Nanocomposite
of
the
CoFe2o4/NiFe2o4
Core/Shell
Magnetic
CoFe2O4/NiFe2O4 core/shell magnetic nanocompositeis synthesis by two steps:
First, Surface modification of cobalt ferrite nanoparticles was taken place by the reaction of sodium citrate, coating (0.5 g) of Cobalt ferrite and (2.58 g) sodium citrate solution were mixed together in a flask and Stirr well for 1 hour. To reduce the aggregation of nanoparticles, the mixture was kept in ultrasonic bath for 1 hour at 60 ºC. Then, the precipitates were collected by a magnet and washed with acetone to remove remnant sodium citrate. Second, NiFe2O4 (shell) was prepared in the presence of surface modified CoFe2O4 (core) using hydrothermal method. The surface modified cobalt ferrite nanoparticles were re-dispersed in the mixed solution 1 mole of Ni(NO3)2 and 2 moles of Fe (NO3) 3.9H2O and Under vigorous stirring, the pH of the precursor solution was adjusted to pH 12 as before. The same above steps for preparing CoFe2O4 using hydrothermal method are repeated; the obtained product was filtered and washed by water several times until the pH reached to 7.0. To reduce the aggregation of nanoparticles, ultrasonic bath is used for 1 hr at 60oC. 3. Sample Characterizations: Phase identification of the powder samples was performed using X–ray diffraction (XRD), Philips diffractometer (X’pert MPD) with Cu–Kα radiation (λ = 1.5405 Å) at room temperature.IR spectra were recorded in the range from 200 to 4000 cm−1, using Perkin Elmer Spectrum One FT–IR spectrophotometer on KBr pellets. The particle morphology and crystal planes examined with High resolution transmission electron microscopy (HR–TEM, Tecnai G20, FEI, Netherland) Model, Super Twin TEM, with point to point resolution of 0.23 nm and line resolution of 0.14 nm. The magnetic measurements of the prepared powder were determined at room temperature by using the vibrating sample magnetometer (VSM). 4. Results and Discussion 4.1 X-Ray Diffraction Analysis The X–ray diffraction patterns of CoFe2O4, NiFe2O4 and CoFe2O4/NiFe2O4 ferrite nanoparticles are shown in Fig. (1). The reflection peaks from the planes (111), (311), (220), (222), (400), (422), (511), (440) and (533) correspond to the face centered cubic close packed fcc system of CoFe2O4
and NiFe2O4 are indexed with the JCPDS card (reference code: 01–1121) and (reference code: 10–325) respectively [9, 13]. It is obvious that all prepared samples could be refined using the Fd3m space group and have single phase spinelcubic structure. The lattice parameters of prepared ferrite samples are listed in the table (1). The values of the lattice parameters of CoFe2O4 and NiFe2O4 are in agreement with that reported by [13-16]. Furthermore the lattice parameter of CoFe2O4/NiFe2O4 is close to that of bare CoFe2O4. This observation could be attributed to the lattice matching between NiFe2O4 of CoFe2O4. Where: (
)
(1)
This small lattice mismatch between NiFe2O4 and CoFe2O4 causes the crystal growth of NiFe2O4 as a shell on CoFe2O4. The crystallite size and strain of the prepared samples using (Williamson–Hall, Debye–Scherrer methods and Bragg equation) [7, 8, 15, 17] are calculated and listed in table 1. One can observe that CoFe2O4 ferrite has lowest crystallite size and lattice strain while NiFe2O4 ferrite has larger crystallite size and lattice strain. Furthermore, comparing the crystallite size of CoFe2O4/NiFe2O4 with that of bare CoFe2O4, it is obvious that, the crystallite size of the core/shell sample is greater than that of bare CoFe2O4 by about 3 nm which supports the formation of core/shell nanocomposite. Moreover, the lattice strain of CoFe2O4/NiFe2O4 is greater than that of bare CoFe2O4; the increase of lattice strain is coming from the small lattice mismatch between NiFe2O4 with CoFe2O4 which also, confirms the presence of NiFe2O4 as a shell on CoFe2O4. 4.2 FT–IR Spectra Fig. (2) shows the FT–IR spectrum of the three prepared samples in the wave-number range between 200 and 4000 cm-1. It is obvious that, Fig. 2 (a, b) the IR spectra of have the two principle broad absorption bands (v1& v2) at 373.16 cm-1 and 581.15 cm-1 respectively while for NiFe2O4 they are at 400.56 cm-1 and 601.18 cm-1 respectively. These two vibration bands are corresponded to the intrinsic vibrations of octahedral and tetrahedral sites
in the spinel structure respectively [18, 19]. Using a software program, the peaks could be analyzed to their principle peaks as shown in the inset figures. The analyzed peaks are listed in table 2. It is clear that, for CoFe2O4 the octahedral band has two large peaks at 336 cm-1 and 400 cm-1 which is corresponding to the Me2+–O and Fe3+–O bonds respectively. On the other hand the tetrahedral band has two peaks, the large one at 579 cm-1 which corresponding to Fe3+–O bond and small peak at 629 cm-1 which corresponding to Co2+–O bond in the tetrahedral site [20, 21]. From these results, one can conclude that there is a small Co ions in the tetrahedral site i.e. the prepared CoFe2O4 is partially inverse ferrite [8]. On the other hand, for NiFe2O4, the two vibrational modes which are observed in the octahedral site are corresponding to the Fe3+–O and Ni2+–O bonds respectively, while there is no M2+–O in the tetrahedral site which confirms the complete inverse structure of NiFe2O4 [22]. Finally, for CoFe2O4/NiFe2O4 the FT–IR peaks lie between that of the bare CoFe2O4 and NiFe2O4. Also, the peak analysis shows the presence of vibrations of Fe3+–O and M2+–O in both of tetrahedral and octahedral sites. Furthermore, the peak positions are near to that of CoFe2O4 than that of NiFe2O4. These observations support the formation of NiFe2O4 on the surface of CoFe2O4 [23]. The presence of peaks at high wave numbers is corresponding to the following vibrational modes: •
the broad peak at 3441 cm-1 is characteristic of O–H bonds [24] and the broad band centered at 1611 cm-1 was assigned to the H–O–H bending vibration of the absorbed water [20,25].
•
The peaks at 2923 cm−1, 1641 cm−1, 1388 cm−1 and 1029 cm−1 are ascribed to C–H of alkyl group, symmetric COO–Fe, asymmetric COO–Fe and C–OH bond, respectively [26].
The above results reveal that sodium citrate has been grafted onto the surface of CoFe2O4 nanoparticles through the reaction of hydroxide radical groups (–OH) on the surface of CoFe2O4 nanoparticles and carboxylate anion (R–COO−) of sodium citrate [23]. Finally, the presence of small
absorption broad band in the range 200 –300 cm-1 is assigned to the lattice vibration of the ferrite [27].
4.3 TEM and EDX Spectrum Fig.(3) shows the TEM images for the samples CoFe2O4 and CoFe2O4/NiFe2O4 nanocomposite respectively. It is obvious that for bare CoFe2O4 sample Fig. 3(a) the particle size of is about 17 nm while for CoFe2O4/NiFe2O4 Fig. 3(b & c) there are an obvious shell around the core with thickness about 3 nm (light shadow). The selected area electron diffraction (SAED) patterns indicate that CoFe2O4/NiFe2O4 nanocomposite is found in well-crystalline nature. The particle size distribution obtained from analysis of bare CoFe2O4 and CoFe2O4/NiFe2O4 images is presented in Fig. 4 (a & b). This histogram is well described by the Lorentz distribution function, shown by a continuous curve in the figure. This analysis establishes a diameter for CoFe2O4/NiFe2O4 (20 nm) which is distinctly larger than, that the diameter of bare CoFe2O4 (15 nm) nanoparticles suggesting the growth of NiFe2O4 shell over the core CoFe2O4 particles [23, 28] and also which are in excellent agreement with the results of XRD. Fig.(5) shows the EDX analysis in the core and shell regions respectively. By elemental analysis in both of the core and shell, there are Co, Ni and Fe elements in the regions of the core while the shell has only Ni, Fe elements which confirm the core/shell formation. 4.4 The Magnetic Properties 4.4.1 The Hysteresis loops Fig.(6) displays magnetization hysteresis of the CoFe2O4, NiFe2O4 nanoparticles and CoFe2O4/NiFe2O4 core/shell magnetic nanocomposites. The hysteresis curves show characteristics of CoFe2O4 hard ferrites with high coercive field and magnetization, and characteristics of NiFe2O4 soft ferrites with low coercive field and magnetization. Furthermore, the hysteresis loop of the CoFe2O4/NiFe2O4core/shell magnetic nanocomposite shows is smooth hysteresis, indicative a single-phase-like magnetic material behavior. This behavior reveals that the NiFe2O4 shell has been
tailored to CoFe2O4 core surfaces perfectly, according to the formation of a coherent interface between these two phases. This is due to the similar crystal structure and lattice parameters of the core and shell materials as discussed before. The saturation magnetization (Ms), coercive field (Hc) and remnant magnetization (Mr) in table 3. It is obvious that both of magnetization and the coercive field of CoFe2O4 is greater than that of NiFe2O4 which may be attributed to the magnitude of the magnetic moments of Co+2 (3µB) which is greater than that of Ni+2 (2µB). Furthermore the high value of Hc for cobalt ferrite could be interpreted in the light of one ion model [29]. Also, one can see that, in table 3, the magnetic properties of the synthesized core/shell material was changed between the magnetic properties of the core (CoFe2O4) and shell (NiFe2O4) materials, where both of the saturation magnetization and coercive field are decreased compared with bare CoFe2O4.The decreases of the saturation magnetization and coercive field can be explained in terms of the existence of NiFe2O4 on the surface of CoFe2O4 nanoparticles[30], which causes an exchange interaction between Co ions and Ni ions at the interface. As, the Co–Ni exchange interaction is less than that of Co–Fe, this leads to decreases the magnetization[31]. Also, in the nanoscale the surface to volume ratio is large.i.e. most of magnetic moments are at the surface which leads to surface pinning of the moment, i.e. It is difficult to rotate with the external field which results further decreases in the magnetization. Furthermore, the absence of the oxygen ion at the surface or the presence of another atom (ion) in the form of an impurity leads to a break of the superexchange bonds between the magnetic cations which induce surface spin disorder. Due to the above-mentioned effects, the magnetization in the shell is lower than that of bare cobalt ferrite [32]. 4.4.2 Change of Hysteresis Parameters with Temperature Fig. (7) shows the change of Ms with temperature for bare CoFe2O4 and CoFe2O4/NiFe2O4 core/shell samples. It is obvious that Ms of bare CoFe2O4 and CoFe2O4/NiFe2O4 core/shell decreases with increasing the temperature which is attributed to thermal agitation of magnetic moments with increasing temperature [33]. But Ms in core/shell is more
thermally stable than bare CoFe2O4. This means that there is a pinning of the magnetic moments at the interface between core and shell. Fig. (8) shows change of Hc the with temperature. It is obvious that, the rate change of Hc with temperature for bare CoFe2O4 is faster than that of CoFe2O4/NiFe2O4 core/shell nanocomposite, which means that, the anisotropy barrier in bare CoFe2O4 is lower than that in case of CoFe2O4/NiFe2O4 core/shell nanocomposite. Furthermore, the value of Hc for bare CoFe2O4 vanishes at temperature less than that of CoFe2O4/NiFe2O4 core/shell nanocomposite. This could be understood in terms of the surface defects and the pinning of the spins at the interface between core and the shell [34]. Moreover the magnitude of Hc for bare CoFe2O4 vanishes at temperature less than that of CoFe2O4/NiFe2O4 core/shell nanocomposite. It is valuable to note that, the thermal stability of the magnetic parameters for CoFe2O4 is enhanced by covering it with NiFe2O4. To determine the effective anisotropy constant CoFe2O4/NiFe2O4 core/shell, the variation of the initial permeability of CoFe2O4/NiFe2O4 core/shell with the temperature at a several frequencies (5, 10, 30, 50) KHz is measured as shown in Fig. (9). It is obvious that the curve could be divided into two regions. In region I: - The initial permeability increase with increasing temperature up to certain value which is called blocking temperature (TB). In region II: - The initial permeability decreases with increase temperature. This behavior could be discussed in terms of the magnetic transition from ferromagnetic to superparamagnetic phase. At T < TB the magnitude of the anisotropy field (coercive field) is large, so the magnetic moment can’t rotate in phase with the external applied field. As the temperature increases the thermal energy reduced the anisotropy field and then the magnetic moments can break the barrier of the anisotropy fieldand rotate with the applied magnetic field to contribute in the magnetization process. The temperature at the maximum value of permeability is called Blocking temperature (TB) at which the anisotropy barrier is vanished by the thermal agitation energy. It is also obvious that, the temperature at the maximum value of permeability is shifted to higher value with increasing
the frequency. In fact the blocking temperature depends on the frequency of the applied field according to [32]. τ = τ0exp (Keff V / kBT)
(2)
Where τ is the relaxation time (τ = 1/F; F is the frequency), τ0 is the relaxation time of non-interacting Magnetic nanoparticles (10−9 to 10−12 s), Keff is the anisotropy constant, kB is the Boltzmann constant, V is the volume of nanoparticle, where τ0 depends weakly on temperature [30, 32, 35, 36].This means that as frequency increases TB increases which means the moments need more thermal energy to overcome the anisotropy barrier and rotate with the oscillating applied field [32]. From equation (2), The effective anisotropy could be calculated by plotting the a straight line relation between ln(1/F)with 1/T . Fig. (10) shows the relation between ln (1/F) & 1/T which is a straight line. From the slope, the magnitude of Keff for CoFe2O4/NiFe2O4 core/shell is found to be 45 × 104 J/m3. Comparing this value with the anisotropy constants of nano CoFe2O4 (20-39 × 104 J/m3) [37,38] and nano NiFe2O4 (1.64 × 104 J/m3) [39], it is clear that Keff (CoFe2O4/NiFe2O4) >Keff (CoFe2O4) which indicates that, the presence of exchange coupling between CoFe2O4 and NiFe2O4 at the interface between the core and shell [20,33]. Conclusions: Core/shell nanocomposite of CoFe2O4/NiFe2O4 sample could be prepared by hydrothermal method. The formation of the core/shell is confirmed by using HRTEM and EDX analysis. The magnetic properties of CoFe2O4 could be modified by coating it with NiFe2O4. The thermal stability of the magnetic properties is enhanced by using only 3 nm thick of NiFe2O4 as a shell. The superparamagnetic phase is established at temperatures higher than the room temperature. Using of NiFe2O4 as a shell with different thickness may be used to tune the magnetic parameters of CoFe2O4 for the desired application.
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Figure Captions: Fig. 1: XRD patterns of (a) CoFe2O4 (b) NiFe2O4 (c) CoFe2O4/NiFe2O4 nanoparticle. Fig. 2: FT-IR spectra of (a) CoFe2O4, (b) NiFe2O4 and (c) CoFe2O4/NiFe2O4 core/shell nanoparticles. Fig. 3: TEM images of (a) CoFe2O4 nanoparticles and (b)&(c) CoFe2O4/NiFe2O4nanocomposite, the insets show the selective area electron diffraction (SAED) analyses of the respective samples. Fig. 4: The particles size distribution of (a) CoFe2O4 nanoparticles and (b) CoFe2O4/NiFe2O4nanocomposite. Fig. 5: The energy dispersive X-ray (EDX) analysis of CoFe2O4/NiFe2O4 core and shell regions nanocomposite respectively. The inset figure illustrates where the electron beam was aligned when the corresponding EDX spectrum was acquired. Fig. 6: Hysteresis curves of (a) CoFe2O4 (b) NiFe2O4 and (c) CoFe2O4/NiFe2O4. Fig. 7: The change of Ms both of bare CoFe2O4 and CoFe2O4/NiFe2O4 core/shell with temperature. Fig. 8: The change of Hc for both of bare CoFe2O4 and CoFe2O4/NiFe2O4 core/shell with temperature. Fig. 9: Variation of the initial permeability of CoFe2O4/NiFe2O4 core/shell with the temperature at a several frequencies. Fig. 10: Plot of ln(1/F) versus (1/T) for CoFe2O4/NiFe2O4 core/shell.
Table 1. The crystallite size (D), Lattice parameter and Lattice strain of the samples. Sample Lattice parameter Å Crystalline size (nm) Lattice strain
Bare CoFe2O4 Bare NiFe2O4
CoFe2O4/NiFe2O4 core/shell
8.3957
8.3377
8.3831
13
15
17.17
0.0049
0.0060
0.0056
Table 2. Infrared absorption bands (ν1 and ν2) Sample
cm-1
CoFe2O4
247
NiFe2O4
272
CoFe2O4/NiFe2O4
274
1 (MOcta-O)
2 (Mtetra-O) cm-1
cm-1 373.16 581.15 3+ -2 2+ -2 3+ Fe – O M – O Fe – O-2 M2+– O-2 400 336 579 629 400.56 601.18 407 386 599 – 383.94 586.98 394 368 568 606
Table 3. Magnetic parameters of the prepared samples. Sample
CoFe2O4 core
NiFe2O4 shell
CoFe2O4/NiFe2O4 core/shell
MS (emu/g)
36.7890
24.9126
28.3130
MR(emu/g)
17.8397
7.4988
10.7732
HC (Oe)
921.8
52.44
867.9
(311)
(533)
(440)
(422) (511)
(222)
(220)
33
(400)
CoFe2O4 / NiFe2O4
66
(111)
intensity
99
0
NiFe2O4
intensity
99 66 33 0
CoFe2O4
intensity
99 66 33 0 20
25
30
35
40
45
50
Fig. 1
55
60
65
70
75
80
Fig. 2
Fig. 3
10
N tot Mean Stand Sum Mini Medi Maxi 31 14.80 2.545 459 10 14.5 21.5
partic 9
(a) CoFe2O4
particles size
8
7
5
4
3
2
1
0 8
10
12
14
16
18
20
22
particles size (nm)
6
partic
N tot Mean Stan Sum Mini Medi Maxi 21 20.2 3.88 426 15 20 28
(b) CoFe2O4 / NiFe2O4
particlse size
5
4
acount
acount
6
3
2
1
0 14
16
18
20
22
24
particles size (nm)
Fig. 4
26
28
30
Core
3000
2500 Fe
acount
2000
1500 Cu
1000
Co Fe
500 Ni Co
Ni
Cu
0 0
2000
4000
6000
8000
10000
Energy (Kev)
Shell
3500 3000
acount
2500 2000 1500 Cu
1000 Fe
500
Fe
Ni
Cu
0 0
2000
4000
6000
Energy (Kev)
Fig. 5
8000
10000
40 35
CoFe2O4
30
NiFe2O4
25
CoFe2O4 / NiFe2O4
20 15
M (emu/g)
10 5 0 -5 -10 -15 -20 -25 -30 -35 -40 -8000
-6000
-4000
-2000
0
H (Oe)
Fig. 6
2000
4000
6000
8000
38 CoFe2O4
36
CoFe2O4 / NiFe2O4
34
Ms (emu/g)
32 30 28 26 24 22 20 18 275 300 325 350 375 400 425 450 475 500 525 550 575
T (K)
Fig. 7
1000
CoFe2O4 CoFe2O4 / NiFe2O4
Hc (Oe)
800
600
400
200
0 300
350
400
450
T (K)
Fig. 8
500
550
2.05
Initial Permeability ( r )
2.00 1.95
5 KHZ 10 KHZ 30 KHZ 50 KHZ
II
I
1.90 1.85 1.80 1.75 1.70 1.65 1.60 1.55 250 300 350 400 450 500 550 600 650 700 750 800 850 900
T (K)
Fig. 9
-8.25 -8.50
CoFe2O4 / NiFe2O4
-8.75 -9.00 -9.25
ln(1/F)
-9.50 -9.75 -10.00 -10.25 -10.50 -10.75 -11.00 -11.25 0.001420 0.001425 0.001430 0.001435 0.001440 0.001445 0.001450
1/T (k-1)
Fig. 10
Highlights 1- CoFe2O4/NiFe2O4 core/shell could be prepared by hydrothermal method. 2- The structural analysis proved the formation of NiFe 2O4 shell with thickness 3 nm. 3- The thermal stability of Ms and Hc is enhanced due to the presence of NiFe2O4 as a shell. 4- Super paramagnetic transition is confirmed and the effective magnetic anisotropy was calculated.
PII: DOI: Reference:
S0304-8853(15)30345-0 http://dx.doi.org/10.1016/j.jmmm.2015.07.039 MAGMA60404
To appear in: Journal of Magnetism and Magnetic Materials Received date: 15 June 2015 Accepted date: 13 July 2015 Cite this article as: A.A. Sattar, H.M. EL-Sayed and Ibrahim ALsuqia, Structural and magnetic properties of CoFe2O4/NiFe2O4 CORE/shell nanocomposite prepared by Hydrothermal method, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2015.07.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and Magnetic Properties of CoFe2O4/NiFe2O4 Core/Shell Nanocomposite Prepared By Hydrothermal Method A.A.Sattar1, H.M.EL-Sayed1 and Ibrahim.ALsuqia2 1
Department of Physics, Faculty of Science, Ain Shams University, 11566 Abbasia, Cairo, Egypt. 2
Department of Physics, Faculty of Education and Applied Science, Hajjah University, Alshahli, Hajjah, Yemen. E-mail of Corresponding Author : [email protected] Abstract CoFe2O4/NiFe2O4 core/shell magnetic nanocomposite was synthesized by using hydrothermal method.The analysis of XRD indicated the coexistence of CoFe2O4, NiFe2O4as core/shell composite. The core/shell structure of the composite sample has been confirmed by HR–TEM images, EDX and FT–IR measurements. The size of obtained core/shell nanoparticles was 17 nm in core diameter and about 3 nm in shell thickness. The magnetization measurements showed that both the coercive field and the saturation magnetization of the resulting core/shell nanocomposite were slightly decreased compared to those of the CoFe2O4 core but the thermal stability is of the magnetization parameter was enhanced. Furthermore, superparamagnetic phase is established at temperatures higher than the room temperature. The results were discussed in terms of the surface pinning and the magnetic interaction at the interface between the core and shell. Keywords: CoFe2O4/NiFe2O4,Core/Shell Magnetic Hydrothermal Method, Magnetic Properties.
Nanocomposite,
1. Introduction Core/shell magnetic nanocomposites have attracted considerable attention because of their unique physical and chemical properties. They have the advantage of tuning and tailoring their physical properties by changing the chemical composition and size of the core and shell [1].
Recent studies have demonstrated some merits of bimagnetic core/shell nanocrystals in improving the energy product of permanent magnets and in enhancing the thermal stability of magnetic nanocrystals to overcome the “superparamagnetic limitation” in recording media [2]. The exploration of various core−shell combinations of different magnetic materials provided a better fundamental understanding of magnetic interactions and achieved the desirable magnetic characteristics for specific applications [3]. One of the most important ferrites is Cobalt ferrite (CoFe2O4) because of its high magnetic and thermal stability and high anisotropy field. These properties, along with their greatest physical and chemical stability, make CoFe2O4 nanoparticles suitable for high density information recording, ferrofluid technology, storage systems, clinical and biological applications [1], [4-8]. On the other hand,Nickel ferrite (NiFe2O4) is one of the versatile and technologically important soft ferrite (low anisotropy field) materials with spinel structure because of its typical ferromagnetic properties, low electrical conductivity and thus lower eddy current losses, high electrochemical stability, catalytic behavior and abundance in nature [9-12]. The combination of these two ferrites as a core/shell structure may improve their magnetic properties [1]. The purpose of this work is to prepare and characterize high coercive field/low coercive field magnetic core/shell nanocomposites based on cobalt and nickel ferrites, respectively, by the hydrothermal method.
2. Experimental details: 2.1 Materials Analytical grades of Iron (III) nitrate nonahydrate (Fe(NO3)3.9H2O), Cobalt (II) nitrate hexahydrate (Co(NO3)2.6H2O) and sodium hydroxide (NaOH) were used as precursors and deionized water as solvent for preparing the core. Sodium citrate solution for surface modification of the core and Nickel (II) nitrate hexahydrate (Ni(NO3)2.6H2O) for preparing the shell.
2.2 Synthesis of Cobalt Ferrite and Nickel Ferrite by Hydrothermal Method: Cobalt ferrite nanoparticles were prepared by dissolving 1 mole of (Co(NO3) 2.6H2O) and 2 moles of (Fe (NO3) 3.9H2O) in distilled water. Under vigorous stirring, the pH of the precursor solution was adjusted to pH 12 with the drop–wise addition of (NaOH) solution. The cobalt ferrite (CoFe2O4) nanoparticles are prepared according to the following chemical reaction: Co(NO3)2 + 2 Fe(NO3)3 + 8 NaOH →
CoFe2O4 + 8 Na(NO3) + 4H2O
The same method is used for preparing Nickel ferrite nanoparticles by the chemical reaction: Ni(NO3)2 + 2 Fe(NO3)3 + 8 NaOH →
NiFe2O4 + 8 Na(NO3) + 4H2O
After continued stirring for 1 h, the solution transferred into 100 mL Teflon-lined steel autoclave. The autoclave was sealed and treated at 180 ºC for 15 hours. The pressure in the sealed autoclave was lower than the equilibrium vapor tension of water at the same temperature because the solution contained NaOH. After the hydrothermal reaction, the autoclave was taken out and cooled at room temperature. The obtained product of CoFe2O4 and NiFe2O4 were filtered and washed by water several times until the pH decreased to 7.0. The solution of CoFe2O4 was separated into two equal quantities; the first half was dried at 90 ºC for 1 hour and the second half will be used as a core for core/shell composite sample. 2.3 Synthesis Nanocomposite
of
the
CoFe2o4/NiFe2o4
Core/Shell
Magnetic
CoFe2O4/NiFe2O4 core/shell magnetic nanocompositeis synthesis by two steps:
First, Surface modification of cobalt ferrite nanoparticles was taken place by the reaction of sodium citrate, coating (0.5 g) of Cobalt ferrite and (2.58 g) sodium citrate solution were mixed together in a flask and Stirr well for 1 hour. To reduce the aggregation of nanoparticles, the mixture was kept in ultrasonic bath for 1 hour at 60 ºC. Then, the precipitates were collected by a magnet and washed with acetone to remove remnant sodium citrate. Second, NiFe2O4 (shell) was prepared in the presence of surface modified CoFe2O4 (core) using hydrothermal method. The surface modified cobalt ferrite nanoparticles were re-dispersed in the mixed solution 1 mole of Ni(NO3)2 and 2 moles of Fe (NO3) 3.9H2O and Under vigorous stirring, the pH of the precursor solution was adjusted to pH 12 as before. The same above steps for preparing CoFe2O4 using hydrothermal method are repeated; the obtained product was filtered and washed by water several times until the pH reached to 7.0. To reduce the aggregation of nanoparticles, ultrasonic bath is used for 1 hr at 60oC. 3. Sample Characterizations: Phase identification of the powder samples was performed using X–ray diffraction (XRD), Philips diffractometer (X’pert MPD) with Cu–Kα radiation (λ = 1.5405 Å) at room temperature.IR spectra were recorded in the range from 200 to 4000 cm−1, using Perkin Elmer Spectrum One FT–IR spectrophotometer on KBr pellets. The particle morphology and crystal planes examined with High resolution transmission electron microscopy (HR–TEM, Tecnai G20, FEI, Netherland) Model, Super Twin TEM, with point to point resolution of 0.23 nm and line resolution of 0.14 nm. The magnetic measurements of the prepared powder were determined at room temperature by using the vibrating sample magnetometer (VSM). 4. Results and Discussion 4.1 X-Ray Diffraction Analysis The X–ray diffraction patterns of CoFe2O4, NiFe2O4 and CoFe2O4/NiFe2O4 ferrite nanoparticles are shown in Fig. (1). The reflection peaks from the planes (111), (311), (220), (222), (400), (422), (511), (440) and (533) correspond to the face centered cubic close packed fcc system of CoFe2O4
and NiFe2O4 are indexed with the JCPDS card (reference code: 01–1121) and (reference code: 10–325) respectively [9, 13]. It is obvious that all prepared samples could be refined using the Fd3m space group and have single phase spinelcubic structure. The lattice parameters of prepared ferrite samples are listed in the table (1). The values of the lattice parameters of CoFe2O4 and NiFe2O4 are in agreement with that reported by [13-16]. Furthermore the lattice parameter of CoFe2O4/NiFe2O4 is close to that of bare CoFe2O4. This observation could be attributed to the lattice matching between NiFe2O4 of CoFe2O4. Where: (
)
(1)
This small lattice mismatch between NiFe2O4 and CoFe2O4 causes the crystal growth of NiFe2O4 as a shell on CoFe2O4. The crystallite size and strain of the prepared samples using (Williamson–Hall, Debye–Scherrer methods and Bragg equation) [7, 8, 15, 17] are calculated and listed in table 1. One can observe that CoFe2O4 ferrite has lowest crystallite size and lattice strain while NiFe2O4 ferrite has larger crystallite size and lattice strain. Furthermore, comparing the crystallite size of CoFe2O4/NiFe2O4 with that of bare CoFe2O4, it is obvious that, the crystallite size of the core/shell sample is greater than that of bare CoFe2O4 by about 3 nm which supports the formation of core/shell nanocomposite. Moreover, the lattice strain of CoFe2O4/NiFe2O4 is greater than that of bare CoFe2O4; the increase of lattice strain is coming from the small lattice mismatch between NiFe2O4 with CoFe2O4 which also, confirms the presence of NiFe2O4 as a shell on CoFe2O4. 4.2 FT–IR Spectra Fig. (2) shows the FT–IR spectrum of the three prepared samples in the wave-number range between 200 and 4000 cm-1. It is obvious that, Fig. 2 (a, b) the IR spectra of have the two principle broad absorption bands (v1& v2) at 373.16 cm-1 and 581.15 cm-1 respectively while for NiFe2O4 they are at 400.56 cm-1 and 601.18 cm-1 respectively. These two vibration bands are corresponded to the intrinsic vibrations of octahedral and tetrahedral sites
in the spinel structure respectively [18, 19]. Using a software program, the peaks could be analyzed to their principle peaks as shown in the inset figures. The analyzed peaks are listed in table 2. It is clear that, for CoFe2O4 the octahedral band has two large peaks at 336 cm-1 and 400 cm-1 which is corresponding to the Me2+–O and Fe3+–O bonds respectively. On the other hand the tetrahedral band has two peaks, the large one at 579 cm-1 which corresponding to Fe3+–O bond and small peak at 629 cm-1 which corresponding to Co2+–O bond in the tetrahedral site [20, 21]. From these results, one can conclude that there is a small Co ions in the tetrahedral site i.e. the prepared CoFe2O4 is partially inverse ferrite [8]. On the other hand, for NiFe2O4, the two vibrational modes which are observed in the octahedral site are corresponding to the Fe3+–O and Ni2+–O bonds respectively, while there is no M2+–O in the tetrahedral site which confirms the complete inverse structure of NiFe2O4 [22]. Finally, for CoFe2O4/NiFe2O4 the FT–IR peaks lie between that of the bare CoFe2O4 and NiFe2O4. Also, the peak analysis shows the presence of vibrations of Fe3+–O and M2+–O in both of tetrahedral and octahedral sites. Furthermore, the peak positions are near to that of CoFe2O4 than that of NiFe2O4. These observations support the formation of NiFe2O4 on the surface of CoFe2O4 [23]. The presence of peaks at high wave numbers is corresponding to the following vibrational modes: •
the broad peak at 3441 cm-1 is characteristic of O–H bonds [24] and the broad band centered at 1611 cm-1 was assigned to the H–O–H bending vibration of the absorbed water [20,25].
•
The peaks at 2923 cm−1, 1641 cm−1, 1388 cm−1 and 1029 cm−1 are ascribed to C–H of alkyl group, symmetric COO–Fe, asymmetric COO–Fe and C–OH bond, respectively [26].
The above results reveal that sodium citrate has been grafted onto the surface of CoFe2O4 nanoparticles through the reaction of hydroxide radical groups (–OH) on the surface of CoFe2O4 nanoparticles and carboxylate anion (R–COO−) of sodium citrate [23]. Finally, the presence of small
absorption broad band in the range 200 –300 cm-1 is assigned to the lattice vibration of the ferrite [27].
4.3 TEM and EDX Spectrum Fig.(3) shows the TEM images for the samples CoFe2O4 and CoFe2O4/NiFe2O4 nanocomposite respectively. It is obvious that for bare CoFe2O4 sample Fig. 3(a) the particle size of is about 17 nm while for CoFe2O4/NiFe2O4 Fig. 3(b & c) there are an obvious shell around the core with thickness about 3 nm (light shadow). The selected area electron diffraction (SAED) patterns indicate that CoFe2O4/NiFe2O4 nanocomposite is found in well-crystalline nature. The particle size distribution obtained from analysis of bare CoFe2O4 and CoFe2O4/NiFe2O4 images is presented in Fig. 4 (a & b). This histogram is well described by the Lorentz distribution function, shown by a continuous curve in the figure. This analysis establishes a diameter for CoFe2O4/NiFe2O4 (20 nm) which is distinctly larger than, that the diameter of bare CoFe2O4 (15 nm) nanoparticles suggesting the growth of NiFe2O4 shell over the core CoFe2O4 particles [23, 28] and also which are in excellent agreement with the results of XRD. Fig.(5) shows the EDX analysis in the core and shell regions respectively. By elemental analysis in both of the core and shell, there are Co, Ni and Fe elements in the regions of the core while the shell has only Ni, Fe elements which confirm the core/shell formation. 4.4 The Magnetic Properties 4.4.1 The Hysteresis loops Fig.(6) displays magnetization hysteresis of the CoFe2O4, NiFe2O4 nanoparticles and CoFe2O4/NiFe2O4 core/shell magnetic nanocomposites. The hysteresis curves show characteristics of CoFe2O4 hard ferrites with high coercive field and magnetization, and characteristics of NiFe2O4 soft ferrites with low coercive field and magnetization. Furthermore, the hysteresis loop of the CoFe2O4/NiFe2O4core/shell magnetic nanocomposite shows is smooth hysteresis, indicative a single-phase-like magnetic material behavior. This behavior reveals that the NiFe2O4 shell has been
tailored to CoFe2O4 core surfaces perfectly, according to the formation of a coherent interface between these two phases. This is due to the similar crystal structure and lattice parameters of the core and shell materials as discussed before. The saturation magnetization (Ms), coercive field (Hc) and remnant magnetization (Mr) in table 3. It is obvious that both of magnetization and the coercive field of CoFe2O4 is greater than that of NiFe2O4 which may be attributed to the magnitude of the magnetic moments of Co+2 (3µB) which is greater than that of Ni+2 (2µB). Furthermore the high value of Hc for cobalt ferrite could be interpreted in the light of one ion model [29]. Also, one can see that, in table 3, the magnetic properties of the synthesized core/shell material was changed between the magnetic properties of the core (CoFe2O4) and shell (NiFe2O4) materials, where both of the saturation magnetization and coercive field are decreased compared with bare CoFe2O4.The decreases of the saturation magnetization and coercive field can be explained in terms of the existence of NiFe2O4 on the surface of CoFe2O4 nanoparticles[30], which causes an exchange interaction between Co ions and Ni ions at the interface. As, the Co–Ni exchange interaction is less than that of Co–Fe, this leads to decreases the magnetization[31]. Also, in the nanoscale the surface to volume ratio is large.i.e. most of magnetic moments are at the surface which leads to surface pinning of the moment, i.e. It is difficult to rotate with the external field which results further decreases in the magnetization. Furthermore, the absence of the oxygen ion at the surface or the presence of another atom (ion) in the form of an impurity leads to a break of the superexchange bonds between the magnetic cations which induce surface spin disorder. Due to the above-mentioned effects, the magnetization in the shell is lower than that of bare cobalt ferrite [32]. 4.4.2 Change of Hysteresis Parameters with Temperature Fig. (7) shows the change of Ms with temperature for bare CoFe2O4 and CoFe2O4/NiFe2O4 core/shell samples. It is obvious that Ms of bare CoFe2O4 and CoFe2O4/NiFe2O4 core/shell decreases with increasing the temperature which is attributed to thermal agitation of magnetic moments with increasing temperature [33]. But Ms in core/shell is more
thermally stable than bare CoFe2O4. This means that there is a pinning of the magnetic moments at the interface between core and shell. Fig. (8) shows change of Hc the with temperature. It is obvious that, the rate change of Hc with temperature for bare CoFe2O4 is faster than that of CoFe2O4/NiFe2O4 core/shell nanocomposite, which means that, the anisotropy barrier in bare CoFe2O4 is lower than that in case of CoFe2O4/NiFe2O4 core/shell nanocomposite. Furthermore, the value of Hc for bare CoFe2O4 vanishes at temperature less than that of CoFe2O4/NiFe2O4 core/shell nanocomposite. This could be understood in terms of the surface defects and the pinning of the spins at the interface between core and the shell [34]. Moreover the magnitude of Hc for bare CoFe2O4 vanishes at temperature less than that of CoFe2O4/NiFe2O4 core/shell nanocomposite. It is valuable to note that, the thermal stability of the magnetic parameters for CoFe2O4 is enhanced by covering it with NiFe2O4. To determine the effective anisotropy constant CoFe2O4/NiFe2O4 core/shell, the variation of the initial permeability of CoFe2O4/NiFe2O4 core/shell with the temperature at a several frequencies (5, 10, 30, 50) KHz is measured as shown in Fig. (9). It is obvious that the curve could be divided into two regions. In region I: - The initial permeability increase with increasing temperature up to certain value which is called blocking temperature (TB). In region II: - The initial permeability decreases with increase temperature. This behavior could be discussed in terms of the magnetic transition from ferromagnetic to superparamagnetic phase. At T < TB the magnitude of the anisotropy field (coercive field) is large, so the magnetic moment can’t rotate in phase with the external applied field. As the temperature increases the thermal energy reduced the anisotropy field and then the magnetic moments can break the barrier of the anisotropy fieldand rotate with the applied magnetic field to contribute in the magnetization process. The temperature at the maximum value of permeability is called Blocking temperature (TB) at which the anisotropy barrier is vanished by the thermal agitation energy. It is also obvious that, the temperature at the maximum value of permeability is shifted to higher value with increasing
the frequency. In fact the blocking temperature depends on the frequency of the applied field according to [32]. τ = τ0exp (Keff V / kBT)
(2)
Where τ is the relaxation time (τ = 1/F; F is the frequency), τ0 is the relaxation time of non-interacting Magnetic nanoparticles (10−9 to 10−12 s), Keff is the anisotropy constant, kB is the Boltzmann constant, V is the volume of nanoparticle, where τ0 depends weakly on temperature [30, 32, 35, 36].This means that as frequency increases TB increases which means the moments need more thermal energy to overcome the anisotropy barrier and rotate with the oscillating applied field [32]. From equation (2), The effective anisotropy could be calculated by plotting the a straight line relation between ln(1/F)with 1/T . Fig. (10) shows the relation between ln (1/F) & 1/T which is a straight line. From the slope, the magnitude of Keff for CoFe2O4/NiFe2O4 core/shell is found to be 45 × 104 J/m3. Comparing this value with the anisotropy constants of nano CoFe2O4 (20-39 × 104 J/m3) [37,38] and nano NiFe2O4 (1.64 × 104 J/m3) [39], it is clear that Keff (CoFe2O4/NiFe2O4) >Keff (CoFe2O4) which indicates that, the presence of exchange coupling between CoFe2O4 and NiFe2O4 at the interface between the core and shell [20,33]. Conclusions: Core/shell nanocomposite of CoFe2O4/NiFe2O4 sample could be prepared by hydrothermal method. The formation of the core/shell is confirmed by using HRTEM and EDX analysis. The magnetic properties of CoFe2O4 could be modified by coating it with NiFe2O4. The thermal stability of the magnetic properties is enhanced by using only 3 nm thick of NiFe2O4 as a shell. The superparamagnetic phase is established at temperatures higher than the room temperature. Using of NiFe2O4 as a shell with different thickness may be used to tune the magnetic parameters of CoFe2O4 for the desired application.
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Figure Captions: Fig. 1: XRD patterns of (a) CoFe2O4 (b) NiFe2O4 (c) CoFe2O4/NiFe2O4 nanoparticle. Fig. 2: FT-IR spectra of (a) CoFe2O4, (b) NiFe2O4 and (c) CoFe2O4/NiFe2O4 core/shell nanoparticles. Fig. 3: TEM images of (a) CoFe2O4 nanoparticles and (b)&(c) CoFe2O4/NiFe2O4nanocomposite, the insets show the selective area electron diffraction (SAED) analyses of the respective samples. Fig. 4: The particles size distribution of (a) CoFe2O4 nanoparticles and (b) CoFe2O4/NiFe2O4nanocomposite. Fig. 5: The energy dispersive X-ray (EDX) analysis of CoFe2O4/NiFe2O4 core and shell regions nanocomposite respectively. The inset figure illustrates where the electron beam was aligned when the corresponding EDX spectrum was acquired. Fig. 6: Hysteresis curves of (a) CoFe2O4 (b) NiFe2O4 and (c) CoFe2O4/NiFe2O4. Fig. 7: The change of Ms both of bare CoFe2O4 and CoFe2O4/NiFe2O4 core/shell with temperature. Fig. 8: The change of Hc for both of bare CoFe2O4 and CoFe2O4/NiFe2O4 core/shell with temperature. Fig. 9: Variation of the initial permeability of CoFe2O4/NiFe2O4 core/shell with the temperature at a several frequencies. Fig. 10: Plot of ln(1/F) versus (1/T) for CoFe2O4/NiFe2O4 core/shell.
Table 1. The crystallite size (D), Lattice parameter and Lattice strain of the samples. Sample Lattice parameter Å Crystalline size (nm) Lattice strain
Bare CoFe2O4 Bare NiFe2O4
CoFe2O4/NiFe2O4 core/shell
8.3957
8.3377
8.3831
13
15
17.17
0.0049
0.0060
0.0056
Table 2. Infrared absorption bands (ν1 and ν2) Sample
cm-1
CoFe2O4
247
NiFe2O4
272
CoFe2O4/NiFe2O4
274
1 (MOcta-O)
2 (Mtetra-O) cm-1
cm-1 373.16 581.15 3+ -2 2+ -2 3+ Fe – O M – O Fe – O-2 M2+– O-2 400 336 579 629 400.56 601.18 407 386 599 – 383.94 586.98 394 368 568 606
Table 3. Magnetic parameters of the prepared samples. Sample
CoFe2O4 core
NiFe2O4 shell
CoFe2O4/NiFe2O4 core/shell
MS (emu/g)
36.7890
24.9126
28.3130
MR(emu/g)
17.8397
7.4988
10.7732
HC (Oe)
921.8
52.44
867.9
(311)
(533)
(440)
(422) (511)
(222)
(220)
33
(400)
CoFe2O4 / NiFe2O4
66
(111)
intensity
99
0
NiFe2O4
intensity
99 66 33 0
CoFe2O4
intensity
99 66 33 0 20
25
30
35
40
45
50
Fig. 1
55
60
65
70
75
80
Fig. 2
Fig. 3
10
N tot Mean Stand Sum Mini Medi Maxi 31 14.80 2.545 459 10 14.5 21.5
partic 9
(a) CoFe2O4
particles size
8
7
5
4
3
2
1
0 8
10
12
14
16
18
20
22
particles size (nm)
6
partic
N tot Mean Stan Sum Mini Medi Maxi 21 20.2 3.88 426 15 20 28
(b) CoFe2O4 / NiFe2O4
particlse size
5
4
acount
acount
6
3
2
1
0 14
16
18
20
22
24
particles size (nm)
Fig. 4
26
28
30
Core
3000
2500 Fe
acount
2000
1500 Cu
1000
Co Fe
500 Ni Co
Ni
Cu
0 0
2000
4000
6000
8000
10000
Energy (Kev)
Shell
3500 3000
acount
2500 2000 1500 Cu
1000 Fe
500
Fe
Ni
Cu
0 0
2000
4000
6000
Energy (Kev)
Fig. 5
8000
10000
40 35
CoFe2O4
30
NiFe2O4
25
CoFe2O4 / NiFe2O4
20 15
M (emu/g)
10 5 0 -5 -10 -15 -20 -25 -30 -35 -40 -8000
-6000
-4000
-2000
0
H (Oe)
Fig. 6
2000
4000
6000
8000
38 CoFe2O4
36
CoFe2O4 / NiFe2O4
34
Ms (emu/g)
32 30 28 26 24 22 20 18 275 300 325 350 375 400 425 450 475 500 525 550 575
T (K)
Fig. 7
1000
CoFe2O4 CoFe2O4 / NiFe2O4
Hc (Oe)
800
600
400
200
0 300
350
400
450
T (K)
Fig. 8
500
550
2.05
Initial Permeability ( r )
2.00 1.95
5 KHZ 10 KHZ 30 KHZ 50 KHZ
II
I
1.90 1.85 1.80 1.75 1.70 1.65 1.60 1.55 250 300 350 400 450 500 550 600 650 700 750 800 850 900
T (K)
Fig. 9
-8.25 -8.50
CoFe2O4 / NiFe2O4
-8.75 -9.00 -9.25
ln(1/F)
-9.50 -9.75 -10.00 -10.25 -10.50 -10.75 -11.00 -11.25 0.001420 0.001425 0.001430 0.001435 0.001440 0.001445 0.001450
1/T (k-1)
Fig. 10
Highlights 1- CoFe2O4/NiFe2O4 core/shell could be prepared by hydrothermal method. 2- The structural analysis proved the formation of NiFe 2O4 shell with thickness 3 nm. 3- The thermal stability of Ms and Hc is enhanced due to the presence of NiFe2O4 as a shell. 4- Super paramagnetic transition is confirmed and the effective magnetic anisotropy was calculated.