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Structural, elastic and magnetic studies of the as-synthesized Co1−xSrxFe2O4 nanoparticles
Journal of Alloys and Compounds 690 (2017) 293e303
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Structural, elastic and magnetic studies of the as-synthesized Co1xSrxFe2O4 nanoparticles E.H. El-Ghazzawy, M.A. Amer* Physics Department, Faculty of Science, Tanta University, Tanta 31527, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2016 Received in revised form 12 August 2016 Accepted 16 August 2016 Available online 17 August 2016
A series of as-synthesized Co1xSrxFe2O4 nano-ferrites, 0.0 x 0.6, were prepared by the chemical coprecipitation method. The techniques used in this investigation were x-ray diffraction, transmission electron microscope, infrared spectrometer and vibrating sample magnetometer. The results proved that the samples have single phase cubic spinel structure. This study revealed that the crystallite size, strain and elastic wave velocities were decreased by increasing strontium content x, whereas the density, theoretical density, specific surface area, force constants, Debye temperature and elastic moduli were increased. The true lattice constant and porosity proved dependence on Sr2þion content x. TEM images showed agglomeration of the nanoparticles, where the average particle size values ranged 10.56 e23.66 nm which agree with R values. Six absorption bands were observed in the IR spectra and assigned to the corresponding site bonds. The saturation magnetization (Ms), coercivity, magnetic moment, anisotropy constant and remnant magnetization were affected by Sr2þ ion content x. The vibrational frequencies; n1 and n2 showed decrease with increasing Ms. On the other hand, Ms proved dependence on the crystallite size R, whereas the elastic wave velocities were affected by the sample density. © 2016 Elsevier B.V. All rights reserved.
Keywords: Co1xSrxFe2O4 nano-ferrites Structural and magnetic properties Elastic moduli and wave velocities Resonant frequencies
1. Introduction The field of nanostructure science and technology has a broad area of research that has been grown rapidly in the past few years. There is a wide range of new applications in high capacity uses for chemical and electrical energy storage, or in sensors and other applications that take numerous advantages of this feature. Already there are multiple commercial applications in porous membranes or molecular sieves, drug delivery, tailored catalysts and absorption/desorption materials [1]. Nano magnetic materials constitute a very important group of nano-materials. Their nano-size and surfaceeto-volume ratio play a very important rule in governing their magnetic properties. The fine particle behaves as a single domain magnet because its diameter would be even smaller than domain wall thickness. When the particle size shrinks further the superparamagnetic limit is achieved. The spin movement can take place either via thermal excitation or quantum tunneling [2]. The spinel magnetic ferrites have been investigated for their useful electrical and magnetic properties which can be used for
* Corresponding author. E-mail addresses: [email protected], [email protected]. eg (M.A. Amer). http://dx.doi.org/10.1016/j.jallcom.2016.08.135 0925-8388/© 2016 Elsevier B.V. All rights reserved.
different technical, medical and industrial applications [3e5]. The properties of spinel ferrite nanoparticles are sensitive to their preparation route, composition, crystallite and particle sizes and substitution process, where the substitution of larger ions can affect their properties greatly. The elastic properties of ferrites are important in industry because of their elastic data. They are very much useful to determine the strength of the materials under various strained conditions. In basic research these data are useful for obtaining an insight into the structure of the inter-atomic and inter-ionic forces in solids especially of the long-range type forces [6]. A new method based on infrared spectroscopy has been developed by Modi et al. [7] to study the elastic properties of spinel ferrites. Cobalt ferrite (CoFe2O4) is one of the best candidates among the other spinel ferrites for various applications [3], where the incorporation of cobalt ions improves coercivity [8]. Also, Patil Basavani et al. [9] have synthesized cobalt nickel ferrite (CoxNi1xFe2O4) nanoparticles by a sol-gel combustion technique. They found that the magnetic properties and grain size of the samples show observable variations with change of Co2þ content. On the other hand, Baharuddin et al. [10] reported that among many types of cathode materials, lanthanum-based materials such as lanthanum strontium cobalt ferrite (La1xSrxCo1yFeyO3d) have been
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synthesized to offer great compatibility with electrolyte materials in performing as composite cathode materials. Some researchers have synthesized Mg0.2xSrxMn0.8Fe2O4 nanoparticle system with low concentration of Sr2þ, 0.0 x 0.2, using the co-precipitation method [5]. It has been reported that the larger Sr2þ ions occupy the tetrahedral A-site interstices causing their expansion and shrinkage of the octahedral B-sites. The saturation magnetization showed dependence on the crystallite size and porosity where ferromagnetically ordered nano clusters may be existed in the samples. It was found that all the parameters were affected by the substitution process and showed nonlinear dependence on the Sr2þ ion content x. Loganathan et al. [11] studied the effect of calcination temperatures on Sr-ferrites. They found that the lattice constants and particle size and the optical, electrical, and magnetic properties of ferrite compositions were strongly dependant on calcination temperatures. The Mg0.1Sr0.1Mn0.8Fe2O4 nanoferrites [12] were synthesized and annealed at different temperatures. It was found that the structural, magnetic and vibrational frequency parameters were strongly affected by the annealing temperatures, where the structural phase of the samples was changed from cubic to hexagonal. Therefore, the aim of this investigation is to study the effect of substitution of higher concentration of diamagnetic and larger Sr2þ ions for Co2þ ions in the crystal lattice of Co1xSrxFe2O4 nano-ferrite system, 0.0 x 0.6, on its structural, magnetic, vibrational and mechanical properties. The techniques used to characterize the samples were x-ray diffraction, Transmission electron microscope, Fourier transform infrared spectrometer and vibrating sample magnetometer. 2. Experimental 2.1. Sample preparation The spinel Co1xSrxFe2O4 nano-particles, x ¼ 0.0, 0.2, 0.3, 0.4, 0.5 and 0.6, were synthesized by co-precipitating route as explained previously [5,12]. Stoichiometric amounts of the high purity salts; Co(NO3)2$6H2O, SrCl2 and Fe(NO3)3$9H2O were weight and dissolved in distilled water under stirring, the weights of the precursors are listed in Table 1. The salt solutions were mixed together and placed in a cold solution bath with continuous stirring. NaOH solution was added to the mixture solution drop wise under constant stirring, until pH value was 11. Then, the solution was heated and maintained at about 90 C for 2 h under continuous stirring till the precipitation would occur. The precipitate was thoroughly washed many times with distilled water till the precipitated materials become free of unwanted salt residuals. The precipitates were dried at 70 C and ground using an agate mortar to obtain fine powder samples. 2.2. Characterization The powder samples were investigated using a GNR APD 2000 Pro x-ray diffractometer step scan type and CuKa radiation
(l ¼ 1.540598 Å). The lattice constant a was calculated using the relation; a ¼ d(h2 þ k2 þ l2)1/2, where d is the inter-planer distance obtained by the Bragg's relation; 2dsinq ¼ nl, where q is the diffraction angle. The crystallite size R was calculated using the prominent peak (311) and Sherrer's formula [13]:
. R ¼ 0$9l b1=2 cosq where b1/2 is the full width at half maximum of the peak (311). The porosity (P) of the samples was calculated using equation:
P ¼ 1 D=Dx where D and Dx are the experimental and theoretical (X-ray) densities, respectively. The IR spectra were recorded in the range of 200e2000 cm1 by the Fourier-transform infrared spectrometer of the type Bruker Tensor 27. The magnetization measurements were carried out at room temperature by a vibrating sample magnetometer with a maximum magnetic field up to 8 kOe [14]. The average particle size of the samples was measured using a transmission electron microscope (TEM) and high resolution TEM of the kind JEOL JEM-2100.
3. Results and discussion 3.1. X-ray diffraction (XRD) analysis XRD patterns of as-synthesized Co1xSrxFe2O4 ferrite nanoparticles, x ¼ 0, 0.2, 0.3, 0.4, 0.5 and 0.6, are shown in Fig. 1. It is shown that the reflection planes (111), (220), (311), (400), (422), (511), (440) and (533) appeared in all the spectra, which proves that the samples have a single phase of cubic spinel structure (JCPDS card no. 22-1086). The observed peak at about 2q z 25 in the compositions for x > 0.2 may be assigned to the presence of strontium ions (JCPDS card file no. 15-305). It has been reported that the relatively larger Sr2þ ions occupy the tetrahedral A-site interstices in spinel nano-ferrites [5,11], which causes their expansion to accommodate these ions. This expansion leads to an induced perturbation on the nearest-neighbor ions at the B-sites [5,11]. The larger strontium ions occupy the tetrahedral A-sublattice for x 0.2, but its ionic radius (1.18 Å) is larger than the mean ionic radius of A-site. Therefore, for x > 0.2 some Sr2þ ions may deposit at the grain boundaries instead of occupying the A-sites [11,15]. The true lattice constant at can be determined by graphing the obtained lattice constant a values for each sample against the Nelson-Riley extrapolating function F(q) [16]:
FðqÞ ¼
1 cos2 q cos2 q þ 2 sinq q
The relation between F(q) and a displays a straight line, where the value of at can be determined by extrapolating the straight line to F(q) ¼ 0 at q ¼ 90 . Fig. 2 displays variation of both the crystallite size R and true
Table 1 The weights of the precursors. Sample
Weight (g) Fe(NO3)3$9H2O
Weight (g) Co(NO3)2$6H2O
Weight (g) SrCl2
Weight (g) NaOH
CoFe2O4 Co0.8Sr0.2Fe2O4 Co0.7Sr0.3Fe2O4 Co0.6Sr0.4Fe2O4 Co0.5Sr0.5Fe2O4 Co0.4Sr0.6Fe2O4
103.300 101.102 99.658 98.497 97.349 96.261
37.200 29.094 25.120 21.280 17.527 13.864
e 6.665 9.865 13.103 16.060 19.058
40.9 39.997 39.466 39.127 38.550 38.120
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Fig. 1. XRD patterns of as-synthesized Co1-xSrxFe2O4 nano-ferrites.
lattice constant at against x. It is shown that R decreases slightly with x. This may be due to that the deposition of the larger Sr2þ ions on the grain boundaries produces a surface-induced stress causing a reduction of the crystallite size. On the other hand, the amount of cobalt ions in the B sublattice decreases that leads to its shrinkage, i.e. decreasing R. Heiba et al. [17] explained that adding bigger radius ions into the crystal sublattice may cause a compress stress and hinder the grain growth. This produces a lattice microstrain that prevent the grain growth. It is displayed that at increases up to x z 0.3 and decreases thereafter. The increase of at may be due the substitution of larger Sr2þ ions in the tetrahedral A-sites and the decrease occurs because of the deposition of Sr2þ ions on the grain boundaries and shrinkage of the B-sites [5,17]. The decrease in a and R may point to the formation of ferromagnetically ordered nanoclusters in the samples that leads to shrinkage of the crystal lattice [5]. The crystalline structure of the nanocluster is the same as the bulk material with little different lattice parameters. Variation of the bulk density D, theoretical density (x-ray density) Dx and porosity P against x is illustrated in Fig. 3. It is illustrated that Dx and D slightly increase with x, whereas P has random
behavior and decrease only in the range of 0.2 < x < 0.5. The increase of Dx and D may be attributed to the substitution of the heavier atomic weight Srþ2 ions (87.62) for Co2þ ions (58.9) in the crystal lattice. The nonmonotonical variation of P may be a result of inaccuracies of the Dx and D values. The decrease in P may be due to densification of the sample materials and increase in packing. The increase of densities and decrease in porosity may be due to increasing the absolute values of compressive strain (Fig. 4). The strain 3 of the samples was calculated by using the following equation [18]:
b1=2 cosq ¼
0:9l þ 4ε sin q R
The relation between 3 and x is plotted in Fig. 4. Variation of 3 reflects the mechanical properties of the samples, where its negative values prove that the strain is compressive [5,11,18]. This confirms the relation between the reduction of crystallite size and the compressibility of the samples by increasing the Sr2þ ion content x [5,11]. The specific surface area S (m2/g) was calculated using the
Fig. 2. Variation of the true lattice constant at and crystallite size R against composition x.
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Fig. 3. Composition x dependence of the density D, theoretical density Dx and porosity P.
Fig. 4. Dependence of the strain
3
and specific surface area S on Sr2þ ion content x.
relation [19]:
LAA ¼ a
pffiffiffi pffiffiffi pffiffiffiffiffiffi 3 2 11 ; LBB ¼ a andLAB ¼ a 4 4 8
The obtained data are presented in Table 2.
S¼
particle surface area Particle surface area 6000 ¼ ¼ particle mass Density volume RDx
The values of S increase versus x as expected because of decreasing the crystallite size that leads to increasing the surface to volume ratio [5,11,19]. The hopping length is known as the distance between the centers of adjacent ions. So, the presence of larger ions results in an increase of the distance between the magnetic ions [20]. Thus, the distance between magnetic ions (hopping length), L, in the A-sites (LA-A), B-sites (LB-B) and shared sites (LA-B) can be evaluated using the following equations [5,6,21]:
Table 2 The obtained distance between magnetic ions (hopping length) in the A-sites (LA-A), B-sites (LB-B), shared sites (LA-B) and true lattice constant at for Co1x SrxFe2O4 samples. x
LA-A (Å)
LB-B (Å)
LAeB (Å)
at(Å)
0 0.2 0.3 0.4 0.5 0.6
3.627 3.636 3.636 3.627 3.620 3.605
2.962 2.969 2.969 2.961 2.956 2.943
3.473 3.481 3.481 3.472 3.466 3.451
8.377 8.397 8.397 8.375 8.361 8.326
E.H. El-Ghazzawy, M.A. Amer / Journal of Alloys and Compounds 690 (2017) 293e303 Table 3 IR absorption band values. x
n4 (cm1)
n2 (cm1)
n1 (cm1)
nA (cm1)
nB (cm1)
0 0.2 0.3 0.4 0.5 0.6
246.88 246.88 237.24 235.31 235.31 235.3
410.83 410.83 410.83 414.68 416.61 424.33
592.13 594.06 596 596 597.9 597.92
896.88 894.95 894.94 896.87 e e
e 1014.53 1014.53 1016.64 1020.32 1020.32
297
agglomerated. The obtained average particle size of the samples were 23.66, 16.05 and 10.53 nm for x ¼ 0,0, 0,4, 0.6, respectively. These values agree well with the calculated values of crystallite size R. The lattice spacing (the distance between atom centers) obtained from HRTEM image for Co0.6Sr0.4Fe2O4 sample was determined for certain direction in the lattice as 0.21 nm. 3.3. Infrared spectra (IR)
It is presented that LA-A, LB-B and LA-B behave similar to at against x. The values of LA-A are higher than LB-B values, which may be due to the expansion of the A-sites and shrinkage of the B-sites by the addition of Sr2þ ions [14]. Table 3 presents the obtained values of interplaner distance d calculated from TEM. These values are comparable with those calculated from XRD.
3.2. Transmission electron microscope (TEM) images TEM images for Co1xSrxFe2O4 system are depicted in Fig. 5. The images show that the particles are in the nano-scale and
Fig. 6 depicts the IR absorption spectra of as-prepared Co1nano-ferrites recorded in the range 200e2000 cm1. The absorption bands; n4, n2, n1, nA, nB and nT are seen in the IR spectra. The obtained results from IR analysis are listed in Table 3. The two characteristic absorption bands of spinel nano-ferrites, n1 and n2, are observed, n1 in the range of 592e598 cm1 and n2 in the range of 410e424 cm1. The band n1 is assigned to intrinsic stretching vibrations of the tetrahedral A-site metal ion-oxygen bonding corresponding to the highest restoring force [8,18], and n2 to intrinsic vibrations of the octahedral B-site metal ion-oxygen vibrations, which are bond-bending vibrations [8,18]. The values of n1 are higher compared to n2, which indicate that the normal mode
xSrxFe2O4
Fig. 5. TEM images for as-prepared Co1xSrxFe2O4 nano-ferrite system.
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Fig. 6. IR absorption spectra of as-prepared Co1xSrxFe2O4 nano-ferrites.
Fig. 7. Force constants, FA and FB, at the A- and B-sites, respectively, as functions of x.
of vibration of the tetrahedral cluster is higher than that of the octahedral cluster. This is attributed to the shorter bond length of the A-site clusters than that of the B-site clusters [18,22]. The prominent shoulder appeared at the band n2 at around 290 cm1 may assign to the existing of the divalent ions Co2þ and/ or Fe2þ among the B-sites. The band n4 appeared in the range of
235e247 cm1 is attributed to the lattice vibrations of the system and depends on the mass of the divalent ions among the A-sites and their complexes, Fe2þ-O2, Co2þ-O2 and/or Sr2-O2 [5,14]. Two weak absorption bands nA and nB are observed in the IR spectra. The band nA may exist because of the presence of divalent ions Co2þ, Fe2þand/or Sr2þ in the A-sites. The band nB may be
Table 4 The stiffness constants, C11, C12, elastic moduli, B, G and E, elastic wave velocities, VL and Vs and Poisson's ratio, s. x
C11 (GPa)
C12 (GPa)
B (GPa)
G (GPa)
E (GPa)
VL (103m/s)
Vs (103m/s)
s
0 0.2 0.3 0.4 0.5 0.6
227.144 228.236 229.714 229.699 232.295 236.827
34.760 33.481 34.410 35.649 35.765 36.037
98.888 98.399 99.511 100.332 101.275 102.967
96.192 97.378 97.652 97.025 98.265 100.395
217.918 219.670 220.748 220.120 222.751 227.308
6.530 6.463 6.426 6.447 6.408 6.365
3.770 3.731 3.710 3.722 3.670 3.674
0.1327 0.1279 0.1302 0.1343 0.1334 0.1320
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Fig. 8. Effect of sample density D on the elastic wave velocities VL and Vs.
assigned to the tetravalent metal ioneoxygen, i.e. the complexes Fe4þeO2 and/or Co4þ- O2. The tetravalent ions may result from the electron hopping between the Co2þ and Fe3þ ions, where Fe2þ, Fe4þand Co4þmay be produced [5,11,14]. The triple band nT at around 1542 cm1 in all IR spectra represents the characteristic hydroxyl group (OH) that can be attributed to the humidity in the samples. The presence of hydroxyl groups in the samples may help in the conjugation and dispersion of nano-materials [23]. The nonlinear and triatomic molecule of water has three main vibration modes: symmetric stretching, asymmetric stretching and scissoring vibration modes. The scissoring vibration exists for this nonlinear molecule which proves that the band nT may assign to O-H group [24]. It is known that the Fe3þ-O2 band frequency is proportional to the force constant (Fc) at the A- and B-sites, FA and FB, respectively. Therefore, Fc can be calculated by using the relation [5,14,19]:
Fc ¼ 4p2 c2 v2 m Where m is the reduced mass of Fe3þ and O2 ions, m ¼ 2.061 1023 g. The calculated values of FA and FB are seen as a function of x in Fig. 7. It is seen that FA increases against x but FB increases only from x ˃ 0.3. This increase proves that the force constant depends on the resonant frequency and substitution of heavier ions [5,14]. It is seen that FA > FB for x > 0.0, which may assign to the shorter bond length of the A-site as mentioned above [18,24]. This is attributed to the inverse proportionality between the bond length and force constant [9,18]. 3.4. Elastic properties The elastic constants and Debye temperature of spinel ferrite system can be deduced using x-ray and IR data. The stiffness
Fig. 9. The relation between Debye temperature ӨD and the composition x.
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Fig. 10. Magnetic hysteresis loops for as-prepared Co1-xSrxFe2O4 nano-ferrites.
Fig. 11. Composition x dependence of the deduced saturation magnetization Ms and coercivity Hc.
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Fig. 12. The correlation between the saturation magnetization Ms and crystallite size R.
Table 5 The saturation magnetization (Ms), coercivity (Hc), magnetic moment (nB),anisotropy constant (K) and remnant magnetization (Mr). x
Ms(emu/g)
Hc (Oe)
nB (mB)
K(erg/Gauss)
Mr(emu/g)
0 0.2 0.3 0.4 0.5 0.6
46.420 39.215 41.642 41.194 37.874 33.901
411 557.5 707.5 557.5 411 261.5
1.95 1.688 1.813 1.815 1.688 1.528
18315.53 20987.92 28283.14 22047.03 14943.57 8510.46
9.12 10.66 14.44 11.76 8.29 5.36
constants C11 and C12 are calculated using the following relations [25]:
C11 ¼
F s C11 and C12 ¼ a ð1 sÞ
Where F ¼ (FA þ FB)/2 and s is the Poisson's ratio that is a function of pore fraction (porosity P) [26]:
s ¼ 0:324ð1 1:043PÞ Therefore, the three elastic moduli of solids for cubic structure in terms of stiffness constants are calculated using the following relations [7,26]:
Bulk modulus ðBÞ ¼
1 ðC þ 2C12 Þ 3 11
Young0 s modulus ðEÞ ¼
ðC11 C12 ÞðC11 þ 2C12 Þ ðC11 þ C12 Þ
Rigidity modulus ðGÞ ¼
E 2ðs þ 1Þ
The obtained data of elastic moduli and stiffness constants are presented in Table 4. The X-ray density Dx and stiffness constant C11 are further used to estimate the elastic wave velocity VL using the forqffiffiffiffiffilongitudinal ffi mula; VL ¼ CD11x , and the transverse elastic wave velocity Vs by general approximation; Vs ¼ pVLffiffiffi [7,24]. 3 The calculated values of elastic wave velocities are presented in
Table 4. Table 4 presents that the stiffness constants, C11 and C12, and elastic moduli, B, G and E, increase with increasing Sr2þ content, whereas the elastic wave velocities, VL and Vs decrease. This may be interpreted in terms of the interatomic bonding. So, this behavior of elastic moduli is attributed to the strengthening of the inter-atomic bonding between various atoms of the spinel lattice [9,26]. It is worth noting that the density has a great effect on elastic moduli and wave velocities, which may be due to that the ferrite samples have different densities. Since the density increases with increasing Sr2þ content x, the elastic moduli increase and wave velocities decrease as functions of density [27]. Fig. 8 explains the effect of density on the elastic wave velocities. Debye temperature WD can be calculated using the relation yAV [28], where h is Plank's constant, k is Boltzmann's conWD ¼ hc 2pk stant, c is the velocity of light (c ¼ 3 1010 cm/s) and nAV ¼ (n1 þ n2)/2 is the average value of wavenumbers for the Aand B-site. Fig. 9 illustrates the relation between WD and the additional factor x. It is illustrated that WD increases against x. This increase implies the increase in rigidity of the samples and lattice vibrations [7,26]. According to specific heat theory, the increase in ӨD proves a decrease in the number of conduction electrons (n-type) which can absorb part of the heat causing increase of ӨD. The contribution of holes (p-type) to conductivity increases causing a decrease in the absorption part of heat by electrons and hence ӨD increases [11,25,26]. Increase of ӨD points to change of the conductivity of these samples from n-type to p-type [11,25,26].
3.5. Vibrating sample magnetometer (VSM) measurements The obtained magnetic hysteresis loops by VSM for the asprepared Co1-xSrxFe2O4 nano-ferrite samples are depicted in Fig. 10. It is depicted that the curves have a relatively large coercivity and small saturation magnetization, which is characteristic of the hard magnetic materials. Among the ferrites, CoFe2O4 has the largest positive anisotropy value due to the strong spin orbit coupling at the Co2þ lattice sites. Although the smallness of the crystallite size, the coercivity of CoFe2O4 is relatively high and it has a ferromagnetic nature [29]. Fig. 11 shows the variation of deduced saturation magnetization
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Fig. 13. Dependence of n1 and n2 on Ms.
Ms and coercivity Hc against x. It is shown that the trend of Ms is a slight decrease with x, whereas Hc increases for x 0.3 and decreases thereafter. The decrease of Ms may be attributed to the substitution process of the diamagnetic Sr2þ ions for magnetic Co2þ ions. This leads to decreasing the magnetization of the sample, i.e. weakening the super-exchange magnetic interactions between the sublattices, MB-MA, which results in decreasing the total magnetization [30e33]. On the other hand, Ms decreases as the crystallite size decreases (Fig. 2) which can be explained on the basis of nonlinear or canted spin disordering at the surface layer of nanoparticles due to local chemical disorder and broken exchange interaction among magnetic spins at the surface. Also, the lack of Co ions in the B-sublattice of ferrimagnetic samples leads to spin canting in the other sublattice [30e33]. The correlation between Ms and the crystallite size R is seen in Fig. 12. The increase in coercivity for x 0.3 (Fig. 11) may be due to increase in magnetic crystalline anisotropy. In ferrite nanoparticles, the surface anisotropy has a great effect on the coercivity. Therefore, for a given composition of ferrite, when the crystallite size decreases, the coercivity increases as the surface effect becomes more dominant [34]. But for x 0.3 the effect of decreasing Co2þ concentration (which has large positive anisotropy, as mentioned above) will prevail, thus the coercivity decreases. This may be attributed to decrease in the anisotropy field, which in turn decreases the domain wall energy [35]. The anisotropy constant K can be evaluated using the following equation [5,11]:
Hc ¼
0$96K Ms
The obtained values of K and remnant magnetization Mr are listed in Table 5. Evidently, the large magneto-crystalline anisotropy of CoFe2O4 nanoparticles is due to the strong LeS couplings on the Co2þ cation sites. Energy barriers in the individual nanoparticles are considered and introduced by the magneto-crystalline anisotropy energy of cobalt ferrite [3,36]. It is expected that the remnant magnetization Mr has the same behavior of the coercivity Hc as presented in Table 5. The larger the remnant magnetization the larger the reverse field needed to reduce it. The magnetic moment nB per atom in Bohr magneton units (mB) could be calculated using the following relation [37]:
nB ¼
M Ms Nb
where M is the molecular weight, N is Avogadro's number and b is the conversion factor to express the magnetic moment per atom in Bohr magnetons (its value is 9.27 1021 erg/gauss). The obtained values of nB are given in Table 5. It is clear that nB varies similar to Ms, where its trend shows decrease against x. This is because the individual grain acts as magnetic material and has its magnetization. This is considered as an indicator of the total magnetization of the samples [5,11,37e39]. Fig. 13 explains the dependence of n1 and n2 on the saturation magnetization Ms. It is clear that n1 and n2 decreases against Ms. As the saturation magnetization increases, the A-A and B-B supertransferred magnetic interactions inside the crystal sublattices increase, which have a great effect in hindering the vibrational frequency of tetrahedral and octahedral sites and leads to decrease in n1 and n2. 4. Conclusion This investigation revealed that as-synthesized Co1-xSrxFe2O4 nanoparticles, 0.0 x 0.6, by the chemical co-precipitation method have a single phase of cubic spinel structure. It is proved that the larger strontium ions occupy the A-sites at low concentration and deposit at the grain boundaries at high concentration. This deposition suppresses the growth of the nanoparticle and presses the crystallite size causing a decrease in the lattice constant for x 0.3 and average and crystallite size. The obtained parameters showed dependence on Sr2þ ion content x. Five absorption bands were observed in IR spectra and assigned to the corresponding site bonds. They pointed to the existence of Fe2þ and Fe4þ ions among the crystal sublattices. The resonant frequencies; n1 and n2 proved dependence on Ms, whereas Ms showed dependence on R. The negative values of the strain explain that it is compressive. The increase in stiffness constants, elastic moduli and Debye temperature implies an increase in the rigidity of the samples where the sample density revealed an effect on the elastic wave velocities. References [1] R.W. Siegel, E. Hu, M.C. Roco, Nanostructure Science and Technology, Loyola College in Maryland, 1999.
E.H. El-Ghazzawy, M.A. Amer / Journal of Alloys and Compounds 690 (2017) 293e303 [2] B. Aktas, Nanostructured Magnetic Materials and Their Applications, Springer Verlag Berlin, Heidelberg, 2002. [3] V. Vinayak, P.P. Khirade, S.D. Birajdar, P.K. Gaikwad, N.D. Shinde, K.M. Jadhav, Intern. Adv. Res. J. Sci. Engin. Tech. 2 (2015) 55e58. [4] P. Amalthi, J.J. Vijaya, L.J. Kennedy, M. Bououdina, Inter. J. Chem. Tech. Res. 7 (2015) 1237e1246. [5] M.A. Amer, T.M. Meaz, S.S. Attalah, A.I. Ghoneim, J. Magn. Magn. Mater. 363 (2014) 60e65. [6] M.V.K. Meher, Inter. J. Sci. Res. 39 (2014) 144e147. [7] K.B. Modi, U.N. Trivedi, P.U. Sharma, V.K. Lakhani, M.C. Chhantbar, H.H. Joshi, Ind. J. Pure Appl. Phys. 44 (2006) 165e168. [8] J. Balavijayalakshmi, T. Sudha, K. Karthika, Inter. J. ChemTech Res. 7 (2015) 1279e1283. [9] K. Patil Basavani, M. Rathod Sopan, Inter. J. Innovt. Res. Sci. Engin. Technol. 3 (2014) 13404e13409. [10] N.A. Baharuddin, H. Abd Rahman, A. Muchtar, A.B. Sulong, H. Abdullah, J. Zhejiang Univ. Sci. A Appl. Phys. Eng. 14/1 (2013) 11e24. [11] A. Loganathan, K. Kumar, Appl. Nanosci. 6/5 (2016) 629e639. [12] M.A. Amer, T.M. Meaz, S.S. Attalah, A.I. Ghoneim, J. All. Compd. 654 (2016) 45e55. [13] B.D. Cullity, Elements of X-Ray Diffraction, second ed., Addison-Wesley Publishing Company, INC, United States of America, 1978. Congress catalog No 56e10137. [14] T.M. El-Alaily, M.K. El-Nimr, S.A. Saafan, M.M. Kamel, T.M. Meaz, S.T. Assar, J. Magn. Magn. Mater. 386 (2015) 25e30. [15] A. Goldman, Modern Ferrite Technology, Marcel Dekker, Inc., New York, 1993. [16] S.S. Bhatu, V.K. Lakhani, A.R. Tanna, N.H. Vasoya, J.U. Buch, P.U. Sharma, U.N. Trivedi, H.H. Joshi, K.B. Modi, Ind. J. Pure Appl. Phys. 45 (2007) 596e608. [17] Zein K. Heiba, Mohamed Bakr Mohamed, Adel Maher Wahba, J. Mol.Struct. 1108 (2016) 347e351. [18] M.A. Amer, T. Meaz, M. Yehia, S.S. Attalah, F. Fakhry, J. All. Compd. 633 (2015) 448e455. [19] S.T. Assar, H.F. Abosheiasha, J. Magn. Magn. Mater. 324 (2012) 3846e3852. [20] A.M. Alakrmi, A.S. Alzawaly, M.M. Elakrami, A.M. Algamoudi, Aljapal Algharbi
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Univ. Univ. Bull. 3 (2015). [21] E.H. El-Ghazzawy, S.N. Alamri, Bull. Mater. Sci. 38 (2015) 915e924. [22] S.A. Saafan, T.M. Meaz, E.H. El-Ghazzawy, M.K. El Nimr, M.M. Ayad, M. Bakr, J. Magn. Magn. Mater. 322 (2010) 2369e2374. [23] M.S. Al-Hoshan, J.P. Singh, A.M. Al-Mayouf, A.A. Al-Suhybani, M.N. Shaddad, Inter. J. Electrochem. Sci. 7 (2012) 4959e4973. [24] D.A. Skoog, Principles of Instrumental Analysis”, Saunders Golden Sunburst Series, 1985. [25] S.A. Mazen, T.A. Elmosalami, Inter. Sch. Res. Netw., doi:10.5402/2011/820726. [26] S.M. Patange, S.E. Shirsath, K.S. Lohar, S.G. Algude, S.R. Kamble, N. Kulkarni, D.R. Mane, K.M. Jadhav, J. Magn. Magn. Mater. 325 (2013) 107e111. [27] N. Varalaxmi, K.V. Sivakumar, Ind. J. Appl. Res. 4 (2014) 537e549. [28] M.A. Amer, T.M. Meaz, A.G. Mostafa, H.F. El-Ghazally, Mater. Sci. Semicond. Proc. 36 (2015) 49e56. [29] M.A. Ahmed, N. Okasha, S.F. Mansour, S.I. El-dek, J. All. Compd. 496 (2010) 345e350. [30] V. Naidu, S. Chandra, K.S.P. Durairaj, J. KalyanaSundar, M. Sivabharathy, Inter J. ChemTech Res. 6 (2014) 5595e5603. [31] A.M. Kumar, M.C. Varma, G. Choudary, K.S. Rao, K.H. Rao, G. Gopalakrishna, J. Optoelectron. Advan. Mater. 12 (2010) 2386e2390. [32] H.E. Zhang, B.F. Zhang, G.F. Wang, X.H. Dong, Y. Gao, J. Magn. Magn. Mater. 312 (2007) 126e130. € seog lu, Ceram. Inter. 39 (2013) 4221e4230. [33] Y. Ko [34] M. Raghasudha, D. Ravinder, P. Veerasomaiah, J. Nanostr. Chem. 3 (2013) 1e6. [35] S. Singhal, S. Bhukal, J. Singh, K. Chandra, S. Bansal, J. Nanotech., doi:10.1155/ 2011/930243. [36] I. Sharifi, H. Shokrollahi, M.M. Doroodmand, R. Safi, J. Magn. Magn. Mater. 324 (2012) 1854e1861. [37] R. Peelamedu, Craig Grimes, Dinesh Agrawal, Rustum Roy, J. Mater. Res. 18 (2003) 2292e2295. [38] M.A. Amer, T.M. Meaz, S.S. Attalah, F. Fakhry, J. Magn. Magn. Mater. 401 (2016) 150e158. [39] A.M. Cojocariu, M. Soroceanu, L. Hrib, V. Nica, O.F. Caltun, Mater. Chem. Phys. 135 (2012) 728e732.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Structural, elastic and magnetic studies of the as-synthesized Co1xSrxFe2O4 nanoparticles E.H. El-Ghazzawy, M.A. Amer* Physics Department, Faculty of Science, Tanta University, Tanta 31527, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2016 Received in revised form 12 August 2016 Accepted 16 August 2016 Available online 17 August 2016
A series of as-synthesized Co1xSrxFe2O4 nano-ferrites, 0.0 x 0.6, were prepared by the chemical coprecipitation method. The techniques used in this investigation were x-ray diffraction, transmission electron microscope, infrared spectrometer and vibrating sample magnetometer. The results proved that the samples have single phase cubic spinel structure. This study revealed that the crystallite size, strain and elastic wave velocities were decreased by increasing strontium content x, whereas the density, theoretical density, specific surface area, force constants, Debye temperature and elastic moduli were increased. The true lattice constant and porosity proved dependence on Sr2þion content x. TEM images showed agglomeration of the nanoparticles, where the average particle size values ranged 10.56 e23.66 nm which agree with R values. Six absorption bands were observed in the IR spectra and assigned to the corresponding site bonds. The saturation magnetization (Ms), coercivity, magnetic moment, anisotropy constant and remnant magnetization were affected by Sr2þ ion content x. The vibrational frequencies; n1 and n2 showed decrease with increasing Ms. On the other hand, Ms proved dependence on the crystallite size R, whereas the elastic wave velocities were affected by the sample density. © 2016 Elsevier B.V. All rights reserved.
Keywords: Co1xSrxFe2O4 nano-ferrites Structural and magnetic properties Elastic moduli and wave velocities Resonant frequencies
1. Introduction The field of nanostructure science and technology has a broad area of research that has been grown rapidly in the past few years. There is a wide range of new applications in high capacity uses for chemical and electrical energy storage, or in sensors and other applications that take numerous advantages of this feature. Already there are multiple commercial applications in porous membranes or molecular sieves, drug delivery, tailored catalysts and absorption/desorption materials [1]. Nano magnetic materials constitute a very important group of nano-materials. Their nano-size and surfaceeto-volume ratio play a very important rule in governing their magnetic properties. The fine particle behaves as a single domain magnet because its diameter would be even smaller than domain wall thickness. When the particle size shrinks further the superparamagnetic limit is achieved. The spin movement can take place either via thermal excitation or quantum tunneling [2]. The spinel magnetic ferrites have been investigated for their useful electrical and magnetic properties which can be used for
* Corresponding author. E-mail addresses: [email protected], [email protected]. eg (M.A. Amer). http://dx.doi.org/10.1016/j.jallcom.2016.08.135 0925-8388/© 2016 Elsevier B.V. All rights reserved.
different technical, medical and industrial applications [3e5]. The properties of spinel ferrite nanoparticles are sensitive to their preparation route, composition, crystallite and particle sizes and substitution process, where the substitution of larger ions can affect their properties greatly. The elastic properties of ferrites are important in industry because of their elastic data. They are very much useful to determine the strength of the materials under various strained conditions. In basic research these data are useful for obtaining an insight into the structure of the inter-atomic and inter-ionic forces in solids especially of the long-range type forces [6]. A new method based on infrared spectroscopy has been developed by Modi et al. [7] to study the elastic properties of spinel ferrites. Cobalt ferrite (CoFe2O4) is one of the best candidates among the other spinel ferrites for various applications [3], where the incorporation of cobalt ions improves coercivity [8]. Also, Patil Basavani et al. [9] have synthesized cobalt nickel ferrite (CoxNi1xFe2O4) nanoparticles by a sol-gel combustion technique. They found that the magnetic properties and grain size of the samples show observable variations with change of Co2þ content. On the other hand, Baharuddin et al. [10] reported that among many types of cathode materials, lanthanum-based materials such as lanthanum strontium cobalt ferrite (La1xSrxCo1yFeyO3d) have been
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synthesized to offer great compatibility with electrolyte materials in performing as composite cathode materials. Some researchers have synthesized Mg0.2xSrxMn0.8Fe2O4 nanoparticle system with low concentration of Sr2þ, 0.0 x 0.2, using the co-precipitation method [5]. It has been reported that the larger Sr2þ ions occupy the tetrahedral A-site interstices causing their expansion and shrinkage of the octahedral B-sites. The saturation magnetization showed dependence on the crystallite size and porosity where ferromagnetically ordered nano clusters may be existed in the samples. It was found that all the parameters were affected by the substitution process and showed nonlinear dependence on the Sr2þ ion content x. Loganathan et al. [11] studied the effect of calcination temperatures on Sr-ferrites. They found that the lattice constants and particle size and the optical, electrical, and magnetic properties of ferrite compositions were strongly dependant on calcination temperatures. The Mg0.1Sr0.1Mn0.8Fe2O4 nanoferrites [12] were synthesized and annealed at different temperatures. It was found that the structural, magnetic and vibrational frequency parameters were strongly affected by the annealing temperatures, where the structural phase of the samples was changed from cubic to hexagonal. Therefore, the aim of this investigation is to study the effect of substitution of higher concentration of diamagnetic and larger Sr2þ ions for Co2þ ions in the crystal lattice of Co1xSrxFe2O4 nano-ferrite system, 0.0 x 0.6, on its structural, magnetic, vibrational and mechanical properties. The techniques used to characterize the samples were x-ray diffraction, Transmission electron microscope, Fourier transform infrared spectrometer and vibrating sample magnetometer. 2. Experimental 2.1. Sample preparation The spinel Co1xSrxFe2O4 nano-particles, x ¼ 0.0, 0.2, 0.3, 0.4, 0.5 and 0.6, were synthesized by co-precipitating route as explained previously [5,12]. Stoichiometric amounts of the high purity salts; Co(NO3)2$6H2O, SrCl2 and Fe(NO3)3$9H2O were weight and dissolved in distilled water under stirring, the weights of the precursors are listed in Table 1. The salt solutions were mixed together and placed in a cold solution bath with continuous stirring. NaOH solution was added to the mixture solution drop wise under constant stirring, until pH value was 11. Then, the solution was heated and maintained at about 90 C for 2 h under continuous stirring till the precipitation would occur. The precipitate was thoroughly washed many times with distilled water till the precipitated materials become free of unwanted salt residuals. The precipitates were dried at 70 C and ground using an agate mortar to obtain fine powder samples. 2.2. Characterization The powder samples were investigated using a GNR APD 2000 Pro x-ray diffractometer step scan type and CuKa radiation
(l ¼ 1.540598 Å). The lattice constant a was calculated using the relation; a ¼ d(h2 þ k2 þ l2)1/2, where d is the inter-planer distance obtained by the Bragg's relation; 2dsinq ¼ nl, where q is the diffraction angle. The crystallite size R was calculated using the prominent peak (311) and Sherrer's formula [13]:
. R ¼ 0$9l b1=2 cosq where b1/2 is the full width at half maximum of the peak (311). The porosity (P) of the samples was calculated using equation:
P ¼ 1 D=Dx where D and Dx are the experimental and theoretical (X-ray) densities, respectively. The IR spectra were recorded in the range of 200e2000 cm1 by the Fourier-transform infrared spectrometer of the type Bruker Tensor 27. The magnetization measurements were carried out at room temperature by a vibrating sample magnetometer with a maximum magnetic field up to 8 kOe [14]. The average particle size of the samples was measured using a transmission electron microscope (TEM) and high resolution TEM of the kind JEOL JEM-2100.
3. Results and discussion 3.1. X-ray diffraction (XRD) analysis XRD patterns of as-synthesized Co1xSrxFe2O4 ferrite nanoparticles, x ¼ 0, 0.2, 0.3, 0.4, 0.5 and 0.6, are shown in Fig. 1. It is shown that the reflection planes (111), (220), (311), (400), (422), (511), (440) and (533) appeared in all the spectra, which proves that the samples have a single phase of cubic spinel structure (JCPDS card no. 22-1086). The observed peak at about 2q z 25 in the compositions for x > 0.2 may be assigned to the presence of strontium ions (JCPDS card file no. 15-305). It has been reported that the relatively larger Sr2þ ions occupy the tetrahedral A-site interstices in spinel nano-ferrites [5,11], which causes their expansion to accommodate these ions. This expansion leads to an induced perturbation on the nearest-neighbor ions at the B-sites [5,11]. The larger strontium ions occupy the tetrahedral A-sublattice for x 0.2, but its ionic radius (1.18 Å) is larger than the mean ionic radius of A-site. Therefore, for x > 0.2 some Sr2þ ions may deposit at the grain boundaries instead of occupying the A-sites [11,15]. The true lattice constant at can be determined by graphing the obtained lattice constant a values for each sample against the Nelson-Riley extrapolating function F(q) [16]:
FðqÞ ¼
1 cos2 q cos2 q þ 2 sinq q
The relation between F(q) and a displays a straight line, where the value of at can be determined by extrapolating the straight line to F(q) ¼ 0 at q ¼ 90 . Fig. 2 displays variation of both the crystallite size R and true
Table 1 The weights of the precursors. Sample
Weight (g) Fe(NO3)3$9H2O
Weight (g) Co(NO3)2$6H2O
Weight (g) SrCl2
Weight (g) NaOH
CoFe2O4 Co0.8Sr0.2Fe2O4 Co0.7Sr0.3Fe2O4 Co0.6Sr0.4Fe2O4 Co0.5Sr0.5Fe2O4 Co0.4Sr0.6Fe2O4
103.300 101.102 99.658 98.497 97.349 96.261
37.200 29.094 25.120 21.280 17.527 13.864
e 6.665 9.865 13.103 16.060 19.058
40.9 39.997 39.466 39.127 38.550 38.120
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295
Fig. 1. XRD patterns of as-synthesized Co1-xSrxFe2O4 nano-ferrites.
lattice constant at against x. It is shown that R decreases slightly with x. This may be due to that the deposition of the larger Sr2þ ions on the grain boundaries produces a surface-induced stress causing a reduction of the crystallite size. On the other hand, the amount of cobalt ions in the B sublattice decreases that leads to its shrinkage, i.e. decreasing R. Heiba et al. [17] explained that adding bigger radius ions into the crystal sublattice may cause a compress stress and hinder the grain growth. This produces a lattice microstrain that prevent the grain growth. It is displayed that at increases up to x z 0.3 and decreases thereafter. The increase of at may be due the substitution of larger Sr2þ ions in the tetrahedral A-sites and the decrease occurs because of the deposition of Sr2þ ions on the grain boundaries and shrinkage of the B-sites [5,17]. The decrease in a and R may point to the formation of ferromagnetically ordered nanoclusters in the samples that leads to shrinkage of the crystal lattice [5]. The crystalline structure of the nanocluster is the same as the bulk material with little different lattice parameters. Variation of the bulk density D, theoretical density (x-ray density) Dx and porosity P against x is illustrated in Fig. 3. It is illustrated that Dx and D slightly increase with x, whereas P has random
behavior and decrease only in the range of 0.2 < x < 0.5. The increase of Dx and D may be attributed to the substitution of the heavier atomic weight Srþ2 ions (87.62) for Co2þ ions (58.9) in the crystal lattice. The nonmonotonical variation of P may be a result of inaccuracies of the Dx and D values. The decrease in P may be due to densification of the sample materials and increase in packing. The increase of densities and decrease in porosity may be due to increasing the absolute values of compressive strain (Fig. 4). The strain 3 of the samples was calculated by using the following equation [18]:
b1=2 cosq ¼
0:9l þ 4ε sin q R
The relation between 3 and x is plotted in Fig. 4. Variation of 3 reflects the mechanical properties of the samples, where its negative values prove that the strain is compressive [5,11,18]. This confirms the relation between the reduction of crystallite size and the compressibility of the samples by increasing the Sr2þ ion content x [5,11]. The specific surface area S (m2/g) was calculated using the
Fig. 2. Variation of the true lattice constant at and crystallite size R against composition x.
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Fig. 3. Composition x dependence of the density D, theoretical density Dx and porosity P.
Fig. 4. Dependence of the strain
3
and specific surface area S on Sr2þ ion content x.
relation [19]:
LAA ¼ a
pffiffiffi pffiffiffi pffiffiffiffiffiffi 3 2 11 ; LBB ¼ a andLAB ¼ a 4 4 8
The obtained data are presented in Table 2.
S¼
particle surface area Particle surface area 6000 ¼ ¼ particle mass Density volume RDx
The values of S increase versus x as expected because of decreasing the crystallite size that leads to increasing the surface to volume ratio [5,11,19]. The hopping length is known as the distance between the centers of adjacent ions. So, the presence of larger ions results in an increase of the distance between the magnetic ions [20]. Thus, the distance between magnetic ions (hopping length), L, in the A-sites (LA-A), B-sites (LB-B) and shared sites (LA-B) can be evaluated using the following equations [5,6,21]:
Table 2 The obtained distance between magnetic ions (hopping length) in the A-sites (LA-A), B-sites (LB-B), shared sites (LA-B) and true lattice constant at for Co1x SrxFe2O4 samples. x
LA-A (Å)
LB-B (Å)
LAeB (Å)
at(Å)
0 0.2 0.3 0.4 0.5 0.6
3.627 3.636 3.636 3.627 3.620 3.605
2.962 2.969 2.969 2.961 2.956 2.943
3.473 3.481 3.481 3.472 3.466 3.451
8.377 8.397 8.397 8.375 8.361 8.326
E.H. El-Ghazzawy, M.A. Amer / Journal of Alloys and Compounds 690 (2017) 293e303 Table 3 IR absorption band values. x
n4 (cm1)
n2 (cm1)
n1 (cm1)
nA (cm1)
nB (cm1)
0 0.2 0.3 0.4 0.5 0.6
246.88 246.88 237.24 235.31 235.31 235.3
410.83 410.83 410.83 414.68 416.61 424.33
592.13 594.06 596 596 597.9 597.92
896.88 894.95 894.94 896.87 e e
e 1014.53 1014.53 1016.64 1020.32 1020.32
297
agglomerated. The obtained average particle size of the samples were 23.66, 16.05 and 10.53 nm for x ¼ 0,0, 0,4, 0.6, respectively. These values agree well with the calculated values of crystallite size R. The lattice spacing (the distance between atom centers) obtained from HRTEM image for Co0.6Sr0.4Fe2O4 sample was determined for certain direction in the lattice as 0.21 nm. 3.3. Infrared spectra (IR)
It is presented that LA-A, LB-B and LA-B behave similar to at against x. The values of LA-A are higher than LB-B values, which may be due to the expansion of the A-sites and shrinkage of the B-sites by the addition of Sr2þ ions [14]. Table 3 presents the obtained values of interplaner distance d calculated from TEM. These values are comparable with those calculated from XRD.
3.2. Transmission electron microscope (TEM) images TEM images for Co1xSrxFe2O4 system are depicted in Fig. 5. The images show that the particles are in the nano-scale and
Fig. 6 depicts the IR absorption spectra of as-prepared Co1nano-ferrites recorded in the range 200e2000 cm1. The absorption bands; n4, n2, n1, nA, nB and nT are seen in the IR spectra. The obtained results from IR analysis are listed in Table 3. The two characteristic absorption bands of spinel nano-ferrites, n1 and n2, are observed, n1 in the range of 592e598 cm1 and n2 in the range of 410e424 cm1. The band n1 is assigned to intrinsic stretching vibrations of the tetrahedral A-site metal ion-oxygen bonding corresponding to the highest restoring force [8,18], and n2 to intrinsic vibrations of the octahedral B-site metal ion-oxygen vibrations, which are bond-bending vibrations [8,18]. The values of n1 are higher compared to n2, which indicate that the normal mode
xSrxFe2O4
Fig. 5. TEM images for as-prepared Co1xSrxFe2O4 nano-ferrite system.
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Fig. 6. IR absorption spectra of as-prepared Co1xSrxFe2O4 nano-ferrites.
Fig. 7. Force constants, FA and FB, at the A- and B-sites, respectively, as functions of x.
of vibration of the tetrahedral cluster is higher than that of the octahedral cluster. This is attributed to the shorter bond length of the A-site clusters than that of the B-site clusters [18,22]. The prominent shoulder appeared at the band n2 at around 290 cm1 may assign to the existing of the divalent ions Co2þ and/ or Fe2þ among the B-sites. The band n4 appeared in the range of
235e247 cm1 is attributed to the lattice vibrations of the system and depends on the mass of the divalent ions among the A-sites and their complexes, Fe2þ-O2, Co2þ-O2 and/or Sr2-O2 [5,14]. Two weak absorption bands nA and nB are observed in the IR spectra. The band nA may exist because of the presence of divalent ions Co2þ, Fe2þand/or Sr2þ in the A-sites. The band nB may be
Table 4 The stiffness constants, C11, C12, elastic moduli, B, G and E, elastic wave velocities, VL and Vs and Poisson's ratio, s. x
C11 (GPa)
C12 (GPa)
B (GPa)
G (GPa)
E (GPa)
VL (103m/s)
Vs (103m/s)
s
0 0.2 0.3 0.4 0.5 0.6
227.144 228.236 229.714 229.699 232.295 236.827
34.760 33.481 34.410 35.649 35.765 36.037
98.888 98.399 99.511 100.332 101.275 102.967
96.192 97.378 97.652 97.025 98.265 100.395
217.918 219.670 220.748 220.120 222.751 227.308
6.530 6.463 6.426 6.447 6.408 6.365
3.770 3.731 3.710 3.722 3.670 3.674
0.1327 0.1279 0.1302 0.1343 0.1334 0.1320
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299
Fig. 8. Effect of sample density D on the elastic wave velocities VL and Vs.
assigned to the tetravalent metal ioneoxygen, i.e. the complexes Fe4þeO2 and/or Co4þ- O2. The tetravalent ions may result from the electron hopping between the Co2þ and Fe3þ ions, where Fe2þ, Fe4þand Co4þmay be produced [5,11,14]. The triple band nT at around 1542 cm1 in all IR spectra represents the characteristic hydroxyl group (OH) that can be attributed to the humidity in the samples. The presence of hydroxyl groups in the samples may help in the conjugation and dispersion of nano-materials [23]. The nonlinear and triatomic molecule of water has three main vibration modes: symmetric stretching, asymmetric stretching and scissoring vibration modes. The scissoring vibration exists for this nonlinear molecule which proves that the band nT may assign to O-H group [24]. It is known that the Fe3þ-O2 band frequency is proportional to the force constant (Fc) at the A- and B-sites, FA and FB, respectively. Therefore, Fc can be calculated by using the relation [5,14,19]:
Fc ¼ 4p2 c2 v2 m Where m is the reduced mass of Fe3þ and O2 ions, m ¼ 2.061 1023 g. The calculated values of FA and FB are seen as a function of x in Fig. 7. It is seen that FA increases against x but FB increases only from x ˃ 0.3. This increase proves that the force constant depends on the resonant frequency and substitution of heavier ions [5,14]. It is seen that FA > FB for x > 0.0, which may assign to the shorter bond length of the A-site as mentioned above [18,24]. This is attributed to the inverse proportionality between the bond length and force constant [9,18]. 3.4. Elastic properties The elastic constants and Debye temperature of spinel ferrite system can be deduced using x-ray and IR data. The stiffness
Fig. 9. The relation between Debye temperature ӨD and the composition x.
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Fig. 10. Magnetic hysteresis loops for as-prepared Co1-xSrxFe2O4 nano-ferrites.
Fig. 11. Composition x dependence of the deduced saturation magnetization Ms and coercivity Hc.
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301
Fig. 12. The correlation between the saturation magnetization Ms and crystallite size R.
Table 5 The saturation magnetization (Ms), coercivity (Hc), magnetic moment (nB),anisotropy constant (K) and remnant magnetization (Mr). x
Ms(emu/g)
Hc (Oe)
nB (mB)
K(erg/Gauss)
Mr(emu/g)
0 0.2 0.3 0.4 0.5 0.6
46.420 39.215 41.642 41.194 37.874 33.901
411 557.5 707.5 557.5 411 261.5
1.95 1.688 1.813 1.815 1.688 1.528
18315.53 20987.92 28283.14 22047.03 14943.57 8510.46
9.12 10.66 14.44 11.76 8.29 5.36
constants C11 and C12 are calculated using the following relations [25]:
C11 ¼
F s C11 and C12 ¼ a ð1 sÞ
Where F ¼ (FA þ FB)/2 and s is the Poisson's ratio that is a function of pore fraction (porosity P) [26]:
s ¼ 0:324ð1 1:043PÞ Therefore, the three elastic moduli of solids for cubic structure in terms of stiffness constants are calculated using the following relations [7,26]:
Bulk modulus ðBÞ ¼
1 ðC þ 2C12 Þ 3 11
Young0 s modulus ðEÞ ¼
ðC11 C12 ÞðC11 þ 2C12 Þ ðC11 þ C12 Þ
Rigidity modulus ðGÞ ¼
E 2ðs þ 1Þ
The obtained data of elastic moduli and stiffness constants are presented in Table 4. The X-ray density Dx and stiffness constant C11 are further used to estimate the elastic wave velocity VL using the forqffiffiffiffiffilongitudinal ffi mula; VL ¼ CD11x , and the transverse elastic wave velocity Vs by general approximation; Vs ¼ pVLffiffiffi [7,24]. 3 The calculated values of elastic wave velocities are presented in
Table 4. Table 4 presents that the stiffness constants, C11 and C12, and elastic moduli, B, G and E, increase with increasing Sr2þ content, whereas the elastic wave velocities, VL and Vs decrease. This may be interpreted in terms of the interatomic bonding. So, this behavior of elastic moduli is attributed to the strengthening of the inter-atomic bonding between various atoms of the spinel lattice [9,26]. It is worth noting that the density has a great effect on elastic moduli and wave velocities, which may be due to that the ferrite samples have different densities. Since the density increases with increasing Sr2þ content x, the elastic moduli increase and wave velocities decrease as functions of density [27]. Fig. 8 explains the effect of density on the elastic wave velocities. Debye temperature WD can be calculated using the relation yAV [28], where h is Plank's constant, k is Boltzmann's conWD ¼ hc 2pk stant, c is the velocity of light (c ¼ 3 1010 cm/s) and nAV ¼ (n1 þ n2)/2 is the average value of wavenumbers for the Aand B-site. Fig. 9 illustrates the relation between WD and the additional factor x. It is illustrated that WD increases against x. This increase implies the increase in rigidity of the samples and lattice vibrations [7,26]. According to specific heat theory, the increase in ӨD proves a decrease in the number of conduction electrons (n-type) which can absorb part of the heat causing increase of ӨD. The contribution of holes (p-type) to conductivity increases causing a decrease in the absorption part of heat by electrons and hence ӨD increases [11,25,26]. Increase of ӨD points to change of the conductivity of these samples from n-type to p-type [11,25,26].
3.5. Vibrating sample magnetometer (VSM) measurements The obtained magnetic hysteresis loops by VSM for the asprepared Co1-xSrxFe2O4 nano-ferrite samples are depicted in Fig. 10. It is depicted that the curves have a relatively large coercivity and small saturation magnetization, which is characteristic of the hard magnetic materials. Among the ferrites, CoFe2O4 has the largest positive anisotropy value due to the strong spin orbit coupling at the Co2þ lattice sites. Although the smallness of the crystallite size, the coercivity of CoFe2O4 is relatively high and it has a ferromagnetic nature [29]. Fig. 11 shows the variation of deduced saturation magnetization
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Fig. 13. Dependence of n1 and n2 on Ms.
Ms and coercivity Hc against x. It is shown that the trend of Ms is a slight decrease with x, whereas Hc increases for x 0.3 and decreases thereafter. The decrease of Ms may be attributed to the substitution process of the diamagnetic Sr2þ ions for magnetic Co2þ ions. This leads to decreasing the magnetization of the sample, i.e. weakening the super-exchange magnetic interactions between the sublattices, MB-MA, which results in decreasing the total magnetization [30e33]. On the other hand, Ms decreases as the crystallite size decreases (Fig. 2) which can be explained on the basis of nonlinear or canted spin disordering at the surface layer of nanoparticles due to local chemical disorder and broken exchange interaction among magnetic spins at the surface. Also, the lack of Co ions in the B-sublattice of ferrimagnetic samples leads to spin canting in the other sublattice [30e33]. The correlation between Ms and the crystallite size R is seen in Fig. 12. The increase in coercivity for x 0.3 (Fig. 11) may be due to increase in magnetic crystalline anisotropy. In ferrite nanoparticles, the surface anisotropy has a great effect on the coercivity. Therefore, for a given composition of ferrite, when the crystallite size decreases, the coercivity increases as the surface effect becomes more dominant [34]. But for x 0.3 the effect of decreasing Co2þ concentration (which has large positive anisotropy, as mentioned above) will prevail, thus the coercivity decreases. This may be attributed to decrease in the anisotropy field, which in turn decreases the domain wall energy [35]. The anisotropy constant K can be evaluated using the following equation [5,11]:
Hc ¼
0$96K Ms
The obtained values of K and remnant magnetization Mr are listed in Table 5. Evidently, the large magneto-crystalline anisotropy of CoFe2O4 nanoparticles is due to the strong LeS couplings on the Co2þ cation sites. Energy barriers in the individual nanoparticles are considered and introduced by the magneto-crystalline anisotropy energy of cobalt ferrite [3,36]. It is expected that the remnant magnetization Mr has the same behavior of the coercivity Hc as presented in Table 5. The larger the remnant magnetization the larger the reverse field needed to reduce it. The magnetic moment nB per atom in Bohr magneton units (mB) could be calculated using the following relation [37]:
nB ¼
M Ms Nb
where M is the molecular weight, N is Avogadro's number and b is the conversion factor to express the magnetic moment per atom in Bohr magnetons (its value is 9.27 1021 erg/gauss). The obtained values of nB are given in Table 5. It is clear that nB varies similar to Ms, where its trend shows decrease against x. This is because the individual grain acts as magnetic material and has its magnetization. This is considered as an indicator of the total magnetization of the samples [5,11,37e39]. Fig. 13 explains the dependence of n1 and n2 on the saturation magnetization Ms. It is clear that n1 and n2 decreases against Ms. As the saturation magnetization increases, the A-A and B-B supertransferred magnetic interactions inside the crystal sublattices increase, which have a great effect in hindering the vibrational frequency of tetrahedral and octahedral sites and leads to decrease in n1 and n2. 4. Conclusion This investigation revealed that as-synthesized Co1-xSrxFe2O4 nanoparticles, 0.0 x 0.6, by the chemical co-precipitation method have a single phase of cubic spinel structure. It is proved that the larger strontium ions occupy the A-sites at low concentration and deposit at the grain boundaries at high concentration. This deposition suppresses the growth of the nanoparticle and presses the crystallite size causing a decrease in the lattice constant for x 0.3 and average and crystallite size. The obtained parameters showed dependence on Sr2þ ion content x. Five absorption bands were observed in IR spectra and assigned to the corresponding site bonds. They pointed to the existence of Fe2þ and Fe4þ ions among the crystal sublattices. The resonant frequencies; n1 and n2 proved dependence on Ms, whereas Ms showed dependence on R. The negative values of the strain explain that it is compressive. The increase in stiffness constants, elastic moduli and Debye temperature implies an increase in the rigidity of the samples where the sample density revealed an effect on the elastic wave velocities. References [1] R.W. Siegel, E. Hu, M.C. Roco, Nanostructure Science and Technology, Loyola College in Maryland, 1999.
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