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Microstructure and magnetic properties of substituted (Cr, Mn) - cobalt ferrite nanoparticles
Materials Chemistry and Physics 135 (2012) 728e732
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Microstructure and magnetic properties of substituted (Cr, Mn) - cobalt ferrite nanoparticles Alina Mihaela Cojocariu a, Marius Soroceanu a, b, Luminita Hrib a, Valentin Nica a, Ovidiu Florin Caltun a, * a b
Faculty of Physics and Carpath Center of Excellence, Alexandru Ioan Cuza University, Bd. Carol I nr. 11, 700506 Iasi, Romania Institute for Macromolecular Chemistry P. Poni, Aleea Grigore Ghica Voda, nr. 41A, 700487 Iasi, Romania
h i g h l i g h t s < The influence of manganese and chromium substitutions on the coprecipitated cobalt ferrite nanoparticles is described. < The microstructure of the as synthesized powders is correlated with the substitution. < Development of the spinel phase enhanced by thermal treatment is monitored by FTIR and XRD analyses. < The magnetic properties of the mixed ferrites are correlated with phase content and cation distribution. < The influence of the annealing process on the magnetic properties of the mixed ferrites is discussed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 November 2011 Received in revised form 13 May 2012 Accepted 16 May 2012
Three mixed ferrite systems CoFe2O4, CoCr0.2Fe1.8O4 and CoMn0.2Fe1.8O4 were prepared by the coprecipitation method. X-ray diffraction (XRD) of the calcined powders at 900 C confirms the accomplishment of the cubic spinel phase formation without any secondary phase. The cation distribution was determined from XRD data and suggested a mixed spinel structure. The higher value of the lattice parameter for Mn comparing with Cr substituted cobalt ferrite could be explained by migration of Co2þ cation from octahedral to tetrahedral sites. Infrared spectroscopy (FTIR) showed two absorption bands attributed to intrinsic vibrations of tetrahedral and octahedral complexes. The bands shifted toward lower frequency for doped samples comparing with corresponding Co-host ferrite frequencies. The microstructure was investigated by scanning electron microscopy (SEM) and suggested that the nanoparticles are agglomerated and have polygonal faced surfaces. The effect of Mn3þ and Cr3þ cation distribution among the tetrahedral (A) - and octahedral [B] - sites of Co substituted ferrite on magnetization and coercive field was investigated by vibrating sample magnetometer technique. Higher values of the coercive field of manganese and chromium substituted cobalt ferrite was explained by lower value of the average particle size. The small increase of saturation magnetization for doped ferrites was explained considering the oxidation state of the substituting ions. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Polycrystalline nanoparticles Substituted cobalt ferrite Co-precipitation method Cation distribution Magnetization Curie temperature
1. Introduction It is known that ferrospinels have attracted considerable attention due to their technological importance in different applications such as high frequency devices, biomedicine, catalysis and magnetic refrigerators [1], [2]. Several simple and low cost methods based on co-precipitation [3], intense ball milling [4], solegel [5], [6], hydrothermal [7], reverse micelle [8], [9] and combustion
* Corresponding author. Tel./fax: þ40 232201174. E-mail address: [email protected] (O.F. Caltun). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.051
synthesis [10] processes are used for synthesizing stoichiometric and doped ferrite nanoparticles. The site occupancy in spinels can be represented by the formula (M1iFex)A[Mi Fe2i]BO4, where round and square brackets represent the A- and B-sites respectively, M stands for a metal cation and ‘i’ specifies the fraction of A-sites occupied by the majority ions. For a normal spinel the inversion parameter i ¼ 0 while for an inverted spinel i ¼ 1 and 0 < i < 1 for partially inverted spinel [11]. The cation distribution among the tetrahedral (A) and octahedral [B]sites depends on the type of metallic ions and synthesis method [12]. CoFe2O4 can be a normal or an inverse spinel-type structure. It is known that the magnetic and magnetostrictive properties of Co based ferrites. depends on the concentration of Co2þ at the
A.M. Cojocariu et al. / Materials Chemistry and Physics 135 (2012) 728e732
729
octahedral [B]-sites [13]. The addition of metal cations with different valence state to parent oxide can tailor magnetic properties and Curie temperature of the system for a particular application. Some studies regarding the substitution of trivalent cations such as Cr3þ and Mn3þ in cobalt ferrites have been reported [14,15]. Therefore, the knowledge of cation distribution [16] is important to understand the magnetic properties of nanoparticles. Up to now CoCr0.2Fe1.8O4 and CoMn0.2Fe1.8O4 were prepared by standard powder ceramic techniques [17], that involve the preparation of the powder from metal oxides or carbonates by crushing, grinding and milling. This paper reports the experimental investigations of the crystal structure, cation distribution and magnetic properties of (Cr, Mn) - substituted CoFe2O4 prepared by coprecipitation method. 2. Experimental Powders of CoFe2O4 and CoMexFe2xO4 (where Me ¼ Cr, Mn; and x ¼ 0.2) were synthesized by the co-precipitation method. Solutions of FeCl2$6H2O (0.4 M), CoCl2$6H2O (0.2 M), CrCl2$6H2O (0.3 M) and MnCl2$4H2O (0.3 M) from Aldrich in the appropriate ratio were dissolved in double distilled water and heated to 60 C. Aqueous NaOH (3 M) at 60 C was added to the reagent solutions to precipitate the powders; solutions were stirred at 400 rpm for 1 h to homogenize the products. The final pH of these mixed solutions was between 11 and 12. The precipitate was washed with double distilled water, filtered and dried at 70 C for 12 h in air. Powders were calcined in three steps at 400, 650 and 900 C for 5 h. XRD spectra of the products were recorded using Shimadzu LabX 6000 diffractometer (CuKa l ¼ 0.15406 nm) equipped with a graphite monochromator. The specimens mounted in reflection mode were analyzed at ambient temperature and pressure with scanning rate of 0.02 and counting time of 2s/step over the 2q ¼ 20e80 range. The crystallite size, theoretical and experimental lattice parameters, theoretical density, atomic co-ordinates, ionic radii, bond lengths and the lengths of the tetrahedral (A) and octahedral [B] edges were determined from powder diffraction patterns. Samples for FTIR were prepared by mixing a small amount of KBr into the powder and pressed into a pellet with a diameter of 13 mm. The FTIR spectra (Spectrum One, PerkinElmer, USA) were recorded at 25 C in the range of 400e900 cm1. The surface features and the morphology of the calcined powders at 973 K were investigated by Scanning Electron Microscopy (SEM) using a Vega Tescan Scanning Electron Microscope. Magnetic measurements were carried out at room temperature using a dual AGM/VSM 2009 Princeton Magnetometer. Curie temperature was measured by using a homemade experimental device based on Soohoo’s method [18]. 3. Results The XRD spectra of the precipitates and calcined powders at different temperature suggested the temperature of 900 C as the final stage for spinel phase formation. The spectra shown in Fig. 1a), for the as synthetized powders, suggest that the presence of the manganese and chromium cations, in the bath, have inhibited the spinel phase formation, even if the synthesis conditions were kept the same. The XRD spectra recorded at different increasing calcination temperatures demonstrated a continuous enhancement of the spinel phase content and an increase of the crystallite size. The patterns plotted in (Fig. 1b) confirmed the presence of spinel phase without secondary phase or segregation to the nanoparticle surface. The Bragg reflections fully match the reported values of face-centered cubic structure with the space group Fd3m (Powder Diffraction Files, card no 22e1086). No obvious secondary phase
Fig. 1. XRD patterns of the powders: a) as synthetized and b) calcined at 900 C for 5 h in air.
was detected, showing the purity of the particles. The average crystallite size of the particles for each sample was calculated using a Lorentzian profile analysis of the individual peaks in a method implemented in the Topas Academic v4.1 software. The data inserted in Table 2 confirmed highest crystallite size for CoMn0.2Fe1.8O4. The lattice parameters for the powders calcined in air at 900 C were calculated using the linear multiple regressions based on the Least Squares Procedure with a Si standard powder correction. The cation distribution of the spinel ferrite systems was estimated from XRD data. The best result for the cationic distribution was estimated to agree with both theoretical and experimental lattice parameters (Table 1). The proposed cation distribution for CoFe2O4 ferrite indicates that the Co2þ ions have preference for the B-sites. This corresponds to a mixed spinel in agreement with previously reported Mössbauer studies [16]. Cr3þ and Mn3þ have a strong tendency to occupy the octahedral site [19,20] and for binary ferrites, the cation distribution can be represented by the formula:
3þ Co2þ x Fe1x
A h
3þ 3þ Co2þ 1x Mey Fe2xy
iB
O2 4
The lattice parameter (aexp) of Cr-substituted Co ferrite was expected to be higher than that of Mn-substituted Co ferrite due to
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Table 1 The values of crystallographic parameters evaluated from X-ray diffraction patterns. The accuracy is 0.002 Å. X-ray density (g cm3)
Lattice constant (Å)
Bond length (Å)
Site radii (Å)
Oxygen parameter (Å)
Samples
dx
aexp
ath
RA
RB
ra
rb
uexp
uth
RAE
RBE
RBEU
CoFe2O4 CoMn0.2Fe1.8O4 CoCr0.2Fe1.8O4
5.218 5.304 5.296
8.379 8.397 8.370
8.410 8.408 8.403
1.874 1.892 1.885
2.072 2.062 2.063
0.494 0.512 0.505
0.692 0.682 0.683
0.379 0.380 0.380
0.378 0.379 0.379
3.057 3.087 3.077
2.867 2.850 2.840
2.963 2.969 2960
the smaller ionic radius of the substituting octahedral Cr3þ (0.615 Å) as compared to Mn3þ (0.645 Å) and Fe3þ radii (0.645 Å) [21]. The higher value of the lattice parameter for Mn substituted Co ferrite could be explained by migration of Co2þ cation from octahedral (0.580 Å) to tetrahedral (0.745 Å) sites [22]. The theoretical lattice parameter (ath) was calculated according to the equation:
ath ¼
pffiffiffi 8 pffiffiffi ðrA þ R0 Þ þ 3ðrB þ R0 Þ 3 3
The theoretical density (dx) was calculated according to the relation: dx ¼ 8M=ðNA a3exp Þ where NA is the Avogadro’s number and M is the molecular weight of the sample. The change of dx can be attributed to the atomic weight and the radii of constituent cations. Therefore, the shift of reflections towards higher angle values, the lattice parameter and theoretical density confirm that Cr3þ and Mn3þ cations, respectively, have replaced the Fe3þ cations at octahedral B-sites into spinel cubic structure of the Co-host ferrite [23]. Using the relation [24]:
Edge lengths (Å)
Infrared absorption (FTIR) spectra were recorded after each calcination step. In Fig. 2a) and b) are the spectra for the precipitates and calcined powders at 900 C in air for 5 h. In the FTIR spectra, of the precipitated powders, the vibrational bands of the spinel phase and some bands corresponding to the water and secondary phase are observed. The spectra of the calcined powders exhibit two vibrational bands, the higher frequency band located around 600 cm1 (n1) and the lower frequency band ranged around 400 cm1 (n2). The n1 is assigned to the intrinsic vibrations of metaleanion complexes at the tetrahedral sites (MtheO) and n2 corresponds to octahedral metal stretching (MoheO) of the spinel lattice [29]. MtheO and MoheO correspond to the tetrahedral and octahedral metal-oxygen vibrations of the spinel lattice respectively. The bands n1 and n2 slightly shift to lower frequency for doped samples comparing with corresponding Co-host ferrite
pffiffiffi rA ¼ ðu 1=4Þaexp 3 R0 it was calculated the oxygen atomic co-ordinate “u”, where R0 ¼ 1.38 Å is the radius of oxygen ion and “a” corresponds to the experimental value of the lattice parameter (Table 2). The decrease of rB values (Table 1) for doped ferrites can be explained in terms of Co2þ migration from the octahedral [B]- to the tetrahedral (A) - sites [25]. The oxygen atomic co-ordinate u for all the samples is slightly different to that of the ideal spinel ferrite (0.375) due to synthesis conditions [26]. Cationeoxygen bond distance for tetrahedral (RA) and octahedral (RB) sites, tetrahedral edge length (RAE), shared (RBE) and unshared octahedral edge lengths (RBEU) were computed according with Otero Arean et al. [27]. The increase in the cation-oxygen bond length RA for doped samples can be attributed to the substitution processes due to the displacement of Fe3þ ions from the B-sites to the A-sites. The introduction of larger ionic radius Co2þ (0.745 Å) compared to Fe3þ (0.490 Å) at the tetrahedral sites for chromite and manganese ferrites increases the tetrahedral edge lengths RAE of the both samples compared to that of CoFe2O4. The values of RAE, RBE and RBEU are common for the ferrospinel systems with distributed cations [27,28].
Table 2 Crystallite size, magnetic properties and Curie temperature of calcined powders.
Cation distribution (Co0.05Fe0.95)A [Co0.95Fe1.05)]B (Co0.25Fe0.75)A [Co0.75Fe1.05Mn0.2)]B (Co0.17Fe0.83)A [Co0.83Fe0.97Cr0.2)]B
Crystallite size D (nm)
Ms (emu g1)
Mr (emu g1)
Hc (Oe)
T ( C)
41
56
14
167
356
79
60
23
552
407
44
59
32
783
387
Fig. 2. FTIR spectra of the powders: a) as synthetized and b) calcined at 900 C for 5 h in air.
A.M. Cojocariu et al. / Materials Chemistry and Physics 135 (2012) 728e732
frequencies. The change in band position may be due to the cationanion distances at the octahedral and tetrahedral lattice sites [30]. The SEM micrographs shown in Fig. 3 suggest that nanoparticles are agglomerated and have polygonal faced surfaces. It is observed that the average particle size of the CoMn0,2Fe1,8O4 and CoCr0,2Fe1,8O4 samples is lower than that of CoFe2O4 sample in correlation with the values obtained for the coercive field during the magnetic measurement. Magnetic hysteresis loops at room temperature of as synthetized and calcined samples are shown in Fig. 4. The coercive field (Hc), saturation magnetization (Ms) and remnant (Mr) values for undoped and doped ferrite powders calcined at 900 C for 5 h in air are summarized in Table 2. Comparing the value obtained for undoped cobalt ferrite with the values obtained for manganese and chromium doped ferrites a small increase of the saturation magnetization was observed. The net magnetization of a ferrimagnetic material depends on cation distribution among octahedral and tetrahedral sublattice. According to molecular field theory, the net magnetization at temperature T can be written as:
MðTÞ ¼ MB ðTÞ MA ðTÞ where, MB(T) and MA(T) are the magnetization of B and A sublattices, respectively at temperature T [31].
731
The net magnetic moment per ion for Co2þ, Mn3þ, Cr3þ and Fe3þ are 3mB, 4 mB, 3mB and 5mB. From spectroscopic measurements, it was found that Co2þ, Mn3þ, Cr3þ ions have strong site preference for octahedral sites while Fe3þ have no particular preference for either coordination. For CoMn0,2Fe1,8O4 the small increase of saturation magnetization can be explained considering the oxidation state of the manganese ions. The Mn2þ (5mB) ions have no particular preference for octahedral or tetrahedral sites. If the amount of Mn2þ ions from octahedral sites exceeds that of Mn3þ, a large increase in the magnetization is expected, whereas if the amount of Mn3þ ions is larger, the increase in the magnetic moment will be smaller [15,32]. The saturation magnetization value obtained for CoCr0,2Fe1,8O4 is similar to that obtained for CoMn0,2Fe1,8O4. From cation distribution obtained from XRD analysis, one can see that for CoMn0,2Fe1,8O4 and CoCr0,2Fe1,8O4, some cobalt ions from octahedral migrate to tetrahedral sites. In cobalt ferrite the magnetic anisotropy predominantly arises from the single-ion anisotropy of Co2þ on the B sites, and the displacement of these ions should lead to a decrease of magnetic anisotropy and consequently to the decrease of the coercive field. In this study the values of Hc obtained for manganese substituted cobalt ferrite and chromium substituted cobalt ferrite are higher than that of CoFe2O4. A
Fig. 3. SEM micrographs of the calcined powders.
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of Hc obtained for CoMn0.2Fe1.8O4 manganese substituted cobalt ferrite and chromium substituted cobalt ferrite are higher than that of CoFe2O4 in correlation with the average grain size suggested by micrographs. Acknowledgments This work was supported by the European Social Fund in Romania, under the responsibility of the Managing Authority for the Sectoral Operational Programme for Human Resources Development 2007-2013 [grant POSDRU/107/1.5/S/78342 and POSDRU/ 89/1.5/S/49944]. The authors are thankful to Prof. D. Creanga and Prof. M. Palamaru for their kind support during sample preparation and to Mrs. Slatineanu for useful discussion concerning cation distribution. Special thanks to Robert Lowndes from National Institute for Material Physics for useful comments and English spelling check. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] Fig. 4. Magnetic hysteresis loops of the powders: a) as synthetized and b) calcined at 900 C for 5 h in air.
[17] [18] [19]
possible explanation of the increase of the coercive field for substituted ferrites could be the lower value of the average particle size comparing with that of stoichiometric cobalt ferrite. With the reduction of the particle size, the surface to volume ratio increases and surface effects may induce a spin disorder in the surface layer as well as an enhanced surface anisotropy [27,33].
[23] [24]
4. Conclusions
[25]
Mn and Cr substituted cobalt ferrite were prepared by chemical co-precipitation method to study the influence of these substitutions on the microstructure and magnetic properties. The XRD patterns confirmed the formation of the spinel phase after thermal treatment at 900 C. In substituted ferrite some cobalt ions from the octahedral site migrated to the tetrahedral sites. The crystallite size ranges from 44 to 79 nm, manganese substitution favoring higher crystallite size. Magnetic measurements show that manganese substitution for iron, increased the saturation magnetization, which can be explained by the oxidation state of the manganese ions. The values
[20] [21] [22]
[26] [27] [28] [29] [30] [31] [32] [33]
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Microstructure and magnetic properties of substituted (Cr, Mn) - cobalt ferrite nanoparticles Alina Mihaela Cojocariu a, Marius Soroceanu a, b, Luminita Hrib a, Valentin Nica a, Ovidiu Florin Caltun a, * a b
Faculty of Physics and Carpath Center of Excellence, Alexandru Ioan Cuza University, Bd. Carol I nr. 11, 700506 Iasi, Romania Institute for Macromolecular Chemistry P. Poni, Aleea Grigore Ghica Voda, nr. 41A, 700487 Iasi, Romania
h i g h l i g h t s < The influence of manganese and chromium substitutions on the coprecipitated cobalt ferrite nanoparticles is described. < The microstructure of the as synthesized powders is correlated with the substitution. < Development of the spinel phase enhanced by thermal treatment is monitored by FTIR and XRD analyses. < The magnetic properties of the mixed ferrites are correlated with phase content and cation distribution. < The influence of the annealing process on the magnetic properties of the mixed ferrites is discussed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 November 2011 Received in revised form 13 May 2012 Accepted 16 May 2012
Three mixed ferrite systems CoFe2O4, CoCr0.2Fe1.8O4 and CoMn0.2Fe1.8O4 were prepared by the coprecipitation method. X-ray diffraction (XRD) of the calcined powders at 900 C confirms the accomplishment of the cubic spinel phase formation without any secondary phase. The cation distribution was determined from XRD data and suggested a mixed spinel structure. The higher value of the lattice parameter for Mn comparing with Cr substituted cobalt ferrite could be explained by migration of Co2þ cation from octahedral to tetrahedral sites. Infrared spectroscopy (FTIR) showed two absorption bands attributed to intrinsic vibrations of tetrahedral and octahedral complexes. The bands shifted toward lower frequency for doped samples comparing with corresponding Co-host ferrite frequencies. The microstructure was investigated by scanning electron microscopy (SEM) and suggested that the nanoparticles are agglomerated and have polygonal faced surfaces. The effect of Mn3þ and Cr3þ cation distribution among the tetrahedral (A) - and octahedral [B] - sites of Co substituted ferrite on magnetization and coercive field was investigated by vibrating sample magnetometer technique. Higher values of the coercive field of manganese and chromium substituted cobalt ferrite was explained by lower value of the average particle size. The small increase of saturation magnetization for doped ferrites was explained considering the oxidation state of the substituting ions. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Polycrystalline nanoparticles Substituted cobalt ferrite Co-precipitation method Cation distribution Magnetization Curie temperature
1. Introduction It is known that ferrospinels have attracted considerable attention due to their technological importance in different applications such as high frequency devices, biomedicine, catalysis and magnetic refrigerators [1], [2]. Several simple and low cost methods based on co-precipitation [3], intense ball milling [4], solegel [5], [6], hydrothermal [7], reverse micelle [8], [9] and combustion
* Corresponding author. Tel./fax: þ40 232201174. E-mail address: [email protected] (O.F. Caltun). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.051
synthesis [10] processes are used for synthesizing stoichiometric and doped ferrite nanoparticles. The site occupancy in spinels can be represented by the formula (M1iFex)A[Mi Fe2i]BO4, where round and square brackets represent the A- and B-sites respectively, M stands for a metal cation and ‘i’ specifies the fraction of A-sites occupied by the majority ions. For a normal spinel the inversion parameter i ¼ 0 while for an inverted spinel i ¼ 1 and 0 < i < 1 for partially inverted spinel [11]. The cation distribution among the tetrahedral (A) and octahedral [B]sites depends on the type of metallic ions and synthesis method [12]. CoFe2O4 can be a normal or an inverse spinel-type structure. It is known that the magnetic and magnetostrictive properties of Co based ferrites. depends on the concentration of Co2þ at the
A.M. Cojocariu et al. / Materials Chemistry and Physics 135 (2012) 728e732
729
octahedral [B]-sites [13]. The addition of metal cations with different valence state to parent oxide can tailor magnetic properties and Curie temperature of the system for a particular application. Some studies regarding the substitution of trivalent cations such as Cr3þ and Mn3þ in cobalt ferrites have been reported [14,15]. Therefore, the knowledge of cation distribution [16] is important to understand the magnetic properties of nanoparticles. Up to now CoCr0.2Fe1.8O4 and CoMn0.2Fe1.8O4 were prepared by standard powder ceramic techniques [17], that involve the preparation of the powder from metal oxides or carbonates by crushing, grinding and milling. This paper reports the experimental investigations of the crystal structure, cation distribution and magnetic properties of (Cr, Mn) - substituted CoFe2O4 prepared by coprecipitation method. 2. Experimental Powders of CoFe2O4 and CoMexFe2xO4 (where Me ¼ Cr, Mn; and x ¼ 0.2) were synthesized by the co-precipitation method. Solutions of FeCl2$6H2O (0.4 M), CoCl2$6H2O (0.2 M), CrCl2$6H2O (0.3 M) and MnCl2$4H2O (0.3 M) from Aldrich in the appropriate ratio were dissolved in double distilled water and heated to 60 C. Aqueous NaOH (3 M) at 60 C was added to the reagent solutions to precipitate the powders; solutions were stirred at 400 rpm for 1 h to homogenize the products. The final pH of these mixed solutions was between 11 and 12. The precipitate was washed with double distilled water, filtered and dried at 70 C for 12 h in air. Powders were calcined in three steps at 400, 650 and 900 C for 5 h. XRD spectra of the products were recorded using Shimadzu LabX 6000 diffractometer (CuKa l ¼ 0.15406 nm) equipped with a graphite monochromator. The specimens mounted in reflection mode were analyzed at ambient temperature and pressure with scanning rate of 0.02 and counting time of 2s/step over the 2q ¼ 20e80 range. The crystallite size, theoretical and experimental lattice parameters, theoretical density, atomic co-ordinates, ionic radii, bond lengths and the lengths of the tetrahedral (A) and octahedral [B] edges were determined from powder diffraction patterns. Samples for FTIR were prepared by mixing a small amount of KBr into the powder and pressed into a pellet with a diameter of 13 mm. The FTIR spectra (Spectrum One, PerkinElmer, USA) were recorded at 25 C in the range of 400e900 cm1. The surface features and the morphology of the calcined powders at 973 K were investigated by Scanning Electron Microscopy (SEM) using a Vega Tescan Scanning Electron Microscope. Magnetic measurements were carried out at room temperature using a dual AGM/VSM 2009 Princeton Magnetometer. Curie temperature was measured by using a homemade experimental device based on Soohoo’s method [18]. 3. Results The XRD spectra of the precipitates and calcined powders at different temperature suggested the temperature of 900 C as the final stage for spinel phase formation. The spectra shown in Fig. 1a), for the as synthetized powders, suggest that the presence of the manganese and chromium cations, in the bath, have inhibited the spinel phase formation, even if the synthesis conditions were kept the same. The XRD spectra recorded at different increasing calcination temperatures demonstrated a continuous enhancement of the spinel phase content and an increase of the crystallite size. The patterns plotted in (Fig. 1b) confirmed the presence of spinel phase without secondary phase or segregation to the nanoparticle surface. The Bragg reflections fully match the reported values of face-centered cubic structure with the space group Fd3m (Powder Diffraction Files, card no 22e1086). No obvious secondary phase
Fig. 1. XRD patterns of the powders: a) as synthetized and b) calcined at 900 C for 5 h in air.
was detected, showing the purity of the particles. The average crystallite size of the particles for each sample was calculated using a Lorentzian profile analysis of the individual peaks in a method implemented in the Topas Academic v4.1 software. The data inserted in Table 2 confirmed highest crystallite size for CoMn0.2Fe1.8O4. The lattice parameters for the powders calcined in air at 900 C were calculated using the linear multiple regressions based on the Least Squares Procedure with a Si standard powder correction. The cation distribution of the spinel ferrite systems was estimated from XRD data. The best result for the cationic distribution was estimated to agree with both theoretical and experimental lattice parameters (Table 1). The proposed cation distribution for CoFe2O4 ferrite indicates that the Co2þ ions have preference for the B-sites. This corresponds to a mixed spinel in agreement with previously reported Mössbauer studies [16]. Cr3þ and Mn3þ have a strong tendency to occupy the octahedral site [19,20] and for binary ferrites, the cation distribution can be represented by the formula:
3þ Co2þ x Fe1x
A h
3þ 3þ Co2þ 1x Mey Fe2xy
iB
O2 4
The lattice parameter (aexp) of Cr-substituted Co ferrite was expected to be higher than that of Mn-substituted Co ferrite due to
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Table 1 The values of crystallographic parameters evaluated from X-ray diffraction patterns. The accuracy is 0.002 Å. X-ray density (g cm3)
Lattice constant (Å)
Bond length (Å)
Site radii (Å)
Oxygen parameter (Å)
Samples
dx
aexp
ath
RA
RB
ra
rb
uexp
uth
RAE
RBE
RBEU
CoFe2O4 CoMn0.2Fe1.8O4 CoCr0.2Fe1.8O4
5.218 5.304 5.296
8.379 8.397 8.370
8.410 8.408 8.403
1.874 1.892 1.885
2.072 2.062 2.063
0.494 0.512 0.505
0.692 0.682 0.683
0.379 0.380 0.380
0.378 0.379 0.379
3.057 3.087 3.077
2.867 2.850 2.840
2.963 2.969 2960
the smaller ionic radius of the substituting octahedral Cr3þ (0.615 Å) as compared to Mn3þ (0.645 Å) and Fe3þ radii (0.645 Å) [21]. The higher value of the lattice parameter for Mn substituted Co ferrite could be explained by migration of Co2þ cation from octahedral (0.580 Å) to tetrahedral (0.745 Å) sites [22]. The theoretical lattice parameter (ath) was calculated according to the equation:
ath ¼
pffiffiffi 8 pffiffiffi ðrA þ R0 Þ þ 3ðrB þ R0 Þ 3 3
The theoretical density (dx) was calculated according to the relation: dx ¼ 8M=ðNA a3exp Þ where NA is the Avogadro’s number and M is the molecular weight of the sample. The change of dx can be attributed to the atomic weight and the radii of constituent cations. Therefore, the shift of reflections towards higher angle values, the lattice parameter and theoretical density confirm that Cr3þ and Mn3þ cations, respectively, have replaced the Fe3þ cations at octahedral B-sites into spinel cubic structure of the Co-host ferrite [23]. Using the relation [24]:
Edge lengths (Å)
Infrared absorption (FTIR) spectra were recorded after each calcination step. In Fig. 2a) and b) are the spectra for the precipitates and calcined powders at 900 C in air for 5 h. In the FTIR spectra, of the precipitated powders, the vibrational bands of the spinel phase and some bands corresponding to the water and secondary phase are observed. The spectra of the calcined powders exhibit two vibrational bands, the higher frequency band located around 600 cm1 (n1) and the lower frequency band ranged around 400 cm1 (n2). The n1 is assigned to the intrinsic vibrations of metaleanion complexes at the tetrahedral sites (MtheO) and n2 corresponds to octahedral metal stretching (MoheO) of the spinel lattice [29]. MtheO and MoheO correspond to the tetrahedral and octahedral metal-oxygen vibrations of the spinel lattice respectively. The bands n1 and n2 slightly shift to lower frequency for doped samples comparing with corresponding Co-host ferrite
pffiffiffi rA ¼ ðu 1=4Þaexp 3 R0 it was calculated the oxygen atomic co-ordinate “u”, where R0 ¼ 1.38 Å is the radius of oxygen ion and “a” corresponds to the experimental value of the lattice parameter (Table 2). The decrease of rB values (Table 1) for doped ferrites can be explained in terms of Co2þ migration from the octahedral [B]- to the tetrahedral (A) - sites [25]. The oxygen atomic co-ordinate u for all the samples is slightly different to that of the ideal spinel ferrite (0.375) due to synthesis conditions [26]. Cationeoxygen bond distance for tetrahedral (RA) and octahedral (RB) sites, tetrahedral edge length (RAE), shared (RBE) and unshared octahedral edge lengths (RBEU) were computed according with Otero Arean et al. [27]. The increase in the cation-oxygen bond length RA for doped samples can be attributed to the substitution processes due to the displacement of Fe3þ ions from the B-sites to the A-sites. The introduction of larger ionic radius Co2þ (0.745 Å) compared to Fe3þ (0.490 Å) at the tetrahedral sites for chromite and manganese ferrites increases the tetrahedral edge lengths RAE of the both samples compared to that of CoFe2O4. The values of RAE, RBE and RBEU are common for the ferrospinel systems with distributed cations [27,28].
Table 2 Crystallite size, magnetic properties and Curie temperature of calcined powders.
Cation distribution (Co0.05Fe0.95)A [Co0.95Fe1.05)]B (Co0.25Fe0.75)A [Co0.75Fe1.05Mn0.2)]B (Co0.17Fe0.83)A [Co0.83Fe0.97Cr0.2)]B
Crystallite size D (nm)
Ms (emu g1)
Mr (emu g1)
Hc (Oe)
T ( C)
41
56
14
167
356
79
60
23
552
407
44
59
32
783
387
Fig. 2. FTIR spectra of the powders: a) as synthetized and b) calcined at 900 C for 5 h in air.
A.M. Cojocariu et al. / Materials Chemistry and Physics 135 (2012) 728e732
frequencies. The change in band position may be due to the cationanion distances at the octahedral and tetrahedral lattice sites [30]. The SEM micrographs shown in Fig. 3 suggest that nanoparticles are agglomerated and have polygonal faced surfaces. It is observed that the average particle size of the CoMn0,2Fe1,8O4 and CoCr0,2Fe1,8O4 samples is lower than that of CoFe2O4 sample in correlation with the values obtained for the coercive field during the magnetic measurement. Magnetic hysteresis loops at room temperature of as synthetized and calcined samples are shown in Fig. 4. The coercive field (Hc), saturation magnetization (Ms) and remnant (Mr) values for undoped and doped ferrite powders calcined at 900 C for 5 h in air are summarized in Table 2. Comparing the value obtained for undoped cobalt ferrite with the values obtained for manganese and chromium doped ferrites a small increase of the saturation magnetization was observed. The net magnetization of a ferrimagnetic material depends on cation distribution among octahedral and tetrahedral sublattice. According to molecular field theory, the net magnetization at temperature T can be written as:
MðTÞ ¼ MB ðTÞ MA ðTÞ where, MB(T) and MA(T) are the magnetization of B and A sublattices, respectively at temperature T [31].
731
The net magnetic moment per ion for Co2þ, Mn3þ, Cr3þ and Fe3þ are 3mB, 4 mB, 3mB and 5mB. From spectroscopic measurements, it was found that Co2þ, Mn3þ, Cr3þ ions have strong site preference for octahedral sites while Fe3þ have no particular preference for either coordination. For CoMn0,2Fe1,8O4 the small increase of saturation magnetization can be explained considering the oxidation state of the manganese ions. The Mn2þ (5mB) ions have no particular preference for octahedral or tetrahedral sites. If the amount of Mn2þ ions from octahedral sites exceeds that of Mn3þ, a large increase in the magnetization is expected, whereas if the amount of Mn3þ ions is larger, the increase in the magnetic moment will be smaller [15,32]. The saturation magnetization value obtained for CoCr0,2Fe1,8O4 is similar to that obtained for CoMn0,2Fe1,8O4. From cation distribution obtained from XRD analysis, one can see that for CoMn0,2Fe1,8O4 and CoCr0,2Fe1,8O4, some cobalt ions from octahedral migrate to tetrahedral sites. In cobalt ferrite the magnetic anisotropy predominantly arises from the single-ion anisotropy of Co2þ on the B sites, and the displacement of these ions should lead to a decrease of magnetic anisotropy and consequently to the decrease of the coercive field. In this study the values of Hc obtained for manganese substituted cobalt ferrite and chromium substituted cobalt ferrite are higher than that of CoFe2O4. A
Fig. 3. SEM micrographs of the calcined powders.
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of Hc obtained for CoMn0.2Fe1.8O4 manganese substituted cobalt ferrite and chromium substituted cobalt ferrite are higher than that of CoFe2O4 in correlation with the average grain size suggested by micrographs. Acknowledgments This work was supported by the European Social Fund in Romania, under the responsibility of the Managing Authority for the Sectoral Operational Programme for Human Resources Development 2007-2013 [grant POSDRU/107/1.5/S/78342 and POSDRU/ 89/1.5/S/49944]. The authors are thankful to Prof. D. Creanga and Prof. M. Palamaru for their kind support during sample preparation and to Mrs. Slatineanu for useful discussion concerning cation distribution. Special thanks to Robert Lowndes from National Institute for Material Physics for useful comments and English spelling check. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] Fig. 4. Magnetic hysteresis loops of the powders: a) as synthetized and b) calcined at 900 C for 5 h in air.
[17] [18] [19]
possible explanation of the increase of the coercive field for substituted ferrites could be the lower value of the average particle size comparing with that of stoichiometric cobalt ferrite. With the reduction of the particle size, the surface to volume ratio increases and surface effects may induce a spin disorder in the surface layer as well as an enhanced surface anisotropy [27,33].
[23] [24]
4. Conclusions
[25]
Mn and Cr substituted cobalt ferrite were prepared by chemical co-precipitation method to study the influence of these substitutions on the microstructure and magnetic properties. The XRD patterns confirmed the formation of the spinel phase after thermal treatment at 900 C. In substituted ferrite some cobalt ions from the octahedral site migrated to the tetrahedral sites. The crystallite size ranges from 44 to 79 nm, manganese substitution favoring higher crystallite size. Magnetic measurements show that manganese substitution for iron, increased the saturation magnetization, which can be explained by the oxidation state of the manganese ions. The values
[20] [21] [22]
[26] [27] [28] [29] [30] [31] [32] [33]
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