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Fe molar ratio
Journal of Magnetism and Magnetic Materials 394 (2015) 111–116
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Structural and magnetic properties of CoxFe3 xO4 versus Co/Fe molar ratio Thomas Dippong a,n, Erika Andrea Levei b, Lucian Diamandescu c, Ion Bibicu c, Cristian Leostean d, Gheorghe Borodi d, Lucian Barbu Tudoran e a Technical University of Cluj-Napoca, North University Center of Baia Mare, Department of Chemistry and Biology, 76 Victoriei Street, 430122 Baia Mare, Romania b INCDO-INOE 2000, Research Institute for Analytical Instrumentation, 67 Donath Street, 400293 Cluj-Napoca, Romania c National Institute of Materials Physics, 105bis Atomistilor Street, 077125 Magurele, Romania d National Institute for Research and Development of Isotopic and Molecular Technologies, 65-103 Donath Street, 400293 Cluj-Napoca, Romania e Babes-Bolyai University, Faculty of Biology and Geology, 44 Bilascu Street, 400015 Cluj-Napoca, Romania
art ic l e i nf o
a b s t r a c t
Article history: Received 19 February 2015 Received in revised form 15 June 2015 Accepted 20 June 2015 Available online 23 June 2015
CoxFe3 xO4 (x ¼0.5–2.5) magnetic nanoparticles were synthesized via redox reaction between cobalt nitrate, iron nitrate and 1-4-butanediol using five Co/Fe molar ratios, followed by calcination at 1000 °C. Single phase nanoscaled cobalt ferrite was obtained at x ¼1.0 and at slight Co excess (x ¼1.5), while at high Co/Fe molar ratios (x ¼ 2.0 and x ¼2.5) the prevailing phase was CoO accompanied by CoFe2O4 traces. The highest values of coercive field and saturation magnetization were obtained for the sample at x ¼ 1.0, while the lowest values were obtained in the sample with the highest Co excess (x ¼2.5). The results indicated that the used synthesis route was suitable for the synthesis of cobalt ferrite with moderate saturation magnetization and high coercive field values. & 2015 Elsevier B.V. All rights reserved.
Keywords: Cobalt ferrite nanoparticles Magnetic properties X-ray diffraction analysis Mössbauer spectroscopy
1. Introduction Ferrite nanoparticles are important multifunctional materials with unique optical, electrical and magnetic properties, high physical and chemical stability, used in a wide range of applications: catalysis, electronics, ferrofluids technology, magnetocaloric cooling, magnetic recording, sensors, biology and medicine (drug delivery, diagnostic or therapeutic agents) [1–12]. There are several synthesis techniques for the ferrite nanoparticles: microemulsion [3], thermal or hydrothermal [5,8], self-combustion [13], coprecipitation [9,14], solvothermal [15] methods, etc. The obtained precursors are subjected to different thermal treatments in order to develop the crystalline phase. As both the synthesis and the heating methods strongly influence the particle size and thus the ferrite properties, the synthesis route selection is of great importance [15,16]. Moreover, as metal-ferrites prepared in the classical approach can deviate from the stoichiometric composition, the range over which single phase structure is obtained and how deviation from stoichiometry affects the ferrite properties is of great importance [17]. n
Corresponding author. E-mail addresses: [email protected] (T. Dippong), [email protected] (E.A. Levei), bibicu@infim.ro (I. Bibicu), [email protected] (G. Borodi), [email protected] (L. Barbu Tudoran). http://dx.doi.org/10.1016/j.jmmm.2015.06.055 0304-8853/& 2015 Elsevier B.V. All rights reserved.
Considering its high magnetocrystalline anisotropy, moderate saturation magnetization, high coercivity, chemical stability and mechanical hardness, cobalt ferrite (CoFe2O4) spinel is a promising material for various commercial applications [18,19]. Its magnetic properties are mainly determined by the Co2 þ , and thus the tuning of its magnetic properties by adjusting the Co/Fe ratio become possible [20]. By thermal treatment, the CoxFe3 xO4 is slowly transformed in two iron-rich and cobalt-rich spinel phases [21]. The paramagnetic behavior of CoxFe3 xO4 (x¼ 0.5–1.5) synthesized by redox reaction between Co and Fe nitrates and 1.2etanediol were higher than in case of 1,2-propanediolul and 1,3propanediol, respectively [22–25]. The aim of the present paper was to study the influence of Co/Fe ratio on the phase formation and the corresponding changes of structural and magnetic properties of CoxFe3 xO4 (x¼0.5–2.5) synthesized via redox reaction between cobalt nitrate, iron nitrate and 1-4-butanediol using five Co/Fe molar ratio and calcination at 1000 °C. The Fe and Co content of the nanoparticles were determined by inductively coupled plasma optical emission spectrometry, the formation of the oxide phases was studied by X-ray diffraction (XRD) and Fourier transformed infrared (FT-IR) spectroscopy, while transmission electron microscopy (TEM) was used to determine the nanoparticles shape and clustering. The magnetic properties obtained by Mössbauer spectroscopy were correlated with the XRD data.
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2. Materials and methods The CoxFe3 xO4 system was obtained through a redox reaction between iron nitrate (Fe(NO3)3 9H2O), cobalt nitrate (Co(NO3)2 6H2O) and 1,4-butanediol using a 1/1 nitrate to diol molar ratio and variable Fe/Co molar ratios as shown in Table 1. The formed “carboxylate compounds” were calcined at 1000 °C for 5 h. The presence of the oxidic phases was studied in 1% sample in KBr pellets using the Spectrum BX II (Perkin Elmer) FT-IR Spectrometer. The XRD measurements were performed at room temperature using a XRD-6000 (Shimadzu) X-ray diffractometer operated at 40 kV and 30 mA with graphite monochromator and CuKα radiation (λ ¼1.54178 Å). The Co and Fe contents of the calcined precursors were determined using a 3500 Optima DV (Perkin Elmer) inductively coupled plasma optical emission spectrometer after digestion of 50 mg sample with 21 ml aqua regia (7 ml HNO3 and 21 ml HCl) and dilution in 100 ml ultrapure water. The 57Fe Mössbauer spectra were recorded in transmission geometry at room temperature with a 10 mCi 57Co-Rh source and a conventional constant-acceleration spectrometer (AME-50 Elscint) equipped with CMCA-550 acquisition module. The velocity range was calibrated with α-Fe standard foil. Lorentzian line shapes were used to fit the recorded Mössbauer spectra. All isomer shifts are given relative to that of α-Fe at room temperature. The magnetic measurements were performed with a vibrating sample magnetometer (Cryogenic Limited). The nanoparticles shape and clustering was determined using a JEM1010 (Jeol) TEM equipped with digital image recording system (Mega View III CCD camera, 1.2Mp-Olympus Soft Imaging System, DE), and photographic film image recording system with high resolution scanner (Imacon Flextight X5, 8.000 DPI- Hasselblad).
Table 1 Characteristics of the synthesized precursors. Sample code
x
Co/Fe molar ratio
Nitrate/1.4-butanediol molar ratio
T0.5 T1.0 T1.5 T2.0 T2.5
0.5 1.0 1.5 2.0 2.5
1:5 1:2 1:1 2:1 5:1
1:1 1:1 1:1 1:1 1:1
Fig. 1. X-ray diffraction pattern for T0.5, T1.0, T1.5, T2.0, T2.5.
Table 2 Average crystallites sizes calculated using Scherrer equation.
3. Results and discussion The XRD patterns of CoxFe3 xO4 (x ¼0.5–2.5) are presented in Fig. 1 (T0.5–T2.5). In T0.5 two phases were identified: the crystallized cubic CoFe2O4 spinel (JCPDS file no. 22-1086) and the α-Fe2O3 (JCPDS no. 33-0664), due to iron excess in the composition [20,26]. In case of samples with stoichiometric Fe/Co ratio (T1.0) and slight Co excess (T1.5) the formation of crystallized cubic CoFe2O4 as single phase was observed. In T2.0, due to the higher Co content, CoO (JCPDS file no. 71-1178) peaks appear, while the CoFe2O4 peaks are less intense then in T1.0 and T1.5. The presence of CoO is a consequence of the higher Co excess compared to T1.5 appearing by the reduction of Co3O4 at 950 °C [17, 25,26]. Due to the high Co excess, in T2.5 along the intense peaks of the CoO appear some broad peaks that could be attributed to CoO and CoFe2O4 doublets, with lower intensity than in T2.0. The well-defined peaks of CoO indicate the high crystallization degree following thermal treatment at 1000 °C. In conclusion, the intensity of the cobalt ferrite lines decrease from T1.0 to T2.5, while the CoO lines reflection intensity increase, according to Co/Fe stoichiometric ratio. The crystallites size was calculated using Scherrer equation [27]: D¼ 0.9λ/β cos θ, where λ is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM) after subtracting the instrumental line broadening and θ is the Bragg's angle. The average crystallites size for the samples T0.5–T2.5 is shown in Table 2. The FT-IR spectra (Fig. 2) shows at 500–600 cm 1 an asymmetric peak in T0.5, T1.0 and T1.5 attributed to CoFe2O4, while in T2.0 the peak is wider and was considered to be the overlapping of spinel and CoO peaks [28]. The peak at 460 cm 1 that appears in T0.5 is characteristic for the Fe–O bond vibration in α-Fe2O3, in
Crystallite size [nm] Sample
CoFe2O4
α-Fe2O3
CoO
T0.5 T1.0 T1.5 T2.0 T2.5 Errors
52 73 56 32 30 7 1.5
59 – – – – 71.5
– – – 46 31 71.5
accord with XRD data. In the spectrum of T2.5 two characteristic bands for CoO appears at 660 cm 1 and 570 cm 1 [10,29]. The obtained results are in good agreement with the XRD data. The determined Fe and Co contents of T0.5–T2.5 (Table 3) shows the increase of the Co content from T0.5 to T2.5, the obtained x values being near the nominal values. Fig. 3 shows the Mössbauer spectra of T0.5–T2.5 samples calcined at 1000 °C, together with the computer fit (continuous lines). These are very similar to the spectra of cobalt ferrites obtained by other methods described in literature [21,30]. The Mössbauer spectrum of T0.5 indicates the presence of hematite (α-Fe2O3) in addition to the spinellic phase. The two subspectra corresponding to Fe3 þ in tetrahedral (A) and octahedral coordination (B) in the spectrum of T1.0 confirms the formation of cobalt ferrite, and the Mössbauer fit parameters are close to the bulk values. Similar results for cobalt ferrite were obtained by Feder et al. [31]. At higher x values, Mössbauer spectra show an increase in the line width and some “kinks” because each subspectrum of the spinel phase is a combination of two or three sextets. The hyperfine field at Mössbauer nucleus (H) in both A and B sites gradually decrease with the increase of cobalt concentration as a result of cobalt substituting iron process
T. Dippong et al. / Journal of Magnetism and Magnetic Materials 394 (2015) 111–116
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Fig. 2. FT-IR spectra of T0.5, T1.0, T1.5, T2.0 and T2.5.
in the spinel structure. An additional doublet with increasing intensity is present in case of T2.0 and T2.5. The Mössbauer parameters of the quadrupole doublet are close to those reported by Le Trong et al. [21]. The quadrupole doublet indicates the presence of paramagnetic ferrite. It is known that Curie temperature (Tc) of the CoxFe3 xO4 decreases with the increase of the cobalt content [21]. Mössbauer parameters obtained by fitting the spectra with Lorentzian line shape are given in Table 4. The TEM images for samples T0.5–T2.5 calcined at 1000 °C are presented in Fig. 4. In each case, the obtained nanoparticles are spherical with diameter less than 100 nm and are clustered in agglomerates that may overlap. In T0.5 and T1.0, the nanoparticles are clustered in spherical chains that sometimes overlap, in T1.5 the nanoparticles are clustered in isolated spherical shapes while in T2.0 and T2.5 the nanoparticles are randomly aggregated in wider chains. The ferrite nanoparticles dimensions vary in function of the Co/Fe ratio. The magnetic behavior of the samples is similar to that of hard magnetic ferrites with broad hysteresis cycles and coercive field of hundreds Oersted, due to the high anisotropy of Co ions. Generally, the samples magnetization measured versus magnetic field depends on the calcination temperature that also influences the nanoparticles dimension and the nature of the formed crystalline phases. Fig. 5 displays the hysteresis curves of the CoxFe3 xO4 (x¼ 0.5– 2.5) system and the corresponding magnetization derivative, while the saturation magnetization and the coercive field values together with the crystalline phases present in the system are shown in Table 5.
Fig. 3. Mössbauer spectra of T0.5, T1.0, T1.5, T2.0 and T2.5.
The shape of the magnetization curve is also influenced by the presence of secondary phase along the cobalt ferrite. In T1.5, T2.0 and T2.5 the magnetic effect is determined only by cobalt ferrite, since the cobalt oxide does not have magnetic properties. Sample T1.0 presents the highest saturation magnetization and the highest coercive field due to the formation of CoFe2O4. In sample T0.5 the saturation magnetization decreases as a consequence of the α-Fe2O3 presence in the sample. Due to the comparable evolution of the crystalline phases and presence of CoO without magnetic properties, the magnetic behavior of T1.5 and T2.0 is similar. The lowest value of saturation magnetization was obtained from T2.5 because CoO is the main phase and CoFe2O4 is found in traces, as confirmed by the XRD data and Mössbauer measurements. The two coupled peaks of the magnetization derivative indicate the existence of two magnetic phases in T0.5, while in T1.0 the broad peak indicates that the magnetic phases are fully coupled. The saturation magnetization value of 76.3 emu/g measured for CoFe2O4 (T1.0) is comparable with that obtained by other synthesis
Table 3 Fe and Co content of CoxFe3 xO4. Sample
T0.5 T1.0 T1.5 T2.0 T2.5
Fe [mg/kg]
509,600 480,400 373,800 238,000 123,100
Co [mg/kg]
98,100 234,400 395,400 526,400 657,000
Fe [mol/kg]
9125 8602 6694 4262 2204
Co [mol/kg]
1665 3977 6709 8932 11,148
x
Co/Fe ratio Measured
Theoretical
Measured
Theoretical
0.18 0.46 1.00 2.09 5.06
0.2 0.5 1 2 5
0.46 0.95 1.50 2.03 2.50
0.5 1 1.5 2 2.5
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methods [32]. The CoFe2O4 coercive field value (1.078 kOe) is higher than the bulk ferrite's (980 Oe) [33]. Table 4 Mössbauer parameters of CoxFe3 xO4. Sample
H [kOe]
Δ [mm/s]
IS [mm/s]
A [%]
T0.5
525.9 493.1 512.8 514.8 493.0 489.1 468.3 446.0 390.9 492.0 471.1 447.4 415.3 379.0 – 473.0 445.5 415.1 380.1 320.5 – 7 2.5
0.09 0.05 0.12 0.01 0.02 0.00 0.02 0.02 0.04 0.02 0.01 0.01 0.02 0.04 0.50 0.05 0.01 0.02 0.01 0.06 0.53 7 0.04
0.50 0.28 0.35 0.37 0.28 0.30 0.28 0.33 0.25 0.29 0.30 0.32 0.30 0.28 0.34 0.32 0.31 0.33 0.34 0.23 0.33 7 0.02
16.2 31.9 51. 9 40.1 59.9 32.5 28.6 32.2 06.7 11.6 34.3 28.2 08.5 08.4 09.0 16.4 19.8 12.7 10.9 10.2 30.0 7 2.5
T1.0 T1.5
T2.0
T2.5
Errors
H-hyperfine fields; Δ-quadruple split; IS- isomer shift; A- absorption area.
4. Conclusions Magnetic nanoparticles of CoxFe3 xO4 (x ¼0.5–2.5) were synthesized by redox reaction calcination at 1000 °C, using variable Co/Fe ratio. For x ¼1.0 and x ¼1.5 CoFe2O4 was obtained as single phase, while α-Fe2O3 for x¼ 0.5 and CoO for x ¼2 and x ¼2.5 appear as secondary phases along the cobalt ferrite. The nanoparticles were spherical, clustered in chains, isolated or dispersed with sizes in the range of 30–73 nm. The Mössbauer parameters revealed the peculiarities of magnetic phases and evidenced the presence of paramagnetic phases for x ¼2 and x ¼2.5. The magnetic measurements showed that for x ¼1.0 and x ¼1.5 this synthesis route is suitable for obtaining cobalt ferrite with moderate saturation magnetization and high coercive field values. For x¼ 1.5–2.5 the low values of the coercive field and saturation magnetization are a consequence of the excess of Co distributed in CoFe2O4 and CoO as well.
Acknowledgments The authors from National Institute of Materials Physics thank for financial support from the National Authority for Scientific Research (ANCS) (Core Program contract PN09-45).
Fig. 4. TEM images of T0.5, T1.0, T1.5, T2.0 and T2.5.
T. Dippong et al. / Journal of Magnetism and Magnetic Materials 394 (2015) 111–116
Fig. 5. Magnetic hysteresis loop and magnetization derivative for T0.5, T1.0, T1.5, T2.0, T2.5.
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Table 5 The coercive field and saturation magnetization values of T0.5–T2.5 series. Sample
HC [kOe]
rS [uem/g]
Phases content
T0.5 T1.0 T1.5 T2.0 T2.5
0.850 1.078 0.080 0.110 0.017
40.8 76.3 53.1 32.8 12.7
CoFe2O4 þα-Fe2O3 CoFe2O4 CoFe2O4 CoFe2O4 þCoO CoFe2O4 þCoO
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Structural and magnetic properties of CoxFe3 xO4 versus Co/Fe molar ratio Thomas Dippong a,n, Erika Andrea Levei b, Lucian Diamandescu c, Ion Bibicu c, Cristian Leostean d, Gheorghe Borodi d, Lucian Barbu Tudoran e a Technical University of Cluj-Napoca, North University Center of Baia Mare, Department of Chemistry and Biology, 76 Victoriei Street, 430122 Baia Mare, Romania b INCDO-INOE 2000, Research Institute for Analytical Instrumentation, 67 Donath Street, 400293 Cluj-Napoca, Romania c National Institute of Materials Physics, 105bis Atomistilor Street, 077125 Magurele, Romania d National Institute for Research and Development of Isotopic and Molecular Technologies, 65-103 Donath Street, 400293 Cluj-Napoca, Romania e Babes-Bolyai University, Faculty of Biology and Geology, 44 Bilascu Street, 400015 Cluj-Napoca, Romania
art ic l e i nf o
a b s t r a c t
Article history: Received 19 February 2015 Received in revised form 15 June 2015 Accepted 20 June 2015 Available online 23 June 2015
CoxFe3 xO4 (x ¼0.5–2.5) magnetic nanoparticles were synthesized via redox reaction between cobalt nitrate, iron nitrate and 1-4-butanediol using five Co/Fe molar ratios, followed by calcination at 1000 °C. Single phase nanoscaled cobalt ferrite was obtained at x ¼1.0 and at slight Co excess (x ¼1.5), while at high Co/Fe molar ratios (x ¼ 2.0 and x ¼2.5) the prevailing phase was CoO accompanied by CoFe2O4 traces. The highest values of coercive field and saturation magnetization were obtained for the sample at x ¼ 1.0, while the lowest values were obtained in the sample with the highest Co excess (x ¼2.5). The results indicated that the used synthesis route was suitable for the synthesis of cobalt ferrite with moderate saturation magnetization and high coercive field values. & 2015 Elsevier B.V. All rights reserved.
Keywords: Cobalt ferrite nanoparticles Magnetic properties X-ray diffraction analysis Mössbauer spectroscopy
1. Introduction Ferrite nanoparticles are important multifunctional materials with unique optical, electrical and magnetic properties, high physical and chemical stability, used in a wide range of applications: catalysis, electronics, ferrofluids technology, magnetocaloric cooling, magnetic recording, sensors, biology and medicine (drug delivery, diagnostic or therapeutic agents) [1–12]. There are several synthesis techniques for the ferrite nanoparticles: microemulsion [3], thermal or hydrothermal [5,8], self-combustion [13], coprecipitation [9,14], solvothermal [15] methods, etc. The obtained precursors are subjected to different thermal treatments in order to develop the crystalline phase. As both the synthesis and the heating methods strongly influence the particle size and thus the ferrite properties, the synthesis route selection is of great importance [15,16]. Moreover, as metal-ferrites prepared in the classical approach can deviate from the stoichiometric composition, the range over which single phase structure is obtained and how deviation from stoichiometry affects the ferrite properties is of great importance [17]. n
Corresponding author. E-mail addresses: [email protected] (T. Dippong), [email protected] (E.A. Levei), bibicu@infim.ro (I. Bibicu), [email protected] (G. Borodi), [email protected] (L. Barbu Tudoran). http://dx.doi.org/10.1016/j.jmmm.2015.06.055 0304-8853/& 2015 Elsevier B.V. All rights reserved.
Considering its high magnetocrystalline anisotropy, moderate saturation magnetization, high coercivity, chemical stability and mechanical hardness, cobalt ferrite (CoFe2O4) spinel is a promising material for various commercial applications [18,19]. Its magnetic properties are mainly determined by the Co2 þ , and thus the tuning of its magnetic properties by adjusting the Co/Fe ratio become possible [20]. By thermal treatment, the CoxFe3 xO4 is slowly transformed in two iron-rich and cobalt-rich spinel phases [21]. The paramagnetic behavior of CoxFe3 xO4 (x¼ 0.5–1.5) synthesized by redox reaction between Co and Fe nitrates and 1.2etanediol were higher than in case of 1,2-propanediolul and 1,3propanediol, respectively [22–25]. The aim of the present paper was to study the influence of Co/Fe ratio on the phase formation and the corresponding changes of structural and magnetic properties of CoxFe3 xO4 (x¼0.5–2.5) synthesized via redox reaction between cobalt nitrate, iron nitrate and 1-4-butanediol using five Co/Fe molar ratio and calcination at 1000 °C. The Fe and Co content of the nanoparticles were determined by inductively coupled plasma optical emission spectrometry, the formation of the oxide phases was studied by X-ray diffraction (XRD) and Fourier transformed infrared (FT-IR) spectroscopy, while transmission electron microscopy (TEM) was used to determine the nanoparticles shape and clustering. The magnetic properties obtained by Mössbauer spectroscopy were correlated with the XRD data.
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2. Materials and methods The CoxFe3 xO4 system was obtained through a redox reaction between iron nitrate (Fe(NO3)3 9H2O), cobalt nitrate (Co(NO3)2 6H2O) and 1,4-butanediol using a 1/1 nitrate to diol molar ratio and variable Fe/Co molar ratios as shown in Table 1. The formed “carboxylate compounds” were calcined at 1000 °C for 5 h. The presence of the oxidic phases was studied in 1% sample in KBr pellets using the Spectrum BX II (Perkin Elmer) FT-IR Spectrometer. The XRD measurements were performed at room temperature using a XRD-6000 (Shimadzu) X-ray diffractometer operated at 40 kV and 30 mA with graphite monochromator and CuKα radiation (λ ¼1.54178 Å). The Co and Fe contents of the calcined precursors were determined using a 3500 Optima DV (Perkin Elmer) inductively coupled plasma optical emission spectrometer after digestion of 50 mg sample with 21 ml aqua regia (7 ml HNO3 and 21 ml HCl) and dilution in 100 ml ultrapure water. The 57Fe Mössbauer spectra were recorded in transmission geometry at room temperature with a 10 mCi 57Co-Rh source and a conventional constant-acceleration spectrometer (AME-50 Elscint) equipped with CMCA-550 acquisition module. The velocity range was calibrated with α-Fe standard foil. Lorentzian line shapes were used to fit the recorded Mössbauer spectra. All isomer shifts are given relative to that of α-Fe at room temperature. The magnetic measurements were performed with a vibrating sample magnetometer (Cryogenic Limited). The nanoparticles shape and clustering was determined using a JEM1010 (Jeol) TEM equipped with digital image recording system (Mega View III CCD camera, 1.2Mp-Olympus Soft Imaging System, DE), and photographic film image recording system with high resolution scanner (Imacon Flextight X5, 8.000 DPI- Hasselblad).
Table 1 Characteristics of the synthesized precursors. Sample code
x
Co/Fe molar ratio
Nitrate/1.4-butanediol molar ratio
T0.5 T1.0 T1.5 T2.0 T2.5
0.5 1.0 1.5 2.0 2.5
1:5 1:2 1:1 2:1 5:1
1:1 1:1 1:1 1:1 1:1
Fig. 1. X-ray diffraction pattern for T0.5, T1.0, T1.5, T2.0, T2.5.
Table 2 Average crystallites sizes calculated using Scherrer equation.
3. Results and discussion The XRD patterns of CoxFe3 xO4 (x ¼0.5–2.5) are presented in Fig. 1 (T0.5–T2.5). In T0.5 two phases were identified: the crystallized cubic CoFe2O4 spinel (JCPDS file no. 22-1086) and the α-Fe2O3 (JCPDS no. 33-0664), due to iron excess in the composition [20,26]. In case of samples with stoichiometric Fe/Co ratio (T1.0) and slight Co excess (T1.5) the formation of crystallized cubic CoFe2O4 as single phase was observed. In T2.0, due to the higher Co content, CoO (JCPDS file no. 71-1178) peaks appear, while the CoFe2O4 peaks are less intense then in T1.0 and T1.5. The presence of CoO is a consequence of the higher Co excess compared to T1.5 appearing by the reduction of Co3O4 at 950 °C [17, 25,26]. Due to the high Co excess, in T2.5 along the intense peaks of the CoO appear some broad peaks that could be attributed to CoO and CoFe2O4 doublets, with lower intensity than in T2.0. The well-defined peaks of CoO indicate the high crystallization degree following thermal treatment at 1000 °C. In conclusion, the intensity of the cobalt ferrite lines decrease from T1.0 to T2.5, while the CoO lines reflection intensity increase, according to Co/Fe stoichiometric ratio. The crystallites size was calculated using Scherrer equation [27]: D¼ 0.9λ/β cos θ, where λ is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM) after subtracting the instrumental line broadening and θ is the Bragg's angle. The average crystallites size for the samples T0.5–T2.5 is shown in Table 2. The FT-IR spectra (Fig. 2) shows at 500–600 cm 1 an asymmetric peak in T0.5, T1.0 and T1.5 attributed to CoFe2O4, while in T2.0 the peak is wider and was considered to be the overlapping of spinel and CoO peaks [28]. The peak at 460 cm 1 that appears in T0.5 is characteristic for the Fe–O bond vibration in α-Fe2O3, in
Crystallite size [nm] Sample
CoFe2O4
α-Fe2O3
CoO
T0.5 T1.0 T1.5 T2.0 T2.5 Errors
52 73 56 32 30 7 1.5
59 – – – – 71.5
– – – 46 31 71.5
accord with XRD data. In the spectrum of T2.5 two characteristic bands for CoO appears at 660 cm 1 and 570 cm 1 [10,29]. The obtained results are in good agreement with the XRD data. The determined Fe and Co contents of T0.5–T2.5 (Table 3) shows the increase of the Co content from T0.5 to T2.5, the obtained x values being near the nominal values. Fig. 3 shows the Mössbauer spectra of T0.5–T2.5 samples calcined at 1000 °C, together with the computer fit (continuous lines). These are very similar to the spectra of cobalt ferrites obtained by other methods described in literature [21,30]. The Mössbauer spectrum of T0.5 indicates the presence of hematite (α-Fe2O3) in addition to the spinellic phase. The two subspectra corresponding to Fe3 þ in tetrahedral (A) and octahedral coordination (B) in the spectrum of T1.0 confirms the formation of cobalt ferrite, and the Mössbauer fit parameters are close to the bulk values. Similar results for cobalt ferrite were obtained by Feder et al. [31]. At higher x values, Mössbauer spectra show an increase in the line width and some “kinks” because each subspectrum of the spinel phase is a combination of two or three sextets. The hyperfine field at Mössbauer nucleus (H) in both A and B sites gradually decrease with the increase of cobalt concentration as a result of cobalt substituting iron process
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Fig. 2. FT-IR spectra of T0.5, T1.0, T1.5, T2.0 and T2.5.
in the spinel structure. An additional doublet with increasing intensity is present in case of T2.0 and T2.5. The Mössbauer parameters of the quadrupole doublet are close to those reported by Le Trong et al. [21]. The quadrupole doublet indicates the presence of paramagnetic ferrite. It is known that Curie temperature (Tc) of the CoxFe3 xO4 decreases with the increase of the cobalt content [21]. Mössbauer parameters obtained by fitting the spectra with Lorentzian line shape are given in Table 4. The TEM images for samples T0.5–T2.5 calcined at 1000 °C are presented in Fig. 4. In each case, the obtained nanoparticles are spherical with diameter less than 100 nm and are clustered in agglomerates that may overlap. In T0.5 and T1.0, the nanoparticles are clustered in spherical chains that sometimes overlap, in T1.5 the nanoparticles are clustered in isolated spherical shapes while in T2.0 and T2.5 the nanoparticles are randomly aggregated in wider chains. The ferrite nanoparticles dimensions vary in function of the Co/Fe ratio. The magnetic behavior of the samples is similar to that of hard magnetic ferrites with broad hysteresis cycles and coercive field of hundreds Oersted, due to the high anisotropy of Co ions. Generally, the samples magnetization measured versus magnetic field depends on the calcination temperature that also influences the nanoparticles dimension and the nature of the formed crystalline phases. Fig. 5 displays the hysteresis curves of the CoxFe3 xO4 (x¼ 0.5– 2.5) system and the corresponding magnetization derivative, while the saturation magnetization and the coercive field values together with the crystalline phases present in the system are shown in Table 5.
Fig. 3. Mössbauer spectra of T0.5, T1.0, T1.5, T2.0 and T2.5.
The shape of the magnetization curve is also influenced by the presence of secondary phase along the cobalt ferrite. In T1.5, T2.0 and T2.5 the magnetic effect is determined only by cobalt ferrite, since the cobalt oxide does not have magnetic properties. Sample T1.0 presents the highest saturation magnetization and the highest coercive field due to the formation of CoFe2O4. In sample T0.5 the saturation magnetization decreases as a consequence of the α-Fe2O3 presence in the sample. Due to the comparable evolution of the crystalline phases and presence of CoO without magnetic properties, the magnetic behavior of T1.5 and T2.0 is similar. The lowest value of saturation magnetization was obtained from T2.5 because CoO is the main phase and CoFe2O4 is found in traces, as confirmed by the XRD data and Mössbauer measurements. The two coupled peaks of the magnetization derivative indicate the existence of two magnetic phases in T0.5, while in T1.0 the broad peak indicates that the magnetic phases are fully coupled. The saturation magnetization value of 76.3 emu/g measured for CoFe2O4 (T1.0) is comparable with that obtained by other synthesis
Table 3 Fe and Co content of CoxFe3 xO4. Sample
T0.5 T1.0 T1.5 T2.0 T2.5
Fe [mg/kg]
509,600 480,400 373,800 238,000 123,100
Co [mg/kg]
98,100 234,400 395,400 526,400 657,000
Fe [mol/kg]
9125 8602 6694 4262 2204
Co [mol/kg]
1665 3977 6709 8932 11,148
x
Co/Fe ratio Measured
Theoretical
Measured
Theoretical
0.18 0.46 1.00 2.09 5.06
0.2 0.5 1 2 5
0.46 0.95 1.50 2.03 2.50
0.5 1 1.5 2 2.5
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methods [32]. The CoFe2O4 coercive field value (1.078 kOe) is higher than the bulk ferrite's (980 Oe) [33]. Table 4 Mössbauer parameters of CoxFe3 xO4. Sample
H [kOe]
Δ [mm/s]
IS [mm/s]
A [%]
T0.5
525.9 493.1 512.8 514.8 493.0 489.1 468.3 446.0 390.9 492.0 471.1 447.4 415.3 379.0 – 473.0 445.5 415.1 380.1 320.5 – 7 2.5
0.09 0.05 0.12 0.01 0.02 0.00 0.02 0.02 0.04 0.02 0.01 0.01 0.02 0.04 0.50 0.05 0.01 0.02 0.01 0.06 0.53 7 0.04
0.50 0.28 0.35 0.37 0.28 0.30 0.28 0.33 0.25 0.29 0.30 0.32 0.30 0.28 0.34 0.32 0.31 0.33 0.34 0.23 0.33 7 0.02
16.2 31.9 51. 9 40.1 59.9 32.5 28.6 32.2 06.7 11.6 34.3 28.2 08.5 08.4 09.0 16.4 19.8 12.7 10.9 10.2 30.0 7 2.5
T1.0 T1.5
T2.0
T2.5
Errors
H-hyperfine fields; Δ-quadruple split; IS- isomer shift; A- absorption area.
4. Conclusions Magnetic nanoparticles of CoxFe3 xO4 (x ¼0.5–2.5) were synthesized by redox reaction calcination at 1000 °C, using variable Co/Fe ratio. For x ¼1.0 and x ¼1.5 CoFe2O4 was obtained as single phase, while α-Fe2O3 for x¼ 0.5 and CoO for x ¼2 and x ¼2.5 appear as secondary phases along the cobalt ferrite. The nanoparticles were spherical, clustered in chains, isolated or dispersed with sizes in the range of 30–73 nm. The Mössbauer parameters revealed the peculiarities of magnetic phases and evidenced the presence of paramagnetic phases for x ¼2 and x ¼2.5. The magnetic measurements showed that for x ¼1.0 and x ¼1.5 this synthesis route is suitable for obtaining cobalt ferrite with moderate saturation magnetization and high coercive field values. For x¼ 1.5–2.5 the low values of the coercive field and saturation magnetization are a consequence of the excess of Co distributed in CoFe2O4 and CoO as well.
Acknowledgments The authors from National Institute of Materials Physics thank for financial support from the National Authority for Scientific Research (ANCS) (Core Program contract PN09-45).
Fig. 4. TEM images of T0.5, T1.0, T1.5, T2.0 and T2.5.
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Fig. 5. Magnetic hysteresis loop and magnetization derivative for T0.5, T1.0, T1.5, T2.0, T2.5.
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Table 5 The coercive field and saturation magnetization values of T0.5–T2.5 series. Sample
HC [kOe]
rS [uem/g]
Phases content
T0.5 T1.0 T1.5 T2.0 T2.5
0.850 1.078 0.080 0.110 0.017
40.8 76.3 53.1 32.8 12.7
CoFe2O4 þα-Fe2O3 CoFe2O4 CoFe2O4 CoFe2O4 þCoO CoFe2O4 þCoO
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