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Magnetic properties of MnFe2O4 nano-aggregates dispersed in paraffin wax
Journal of Magnetism and Magnetic Materials 385 (2015) 308–312
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties of MnFe2O4 nano-aggregates dispersed in paraffin wax B. Aslibeiki a,n, P. Kameli b a b
Department of Solid State Physics, Faculty of Physics, University of Tabriz, Tabriz 51666-16471, Iran Department of Physics, Isfahan University of Technology, Isfahan 84156-83111, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 16 December 2014 Received in revised form 2 March 2015 Accepted 6 March 2015 Available online 9 March 2015
Manganese ferrite, MnFe2O4 nanoparticles with average size of ∼6.5 nm were synthesized by using a thermal decomposition method. The nanoparticles were aggregated which was confirmed by FESEM and TEM images. The aggregates with a diameter of ∼50 nm showed interacting superspin glass (SSG) behavior. The powders were dispersed in the molten paraffin wax by using ultrasonic bath. Samples with different paraffin/ferrite weight ratios of P/F ¼ 0, 1, 5, 10 and 20 were prepared. M–H curves of the samples revealed presence of superparamagnetic state at 300 K. Saturation magnetization (Ms) decreased from 26.6 to 1.3 emu/g by increasing the P/F value from 0 to 20, respectively. Furthermore, the VSM measurements showed a decrease in surface spin disorder of paraffin-embedded nanoparticles in comparison with bare particles. The AC magnetic susceptibility peak temperature, TP increased from 230 to 4300 K with increasing the paraffin content in the samples. The present study showed that by dispersing the particles in a non-magnetic matrix, the blocking temperature could be increased. & 2015 Elsevier B.V. All rights reserved.
Keywords: MnFe2O4 Surface spins Superparamagnetic AC susceptibility Paraffin wax
1. Introduction In the past decade, magnetic nanoparticles have been widely investigated because of their potential applications in industry and medicine ranging from microwave absorbance, data recording media and ferro-fluids to drug delivery, magnetic hyperthermia, MRI contrasting, etc. [1–5]. Application of magnetic nanoparticles depends on their coercive field, remanent and saturation magnetization, response to the external AC fields, relaxation times, and anisotropy. MnFe2O4 ferrite with spinel structure shows interesting features such as simple preparation methods, high saturation magnetization (Ms∼83 emu/g at 300 K), high chemical stability and low Curie temperature (Tc∼580 K) among spinel ferrites [6]. The MnFe2O4 nanoparticles show different properties as compared to corresponding bulk sample. The core of nanoparticles consists of almost regular atomic arrangement and shows bulk-like behavior. On the other hand, the shell has structural deficiencies and broken bands. Structural deficiencies lead to an amorphous atomic arrangement in the surface of nanoparticles. Magnetic properties of nanoparticles can change by embedding in a non-magnetic polymer. Interaction between polymer molecules and surface atoms of nanoparticles can change the surface n
Corresponding author. Fax: þ98 41 33341244. E-mail address: [email protected] (B. Aslibeiki).
http://dx.doi.org/10.1016/j.jmmm.2015.03.023 0304-8853/& 2015 Elsevier B.V. All rights reserved.
anisotropy, total anisotropy energy and consequently the coercivity and remanent magnetization [7,8]. Variation of these parameters can change the other properties of system such as blocking temperature and magnetic state. Herein, we present the structural and magnetic characterization of ∼6.5 nm MnFe2O4 nanoparticles embedded in paraffin wax. After structural characterizations, we studied the effect of paraffin matrix on magnetic properties of the ferrite samples.
2. Experimental 2.1. Synthesis MnFe2O4 nanoparticles were synthesized using a thermal decomposition method described elsewhere [9,10]. In a typical synthesis, manganese nitrate (Mn(NO3)2 4H2O, Merk, 99%), iron nitrate (Fe(NO3)3 H2O, Merk, 99%), and citric acid (C6H6O7, Merk, 99.5%) were mixed together by an equal molar ratio of total metal nitrates to citric acid. The powders were ball milled and then were annealed in ambient air at 350 °C for 1 h. The obtained black powders were MnFe2O4 nanoparticles. In the next step, the ferrite sample was dispersed in molten paraffin using an ultrasonic bath. Samples with different paraffin/ferrite weight (P/F) ratios of 0, 1, 5, 10 and 20 were prepared. The samples were named as P0, P1, P5, P10 and P20 according to P/F ratio from 0 to 20 respectively.
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2.2. Experimental techniques and data treatment X-ray diffraction (XRD) pattern of the samples was taken on a Philips X′Pert Pro MPD X-ray diffractometer with Cu-Kα radiation (λ ¼ 0.154 nm). Fourier transform infrared (FTIR) spectra were recorded using a Tensor 27 spectrometer within the range of 400–3500 cm 1. Nanostructural, morphology and elemental composition of the samples were studied using a transmission electron microscope (TEM) and a Hitachi Model S-4160 field emission scanning electron microscope (FESEM) equipped with energy-dispersive X-ray (EDX) spectroscopy. Magnetic hysteresis curves were recorded by a vibrating sample magnetometer (VSM) with a maximum field of 20 kOe. Dynamic magnetic susceptibility measurements were performed by an AC susceptometer (Lake Shore 7000). The measurements were carried out by cooling the sample from room temperature to 80 K in zero magnetic field and then the magnetic susceptibility was measured during the warming process in a magnetic field of 10 Oe and frequency of 333 Hz.
3. Results and discussion 3.1. Structural properties Fig. 1a shows Rietveld refinement of XRD data of the P0 sample. In Fig. 1a the red spheres represent experimental data; the black solid line is calculated values by using Rietveld method, and the blue line shows residuals. Results show that the calculated values are in good accordance with experimental data. Furthermore, there is no noticeable trace of secondary phases in the XRD pattern which confirms single spinel phase of the sample. For comparison, the XRD pattern and Rietveld refinement of bulk MnFe2O4 is presented in Fig. 1b. Average crystallite size and the lattice constant (a) of the MnFe2O4 sample were calculated using the following formula:
⟨D XRD ⟩ =
Kλ β cos θ
(1)
1 h2 + k 2 + l2 = d2 a2
(2)
In Eq. (1) which is called Debye–Scherrer’s formula, β is the full-width at half-maximum (FWHM) of the XRD peaks, θ is the Bragg angle, K is the Scherrer’s constant which is a dimensionless factor related to the shape of crystallites, with a value close to unity (∼0.9) and λ is the wavelength of the radiation. Eq. (2) in which the (h,k,l) are Miller indexes, is used for the cubic spinel structure. Using Scherrer’s formula the average crystallite size 〈D〉¼6.5( 70.5) nm was obtained. The obtained a ¼8.34( 70.03) Å using Eq. (2) was smaller than that of bulk MnFe2O4 (8.51 Å) [6]. Oxidation of Mn2 þ to Mn3 þ and different cation distributions of ferrite nanoparticles can decrease the lattice constant [11,12]. Cation distribution in bulk Mn-ferrite is demonstrated as Mn0.82 +Fe0.23 + Mn0.22 +Fe1.83+ , where A and B denote the
(
)A (
)B
tetrahedral and octahedral sites in spinel structure, respectively. Oxidation of Mn2 þ (0.81 Å) to Mn3 þ (0.72 Å) reduces the lattice parameter. Yang et al. obtained a ¼8.4 Å for ∼7.5 nm MnFe2O4 nanoparticles prepared by a modified co-precipitation method [11]. They justified this result by different cation distribution of nanopartices compared to bulk manganese ferrite. Similar results have been reported in the literature [13–14]. Fig. 2 shows FTIR spectra of the samples. The double peaks at 2919 cm 1 and 2850 cm 1 are assigned to the C–H symmetrical and asymmetrical stretching bonds. The absorption band at
Fig. 1. (a) XRD pattern of MnFe2O4 nanoparticles. Red spheres are experimental data, and the solid black line represents the Rietveld refinement. The blue line shows the residuals. (b) XRD pattern and Rietveld refinement of bulk MnFe2O4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1466 cm 1 is attributed to the C–H bending vibration. The peak at 724 cm 1 is in-plane rocking vibration of the CH2 group of paraffin. It is clear from FTIR spectra that, the intensity of C–H groups increases with increasing the paraffin content in the samples. There is a signature of two important bands below 1000 cm 1 in FTIR spectra. The peak at 560 cm 1 is attributed to the tetrahedral metal–oxygen (M–O)tet bonds and the peak around 452 cm 1 corresponds to vibration of octahedral metal–oxygen (M–O)oct bonds [15]. The presence of these peaks confirmed the formation of metal–oxygen bands in the tetrahedral and octahedral sublattices of the MnFe2O4 nanoparticles. Fig. 3 shows FESEM micrographs of the samples. From this figure, the P0 sample consists of large particles with average diameter of ∼50 nm. It seems that the large particles are aggregates of several smaller particles which was confirmed by TEM image. For P10 and P20 samples, there is no clear trace of ferrite particles in the FESEM images probably due to high wax concentration in these samples. TEM image of P0 sample is shown in Fig. 4a. Particles with size of about 6 nm is seen in the TEM image, which is in good accordance with the crystallite size obtained using Scherrer’s
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formula. Using the EDX analysis (Fig. 4b) the Fe/Mn atomic concentration ratio was obtained as 2.004, which was very close to the stoichiometric value (Fe/Mn¼2). In order to study the effect of wax on static and dynamic magnetic properties of the samples, magnetic hysteresis loops and AC susceptibility were measured. 3.2. Magnetic properties Static magnetic properties of samples were investigated by using a VSM. Fig. 5 shows the magnetic hysteresis (M–H) curves of samples at 300 K. From the inset in Fig. 5, all samples show zero coercivity and remanent magnetization which revealed presence of unblocked SPM particles in the samples. Also the saturation magnetization (Ms) decreases from 26.6 to 1.3 emu/g by increasing the P/F ratio from 0 to 20, respectively. Such a large increment in Ms is due to the non-magnetic nature of paraffin wax. The effect of paraffin on surface spin disorder was studied using high filed region fitting of M–H curves with the following formula: Fig. 2. FTIR spectra of samples. Dashed lines show metal–oxygen bonds in octahedral and tetrahedral sites of spinel structure.
M (H) = M0 + χd H where M0 is zero field saturation magnetization and
Fig. 3. FESEM images of ∼50 nm ferrite nano-aggregates dispersed in the paraffin matrix.
(3)
χd is
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Fig. 4. (a) TEM image and (b) the EDX spectra of MnFe2O4 nanoparticles (sample P0).
Fig. 5. Magnetic hysteresis (M–H) curve of samples at 300 K. The inset shows SPM behavior of nanoparticles.
high-field surface spins susceptibility [16]. Higher χd shows higher surface spin disorder. The normalized M–H curves at high fields are presented in Fig. 6a. Fig. 6b shows the χd vales versus P/F ratio. The χd decreased by increasing the paraffin content in the samples which suggested decrease of surface spin disorder. Oxygen vacancies, structural deficiencies and broken metal–oxygen bonds cause the magnetically disordered (canted spins) layer in the shell of nanoparticles. By dispersing the nanoparticles in the paraffin matrix, the covalent bonding between wax molecules and surface atoms of nanoparticles increase the magnetic order in surface of nanoparticles and consequently decrease the surface spin canting. Desautels et al. reported a significant reduction in intrinsic surface spin disorder in cu coated γ-Fe2O3 nanoparticles, compared with bare γ-Fe2O3 nanoparticles. They showed that interactions between the Fe and Cu atoms in the shell of Cu-coated γ-Fe2O3 nanoparticles alter the non-magnetic state of particles and stabilize the disordered surface spins [17,18]. In order to study the dynamic magnetic properties of samples, the AC magnetic susceptibility measurements were performed. Fig. 7 shows the temperature dependence real part of susceptibility (χ′) at frequency of 333 Hz and field of 10 Oe. The curves show a broad maximum centered at TP. Higher than TP, the nanoparticles show SPM behavior and below of this temperature, the
Fig. 6. (a) Slope of M–H curve at high fields. (b) Variation of χd versus P/F ratio.
blocked nanoparticles appear in the samples. From Fig. 8, it is clear that the peak temperature; TP shifts toward higher temperatures by decreasing the ferrite concentration in the samples. In fact, the bonding between surface atoms and paraffin molecules reduces the surface spins disorder and prevents the particles from becoming superparamagnetic until a higher temperature.
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Acknowledgments Authors thank Dr. Varvaro for his help in VSM measurements.
References
Fig. 7. Temperature dependant behavior of real part of AC susceptibility, χ′ of samples at f¼ 333 Hz and H¼ 10 Oe.
Fig. 8. The normalized χ′. Shift in TP with increasing the paraffin content in samples is clear in this figure.
4. Conclusion In summary, we have studied the magnetic properties of MnFe2O4 nano-aggregated dispersed in paraffin matrix. Samples showed superparamagnetism behavior at room temperature. By decreasing the ferrite concentration in the samples, blocking temperature increased and the saturation magnetization decreased. Bonding between paraffin molecules and nanoparticles surface atoms reduced the surface spins disorder.
[1] S. Rongxiang, Z. Yi, Y. Longgang, X. Haiyan, L. Qingfang, W. Jianbo, Static magnetic and microwave absorption properties of FeCo/Al2O3 composites synthesized by high-energy ball milling method, J. Phys. D: Appl. Phys. 47 (2014) 065001. [2] B. Warne, O.I. Kasyutich, E.L. Mayes, J.A. Wiggins, K.K. Wong, Self assembled nanoparticulate Co: Pt for data storage applications, IEEE Trans. Magn. 36 (2000) 3009–3011. [3] Q.A. Pankhurst, J. Connolly, S. Jones, J. Dobson, Applications of magnetic nanoparticles in biomedicine, J. Phys. D: Appl. Phys. 36 (2003) R167. [4] C. Sun, J.S. Lee, M. Zhang, Magnetic nanoparticles in MR imaging and drug delivery, Adv. Drug Deliv. Rev. 60 (2008) 1252–1265. [5] Y.V. Kolen’ko, M. Bañobre-López, C. Rodríguez-Abreu, E. Carbó-Argibay, A. Sailsman, Y. Piñeiro-Redondo, M.F. Cerqueira, D.Y. Petrovykh, K. Kovnir, O. I. Lebedev, Large-scale synthesis of colloidal Fe3O4 nanoparticles exhibiting high heating efficiency in magnetic hyperthermia, J. Phys. Chem. C 118 (2014) 8691–8701. [6] A. Goldman, Modern Ferrite Technology, Springer, New York, NY, USA, 2005. [7] D. Peddis, F. Orrù, A. Ardu, C. Cannas, A. Musinu, G. Piccaluga, Interparticle interactions and magnetic anisotropy in cobalt ferrite nanoparticles: influence of molecular coating, Chem. Mater. 24 (2012) 1062–1071. [8] S. Dutz, W. Andrä, R. Hergt, R. Müller, C. Oestreich, C. Schmidt, J. Töpfer, M. Zeisberger, M.E. Bellemann, Influence of dextran coating on the magnetic behaviour of iron oxide nanoparticles, J. Magn. Magn. Mater. 311 (2007) 51–54. [9] B. Aslibeiki, P. Kameli, H. Salamati, The effect of grinding on magnetic properties of agglomereted MnFe2O4 nanoparticles, J. Magn. Magn. Mater. 324 (2012) 154–160. [10] B. Aslibeiki, P. Kameli, H. Salamati, M. Eshraghi, T. Tahmasebi, Superspin glass state in MnFe2O4 nanoparticles, J. Magn. Magn. Mater. 322 (2010) 2929–2934. [11] A. Yang, C.N. Chinnasamy, J.M. Greneche, Y. Chen, S.D. Yoon, K. Hsu, C. Vittoria, V.G. Harris, Large tunability of Neel temperature by growth-rate-induced cation inversion in Mn-ferrite nanoparticles, Appl. Phys. Lett. 94 (2009) 113109. [12] P.J. van der Zaag, V.A.M. Brabers, M.T. Johnson, A. Noordermeer, P.F. Bongers, Comment on “Particle-size effects on the value of TC of MnFe2O4: evidence for finite-size scaling”, Phys. Rev. B 51 (1995) 12009–12011. [13] M. Akhtar, M. Younas, Structural and transport properties of nanocrystalline MnFe2O4 synthesized by co-precipitation method, Solid State Sci. 14 (2012) 1536–1542. [14] C. Chinnasamy, A. Yang, S. Yoon, K. Hsu, M. Shultz, E. Carpenter, S. Mukerjee, C. Vittoria, V. Harris, Size dependent magnetic properties and cation inversion in chemically synthesized MnFe 2O4 nanoparticles, J. Appl. Phys. 101 (2007) 09M509. [15] B. Aslibeiki, Nanostructural, magnetic and electrical properties of Ag doped Mn-ferrite nanoparticles, Curr. Appl. Phys. 14 (2014) 1659–1664. [16] P. Guardia, B. Batlle-Brugal, A.G. Roca, O. Iglesias, M.P. Morales, C.J. Serna, A. Labarta, X. Batlle, Surfactant effects in magnetite nanoparticles of controlled size, J. Magn. Magn. Mater. 316 (2007) e756–e759. [17] R.D. Desautels, E. Skoropata, Y.Y. Chen, H. Ouyang, J.W. Freeland, J. van Lierop, Tuning the surface magnetism of gamma-Fe2O3 nanoparticles with a Cu shell, Appl. Phys. Lett. 99 (2011) 262501–262504. [18] R.D. Desautels, E. Skoropata, Y.Y. Chen, H. Ouyang, J.W. Freeland, Jv Lierop, Increased surface spin stability in γ-Fe2O3 nanoparticles with a Cu shell, J. Phys.: Condens. Matter 24 (2012) 146001.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties of MnFe2O4 nano-aggregates dispersed in paraffin wax B. Aslibeiki a,n, P. Kameli b a b
Department of Solid State Physics, Faculty of Physics, University of Tabriz, Tabriz 51666-16471, Iran Department of Physics, Isfahan University of Technology, Isfahan 84156-83111, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 16 December 2014 Received in revised form 2 March 2015 Accepted 6 March 2015 Available online 9 March 2015
Manganese ferrite, MnFe2O4 nanoparticles with average size of ∼6.5 nm were synthesized by using a thermal decomposition method. The nanoparticles were aggregated which was confirmed by FESEM and TEM images. The aggregates with a diameter of ∼50 nm showed interacting superspin glass (SSG) behavior. The powders were dispersed in the molten paraffin wax by using ultrasonic bath. Samples with different paraffin/ferrite weight ratios of P/F ¼ 0, 1, 5, 10 and 20 were prepared. M–H curves of the samples revealed presence of superparamagnetic state at 300 K. Saturation magnetization (Ms) decreased from 26.6 to 1.3 emu/g by increasing the P/F value from 0 to 20, respectively. Furthermore, the VSM measurements showed a decrease in surface spin disorder of paraffin-embedded nanoparticles in comparison with bare particles. The AC magnetic susceptibility peak temperature, TP increased from 230 to 4300 K with increasing the paraffin content in the samples. The present study showed that by dispersing the particles in a non-magnetic matrix, the blocking temperature could be increased. & 2015 Elsevier B.V. All rights reserved.
Keywords: MnFe2O4 Surface spins Superparamagnetic AC susceptibility Paraffin wax
1. Introduction In the past decade, magnetic nanoparticles have been widely investigated because of their potential applications in industry and medicine ranging from microwave absorbance, data recording media and ferro-fluids to drug delivery, magnetic hyperthermia, MRI contrasting, etc. [1–5]. Application of magnetic nanoparticles depends on their coercive field, remanent and saturation magnetization, response to the external AC fields, relaxation times, and anisotropy. MnFe2O4 ferrite with spinel structure shows interesting features such as simple preparation methods, high saturation magnetization (Ms∼83 emu/g at 300 K), high chemical stability and low Curie temperature (Tc∼580 K) among spinel ferrites [6]. The MnFe2O4 nanoparticles show different properties as compared to corresponding bulk sample. The core of nanoparticles consists of almost regular atomic arrangement and shows bulk-like behavior. On the other hand, the shell has structural deficiencies and broken bands. Structural deficiencies lead to an amorphous atomic arrangement in the surface of nanoparticles. Magnetic properties of nanoparticles can change by embedding in a non-magnetic polymer. Interaction between polymer molecules and surface atoms of nanoparticles can change the surface n
Corresponding author. Fax: þ98 41 33341244. E-mail address: [email protected] (B. Aslibeiki).
http://dx.doi.org/10.1016/j.jmmm.2015.03.023 0304-8853/& 2015 Elsevier B.V. All rights reserved.
anisotropy, total anisotropy energy and consequently the coercivity and remanent magnetization [7,8]. Variation of these parameters can change the other properties of system such as blocking temperature and magnetic state. Herein, we present the structural and magnetic characterization of ∼6.5 nm MnFe2O4 nanoparticles embedded in paraffin wax. After structural characterizations, we studied the effect of paraffin matrix on magnetic properties of the ferrite samples.
2. Experimental 2.1. Synthesis MnFe2O4 nanoparticles were synthesized using a thermal decomposition method described elsewhere [9,10]. In a typical synthesis, manganese nitrate (Mn(NO3)2 4H2O, Merk, 99%), iron nitrate (Fe(NO3)3 H2O, Merk, 99%), and citric acid (C6H6O7, Merk, 99.5%) were mixed together by an equal molar ratio of total metal nitrates to citric acid. The powders were ball milled and then were annealed in ambient air at 350 °C for 1 h. The obtained black powders were MnFe2O4 nanoparticles. In the next step, the ferrite sample was dispersed in molten paraffin using an ultrasonic bath. Samples with different paraffin/ferrite weight (P/F) ratios of 0, 1, 5, 10 and 20 were prepared. The samples were named as P0, P1, P5, P10 and P20 according to P/F ratio from 0 to 20 respectively.
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2.2. Experimental techniques and data treatment X-ray diffraction (XRD) pattern of the samples was taken on a Philips X′Pert Pro MPD X-ray diffractometer with Cu-Kα radiation (λ ¼ 0.154 nm). Fourier transform infrared (FTIR) spectra were recorded using a Tensor 27 spectrometer within the range of 400–3500 cm 1. Nanostructural, morphology and elemental composition of the samples were studied using a transmission electron microscope (TEM) and a Hitachi Model S-4160 field emission scanning electron microscope (FESEM) equipped with energy-dispersive X-ray (EDX) spectroscopy. Magnetic hysteresis curves were recorded by a vibrating sample magnetometer (VSM) with a maximum field of 20 kOe. Dynamic magnetic susceptibility measurements were performed by an AC susceptometer (Lake Shore 7000). The measurements were carried out by cooling the sample from room temperature to 80 K in zero magnetic field and then the magnetic susceptibility was measured during the warming process in a magnetic field of 10 Oe and frequency of 333 Hz.
3. Results and discussion 3.1. Structural properties Fig. 1a shows Rietveld refinement of XRD data of the P0 sample. In Fig. 1a the red spheres represent experimental data; the black solid line is calculated values by using Rietveld method, and the blue line shows residuals. Results show that the calculated values are in good accordance with experimental data. Furthermore, there is no noticeable trace of secondary phases in the XRD pattern which confirms single spinel phase of the sample. For comparison, the XRD pattern and Rietveld refinement of bulk MnFe2O4 is presented in Fig. 1b. Average crystallite size and the lattice constant (a) of the MnFe2O4 sample were calculated using the following formula:
⟨D XRD ⟩ =
Kλ β cos θ
(1)
1 h2 + k 2 + l2 = d2 a2
(2)
In Eq. (1) which is called Debye–Scherrer’s formula, β is the full-width at half-maximum (FWHM) of the XRD peaks, θ is the Bragg angle, K is the Scherrer’s constant which is a dimensionless factor related to the shape of crystallites, with a value close to unity (∼0.9) and λ is the wavelength of the radiation. Eq. (2) in which the (h,k,l) are Miller indexes, is used for the cubic spinel structure. Using Scherrer’s formula the average crystallite size 〈D〉¼6.5( 70.5) nm was obtained. The obtained a ¼8.34( 70.03) Å using Eq. (2) was smaller than that of bulk MnFe2O4 (8.51 Å) [6]. Oxidation of Mn2 þ to Mn3 þ and different cation distributions of ferrite nanoparticles can decrease the lattice constant [11,12]. Cation distribution in bulk Mn-ferrite is demonstrated as Mn0.82 +Fe0.23 + Mn0.22 +Fe1.83+ , where A and B denote the
(
)A (
)B
tetrahedral and octahedral sites in spinel structure, respectively. Oxidation of Mn2 þ (0.81 Å) to Mn3 þ (0.72 Å) reduces the lattice parameter. Yang et al. obtained a ¼8.4 Å for ∼7.5 nm MnFe2O4 nanoparticles prepared by a modified co-precipitation method [11]. They justified this result by different cation distribution of nanopartices compared to bulk manganese ferrite. Similar results have been reported in the literature [13–14]. Fig. 2 shows FTIR spectra of the samples. The double peaks at 2919 cm 1 and 2850 cm 1 are assigned to the C–H symmetrical and asymmetrical stretching bonds. The absorption band at
Fig. 1. (a) XRD pattern of MnFe2O4 nanoparticles. Red spheres are experimental data, and the solid black line represents the Rietveld refinement. The blue line shows the residuals. (b) XRD pattern and Rietveld refinement of bulk MnFe2O4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1466 cm 1 is attributed to the C–H bending vibration. The peak at 724 cm 1 is in-plane rocking vibration of the CH2 group of paraffin. It is clear from FTIR spectra that, the intensity of C–H groups increases with increasing the paraffin content in the samples. There is a signature of two important bands below 1000 cm 1 in FTIR spectra. The peak at 560 cm 1 is attributed to the tetrahedral metal–oxygen (M–O)tet bonds and the peak around 452 cm 1 corresponds to vibration of octahedral metal–oxygen (M–O)oct bonds [15]. The presence of these peaks confirmed the formation of metal–oxygen bands in the tetrahedral and octahedral sublattices of the MnFe2O4 nanoparticles. Fig. 3 shows FESEM micrographs of the samples. From this figure, the P0 sample consists of large particles with average diameter of ∼50 nm. It seems that the large particles are aggregates of several smaller particles which was confirmed by TEM image. For P10 and P20 samples, there is no clear trace of ferrite particles in the FESEM images probably due to high wax concentration in these samples. TEM image of P0 sample is shown in Fig. 4a. Particles with size of about 6 nm is seen in the TEM image, which is in good accordance with the crystallite size obtained using Scherrer’s
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formula. Using the EDX analysis (Fig. 4b) the Fe/Mn atomic concentration ratio was obtained as 2.004, which was very close to the stoichiometric value (Fe/Mn¼2). In order to study the effect of wax on static and dynamic magnetic properties of the samples, magnetic hysteresis loops and AC susceptibility were measured. 3.2. Magnetic properties Static magnetic properties of samples were investigated by using a VSM. Fig. 5 shows the magnetic hysteresis (M–H) curves of samples at 300 K. From the inset in Fig. 5, all samples show zero coercivity and remanent magnetization which revealed presence of unblocked SPM particles in the samples. Also the saturation magnetization (Ms) decreases from 26.6 to 1.3 emu/g by increasing the P/F ratio from 0 to 20, respectively. Such a large increment in Ms is due to the non-magnetic nature of paraffin wax. The effect of paraffin on surface spin disorder was studied using high filed region fitting of M–H curves with the following formula: Fig. 2. FTIR spectra of samples. Dashed lines show metal–oxygen bonds in octahedral and tetrahedral sites of spinel structure.
M (H) = M0 + χd H where M0 is zero field saturation magnetization and
Fig. 3. FESEM images of ∼50 nm ferrite nano-aggregates dispersed in the paraffin matrix.
(3)
χd is
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Fig. 4. (a) TEM image and (b) the EDX spectra of MnFe2O4 nanoparticles (sample P0).
Fig. 5. Magnetic hysteresis (M–H) curve of samples at 300 K. The inset shows SPM behavior of nanoparticles.
high-field surface spins susceptibility [16]. Higher χd shows higher surface spin disorder. The normalized M–H curves at high fields are presented in Fig. 6a. Fig. 6b shows the χd vales versus P/F ratio. The χd decreased by increasing the paraffin content in the samples which suggested decrease of surface spin disorder. Oxygen vacancies, structural deficiencies and broken metal–oxygen bonds cause the magnetically disordered (canted spins) layer in the shell of nanoparticles. By dispersing the nanoparticles in the paraffin matrix, the covalent bonding between wax molecules and surface atoms of nanoparticles increase the magnetic order in surface of nanoparticles and consequently decrease the surface spin canting. Desautels et al. reported a significant reduction in intrinsic surface spin disorder in cu coated γ-Fe2O3 nanoparticles, compared with bare γ-Fe2O3 nanoparticles. They showed that interactions between the Fe and Cu atoms in the shell of Cu-coated γ-Fe2O3 nanoparticles alter the non-magnetic state of particles and stabilize the disordered surface spins [17,18]. In order to study the dynamic magnetic properties of samples, the AC magnetic susceptibility measurements were performed. Fig. 7 shows the temperature dependence real part of susceptibility (χ′) at frequency of 333 Hz and field of 10 Oe. The curves show a broad maximum centered at TP. Higher than TP, the nanoparticles show SPM behavior and below of this temperature, the
Fig. 6. (a) Slope of M–H curve at high fields. (b) Variation of χd versus P/F ratio.
blocked nanoparticles appear in the samples. From Fig. 8, it is clear that the peak temperature; TP shifts toward higher temperatures by decreasing the ferrite concentration in the samples. In fact, the bonding between surface atoms and paraffin molecules reduces the surface spins disorder and prevents the particles from becoming superparamagnetic until a higher temperature.
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B. Aslibeiki, P. Kameli / Journal of Magnetism and Magnetic Materials 385 (2015) 308–312
Acknowledgments Authors thank Dr. Varvaro for his help in VSM measurements.
References
Fig. 7. Temperature dependant behavior of real part of AC susceptibility, χ′ of samples at f¼ 333 Hz and H¼ 10 Oe.
Fig. 8. The normalized χ′. Shift in TP with increasing the paraffin content in samples is clear in this figure.
4. Conclusion In summary, we have studied the magnetic properties of MnFe2O4 nano-aggregated dispersed in paraffin matrix. Samples showed superparamagnetism behavior at room temperature. By decreasing the ferrite concentration in the samples, blocking temperature increased and the saturation magnetization decreased. Bonding between paraffin molecules and nanoparticles surface atoms reduced the surface spins disorder.
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