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The effect of Mn2+ substitution on magnetic properties of MnxFe3−xO4 nanoparticles prepared by coprecipitation method
Journal of Magnetism and Magnetic Materials 332 (2013) 157–162
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
The effect of Mn2 þ substitution on magnetic properties of MnxFe3 xO4 nanoparticles prepared by coprecipitation method J. Amighian a, E. Karimzadeh b, M. Mozaffari c,n a
Physics Department, Islamic Azad University—Najafabad Branch, Najafabad, Iran Department of Physics, Faculty of Science, University of Isfahan, Isfahan, Iran c Physics Department, Razi University, Taghbostan, Kermanshah, Iran b
a r t i c l e i n f o
abstract
Article history: Received 20 June 2012 Received in revised form 1 December 2012 Available online 21 December 2012
In this work single phase Mn substituted magnetite (MnxFe3 xO4, x ¼ 0–0.75) nanoparticles were prepared by the coprecipitation method. X-ray diffraction analysis showed that the prepared nanoparticles have a single-phase spinel structure. An average crystallite size of about 20 nm has been obtained for all the samples, using Scherrer’s formula. Field emission scanning electron microscope images of the samples showed that the average particle sizes were about 25 nm. The Curie temperature (TC) of the samples were measured by a Faraday balance and decreased from 610 1C to 510 1C by increasing Mn content from x ¼ 0 to x ¼ 0.75. The M–H curves of the nanoparticles exhibited superparamagnetic behavior for all the samples except for x ¼ 0 and saturation magnetization (ss) decreased with increasing of Mn content. The temperature dependence of AC-susceptibility of samples at different frequencies reveals maxima corresponding to the different blocking temperatures. It was shown that the frequency dependence of the blocking temperature can be described by the Vogel–Fulcher law for superparamagnets, well. & 2012 Elsevier B.V. All rights reserved.
Keywords: Mn substituted magnetite Magnetic nanoparticle Coprecipitation method Magnetic property
1. Introduction Nanocrystalline materials display various unusual and interesting properties compared with their bulk counterparts. These properties originate from reduced size and modification in interparticle interactions [1,2]. It is found that when the particle diameter is reduced to a definite size, magnetic nanoparticles may exhibit the behavior so-called superparamagnet. The remanence (Mr) and coercivity (Hc) of superparamagnetic nanoparticles are zero or negligible [3]. Magnetic moment of each superparamagnetic particle is affected by the thermal fluctuations [4,5]. The temperature where thermal energy overcomes to the magnetic anisotropy energy of a nanoparticle is known as the blocking temperature (TB). Superparamagnetic nanoparticles offer a high potentiality for biomedical applications, such as magnetic cell separation, AC magnetic fieldassisted cancer therapy and magnetically controlled transport of anti cancer drugs [6]. Various phenomenological laws have been used to explain dynamic magnetic behavior of superparamagnetic systems. The Ne´el–Arrhenius law is applicable for an assembly of non-interacting
n
Corresponding author. Tel.: þ98 311 793 4741, þ98 311 973 2409. E-mail address: [email protected] (M. Mozaffari).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.12.005
superparamagnetic nanoparticles. According to the Ne´el– Arrhenius theory, the relaxation time t obeys the following relation [7,10]:
t ¼ t0 exp
DE
ð1Þ
kB T
where f¼1/t stands for the measurement frequency, DE ¼ kef f V is the barrier energy where V denotes the volume of the nanoparticle, kef f the effective magnetic anisotropy constant and t0 is a constant, which has meaning usually when it is within the range of 10 13–10 9 s [11]. On the other hand, for interacting superparamagnetic nanoparticles have been taken into account by Vogel–Fulcher law is applicable
t ¼ t0 exp
DE kB ðTT 0 Þ
ð2Þ
which is a modification of the Ne el–Arrhenius law and where all parameters have the same definition as above and T 0 is an effective temperature [3,7,8]. In this work nanocrystalline MnxFe3 xO4 (x¼ 0.00, 0.25, 0.50 and 0.75) were prepared by the coprecipitation method and their structure and magnetic were studied.
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J. Amighian et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 157–162
Fig. 1. XRD patterns of MnxFe3 xO4 with different x values.
Table 1 Lattice parameters of the samples. x ˚ a0 (A)
0 8.361
0.25 8.382
0.5 8.392
0.75 8.413
Fig. 2. FE-SEM images of the samples (a) x¼ 0, (b) x¼ 0.25, (c) x ¼0.5 and (d) x ¼0.75.
2. Experimental In this work, MnxFe3 xO4 (x ¼0–0.75) nanoparticles were prepared by the coprecipitation method. The raw materials were MnCl2 4H2O, FeCl2 4H2O, FeCl3 6H2O and NH4OH all from Merck Co., with minimum purities of 99%. At first, solutions of iron and manganese salts (all of them 1 M) were prepared and the solutions were mixed and stirred slowly with constant rate at room temperature for 0.5 h to get a homogeneous solution. Then NH4OH was added drop by drop to the solution until the pH of the solution was reached to around 9.5 and precipitation procedure was completed. The precipitate was separated by a magnet and was washed few times with distilled water to remove unwanted ions until the pH became around 7. The washed precipitate was dried at 100 1C for 3 h, finally.
Phase identifications were carried out by an X-ray diffractometer (BRUKER, ADVANCED D8 model), using Cu-Ka radiation ˚ and average crystallite size was estimated from (l ¼1.5406 A) broadening of the main diffraction peaks, using Scherrer’s formula D ¼ 0:9l=B cosy
ð3Þ
where D is the average crystallite size, y is Bragg angle and B is the broadening of the diffraction peak. Morphology of the nanoparticles was investigated, using an FE-SEM unit. M–H curves of the prepared samples were recorded, using a sensitive permeameter and by which saturation magnetizations of the samples were determined by extrapolating of high field part of M 1/H curve when 1/H tends to zero [9]. Curie temperatures were determined by a Faraday balance. To determine blocking temperature of the samples, their AC-susceptibility
J. Amighian et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 157–162
respect to temperature at different frequencies were recorded and their maxima occur at blocking temperature (TB). Self-inductance of the samples was measured, using an LCR meter (Fluke, PM6306).
3. Result and discussion Fig. 1 shows XRD patterns of the samples, MnxFe3 xO4 (x¼ 0– 0.75). As it can be seen, all main peaks are assigned to a singlephase spinel structure. The relatively low intensities and high broadness of the peaks are due to law crystallites of the samples. Regarding XRD patterns, the lattice parameter increases with increasing Mn contents. The average crystallite size of the sample was about 20 nm, using Scherrer’s formula. The ionic radius of Mn2 þ is larger than radii of Fe3 þ and Fe2 þ , then by substitution of Mn in magnetite, lattice parameters increased with increasing Mn content (Table 1). The FE-SEM images of the MnxFe3 xO4 nanoparticles with different x values are shown in Fig. 2. The particle sizes of samples were about 25 nm. Fig. 3 shows the variation of Curie temperature with respect to Mn content. As it can be seen, Curie temperature decreases as Mn
Fig. 3. Variation of Curie temperature (TC) with respect to Mn content.
159
content increases. Curie temperature is mainly determined by the strongest superexchange interaction which in ferrites is A–B one [9,12]. Any factor that decreases this interaction leads to a decrease in Curie temperature. As was mentioned above, by increasing Mn content, the lattice parameter increases. This leads to an increase in ionic distances and then decrease in Curie temperature. Also Curie temperature of the Fe3O4 nanoparticles is about 30 1C higher than that of its bulk counterpart. It can be explained based on the finitesize scaling model, in which relative enhancement in TC is related to the particle size, exponentially [13,14]. Fig. 4a and b shows room temperature M–H curves of the MxFe3 xO4 nanoparticles. As it can be seen from Fig. 4a and its inset, magnetite (x¼0) is not superparamagnet, because of nonzero coercivity, while all other curves have shown in Fig. 4b Table 2 Extrapolated saturation magnetizations of the samples. x
ss (emu/g)
0 86.0
0.25 56.2
0.5 41.1
0.75 4.6
Fig. 5. Variation of theoretical and experimental nB of MnxFe3 xO4 with respect to Mn content.
Fig. 4. M–H curves of (a) Fe3O4, (b) MnxFe3 xO4 with different x values as labeled on the curves, (c) and inset of (a) show low field parts of the corresponding M–H curves.
160
J. Amighian et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 157–162
are S-shape with zero coercivity, which indicates they are superparamagnet. Also, it is clear that all M–H curves are not saturated up to maximum measuring field and then their high field parts were extrapolated and tabulated in Table 2. Magnetic moment per formula unit (nB) can be expressed by nB ¼
AM s
mB N A
Table 3 Values were calculated by fitting experimental data to Eqs. 4 and 5. x
0.25 0.5 0.75
Vogel–Fulcher model t0 ðsÞ
DE (eV)
T0 (K)
t0 ðsÞ 10 133 10 16 10 16
3 10 13 12 10 13 3 10 11
0.0301 0.0138 0.0131
352.5 157 152
Ne el–Arrhenius model
ð2Þ
Fig. 6. AC-susceptibility with respect to temperature at different frequencies (a) x¼ 0.25 (b) x¼ 0.5 and (c) x ¼0.75.
Fig. 7. Variation of ln f with respect to 1/TB for (a) x¼ 0.25 (b) x¼ 0.5 and (c) x ¼0.75.
J. Amighian et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 157–162
161
Fig. 8. Variation of ln f with respect to 1/(TB T0) for (a) x ¼0.25 (b) x¼ 0.5 and (c) x¼ 0.75.
where Ms is saturation magnetization, mB Bohr magneton, NA is Avogadro number and A is formula unit weight [15,16]. According to experimental saturation magnetization and Eq. (2), magnetic moment per formula unit was calculated for all samples, which decreases with increasing Mn content (Fig. 5). Bulk manganese ferrite (MnFe2O4) has a partial inverse structure in which about 20% of Mn2 þ ions occupy octahedral sites that means its cation distribution is ðMn20:8þ Fe30:2þ Fe2 þ ÞA ½Mn20:2þ Fe30:8þ B O4 [17]. It is well known that net magnetic moment of the spinel ferrites is equal to the difference between resultant magnetic moments of A- and B-site [9]. As magnetic moments of Fe2 þ , Fe3 þ and Mn2 þ are equal to 4, 5 and 5 mB, respectively [4], if one supposes Mn2 þ ions substitute for Fe2 þ ones just in B sites, i.e. þ þ2 ðMn2x0:2 Fe30:2þ Fe1x ÞA ½Mn20:2þ Fe30:8þ B O4 , then magnetic moment per formula unit nB should be decreased by increasing Mn content (Fig. 5). As it can be seen both theoretical and experimental results have similar behavior, except for the sample with x¼0 that has inverse spinel structure. To investigate dynamic behavior of superparamagnetic, AC-susceptibility measurement was carried out at four different frequencies on all samples. Fig. 6 shows the variation of w with respect to temperature at four different frequencies. As it can be seen, their maxima occur at blocking temperature. By rewriting Eq. (1), we have 1 DE ð4Þ ln f ¼ ln kB T t0 With depicting ln f 1=T by fitting experimental data and calculating of t0 , unphysical values have found for t0 (Fig. 7). These values for x ¼0.25, 0.5 and 0.75 are much lower than the above quoted range for superparamagnetic systems (Table 3). It means that the nanoparticles are non-interacting. In the case of interacting particles, the frequency dependence of TB should obey the Vogel–Fulcher law which is rewritten as follows [5,11]: 1 DE ln f ¼ ln kB ðT B T 0 Þ t0
ð5Þ
Fig. 8 shows the fit of experimental data to Eq. (5) for calculating t0 (Table 3). There is a good agreement of experimental data with the Vogel–Fulcher law so that t0 is in the range of 10 3 to 10 3 s which has physical meaning. It means that samples with x¼ 0.25, 0.5 and 0.75 are interacting.
4. Conclusion Single phase Mn substituted magnetite (MnxFe1 xFe2O4, x¼0–0.75) nanoparticles have been prepared by the coprecipitation method. The substituted samples show superparamagnetic behavior and their dynamics properties were described by the Volgel–Fulcher law for superparamagnets.
Acknowledgment The authors wish to thank the Office of Graduate Studies of the University of Isfahan for their supports. References [1] M. Zheng, X.C. Wu, B.S. Zou, Y.J. Wang, Magnetic properties of nanosized MnFe2O4 particles, Journal of Magnetism and Magnetic Materials 183 (1998) 152–156. [2] S.K. Sharma, R. Kumar, S. Kumar, V.V.S. Kumar, M. Knobel, V.R. Reddy, A. Banerjee, M. Singh, Magnetic study of Mg0.95Mn0.05Fe2O4 ferrite nanoparticles, Solid State Communications 141 (2007) 203–208. [3] M. Tadic, D. Markovic, V. Spasojevic, V. Kusigerski, M. Remskar, J. Pirnat, Z. Jaglicic, Synthesis and magnetic properties of concentrated a-Fe2O3, Journal of Alloys and Compounds 441 (2007) 291–296. [4] H. Morrish, The Physical Principles of Magnetism, IEEE Press, New York, 2001. [5] A. Rostamnejadi, H. Salamati, P. Kameli, H. Ahmadvand, Superparamagnetic behavior of La0.67Sr0.33MnO3 nanoparticles prepared via sol–gel method, Journal of Magnetism and Magnetic Materials 321 (2009) 3126–3131. [6] J. Giri, P. Pradhan, V. Somani, H. Chelawat, S. Chhatre, R. Banerjee, D. Bahadur, Synthesis and characterization of water-based ferrofluids of substituted ferrites [Fe1 xBxFe2O4, B ¼ Mn, Co (x¼ 0 1)] for biomedical applications, Journal of Magnetism and Magnetic Materials 320 (2008) 724–730. [7] J.A. Mydosh, Spin Glasses: An Experimental IntroductionSecond ed., Taylor and Francis, London, 1993.
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[8] M. Jalaly, M.H. Enayati, F. Karimzadeh, Investigation of structural and magnetic properties of nanocrystalline Ni0.3Zn0.7Fe2O4 prepared by high energy ball milling, Journal of Alloys and Compounds 480 (2009) 737–740. [9] B.D. Culity, C.D. Graham, Introduction to Magnetic Materials, Second ed., John Wiley, New Jersey, 2009. [10] H.M. Widatallaha, A. Al-Omari, F. Sives, M.B. Sturla, S.J. Stewart, Dynamic magnetic behavior of cluster-glass ZnFe2O4 nanosystem, Journal of Magnetism and Magnetic Materials 320 (2008) e324–e326. [11] M.J. Zehetdauer, Y. Theodore, Bulk Nanostructured Materials, Second ed., Wiley-VCH, Weinheim, 2009. [12] Z.J. Zhang, Z.L. Wang, B.C. Chakoumakos, J.S. Yin, Temperature dependence of cation distribution and oxidation state in magnetic Mn–Fe ferrite nanocrystals, Journal of American Chemical Society 120 (1998) 1800–1804.
[13] M. Zheng, X.C. Wu, B.S. Zou, Y.J. Wang, Magnetic properties of nanosized MnFe2O4 particles, Journal of Magnetism and Magnetic Materials 183 (1998) 152–156. [14] J.P. Chen, C.M. Sorensen, K.J. Klabund, G.C. Hadjipanayis, E. Devlin, A. Kostikas, Size-dependent magnetic properties of MnFe2O4 fine particles synthesized by coprecipitation method, Physical Review B 54 (1996) 9288–9296. [15] M. Manjurul, M. Huq, M.A. Hakim, Effect of Zn2 þ substitution on the magnetic properties of Mg1 xZnxFe2O4 ferrites, Physica B 404 (2009) 3915–3921. [16] J. Smit, in: Forth (Ed.), Magnetic Properties of Materials, McGraw Hill Book, London, 1971. [17] R.S. Tebble, Magnetic Materials, First ed., Wiley-Interscience, London, 1969.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
The effect of Mn2 þ substitution on magnetic properties of MnxFe3 xO4 nanoparticles prepared by coprecipitation method J. Amighian a, E. Karimzadeh b, M. Mozaffari c,n a
Physics Department, Islamic Azad University—Najafabad Branch, Najafabad, Iran Department of Physics, Faculty of Science, University of Isfahan, Isfahan, Iran c Physics Department, Razi University, Taghbostan, Kermanshah, Iran b
a r t i c l e i n f o
abstract
Article history: Received 20 June 2012 Received in revised form 1 December 2012 Available online 21 December 2012
In this work single phase Mn substituted magnetite (MnxFe3 xO4, x ¼ 0–0.75) nanoparticles were prepared by the coprecipitation method. X-ray diffraction analysis showed that the prepared nanoparticles have a single-phase spinel structure. An average crystallite size of about 20 nm has been obtained for all the samples, using Scherrer’s formula. Field emission scanning electron microscope images of the samples showed that the average particle sizes were about 25 nm. The Curie temperature (TC) of the samples were measured by a Faraday balance and decreased from 610 1C to 510 1C by increasing Mn content from x ¼ 0 to x ¼ 0.75. The M–H curves of the nanoparticles exhibited superparamagnetic behavior for all the samples except for x ¼ 0 and saturation magnetization (ss) decreased with increasing of Mn content. The temperature dependence of AC-susceptibility of samples at different frequencies reveals maxima corresponding to the different blocking temperatures. It was shown that the frequency dependence of the blocking temperature can be described by the Vogel–Fulcher law for superparamagnets, well. & 2012 Elsevier B.V. All rights reserved.
Keywords: Mn substituted magnetite Magnetic nanoparticle Coprecipitation method Magnetic property
1. Introduction Nanocrystalline materials display various unusual and interesting properties compared with their bulk counterparts. These properties originate from reduced size and modification in interparticle interactions [1,2]. It is found that when the particle diameter is reduced to a definite size, magnetic nanoparticles may exhibit the behavior so-called superparamagnet. The remanence (Mr) and coercivity (Hc) of superparamagnetic nanoparticles are zero or negligible [3]. Magnetic moment of each superparamagnetic particle is affected by the thermal fluctuations [4,5]. The temperature where thermal energy overcomes to the magnetic anisotropy energy of a nanoparticle is known as the blocking temperature (TB). Superparamagnetic nanoparticles offer a high potentiality for biomedical applications, such as magnetic cell separation, AC magnetic fieldassisted cancer therapy and magnetically controlled transport of anti cancer drugs [6]. Various phenomenological laws have been used to explain dynamic magnetic behavior of superparamagnetic systems. The Ne´el–Arrhenius law is applicable for an assembly of non-interacting
n
Corresponding author. Tel.: þ98 311 793 4741, þ98 311 973 2409. E-mail address: [email protected] (M. Mozaffari).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.12.005
superparamagnetic nanoparticles. According to the Ne´el– Arrhenius theory, the relaxation time t obeys the following relation [7,10]:
t ¼ t0 exp
DE
ð1Þ
kB T
where f¼1/t stands for the measurement frequency, DE ¼ kef f V is the barrier energy where V denotes the volume of the nanoparticle, kef f the effective magnetic anisotropy constant and t0 is a constant, which has meaning usually when it is within the range of 10 13–10 9 s [11]. On the other hand, for interacting superparamagnetic nanoparticles have been taken into account by Vogel–Fulcher law is applicable
t ¼ t0 exp
DE kB ðTT 0 Þ
ð2Þ
which is a modification of the Ne el–Arrhenius law and where all parameters have the same definition as above and T 0 is an effective temperature [3,7,8]. In this work nanocrystalline MnxFe3 xO4 (x¼ 0.00, 0.25, 0.50 and 0.75) were prepared by the coprecipitation method and their structure and magnetic were studied.
158
J. Amighian et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 157–162
Fig. 1. XRD patterns of MnxFe3 xO4 with different x values.
Table 1 Lattice parameters of the samples. x ˚ a0 (A)
0 8.361
0.25 8.382
0.5 8.392
0.75 8.413
Fig. 2. FE-SEM images of the samples (a) x¼ 0, (b) x¼ 0.25, (c) x ¼0.5 and (d) x ¼0.75.
2. Experimental In this work, MnxFe3 xO4 (x ¼0–0.75) nanoparticles were prepared by the coprecipitation method. The raw materials were MnCl2 4H2O, FeCl2 4H2O, FeCl3 6H2O and NH4OH all from Merck Co., with minimum purities of 99%. At first, solutions of iron and manganese salts (all of them 1 M) were prepared and the solutions were mixed and stirred slowly with constant rate at room temperature for 0.5 h to get a homogeneous solution. Then NH4OH was added drop by drop to the solution until the pH of the solution was reached to around 9.5 and precipitation procedure was completed. The precipitate was separated by a magnet and was washed few times with distilled water to remove unwanted ions until the pH became around 7. The washed precipitate was dried at 100 1C for 3 h, finally.
Phase identifications were carried out by an X-ray diffractometer (BRUKER, ADVANCED D8 model), using Cu-Ka radiation ˚ and average crystallite size was estimated from (l ¼1.5406 A) broadening of the main diffraction peaks, using Scherrer’s formula D ¼ 0:9l=B cosy
ð3Þ
where D is the average crystallite size, y is Bragg angle and B is the broadening of the diffraction peak. Morphology of the nanoparticles was investigated, using an FE-SEM unit. M–H curves of the prepared samples were recorded, using a sensitive permeameter and by which saturation magnetizations of the samples were determined by extrapolating of high field part of M 1/H curve when 1/H tends to zero [9]. Curie temperatures were determined by a Faraday balance. To determine blocking temperature of the samples, their AC-susceptibility
J. Amighian et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 157–162
respect to temperature at different frequencies were recorded and their maxima occur at blocking temperature (TB). Self-inductance of the samples was measured, using an LCR meter (Fluke, PM6306).
3. Result and discussion Fig. 1 shows XRD patterns of the samples, MnxFe3 xO4 (x¼ 0– 0.75). As it can be seen, all main peaks are assigned to a singlephase spinel structure. The relatively low intensities and high broadness of the peaks are due to law crystallites of the samples. Regarding XRD patterns, the lattice parameter increases with increasing Mn contents. The average crystallite size of the sample was about 20 nm, using Scherrer’s formula. The ionic radius of Mn2 þ is larger than radii of Fe3 þ and Fe2 þ , then by substitution of Mn in magnetite, lattice parameters increased with increasing Mn content (Table 1). The FE-SEM images of the MnxFe3 xO4 nanoparticles with different x values are shown in Fig. 2. The particle sizes of samples were about 25 nm. Fig. 3 shows the variation of Curie temperature with respect to Mn content. As it can be seen, Curie temperature decreases as Mn
Fig. 3. Variation of Curie temperature (TC) with respect to Mn content.
159
content increases. Curie temperature is mainly determined by the strongest superexchange interaction which in ferrites is A–B one [9,12]. Any factor that decreases this interaction leads to a decrease in Curie temperature. As was mentioned above, by increasing Mn content, the lattice parameter increases. This leads to an increase in ionic distances and then decrease in Curie temperature. Also Curie temperature of the Fe3O4 nanoparticles is about 30 1C higher than that of its bulk counterpart. It can be explained based on the finitesize scaling model, in which relative enhancement in TC is related to the particle size, exponentially [13,14]. Fig. 4a and b shows room temperature M–H curves of the MxFe3 xO4 nanoparticles. As it can be seen from Fig. 4a and its inset, magnetite (x¼0) is not superparamagnet, because of nonzero coercivity, while all other curves have shown in Fig. 4b Table 2 Extrapolated saturation magnetizations of the samples. x
ss (emu/g)
0 86.0
0.25 56.2
0.5 41.1
0.75 4.6
Fig. 5. Variation of theoretical and experimental nB of MnxFe3 xO4 with respect to Mn content.
Fig. 4. M–H curves of (a) Fe3O4, (b) MnxFe3 xO4 with different x values as labeled on the curves, (c) and inset of (a) show low field parts of the corresponding M–H curves.
160
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are S-shape with zero coercivity, which indicates they are superparamagnet. Also, it is clear that all M–H curves are not saturated up to maximum measuring field and then their high field parts were extrapolated and tabulated in Table 2. Magnetic moment per formula unit (nB) can be expressed by nB ¼
AM s
mB N A
Table 3 Values were calculated by fitting experimental data to Eqs. 4 and 5. x
0.25 0.5 0.75
Vogel–Fulcher model t0 ðsÞ
DE (eV)
T0 (K)
t0 ðsÞ 10 133 10 16 10 16
3 10 13 12 10 13 3 10 11
0.0301 0.0138 0.0131
352.5 157 152
Ne el–Arrhenius model
ð2Þ
Fig. 6. AC-susceptibility with respect to temperature at different frequencies (a) x¼ 0.25 (b) x¼ 0.5 and (c) x ¼0.75.
Fig. 7. Variation of ln f with respect to 1/TB for (a) x¼ 0.25 (b) x¼ 0.5 and (c) x ¼0.75.
J. Amighian et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 157–162
161
Fig. 8. Variation of ln f with respect to 1/(TB T0) for (a) x ¼0.25 (b) x¼ 0.5 and (c) x¼ 0.75.
where Ms is saturation magnetization, mB Bohr magneton, NA is Avogadro number and A is formula unit weight [15,16]. According to experimental saturation magnetization and Eq. (2), magnetic moment per formula unit was calculated for all samples, which decreases with increasing Mn content (Fig. 5). Bulk manganese ferrite (MnFe2O4) has a partial inverse structure in which about 20% of Mn2 þ ions occupy octahedral sites that means its cation distribution is ðMn20:8þ Fe30:2þ Fe2 þ ÞA ½Mn20:2þ Fe30:8þ B O4 [17]. It is well known that net magnetic moment of the spinel ferrites is equal to the difference between resultant magnetic moments of A- and B-site [9]. As magnetic moments of Fe2 þ , Fe3 þ and Mn2 þ are equal to 4, 5 and 5 mB, respectively [4], if one supposes Mn2 þ ions substitute for Fe2 þ ones just in B sites, i.e. þ þ2 ðMn2x0:2 Fe30:2þ Fe1x ÞA ½Mn20:2þ Fe30:8þ B O4 , then magnetic moment per formula unit nB should be decreased by increasing Mn content (Fig. 5). As it can be seen both theoretical and experimental results have similar behavior, except for the sample with x¼0 that has inverse spinel structure. To investigate dynamic behavior of superparamagnetic, AC-susceptibility measurement was carried out at four different frequencies on all samples. Fig. 6 shows the variation of w with respect to temperature at four different frequencies. As it can be seen, their maxima occur at blocking temperature. By rewriting Eq. (1), we have 1 DE ð4Þ ln f ¼ ln kB T t0 With depicting ln f 1=T by fitting experimental data and calculating of t0 , unphysical values have found for t0 (Fig. 7). These values for x ¼0.25, 0.5 and 0.75 are much lower than the above quoted range for superparamagnetic systems (Table 3). It means that the nanoparticles are non-interacting. In the case of interacting particles, the frequency dependence of TB should obey the Vogel–Fulcher law which is rewritten as follows [5,11]: 1 DE ln f ¼ ln kB ðT B T 0 Þ t0
ð5Þ
Fig. 8 shows the fit of experimental data to Eq. (5) for calculating t0 (Table 3). There is a good agreement of experimental data with the Vogel–Fulcher law so that t0 is in the range of 10 3 to 10 3 s which has physical meaning. It means that samples with x¼ 0.25, 0.5 and 0.75 are interacting.
4. Conclusion Single phase Mn substituted magnetite (MnxFe1 xFe2O4, x¼0–0.75) nanoparticles have been prepared by the coprecipitation method. The substituted samples show superparamagnetic behavior and their dynamics properties were described by the Volgel–Fulcher law for superparamagnets.
Acknowledgment The authors wish to thank the Office of Graduate Studies of the University of Isfahan for their supports. References [1] M. Zheng, X.C. Wu, B.S. Zou, Y.J. Wang, Magnetic properties of nanosized MnFe2O4 particles, Journal of Magnetism and Magnetic Materials 183 (1998) 152–156. [2] S.K. Sharma, R. Kumar, S. Kumar, V.V.S. Kumar, M. Knobel, V.R. Reddy, A. Banerjee, M. Singh, Magnetic study of Mg0.95Mn0.05Fe2O4 ferrite nanoparticles, Solid State Communications 141 (2007) 203–208. [3] M. Tadic, D. Markovic, V. Spasojevic, V. Kusigerski, M. Remskar, J. Pirnat, Z. Jaglicic, Synthesis and magnetic properties of concentrated a-Fe2O3, Journal of Alloys and Compounds 441 (2007) 291–296. [4] H. Morrish, The Physical Principles of Magnetism, IEEE Press, New York, 2001. [5] A. Rostamnejadi, H. Salamati, P. Kameli, H. Ahmadvand, Superparamagnetic behavior of La0.67Sr0.33MnO3 nanoparticles prepared via sol–gel method, Journal of Magnetism and Magnetic Materials 321 (2009) 3126–3131. [6] J. Giri, P. Pradhan, V. Somani, H. Chelawat, S. Chhatre, R. Banerjee, D. Bahadur, Synthesis and characterization of water-based ferrofluids of substituted ferrites [Fe1 xBxFe2O4, B ¼ Mn, Co (x¼ 0 1)] for biomedical applications, Journal of Magnetism and Magnetic Materials 320 (2008) 724–730. [7] J.A. Mydosh, Spin Glasses: An Experimental IntroductionSecond ed., Taylor and Francis, London, 1993.
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