- Email: [email protected]
Superparamagnetic behavior of MnxNi1−xFe2O4 spinel nanoferrites
Journal of Magnetism and Magnetic Materials 361 (2014) 170–174
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Superparamagnetic behavior of MnxNi1 xFe2O4 spinel nanoferrites Hafiz M.I. Abdallah n, Thomas Moyo School of Chemistry and Physics, University of KwaZulu-Natal, Westville campus, P/Bag X5400, Durban 4000, South Africa
art ic l e i nf o
a b s t r a c t
Article history: Received 8 October 2013 Received in revised form 24 December 2013 Available online 5 March 2014
The glycol-thermal technique was used for the synthesis of Mnx Ni1 x Fe2 O4 (x ¼0.1, 0.3 and 0.5) nanoparticle ferrites from high-purity metal chlorides. Structural parameters, morphology and magnetic properties were investigated using X-ray powder diffraction (XRD), high-resolution transmission electron microscopy, high-resolution scanning electron microscopy, energy-dispersive X-ray spectroscopy, 57Fe Mössbauer spectroscopy, vibrating sample magnetometer (VSM) and VSM on a cryogen free measurement system. The XRD results confirm single-phase formation of the as-prepared samples with average crystallite sizes of about 8.5 nm having the Fd3m space group. The microstrains for the as-prepared samples were relieved by increasing the Mn-concentration, x, while the crystallite sizes and lattice parameters increased slightly. The magnetizations for the sample at x ¼0.5 were also measured in external applied fields of up to 5 T and at isothermal temperatures from 2 K to 300 K. Small values of coercive fields ð r 10 OeÞ, remanent magnetizations and broadening of the Mössbauer spectra at room temperature indicate superparamagnetic like-behavior of the nanoparticle compounds. & 2014 Elsevier B.V. All rights reserved.
Keywords: Nanoferrites Langevin function Superparamagnetism Hyperfine interactions
1. Introduction Nanoparticle ferrites have various technological applications which include color magnetic resonance imaging, spintronic devices, magnetic refrigeration, high-density magnetic recording media, biomedical, magnetic coating and ferrofluids [1–3]. A number of preparation methods have been used to synthesize ferrites such as co-precipitation and mechanical alloying [1], glycol-thermal [4], oxalate precursor [5], sol–gel auto-combustion [6–8] and microemulsion [9]. Köseoğlu et al. [10–12] have reported Mnx Ni1 x Fe2 O4 spinel ferrites prepared by polyethylene glycol (PEG)-assisted hydrothermal at synthesis temperatures of 150 1C (12 h) and 180 1C (24 h) with crystallite sizes which are sensitive to reaction temperature and time. In the present case, the samples were synthesized in ethylene glycol at 200 1C (6 h) without the benefit of a surfactant (like PEG). The samples were also washed by filtering instead of centrifugation which can also affect the crystallite sizes. The structure and magnetic properties of ferrites are known to be very sensitive to preparation methods, additive substitutions and the sintering process [3]. Surface spins, spin canting and reduction of particle sizes also play an important role on the magnetization parameters [2]. Mössbauer spectroscopy can be used to provide information of electronic, ion distributions,
n
Corresponding author. Mobile: þ 27 728994179. E-mail address: hafi[email protected] (H.M.I. Abdallah).
http://dx.doi.org/10.1016/j.jmmm.2014.02.077 0304-8853 & 2014 Elsevier B.V. All rights reserved.
magnetic and structure properties within the studied materials. In this communication, we investigate the evolution of the magnetic properties as a function of Mn-concentration, thermal annealing at 500 1C and measuring temperatures (2–300 K) for the MnxNi1 xFe2O4 nanoferrites. We also investigate the superparamagnetism of the compounds using X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRSEM), high-resolution scanning electron microscopy (HRTEM), 57Fe Mössbauer spectroscopy and vibrating sample magnetometer (VSM).
2. Experimental methods High-purity metal chlorides were used as starting raw materials in the synthesis of Mnx Ni1 x Fe2 O4 powder nanoferrites (x ¼0.1, 0.3, and 0.5) via a glycol-thermal route using a Parr stirred pressure reactor type PARR 4842 following the procedure reported elsewhere [4,13]. The phase identification was performed at room temperature by XRD (Model: PANalytical, EMPYREAN) using a monochromatic beam of Co-Kα radiation (λ ¼1.7903067 ̊) in 2θ scanning from 101 to 801. The morphology for the as-prepared Mn0.5Ni0.5Fe2O4 was examined by a Joel_JEM-2100 high-resolution transmission electron microscope (HRTEM) and an Ultra Plus ZEISS-FEG SEM scanning electron microscope (HRSEM). The qualitative real content of the elements for this compound was measured by using energy-dispersive X-ray (EDX) also performed on the Ultra Plus ZEISS-FEG HRSEM instrument. Roomtemperature magnetization curves for the as-prepared samples
H.M.I. Abdallah, T. Moyo / Journal of Magnetism and Magnetic Materials 361 (2014) 170–174
171
and samples annealed at 500 1C under argon atmosphere were carried out by using a vibrating sample magnetometer (VSM, model Lakeshore 735). A mini cryogen free measurement (CFM)– VSM system was also used to characterize the basic magnetization measurements of hysteresis loops for the as-prepared Mn0:5 Ni0:5 Fe2 O4 at different isothermal temperatures from 2 to 300 K in external applied magnetic fields of up to 5 T. Zero field cooling (ZFC) and field cooling (FC) magnetizations were also obtained in a static applied magnetic field of 0.01 T. The hyperfine parameters were deduced from 57Fe Mössbauer spectroscopy measurements at room temperature.
3. Results and discussion Fig. 1 shows room-temperature XRD patterns for the asprepared Mnx Ni1 x Fe2 O4 (x ¼0.1, 0.3 and 0.5) nanoferrites which reveal the formation of single-phase cubic spinel structures. All peaks were successfully indexed using the standard XRD patterns of the cubic spinel structure of MnFe2 O4 (JCPDS card no. 74-2403) [4] and NiFe2 O4 (JCPDS card no. 10-0325) [6]. The mean crystallite diameters (D) were estimated from the full-width at halfmaximum (whkl) of the most intensive (311) peak using Scherrer's formula D ¼ Kλ=ðwhkl cos θÞ where K is the structure factor. For cubic structure, the lattice parameters (a) can be calculated from Miller indices h, k, l and inter-planar spacing (d) using the equation 2 2 2 a ¼ dðh þ k þl Þ1=2 and Bragg's law. The microstrains were also deduced from XRD data by using the formula ε ¼ whkl =ð4 tan θÞ [4,13]. The calculated crystallite sizes, lattice parameters and microstrains for the samples x¼ 0.1, 0.3 and 0.5 are given in Table 1. The results show that the crystallite sizes and lattice parameters increased slightly by Mn substitution, while the microstrains were relieved. Increases in lattice parameters by increasing Mn-concentration can be attributed to replacement of the smaller ionic radii of Ni2 þ (tetrahedral: 0.55 Å; octahedral 0.69 Å) by bigger ionic radii of Mn2 þ (tetrahedral: 0.655 Å; octahedral: 0.80 Å) [7]. Hussain et al. [7] and Hassan et al. [5] have also reported similar trends for a series of MnxNi1 xFe2O4 compounds. The morphology, particle size and elemental content for the asprepared Mn0:5 Ni0:5 Fe2 O4 sample were determined by HRTEM and HRSEM. The HRTEM and HRSEM micrographs are shown in Figs. 2 and 3 respectively. The corresponding average particle size of about 8.5 nm obtained from the images is in good agreement with the XRD result. The particles appear to be uniform, crystalline and nearly spherical in shape and with hardly any agglomeration extent. Fig. 4 illustrates the EDX spectra for the as-prepared Mn0:5 Ni0:5 Fe2 O4 sample. The microanalysis of the raw EDX data indicates qualitatively the content of the constitutive elements in terms of O (at%: 48.76), Mn (at%: 3.78), Fe (at%: 39.28) and Ni (at%: 8.18). The magnetic field dependence of the magnetization measured at room temperature for the as-prepared samples synthesized at 200 1C and the sample annealed at 500 1C is shown in Fig. 5. The magnetic parameters such as coercive fields (Hc), saturation magnetizations (Ms), remanent magnetizations (MR) and squareness ratio of the loops (MR/MS) deduced from the magnetic hysteresis loops are given in Table 2. The saturation magnetizations have been taken at the highest measured field. The experimental magnetic moment per molecule in the Bohr magneton was obtained using the formula μ ¼ M w M S =5585 where MW is the molar mass [3]. Low values of saturation magnetizations for these samples may be attributed to spin canting due to the presence of Jahn–Teller cations which give rise to Yafet–Kittel angles (αY K). These angles can be estimated using the relation μ ¼ ð6 þ xÞ cos αY K 5ð1 xÞ where x is the Mn-concentration
Fig. 1. XRD patterns for the as-prepared Mnx Ni1 x Fe2 O4 nanoferrites.
Table 1 Crystallite sizes (D), lattice constants (a) and microstrains (ε) for the as-prepared nanoparticle Mnx Ni1 x Fe2 O4 . Sample x
D (nm) 7 0.2
a (Å) 7 0.003
ε (nm) 7 0.00008
0.1 0.3 0.5
8.3 8.6 8.7
8.368 8.369 8.375
0.00137 0.00132 0.00130
Fig. 2. HRTEM microstructure for the as-prepared Mn0:5 Ni0:5 Fe2 O4 crystalline nanoferrite.
[14]. The calculated values for the Yafet–Kittel angles (αY K) and experimental magnetic moments (μ) are also displayed in Table 2. Non-zero values of αY K suggest that the magnetization of the samples cannot be explained on the basis of the Nèel model. The rise of the angles indicates that the triangular type spins arrangements are favored which lead to the weakening of the J AB ðA O BÞ superexchange magnetic interactions between magnetic moments at A-site and B-site [15]. The presence of canted spins has also been reported for Cux Zn1 x Fe2 O4 [16] and Li0:5 x=2 Cdx Fe2:5x=2 O4 [15] spinel ferrites.
172
H.M.I. Abdallah, T. Moyo / Journal of Magnetism and Magnetic Materials 361 (2014) 170–174
Fig. 3. HRSEM micrograph for the as-prepared sample of Mn0:5 Ni0:5 Fe2 O4 nanoferrite.
Fig. 5. Room-temperature magnetic hysteresis loops for the as-prepared samples and samples annealed at 500 1C of Mnx Ni1 x Fe2 O4 .
Fig. 4. EDX spectra for the as-prepared Mn0:5 Ni0:5 Fe2 O4 ferrite.
Fig. 6 shows the hysteresis loops for the as-prepared Mn0:5 Ni0:5 Fe2 O4 sample measured at different isothermal temperatures from 2 to 300 K in external applied fields of up to 5 T. Low values of Hc(T) and MR at 300 K suggest superparamagneticlike behavior of the nanoparticles. The Hc(T) increased gradually at lower measuring temperature reaching about 330 Oe at 2 K. The increase of the magnetic hardness at low temperatures is associated with freezing of spins. A typical variation of coercivity as a function of measuring temperature is given in Fig. 7. The Hc(T) data was fitted over the entire temperature range by Kneller's law H C ðTÞ ¼ H C ð0Þ½1 ðT=T B Þα where TB is the blocking temperature and Hc(0) is the coercive field at absolute zero temperature [4]. The saturation magnetizations plotted against measuring temperature are also shown in Fig. 7. MS(T) varies according to the modified Bloch's law M S ðTÞ ¼ M S ð0Þ½1 ðT=T o Þβ where β is Bloch's factor which depends on the size and surface treatment of the sample [4]. This is associated with single-domain nanoparticles [17,18] due to the confinement effects of the spin-wave spectra for magnetic clusters. The modified Bloch's equation appears to fit the saturation magnetization data over the entire temperature range with β 2:14 7 0:07. Good fits with correlation coefficients of about 0.9721 and 0.9979 are obtained based on Kneller's law and the modified Bloch's law respectively. In small ferromagnetic or ferrimagnetic single-domain particles, superparamagnetism occurs above TB because of weakly
interacting and thermal fluctuations of the spins of the nanoparticles. The thermal effects allow flips of spins between the easy magnetization axes which lead to near zero-coercivity and increase in saturation magnetization [19]. Below TB the thermal fluctuations do not dominate and the superparamagnetic nanosized particles cannot rotate freely but freeze in random orientations leading to high-coercive fields. In the superparamagnetic region, T B oT o T C , the magnetization response of the particles under the influence of an applied field H is given by the standard Langevin function M=M S ¼ LðμH=kB TÞ and/or with the modified Langevin function M=M S LðH=γM S Þ. L is the Langevin's function, μ is the moment of the superparamagnetic particles, MS is the saturation magnetization at temperature T and γ is a constant for a given sample [20,21]. In Fig. 8, the relative magnetization (M/MS) is plotted as a function of H/T and H/MS. The results show that the loops did follow the classical superparamagnetic scaling law with H/T above the blocking temperature of 150 K as expected. We also found that the relative magnetizations obeyed the modified Langevin function with H/MS for over a temperature range of 2– 300 K. Similar results have been observed in Cu90 Co10 granular alloys [20] and Ni nanoparticles [21]. Fig. 9 shows typical zero-field cooling and field cooling (ZFC– FC) magnetization curves measured in an external applied field of 0.01 T for Mn0:5 Ni0:5 Fe2 O4 compound. The ZFC magnetization curve increases gradually with increasing temperature and reaches a maximum value at the blocking temperature of about 150 K. The width of the peak in the ZFC curve is associated with particle size distribution [4]. A particle with a particular size has a certain blocking temperature. Wide peaks are observed in our sample which indicate a wide range in the distribution of particle sizes. The slow decrease in magnetization with further increase in temperature is associated with disordering of particle spins. Mössbauer spectra give quantitative information of hyperfine interactions which reflect the interaction between the nuclei and neighboring electrons. Zero-field Mössbauer spectra acquired at 300 K for the as-prepared samples of Mnx Ni1 x Fe2 O4 are presented in Fig. 10. The spectra have been analyzed in terms of two
H.M.I. Abdallah, T. Moyo / Journal of Magnetism and Magnetic Materials 361 (2014) 170–174
173
Table 2 Coercive fields (HC), saturation magnetizations (MS), remanent magnetizations (MR), experimental magnetic moment per molecule (μ), Yafet–Kittel angles (αY K) and squareness ratio of the loops (MR/MS) for the as-prepared (T1 ¼200 1C) and samples annealed at T2 ¼ 500 1C measured at room temperature for Mnx Ni1 x Fe2 O4 nanoferrites. Sample x
0.1 0.3 0.5
HC (Oe)
MS (emu/g)
αY K(o)
m (μB)
MR (emu/g)
MR/MS
(T1)
(T2)
(T1)
(T2)
(T1)
(T2)
(T1)
(T2)
(T1)
(T2)
(T1)
(T2)
6.61 10.76 0.08
5.87 7.67 6.38
26.74 25.32 53.78
23.26 24.27 24.42
0.162 0.301 0.230
0.161 0.284 0.266
1.12 1.06 2.24
0.97 1.01 1.07
22.87 43.66 43.19
26.17 44.25 57.25
0.0061 0.0119 0.1083
0.0069 0.0117 0.0109
Fig. 6. Hysteresis loops for the as-prepared Mn0:5 Ni0:5 Fe2 O4 nanoferrite sample measured at different isothermal temperatures of 2 K–300 K. The inset figure shows magnified view at about 0.05 T.
Fig. 8. The variations of relative magnetization (M/MS) of the as-prepared Mn0:5 Ni0:5 Fe2 O4 sample measured at different isothermal temperatures plotted as a function of (a) H/T of the Langevin function and (b) H/MS of the modified Langevin function.
Fig. 9. ZFC and FC magnetization curves for the as-prepared Mn0:5 Ni0:5 Fe2 O4 sample measured in an external applied field of 0.01 T. Fig. 7. Coercive field and saturation magnetization plotted with measuring temperature for the as-synthesized Mn0:5 Ni0:5 Fe2 O4 sample. The solid lines are best-fits to the experimental data based on (a) Kneller's law and (b) on the modified Bloch's law.
Zeeman splitting sextets which correspond to Fe3 þ ions at tetrahedral (A) and octahedral (B) sites. The spectra were fitted with Lorentzian-shaped lines. The sextet with the lower hyperfine field
174
H.M.I. Abdallah, T. Moyo / Journal of Magnetism and Magnetic Materials 361 (2014) 170–174
populations of Fe3 þ on B-site compared to A-site may also account for the reduction in the coercive fields in the studied samples [23].
4. Conclusions In summary, the glycol-thermal technique was employed to produce Mnx Ni1 x Fe2 O4 nano-magnetic spinel ferrites at low reaction temperature of 200 1C. The average crystallite diameter for the as-prepared samples is about 8.5 nm. Low values of coercive field and remanent magnetization at room temperature in addition to broadened or relaxation of the Mössbauer spectra for all products strongly suggest that the studied samples confirm the characteristic of superparamagnetism.
Acknowledgments
Fig. 10. Room-temperature Mössbauer spectra for the as-prepared sample Mnx Ni1 x Fe2 O4 .
The authors wish to thank the National Research Foundation (NRF) of South Africa for VSM and mini CFMS equipment grants, Electron microscope Unit at Westville campus (UKZN) for HRTEM and HRSEM measurements, and the University of Al Fashir of Sudan for study leave (HMI). References
Table 3 Isomer shifts (δ), hyperfine fields (H), line widths (Γ) and Fe3 þ fraction population (f) on A- and B-sites for the as-prepared Mnx Ni1 x Fe2 O4 samples measured at 300 K. Sample x
δA (mm/s) 7 0.01
δB (mm/s) 7 0.03
HA (T) 7 0.2
HB (T) 7 0.4
ΓA (mm/s) 7 0.03
ΓB (mm/s) 7 0.06
fA (%) 7 0.7
fB (%) 7 0.6
0.1 0.3 0.5
0.34 0.30 0.33
0.34 0.30 0.35
38.3 40.1 37.6
44.7 45.8 43.9
0.52 1.20 0.90
1.07 0.50 0.27
78.0 71.0 84.9
22.0 29.0 15.1
and isomer shift is assigned to tetrahedral site [4]. The hyperfine parameters such as hyperfine magnetic fields, isomer shifts, line widths and fraction of Fe3 þ ions on A- and B-sites are listed in Table 3. The broadening of the spectra suggests the presence of superparamagnetic nanoparticle ferrites [22]. The existence of the superparamagnetism is also confirmed from the data of the magnetizations. The study of the isomer shift can give information on valence states of the absorber material and in the present case the isomer shift values are corresponding with the Fe3 þ ions. No change to the local environments for Fe3 þ on both A-site and B-site for samples x ¼0.1 and 0.3 was observed. The area ratio of each sextet is found to be directly proportional to the fraction population of Fe3 þ at A-site or B-site. Low values of fraction
[1] Y. Shi, J. Ding, X. Liu, J. Wang, J. Magn. Magn. Mater. 205 (1999) 249. [2] D.T.T. Nguyet, N.P. Duong, L.T. Hung, T.D. Hien, T. Satoh, J. Alloys Compd. 509 (2011) 6621. [3] H.M.I. Abdallah, T. Moyo, J.Z Msomi, J. Phys.: Conf. Ser. 217 (2010) 012141. [4] H.M.I. Abdallah, T. Moyo, J. Alloys Compd. 562 (2013) 156. [5] H.E. Hassan, T. Sharshar, M.M. Hessien, O.M. Hemeda, Nucl. Instrum. Methods . Phys. Res. B 304 (2013) 72. [6] P. Sivakumar, R. Ramesh, A. Ramanand, S. Ponnusamy, C. Muthamizhchelvan, Mater. Res. Bull. 46 (2011) 2204. [7] A. Hussain, T. Abbas, S.B. Niazi, Ceram. Int. 39 (2013) 1221. [8] M. Airimioaei, C.E. Ciomaga, N. Apostolescu, L. Leontie, A.R. Iordan, L. Mitoseriu, M.N. Palamaru, J. Alloys. Comp. 509 (2011) 8065. [9] D.O. Yener, H. Giesche, J. Am. Cer. Soc. 84 (2004) 1987. [10] Y. Köseoğlu, Ceram. Int. 39 (2013) 4221. [11] Y. Köseoğlu, İ. Aldemir, F. Bayansal, S. Kahraman, H.A. Ҫetinkara, Mater. Chem. Phys. 139 (2013) 789. [12] Y. Köseoğlu, M. Bay, M. Tan, A. Baykal, H. Sözeri, R. Topkaya, N. Akdoğan, J. Nanopart. Res. 13 (2011) 2235. [13] M. Sugimoto, J. Am. Ceram. 82 (2) (1999) 269. [14] Y. Gao, Y. Zhang, W. Yu, R. Xiong, J. Shi, J. Magn. Magn. Mater. 324 (2012) 2534. [15] M. Ajmal, A. Magsood, J. Magn. Magn. Mater. 460 (2008) 54. [16] S. Singhal, S.K. Barthwal, K Chandra, J. Magn. Magn. Mater. 306 (2006) 233. [17] H. Hiroyoshi, K. Fukamichi, Phys. Lett. A 58 (4) (1981) 242. [18] K. Mazz, A. Mumtaz, S.K. Hasanian, M.F. Bertino, J. Magn. Magn. Mater. 322 (2010) 2199. [19] M. Pileni, Adv. Func. Mater. 11 (5) (2001) 323. [20] P. Allia, M. Coisson, P. Tiberto, F. Vinai, M. Knobel, M.A. Novak, W.C. Nunes, Phys. Rev. B 64 (2001) 144420. [21] F.C. Fonseca, G.F. Goya, R.F. Jardim, R. Muccillo, N.L.V. Carreno, E. Longo, E. R. Liete, Phys. Rev. B 66 (2002) 104406. [22] J. Hochepied, P. Bonville, M. Pileni, J. Phys. Chem. B 104 (2000) 905. [23] H.M.I. Abdallah, T. Moyo, J.Z Msomi, J. Magn. Magn. Mater. 332 (2013) 123.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Superparamagnetic behavior of MnxNi1 xFe2O4 spinel nanoferrites Hafiz M.I. Abdallah n, Thomas Moyo School of Chemistry and Physics, University of KwaZulu-Natal, Westville campus, P/Bag X5400, Durban 4000, South Africa
art ic l e i nf o
a b s t r a c t
Article history: Received 8 October 2013 Received in revised form 24 December 2013 Available online 5 March 2014
The glycol-thermal technique was used for the synthesis of Mnx Ni1 x Fe2 O4 (x ¼0.1, 0.3 and 0.5) nanoparticle ferrites from high-purity metal chlorides. Structural parameters, morphology and magnetic properties were investigated using X-ray powder diffraction (XRD), high-resolution transmission electron microscopy, high-resolution scanning electron microscopy, energy-dispersive X-ray spectroscopy, 57Fe Mössbauer spectroscopy, vibrating sample magnetometer (VSM) and VSM on a cryogen free measurement system. The XRD results confirm single-phase formation of the as-prepared samples with average crystallite sizes of about 8.5 nm having the Fd3m space group. The microstrains for the as-prepared samples were relieved by increasing the Mn-concentration, x, while the crystallite sizes and lattice parameters increased slightly. The magnetizations for the sample at x ¼0.5 were also measured in external applied fields of up to 5 T and at isothermal temperatures from 2 K to 300 K. Small values of coercive fields ð r 10 OeÞ, remanent magnetizations and broadening of the Mössbauer spectra at room temperature indicate superparamagnetic like-behavior of the nanoparticle compounds. & 2014 Elsevier B.V. All rights reserved.
Keywords: Nanoferrites Langevin function Superparamagnetism Hyperfine interactions
1. Introduction Nanoparticle ferrites have various technological applications which include color magnetic resonance imaging, spintronic devices, magnetic refrigeration, high-density magnetic recording media, biomedical, magnetic coating and ferrofluids [1–3]. A number of preparation methods have been used to synthesize ferrites such as co-precipitation and mechanical alloying [1], glycol-thermal [4], oxalate precursor [5], sol–gel auto-combustion [6–8] and microemulsion [9]. Köseoğlu et al. [10–12] have reported Mnx Ni1 x Fe2 O4 spinel ferrites prepared by polyethylene glycol (PEG)-assisted hydrothermal at synthesis temperatures of 150 1C (12 h) and 180 1C (24 h) with crystallite sizes which are sensitive to reaction temperature and time. In the present case, the samples were synthesized in ethylene glycol at 200 1C (6 h) without the benefit of a surfactant (like PEG). The samples were also washed by filtering instead of centrifugation which can also affect the crystallite sizes. The structure and magnetic properties of ferrites are known to be very sensitive to preparation methods, additive substitutions and the sintering process [3]. Surface spins, spin canting and reduction of particle sizes also play an important role on the magnetization parameters [2]. Mössbauer spectroscopy can be used to provide information of electronic, ion distributions,
n
Corresponding author. Mobile: þ 27 728994179. E-mail address: hafi[email protected] (H.M.I. Abdallah).
http://dx.doi.org/10.1016/j.jmmm.2014.02.077 0304-8853 & 2014 Elsevier B.V. All rights reserved.
magnetic and structure properties within the studied materials. In this communication, we investigate the evolution of the magnetic properties as a function of Mn-concentration, thermal annealing at 500 1C and measuring temperatures (2–300 K) for the MnxNi1 xFe2O4 nanoferrites. We also investigate the superparamagnetism of the compounds using X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRSEM), high-resolution scanning electron microscopy (HRTEM), 57Fe Mössbauer spectroscopy and vibrating sample magnetometer (VSM).
2. Experimental methods High-purity metal chlorides were used as starting raw materials in the synthesis of Mnx Ni1 x Fe2 O4 powder nanoferrites (x ¼0.1, 0.3, and 0.5) via a glycol-thermal route using a Parr stirred pressure reactor type PARR 4842 following the procedure reported elsewhere [4,13]. The phase identification was performed at room temperature by XRD (Model: PANalytical, EMPYREAN) using a monochromatic beam of Co-Kα radiation (λ ¼1.7903067 ̊) in 2θ scanning from 101 to 801. The morphology for the as-prepared Mn0.5Ni0.5Fe2O4 was examined by a Joel_JEM-2100 high-resolution transmission electron microscope (HRTEM) and an Ultra Plus ZEISS-FEG SEM scanning electron microscope (HRSEM). The qualitative real content of the elements for this compound was measured by using energy-dispersive X-ray (EDX) also performed on the Ultra Plus ZEISS-FEG HRSEM instrument. Roomtemperature magnetization curves for the as-prepared samples
H.M.I. Abdallah, T. Moyo / Journal of Magnetism and Magnetic Materials 361 (2014) 170–174
171
and samples annealed at 500 1C under argon atmosphere were carried out by using a vibrating sample magnetometer (VSM, model Lakeshore 735). A mini cryogen free measurement (CFM)– VSM system was also used to characterize the basic magnetization measurements of hysteresis loops for the as-prepared Mn0:5 Ni0:5 Fe2 O4 at different isothermal temperatures from 2 to 300 K in external applied magnetic fields of up to 5 T. Zero field cooling (ZFC) and field cooling (FC) magnetizations were also obtained in a static applied magnetic field of 0.01 T. The hyperfine parameters were deduced from 57Fe Mössbauer spectroscopy measurements at room temperature.
3. Results and discussion Fig. 1 shows room-temperature XRD patterns for the asprepared Mnx Ni1 x Fe2 O4 (x ¼0.1, 0.3 and 0.5) nanoferrites which reveal the formation of single-phase cubic spinel structures. All peaks were successfully indexed using the standard XRD patterns of the cubic spinel structure of MnFe2 O4 (JCPDS card no. 74-2403) [4] and NiFe2 O4 (JCPDS card no. 10-0325) [6]. The mean crystallite diameters (D) were estimated from the full-width at halfmaximum (whkl) of the most intensive (311) peak using Scherrer's formula D ¼ Kλ=ðwhkl cos θÞ where K is the structure factor. For cubic structure, the lattice parameters (a) can be calculated from Miller indices h, k, l and inter-planar spacing (d) using the equation 2 2 2 a ¼ dðh þ k þl Þ1=2 and Bragg's law. The microstrains were also deduced from XRD data by using the formula ε ¼ whkl =ð4 tan θÞ [4,13]. The calculated crystallite sizes, lattice parameters and microstrains for the samples x¼ 0.1, 0.3 and 0.5 are given in Table 1. The results show that the crystallite sizes and lattice parameters increased slightly by Mn substitution, while the microstrains were relieved. Increases in lattice parameters by increasing Mn-concentration can be attributed to replacement of the smaller ionic radii of Ni2 þ (tetrahedral: 0.55 Å; octahedral 0.69 Å) by bigger ionic radii of Mn2 þ (tetrahedral: 0.655 Å; octahedral: 0.80 Å) [7]. Hussain et al. [7] and Hassan et al. [5] have also reported similar trends for a series of MnxNi1 xFe2O4 compounds. The morphology, particle size and elemental content for the asprepared Mn0:5 Ni0:5 Fe2 O4 sample were determined by HRTEM and HRSEM. The HRTEM and HRSEM micrographs are shown in Figs. 2 and 3 respectively. The corresponding average particle size of about 8.5 nm obtained from the images is in good agreement with the XRD result. The particles appear to be uniform, crystalline and nearly spherical in shape and with hardly any agglomeration extent. Fig. 4 illustrates the EDX spectra for the as-prepared Mn0:5 Ni0:5 Fe2 O4 sample. The microanalysis of the raw EDX data indicates qualitatively the content of the constitutive elements in terms of O (at%: 48.76), Mn (at%: 3.78), Fe (at%: 39.28) and Ni (at%: 8.18). The magnetic field dependence of the magnetization measured at room temperature for the as-prepared samples synthesized at 200 1C and the sample annealed at 500 1C is shown in Fig. 5. The magnetic parameters such as coercive fields (Hc), saturation magnetizations (Ms), remanent magnetizations (MR) and squareness ratio of the loops (MR/MS) deduced from the magnetic hysteresis loops are given in Table 2. The saturation magnetizations have been taken at the highest measured field. The experimental magnetic moment per molecule in the Bohr magneton was obtained using the formula μ ¼ M w M S =5585 where MW is the molar mass [3]. Low values of saturation magnetizations for these samples may be attributed to spin canting due to the presence of Jahn–Teller cations which give rise to Yafet–Kittel angles (αY K). These angles can be estimated using the relation μ ¼ ð6 þ xÞ cos αY K 5ð1 xÞ where x is the Mn-concentration
Fig. 1. XRD patterns for the as-prepared Mnx Ni1 x Fe2 O4 nanoferrites.
Table 1 Crystallite sizes (D), lattice constants (a) and microstrains (ε) for the as-prepared nanoparticle Mnx Ni1 x Fe2 O4 . Sample x
D (nm) 7 0.2
a (Å) 7 0.003
ε (nm) 7 0.00008
0.1 0.3 0.5
8.3 8.6 8.7
8.368 8.369 8.375
0.00137 0.00132 0.00130
Fig. 2. HRTEM microstructure for the as-prepared Mn0:5 Ni0:5 Fe2 O4 crystalline nanoferrite.
[14]. The calculated values for the Yafet–Kittel angles (αY K) and experimental magnetic moments (μ) are also displayed in Table 2. Non-zero values of αY K suggest that the magnetization of the samples cannot be explained on the basis of the Nèel model. The rise of the angles indicates that the triangular type spins arrangements are favored which lead to the weakening of the J AB ðA O BÞ superexchange magnetic interactions between magnetic moments at A-site and B-site [15]. The presence of canted spins has also been reported for Cux Zn1 x Fe2 O4 [16] and Li0:5 x=2 Cdx Fe2:5x=2 O4 [15] spinel ferrites.
172
H.M.I. Abdallah, T. Moyo / Journal of Magnetism and Magnetic Materials 361 (2014) 170–174
Fig. 3. HRSEM micrograph for the as-prepared sample of Mn0:5 Ni0:5 Fe2 O4 nanoferrite.
Fig. 5. Room-temperature magnetic hysteresis loops for the as-prepared samples and samples annealed at 500 1C of Mnx Ni1 x Fe2 O4 .
Fig. 4. EDX spectra for the as-prepared Mn0:5 Ni0:5 Fe2 O4 ferrite.
Fig. 6 shows the hysteresis loops for the as-prepared Mn0:5 Ni0:5 Fe2 O4 sample measured at different isothermal temperatures from 2 to 300 K in external applied fields of up to 5 T. Low values of Hc(T) and MR at 300 K suggest superparamagneticlike behavior of the nanoparticles. The Hc(T) increased gradually at lower measuring temperature reaching about 330 Oe at 2 K. The increase of the magnetic hardness at low temperatures is associated with freezing of spins. A typical variation of coercivity as a function of measuring temperature is given in Fig. 7. The Hc(T) data was fitted over the entire temperature range by Kneller's law H C ðTÞ ¼ H C ð0Þ½1 ðT=T B Þα where TB is the blocking temperature and Hc(0) is the coercive field at absolute zero temperature [4]. The saturation magnetizations plotted against measuring temperature are also shown in Fig. 7. MS(T) varies according to the modified Bloch's law M S ðTÞ ¼ M S ð0Þ½1 ðT=T o Þβ where β is Bloch's factor which depends on the size and surface treatment of the sample [4]. This is associated with single-domain nanoparticles [17,18] due to the confinement effects of the spin-wave spectra for magnetic clusters. The modified Bloch's equation appears to fit the saturation magnetization data over the entire temperature range with β 2:14 7 0:07. Good fits with correlation coefficients of about 0.9721 and 0.9979 are obtained based on Kneller's law and the modified Bloch's law respectively. In small ferromagnetic or ferrimagnetic single-domain particles, superparamagnetism occurs above TB because of weakly
interacting and thermal fluctuations of the spins of the nanoparticles. The thermal effects allow flips of spins between the easy magnetization axes which lead to near zero-coercivity and increase in saturation magnetization [19]. Below TB the thermal fluctuations do not dominate and the superparamagnetic nanosized particles cannot rotate freely but freeze in random orientations leading to high-coercive fields. In the superparamagnetic region, T B oT o T C , the magnetization response of the particles under the influence of an applied field H is given by the standard Langevin function M=M S ¼ LðμH=kB TÞ and/or with the modified Langevin function M=M S LðH=γM S Þ. L is the Langevin's function, μ is the moment of the superparamagnetic particles, MS is the saturation magnetization at temperature T and γ is a constant for a given sample [20,21]. In Fig. 8, the relative magnetization (M/MS) is plotted as a function of H/T and H/MS. The results show that the loops did follow the classical superparamagnetic scaling law with H/T above the blocking temperature of 150 K as expected. We also found that the relative magnetizations obeyed the modified Langevin function with H/MS for over a temperature range of 2– 300 K. Similar results have been observed in Cu90 Co10 granular alloys [20] and Ni nanoparticles [21]. Fig. 9 shows typical zero-field cooling and field cooling (ZFC– FC) magnetization curves measured in an external applied field of 0.01 T for Mn0:5 Ni0:5 Fe2 O4 compound. The ZFC magnetization curve increases gradually with increasing temperature and reaches a maximum value at the blocking temperature of about 150 K. The width of the peak in the ZFC curve is associated with particle size distribution [4]. A particle with a particular size has a certain blocking temperature. Wide peaks are observed in our sample which indicate a wide range in the distribution of particle sizes. The slow decrease in magnetization with further increase in temperature is associated with disordering of particle spins. Mössbauer spectra give quantitative information of hyperfine interactions which reflect the interaction between the nuclei and neighboring electrons. Zero-field Mössbauer spectra acquired at 300 K for the as-prepared samples of Mnx Ni1 x Fe2 O4 are presented in Fig. 10. The spectra have been analyzed in terms of two
H.M.I. Abdallah, T. Moyo / Journal of Magnetism and Magnetic Materials 361 (2014) 170–174
173
Table 2 Coercive fields (HC), saturation magnetizations (MS), remanent magnetizations (MR), experimental magnetic moment per molecule (μ), Yafet–Kittel angles (αY K) and squareness ratio of the loops (MR/MS) for the as-prepared (T1 ¼200 1C) and samples annealed at T2 ¼ 500 1C measured at room temperature for Mnx Ni1 x Fe2 O4 nanoferrites. Sample x
0.1 0.3 0.5
HC (Oe)
MS (emu/g)
αY K(o)
m (μB)
MR (emu/g)
MR/MS
(T1)
(T2)
(T1)
(T2)
(T1)
(T2)
(T1)
(T2)
(T1)
(T2)
(T1)
(T2)
6.61 10.76 0.08
5.87 7.67 6.38
26.74 25.32 53.78
23.26 24.27 24.42
0.162 0.301 0.230
0.161 0.284 0.266
1.12 1.06 2.24
0.97 1.01 1.07
22.87 43.66 43.19
26.17 44.25 57.25
0.0061 0.0119 0.1083
0.0069 0.0117 0.0109
Fig. 6. Hysteresis loops for the as-prepared Mn0:5 Ni0:5 Fe2 O4 nanoferrite sample measured at different isothermal temperatures of 2 K–300 K. The inset figure shows magnified view at about 0.05 T.
Fig. 8. The variations of relative magnetization (M/MS) of the as-prepared Mn0:5 Ni0:5 Fe2 O4 sample measured at different isothermal temperatures plotted as a function of (a) H/T of the Langevin function and (b) H/MS of the modified Langevin function.
Fig. 9. ZFC and FC magnetization curves for the as-prepared Mn0:5 Ni0:5 Fe2 O4 sample measured in an external applied field of 0.01 T. Fig. 7. Coercive field and saturation magnetization plotted with measuring temperature for the as-synthesized Mn0:5 Ni0:5 Fe2 O4 sample. The solid lines are best-fits to the experimental data based on (a) Kneller's law and (b) on the modified Bloch's law.
Zeeman splitting sextets which correspond to Fe3 þ ions at tetrahedral (A) and octahedral (B) sites. The spectra were fitted with Lorentzian-shaped lines. The sextet with the lower hyperfine field
174
H.M.I. Abdallah, T. Moyo / Journal of Magnetism and Magnetic Materials 361 (2014) 170–174
populations of Fe3 þ on B-site compared to A-site may also account for the reduction in the coercive fields in the studied samples [23].
4. Conclusions In summary, the glycol-thermal technique was employed to produce Mnx Ni1 x Fe2 O4 nano-magnetic spinel ferrites at low reaction temperature of 200 1C. The average crystallite diameter for the as-prepared samples is about 8.5 nm. Low values of coercive field and remanent magnetization at room temperature in addition to broadened or relaxation of the Mössbauer spectra for all products strongly suggest that the studied samples confirm the characteristic of superparamagnetism.
Acknowledgments
Fig. 10. Room-temperature Mössbauer spectra for the as-prepared sample Mnx Ni1 x Fe2 O4 .
The authors wish to thank the National Research Foundation (NRF) of South Africa for VSM and mini CFMS equipment grants, Electron microscope Unit at Westville campus (UKZN) for HRTEM and HRSEM measurements, and the University of Al Fashir of Sudan for study leave (HMI). References
Table 3 Isomer shifts (δ), hyperfine fields (H), line widths (Γ) and Fe3 þ fraction population (f) on A- and B-sites for the as-prepared Mnx Ni1 x Fe2 O4 samples measured at 300 K. Sample x
δA (mm/s) 7 0.01
δB (mm/s) 7 0.03
HA (T) 7 0.2
HB (T) 7 0.4
ΓA (mm/s) 7 0.03
ΓB (mm/s) 7 0.06
fA (%) 7 0.7
fB (%) 7 0.6
0.1 0.3 0.5
0.34 0.30 0.33
0.34 0.30 0.35
38.3 40.1 37.6
44.7 45.8 43.9
0.52 1.20 0.90
1.07 0.50 0.27
78.0 71.0 84.9
22.0 29.0 15.1
and isomer shift is assigned to tetrahedral site [4]. The hyperfine parameters such as hyperfine magnetic fields, isomer shifts, line widths and fraction of Fe3 þ ions on A- and B-sites are listed in Table 3. The broadening of the spectra suggests the presence of superparamagnetic nanoparticle ferrites [22]. The existence of the superparamagnetism is also confirmed from the data of the magnetizations. The study of the isomer shift can give information on valence states of the absorber material and in the present case the isomer shift values are corresponding with the Fe3 þ ions. No change to the local environments for Fe3 þ on both A-site and B-site for samples x ¼0.1 and 0.3 was observed. The area ratio of each sextet is found to be directly proportional to the fraction population of Fe3 þ at A-site or B-site. Low values of fraction
[1] Y. Shi, J. Ding, X. Liu, J. Wang, J. Magn. Magn. Mater. 205 (1999) 249. [2] D.T.T. Nguyet, N.P. Duong, L.T. Hung, T.D. Hien, T. Satoh, J. Alloys Compd. 509 (2011) 6621. [3] H.M.I. Abdallah, T. Moyo, J.Z Msomi, J. Phys.: Conf. Ser. 217 (2010) 012141. [4] H.M.I. Abdallah, T. Moyo, J. Alloys Compd. 562 (2013) 156. [5] H.E. Hassan, T. Sharshar, M.M. Hessien, O.M. Hemeda, Nucl. Instrum. Methods . Phys. Res. B 304 (2013) 72. [6] P. Sivakumar, R. Ramesh, A. Ramanand, S. Ponnusamy, C. Muthamizhchelvan, Mater. Res. Bull. 46 (2011) 2204. [7] A. Hussain, T. Abbas, S.B. Niazi, Ceram. Int. 39 (2013) 1221. [8] M. Airimioaei, C.E. Ciomaga, N. Apostolescu, L. Leontie, A.R. Iordan, L. Mitoseriu, M.N. Palamaru, J. Alloys. Comp. 509 (2011) 8065. [9] D.O. Yener, H. Giesche, J. Am. Cer. Soc. 84 (2004) 1987. [10] Y. Köseoğlu, Ceram. Int. 39 (2013) 4221. [11] Y. Köseoğlu, İ. Aldemir, F. Bayansal, S. Kahraman, H.A. Ҫetinkara, Mater. Chem. Phys. 139 (2013) 789. [12] Y. Köseoğlu, M. Bay, M. Tan, A. Baykal, H. Sözeri, R. Topkaya, N. Akdoğan, J. Nanopart. Res. 13 (2011) 2235. [13] M. Sugimoto, J. Am. Ceram. 82 (2) (1999) 269. [14] Y. Gao, Y. Zhang, W. Yu, R. Xiong, J. Shi, J. Magn. Magn. Mater. 324 (2012) 2534. [15] M. Ajmal, A. Magsood, J. Magn. Magn. Mater. 460 (2008) 54. [16] S. Singhal, S.K. Barthwal, K Chandra, J. Magn. Magn. Mater. 306 (2006) 233. [17] H. Hiroyoshi, K. Fukamichi, Phys. Lett. A 58 (4) (1981) 242. [18] K. Mazz, A. Mumtaz, S.K. Hasanian, M.F. Bertino, J. Magn. Magn. Mater. 322 (2010) 2199. [19] M. Pileni, Adv. Func. Mater. 11 (5) (2001) 323. [20] P. Allia, M. Coisson, P. Tiberto, F. Vinai, M. Knobel, M.A. Novak, W.C. Nunes, Phys. Rev. B 64 (2001) 144420. [21] F.C. Fonseca, G.F. Goya, R.F. Jardim, R. Muccillo, N.L.V. Carreno, E. Longo, E. R. Liete, Phys. Rev. B 66 (2002) 104406. [22] J. Hochepied, P. Bonville, M. Pileni, J. Phys. Chem. B 104 (2000) 905. [23] H.M.I. Abdallah, T. Moyo, J.Z Msomi, J. Magn. Magn. Mater. 332 (2013) 123.