Journal of Magnetism and Magnetic Materials 365 (2014) 83–87
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Structural and magnetic properties of Sr0.5Co0.5Fe2O4 nanoferrite Hafiz M.I. Abdallah n, Thomas Moyo, Itegbeyogene P. Ezekiel, Nadir S.E. Osman 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 25 October 2013 Received in revised form 14 February 2014 Available online 28 April 2014
The nanoparticle Sr0.5Co0.5Fe2O4 powder was produced via glycol-thermal process from high-purity metal chlorides at a low reaction temperature of 200 1C. The phase identification of the as-synthesized powder reveals cubic spinel structure with an average crystallite size of 8 nm. Room-temperature Mössbauer spectra for the as-synthesized sample and samples annealed at different temperatures show different local environments of tetrahedral and octahedral coordinated iron cations. Magnetic properties of the as-synthesized sample and samples annealed at 300, 400, 450, 500, 600, 700 and 800 1C have been investigated using a vibrating sample magnetometer at room-temperature in applied magnetic fields of up to about 1.4 T. A substantial increase in coercive field at 300 K from 0.28 kOe to 2.897 kOe was obtained for the as-synthesized and annealed sample at 800 1C. Magnetic field dependence of magnetization curves measured on a mini-cryogen free VTI system operating at a base temperature of 2 K in magnetic fields of up to 5 T have been investigated. The variation of the saturation magnetization as a function of temperature follows modified Bloch's law. Coercive field increased from about 0.28 kOe and 1.04 kOe at 300 K to 11.14 kOe and 10.43 kOe at 2 K for the as-synthesized sample and sample annealed at 500 1C, respectively because of spin-freezing. The effect of exchange bias and Kneller's law are used to account for the temperature dependence of coercive fields. & 2014 Elsevier B.V. All rights reserved.
Keywords: Glycol-thermal Nanoferrite Hyperfine interaction Exchange bias Spin-freezing Coercivity
1. Introduction
2. Experimental details
Spinel ferrites have been studied for both scientific and technological viewpoints. Some focused efforts have been made to enhance the properties of existing materials, develop new materials and fabricate routes leading to better performance and understanding of sample behavior [1]. Spinel ferrites show a large variety of structural, magnetic, electronic and catalytic properties depending on their method of preparation, particle size and composition [2]. They are interesting materials because of several possible applications which include permanent magnets, ferrofluids, recording media, magneto-optic devices, telecommunications, microwave components, biomedical, high-frequency applications and catalysts [2,3]. The aim of this work is to investigate the effects of heat treatments on the structure, hyperfine interactions and magnetic behavior of a nanosized Sr0.5Co0.5Fe2O4 sample synthesized by glycol-thermal route at 300 K and at low temperatures. Evidence of unusual exchange bias effect is also presented after thermal annealing at 500 1C.
The Sr0.5Co0.5Fe2O4 spinel ferrite was produced by using a glycol-thermal process using metal chlorides (i.e. SrCl2 6H2O (98%), CoCl2 6H2O (99%) and FeCl3 6H2O (99%)) without the use of any surfactants. A typical synthesis procedure is described elsewhere [4,5]. The as-synthesized sample was divided into different specimens thereafter annealed at different temperatures in continuous flow of high purity 99.999% argon gas to investigate the structure and magnetic properties. The structure parameters for the as-synthesized sample and samples annealed at 400, 500, 600, 700 and 800 1C were obtained from powder X-ray diffraction (XRD) (using type: PANalytical-EMPYREAN) with a monochromatic beam of Co-Kα radiation (λ ¼1.7903067 Å) in 2θ scanning between 101 and 901. A high-resolution transmission electron microscopy (HRTEM) type: Joel_JEM-2100 instrument was also used to study the microstructure of the sample after thermal annealing at 500 1C. 57Fe Mössbauer spectra of the as-synthesized and annealed samples at 400, 450, 500, 600, 700 and 800 1C were recorded at 300 K in transmission geometry. Room-temperature magnetization measurements were also performed on the assynthesized and annealed samples by using a vibrating sample magnetometer (VSM, type: Lakeshore 735) in applied fields of up to 1.4 T. A typical sample size of about 0.025 g was loaded into the
n
Corresponding author. Cellphone: +27728994179; Fax: +277312607795. E-mail address: hafi
[email protected] (H.M.I. Abdallah).
http://dx.doi.org/10.1016/j.jmmm.2014.04.041 0304-8853/& 2014 Elsevier B.V. All rights reserved.
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sample holder and secured with some cotton to prevent powder movements before tightening the sample holder. A mini-cryogen free measurement (CFM)–VSM system was used to characterize the magnetization through hysteresis loop measurements for the as-synthesized sample and annealed sample at 500 1C at different isothermal temperatures from 2 to 300 K in external applied magnetic fields of up to 5 T.
3. Results and discussion The XRD patterns of Sr0.5Co0.5Fe2O4 nanoferrites of the assynthesized sample and samples annealed at 400, 500, 600, 700 and 800 1C are shown in Fig. 1 which show consistent single-phase spinel structure with increasing crystallite sizes. The crystallite sizes increase with increasing annealing temperature as expected.
The broadened and indexed reflection peaks also show formation of crystallite nanoferrite. The shape, size and morphology of the Sr0.5Co0.5Fe2O4 sample annealed at 500 1C were examined via direct observation by HRTEM micrograph presented in Fig. 2. No agglomeration of nanoparticles was observed. Using Scherrer's formula D ¼ Kλ=ðW hkl cos θÞ, the average crystallite size (D) was estimated. The shape factor parameter K is taken to be 0.9, Whkl is the full-width at half-maximum of the XRD peaks and θ is Bragg's angle. The average lattice parameter was calculated using Bragg's 2 2 2 law and formula a ¼ dðh þ k þ l Þ1=2 where d is the inter-planar spacing and hkl are the Miller indices. The microstrain was also calculated from XRD data using the formula ζ ¼ W hkl =ð4 tan θÞ [1,4]. The obtained crystallite sizes, lattice parameters and microstrains for the samples are presented in Table 1.
Table 1 Crystallite sizes (G), lattice parameters (a) and microstrains (ζ) of the as-synthesized sample and samples annealed at 400, 500, 600, 700 and 800 1C of Sr0.5Co0.5Fe2O4. TA (1C)
D (nm) 7 0.2
a (Å) 7 0.003
ζ 70.00001
As-prepared 400 500 600 700 800
8.6 9.0 10.5 21.6 26.7 31.6
8.378 8.376 8.378 8.387 8.388 8.400
0.00148 0.00127 0.00108 0.000526 0.00581 0.00397
Fig. 1. XRD pattern for the as-synthesized sample and samples annealed at 400, 500, 600, 700 and 800 1C of Sr0.5Co0.5Fe2O4 nanoferrites.
Fig. 2. HRTEM microstructure for Sr0.5Co0.5Fe2O4 annealed at 500 1C.
Fig. 3. Mössbauer spectra for Sr0.5Co0.5Fe2O4 as a function of annealing temperature recorded at 300 K.
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Table 2 Isomer shifts (δ), hyperfine fields (H), line widths (Γ), electric quadrupole splitting (ε) and Fe3 þ fraction (f) on A and B sites for the as-synthesized sample (200 1C) and sample annealed at different temperatures of Sr0.5Co0.5Fe2O4. TA (oC)
δA (mm/s) 70.01
δB (mm/s) 7 0.02
HA (kOe) 72
HB (kOe) 73
ΓA (mm/s) 7 0.02
ΓB (mm/s) 7 0.01
εA (mm/s) 70.007
εB (mm/s) 7 0.0004
fA (%) 70.5
fB (%) 7 0.6
200 400 450 500 600 700 800
0.29 0.31 0.29 0.28 0.31 0.29 0.28
0.32 0.36 0.31 0.35 0.38 0.37 0.37
456 448 449 487 486 492 492
485 483 488 506 513 521 516
0.30 0.36 0.36 0.21 0.14 0.22 0.17
0.24 0.25 0.24 0.26 0.25 0.23 0.22
0.017 0.012 0.026 0.005 0.006 0.012 0.008
0.0012 0.0130 0.0084 0.0009 0.0778 0.1002 0.0828
52.5 43.7 40.4 35.4 67.4 43.6 38.9
47.5 56.3 59.6 64.4 32.6 56.4 61.1
Fig. 4. Hysteresis loops at 300 K for different annealing temperatures for Sr0.5Co0.5Fe2O4 ferrites.
Table 3 Coercive fields (HC), saturation magnetizations (MS), remanent magnetizations (MR) and squareness of the loops (MR/MS) for the as-synthesized sample (200 1C) and sample annealed at different temperatures of Sr0.5Co0.5Fe2O4 measured at 300 K. TA (1C)
HC (Oe) 7 0.3
MS (emu/g) 7 0.05
MR (emu/g) 70.004
MR/MS 7 0.006
200 300 400 450 500 700 800
275.5 422.6 635.4 619.3 902.6 1391.2 2896.9
28.81 27.52 26.03 25.09 24.48 25.3 28.79
3.771 5.567 6.250 6.840 8.139 12.109 11.649
0.131 0.202 0.240 0.273 0.332 0.479 0.405
The modifications of the Mössbauer spectrum for the assynthesized Sr0.5Co0.5Fe2O4 sample due to thermal annealing are indicated by the spectra given in Fig. 3. The magnetic components of the spectra were fitted using the Lorentzian site analysis consistently with two magnetic sextets due to Fe3 þ ions in tetrahedral (A) and octahedral (B) sites of the spinel structure [1]. The Mössbauer parameters obtained from the fits to the spectra of the assynthesized and annealed samples at 400, 450, 500, 600, 700 and 800 1C are given in Table 2. The sextets are assigned to A- or B-sites based on higher values of isomer shifts and hyperfine fields of Fe3 þ ions at octahedral sites associated with a lower degree of covalent bonding. We have clearly different environments between A and B sites (δA o δB and H A o H B ). The values of the isomer shifts for both sites indicate considerable changes with annealing temperature and only indicate the presence of Fe3 þ ions in the sublattice structures. This result suggests that the s-electron densities at the nucleus of the
Fig. 5. The effect of annealing temperature and crystallite size on coercivity measured at 300 K.
Fe3 þ ions are affected by annealing [5]. The small values of the electric quadrupole splitting indicate that A- and B-sites have nearly cubic symmetry and remain in cubic symmetry after the annealing processes. No significant changes for line widths for studied samples have been observed. Magnetization measurements as a function of magnetic field for the as-prepared and annealed samples at different temperatures 300, 400, 450, 500, 600, 700 and 800 1C measured at 300 K are presented in Fig. 4. The coercive fields (HC), saturation magnetizations (MS), remanent magnetizations (MR) and squareness of the loops (MR/MS) deduced from the hysteresis loops are displayed in Table 3. Fig. 5 shows the variation of HC with TA and D. The magnetic parameters are affected by the changes in TA. The saturation magnetizations at room temperature in Table 3 are lower and comparable but were all obtained in maximum applied field of 1.4 T on the Lakeshore VSM. HC depends critically on the domain structure, particle sizes, crystalline anisotropy and population of Fe3 þ at A and B sites [5]. Increase in HC appears to correlate well with increase in particle size due to increase in D
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and TA as the sample transforms from single-domain to multidomain structure. The increase in HC is significant especially for the sample annealed at 800 1C. For 200 o C r T A r 700 o C, we observe a linear correlation between HC and TA with correlation coefficient χ 2 ¼ 0:98274. Changes in MS can be explained based on Néel's theory and distribution of cations at A- and B-sites [5]. Fig. 6 shows some magnetic hysteresis loops for the asprepared and annealed (500 1C) samples taken at different isothermal temperatures in external applied fields of up to 5 T on a CFM–VSM system. At low temperature, the samples become
magnetically harder due to spin-freezing. The coercive field increases with decreasing temperature for the as-prepared sample and reaches a value of 11.14 kOe at 2 K. Some hysteresis loops are observed to be distorted below 80 K for the as-prepared sample and below 200 K for the sample annealed at 500 1C. We suspect the combination of hard and soft magnetic phases to be responsible for this which may also lead to the shoulder observed in the hysteresis loops. The canting of the surface spins, high anisotropy layer, or the cation site disorder of the magnetic ions in the surface layer of nanoparticles can also lead to distortion of the loops [3]. At low temperatures, nanosized ferrimagnets have been reported to exhibit exchange bias which can lead to coercive field enhancement [7]. The spins-disorder and freeze into spin-glasslike shells at surfaces of the nanosized particles. These surface shells can play the role of antiferromagnetic (AFM) shells surrounding the core ferrimagnetic or ferromagnetic (FM) nanoparticles. The exchange bias (Hexch) is usually associated with magnetic coupling at the interfaces between AFM and FM layers [7,8]. This is defined as H exch ðTÞ ¼ ½H C þ ðTÞ H C ðTÞ=2 where H C þ ðTÞ and H C ðTÞ are the absolute values of positive and negative coercive fields. The variations of the exchange bias with measuring temperature are displayed in Fig. 7. Significant exchange bias is observed at low temperatures for the sample annealed at 500 1C. This may be related to incipient sintering of nanoparticles into larger particles and forming new grain boundaries. This is consistent with previous results which show larger bias at larger particle sizes [9,10]. In the core/shell nanoparticle, the core spin particles are expected to align while the shell spins particles are in randomized state due to broken exchange bonds at the surface of the nanoparticles [7]. This allows the shell to freeze into random directions at the lowest temperatures. Similar behavior has been observed in (Mg, Sr)0.2Mn0.1Co0.7Fe2O4 samples [1]. Variations of HC and MS for the as-synthesized sample and annealed at 500 1C with measuring temperature are shown in Fig. 8. Saturation magnetization as a function of measuring temperature for bulk ferrimagnetic or ferromagnetic assembly below the Curie temperature follows the modified Bloch's law M s ðTÞ ¼ M S ð0Þ½1 ðT=T o Þβ where M S ð0Þ is the spontaneous
Fig. 6. Variation of some hysteresis loops as a function of measuring temperature for the as-synthesized sample and annealed sample at 500 1C of Sr0.5Co0.5Fe2O4 ferrites.
Fig. 7. Exchange bias plotted with measuring temperature for as-synthesized sample and annealed sample at 500 1C.
Fig. 8. Coercivity and magnetization plotted with measuring temperature for assynthesized sample and annealed sample at 500 1C (dots). The solid lines are based on modified Bloch's law and Kneller's law.
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Table 4 The fitting parameters obtained using the best-fit curves to the data based on modified Bloch's law and Kneller's law. Bloch's law Sample
MS(0) (emu/g) 7 0.2
T o (K) 7 17
β 7 0.08
T f (K) 7 0.7
Ao 7 0.0002
χ2
As-prepared 500 1C
71.9 76.7
636 514
2.63 2.48
– 96.6
– 0.0165
0.9929 0.9930
Kneller's law Sample
HC(0) (T) 7 0.6
T B (K) 74
α 7 0.06
χ2
As-prepared 500 1C
12.6 11.3
254 339
0.54 0.55
0.9818 0.9948
magnetization, To is the temperature at which the spontaneous magnetization can vanish and β is an exponent. The temperature dependence of the saturation magnetizations for the as-prepared sample is consistent with confinement effects of the spin-wave spectrum [1]. The fit parameters include M S ð0Þ, To, and β and are given in Table 4. At the lowest temperatures, there are slightly decreased trends in the magnetization especially for the annealed sample which may be due to the spin-glass like layer at the surfaces of nanosized particles which prevent the core spins to follow the direction of external applied magnetic fields [7]. For the sample annealed at 500 1C, the experimental data is successfully fitted by an expression which includes an exponential term in addition to modified Bloch's formula of the form [3,11] M s ðTÞ ¼ M S ð0Þ½1 ðT=T o Þβ þAo expð T=T f Þ where Ao is a pre-exponential factor of the surface contribution and T f is the freezing temperature at which the surface spins become frozen in a spin-glass-like structure [3,11]. The first term shows the contribution of the thermal effect while the second term shows the effect of surface spin to saturation magnetization of the sample. The fit parameters are given in Table 4. The spin-freezing temperature is fitted to be T f ¼ 96:6 K. This confirms that the decrease in saturation magnetization at the lowest temperature for sample annealed at 500 1C is due to the “weak-response from dead-magnetization layer on the particle surface” [11] which is not the case for the as-synthesized sample. Similar results have been observed in nanoparticle systems [3,10,11]. The temperature-dependence of coercive field for the as-prepared sample and sample annealed at 500 1C can be fitted by Kneller's formula [8,12] H C ðTÞ ¼ H C ð0Þ½1 ðT=T B Þα with correlation coefficient of 0.9818 and 0.9948, respectively. H C ð0Þ is the coercive field at 0 K, T B is the blocking temperature and α is the Kneller's exponent. The fit parameters for the constants are given in Table 4. Similar results have also been reported for nanosized particles with uniaxial symmetry [1]. 4. Conclusions Significant differences between the as-synthesized sample and the sample annealed at 500 1C in terms of HC(T), MS(T), exchange bias and hysteresis loops distortions have been observed in Sr0.5Co0.5Fe2O4 nanoferrites. The exchange bias effect in the sample annealed at 500 1C is attributed to increased nanoparticle size and presence of FM ordered core with spin disorder in the surface shell region. The slight decrease trends in the magnetizations especially for the annealed sample is attributed to the spinglass like layer at the surfaces of the nanosized particles which
prevent the core spins to follow the direction of external applied magnetic fields. This result is correlated with the onset of an exponential term (the surface spin effect to saturation magnetization) in the temperature dependence of saturation magnetization in addition to modified Bloch's formula. The hyperfine interactions and magnetization parameters are dependent on the annealing temperature, particle size and fractional population of Fe3 þ ions on sublattices. The coercive field was found to follow Kneller's law.
Acknowledgments The authors wish to thank the National Research Foundation (NRF) of South Africa for VSM and miniCFM equipment grants, Electron Microscope Unit, UKZN for HRTEM measurement and University of Al Fashir, Sudan for study leave (HMI). References [1] H.M.I. Abdallah, T. Moyo, Structural and magnetic studies of (Mg, Sr)0.2Mn0.1Co0.7Fe2O4 nanoferrites, J. Alloys Compd. 562 (2013) 156–163. [2] S.K. Pardeshi, R.Y. Pawar, SrFe2O4 complex oxide an effective and environmentally begin catalyst for selective oxidation of styrene, J. Mol. Catal. A: Chem. 334 (2011) 35–43. [3] C. Vázquez-Vázquez, M.A. López-Quintela, M.C. Buján-Núñez, J. Rivas, Finite size and surface effects on the magnetic properties of cobalt ferrite nanoparticles, J. Nanopart. Res. 13 (2011) 1663–1676. [4] H.M.I. Abdallah, T. Moyo, J.Z. Msomi, Magnetic properties of Mg0.5Mn0.5(RE)0.1Fe1.9O4 ferrites synthesized by glycol-thermal method, J. Magn. Magn. Mater. 332 (2013) 123–129. [5] H.M.I. Abdallah, T. Moyo, The influence of annealing temperature on the magnetic properties of Mn0.5Co0.5Fe2O4 nanoferrites synthesized via mechanical milling method, J. Supercond. Nov. Magn. 26 (2012) 1361–1367. [7] J. Nogués, J. Sort, V. Langlais, V. Skumryev, S. Suriñach., J.S. Muñoz, M.D. Baró, Exchange bias in nanostructures, Phys. Rep. 422 (2005) 65–117. [8] F.G. Silva, R. Aquino, F.A. Tourinho, V.I. Stepanov, Y.L. Raikher, R. Perzynski, J. Depeyrot, The role of magnetic interactions in exchange bias properties of MnFe2O4@γ-Fe2O3 core/shell nanoparticles, J. Phys. D: Appl. Phys. 46 (2013) 285003. [9] K. Mbela, T. Moyo, J.Z. Msomi, M. Öztürk, N. Akdoģan, Synthesis and magnetic properties of Mg0.2Cr1.8 xFexO3 nanoparticles, J. Magn. Magn. Mater. 330 (2013) 159–162. [10] E. Manova, B. Kunev, D. Paneva, I. Mitov, L. Petrov, C. Estournès, C. D'Orlèans, J. Rehsspringer, M. Kurmoo, Mechano-synthesis, characterization and magnetic properties of nanoparticles of cobalt ferrite, CoFe2O4, Chem. Mater. 16 (2004) 5689–5696. [11] R. Gao, Y. Zhang, W. Yu, R. Xiong, J. Shi, Superparamagnetism and spin-glass like state for the MnFe2O4 nanoparticles synthesized by the thermal decomposition method, J. Magn. Magn. Mater. 324 (2012) 2534. [12] X.H. Huang, J.F. Ding, G.Q. Zhang, Y. Hou, Y.P. Yao, X.G. Li, Size-dependent exchange bias in La0.25Ca0.75MnO3 nanoparticles, Phys. Rev. B 78 (2008) 224408.
