The effect of neodymium substitution on the structural and magnetic properties of nickel ferrite

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Progress in Natural Science Materials International Progress in Natural Science: Materials International 25 (2015) 215–221 www.elsevier.com/locate/pnsmi www.sciencedirect.com

Original Research

The effect of neodymium substitution on the structural and magnetic properties of nickel ferrite İsrafil Şabikoğlua,n, Levent Paralıb, Ondrej Malinac, Petr Novakc, Josef Kaslikc, Jiri Tucekc, Jiri Pechousekc, Jakub Navarikc, Oldrich Schneeweissd a

Celal Bayar University, Faculty of Arts&Sciences, Department of Physics 45140 Muradiye, Manisa, Turkey b Celal Bayar University, Department of Electronics and Automation, 45400 Turgutlu, Manisa, Turkey c Regional Centre of Advanced Technologies and Materials, Department of Experimental Physics and Physical Chemistry, Faculty of Science, Palacky University, 17. listopadu 1192/12, 77 146 Olomouc, Czech Republic d Institute of Physics of Materials of the Academy of Sciences of the Czech Republic, Zizkova 22, 61662 Brno, Czech Republic Received 9 October 2014; accepted 13 February 2015 Available online 2 July 2015

Abstract Nickel ferrite NiFe2O4 is a typical soft magnetic ferrite with high electrical resistivity used as high frequency magnetic material. Neodymium (Nd3 þ ) doped NiFe2O4 materials were fabricated using solid state reaction. The properties of the obtained material were investigated by X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), Fourier-Transform Infrared Spectroscopy (FT-IR), magnetic measurements on SQUID and Mössbauer spectroscopy. It was found that the material consists of two different phases: Nd3 þ doped NiFe2O4 and NdFeO3. The Nd3 þ ions occupy cation sites of the NiFe2O4 inverse spinel structure. NdFeO3 phase occurred when the level of Nd3 þ atoms exceed a percolation limit. The presence of both phases was confirmed by SEM observations. The Mössbauer spectra analysis showed two sextets, which can be ascribed to iron atoms in tetrahedral and octahedral positions. From their intensities it is concluded that Nd3 þ occupies octahedral sites in the spinel structure of NiFe2O4, which were originally occupied by Ni2 þ . & 2015 The Authors. Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: NiFe2O4; Neodymium; Solid-state reaction; Magnetic properties; Mössbauer spectroscopy

1. Introduction The spinel ferrites constitute an important group of magnetic materials both from the fundamental and applied reasons. Generally, spinel ferrites are described by a stoichiometric formula given as MeFe2O4, where Me stands for metal or metallic elements with 2þ valence state. These materials crystallize adopting the cubic spinel structure (Fd3m space group) [1,2]. Within the spinel cubic crystal structure two crystallographic non-equivalent cation sites are recognized, i.e., tetrahedral (T) sites and octahedral (O) sites. In the normal spinel structure, the T sites are occupied by divalent metal n

Corresponding author. Tel.: þ90 236 201 31 26; fax: þ 90 236 241 21 58. E-mail address: israfi[email protected] (İ. Şabikoğlu). Peer review under responsibility of Chinese Materials Research Society.

atoms and Fe3 þ locates on the O sites. However, in some cases, divalent metal atoms can replace half of Fe3 þ atoms on the O sites and thus, the T sites are completely filled with Fe3 þ atoms, i.e., (Fe3 þ )T(Me2 þ Fe3 þ )OO4. Me is most frequently Zn, Cu, Co, Ni etc. and it is well known that Ni, Cu, and Co ferrites are materials with inverse spinel structure, while Zn ferrite is a normal spinel [3–6]. The nickel ferrite NiFe2O4 is a ferrimagnetic material when T sites and O sites form magnetic sub-lattices [1,7]. In the NiFe2O4 unit cell, Fe3 þ ions occupy 8 tetrahedral and 8 octahedral crystallographic sites, i.e., it exhibits the inverse spinel structure [8]. It has been shown that the calculated spin moment of Fe ions at T sites is 3.36 μB, while average moment for the O sites is 2.45 μB [9]. Thus, the net moment is oriented in the direction of the T sub-lattice magnetization.

http://dx.doi.org/10.1016/j.pnsc.2015.06.002 1002-0071/& 2015 The Authors. Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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To synthesize NiFe2O4, several preparation routes have been proposed including co-precipitation [3,10], combustion synthesis [11,12], the reverse micelle technique [13], electrospinning [14], thermal decomposition [15–17], synthesis by thermal plasma [18], high-energy milling [19,20], sol–gel method [21,22], microwave-hydrothermal method [23], and the solid-state method [24]. This broad spectrum of syntheses has several advantages and drawbacks. In particular, if the preparation of nickel ferrite by the solid state reaction or by coprecipitation is chosen, high temperatures (600–800 1C) are required in order to yield good crystallinity powder [3,24]. On the other hand, during the sol–gel method, there is a possibility to control either stoichiometry or size distribution of nanoparticles [21]. It seems very interesting to investigate the effect of Fe substitution by the rare earth ions (RE) on structural, magnetic, and electric properties of NiFe2O4. The existence of unusual properties of doped nickel ferrite is consequence of the introduction of the rare earth atoms into the spinel structure, which leads to a change in the spin coupling. The magnetic behavior of such compound is not governed only by Fe–Fe interaction (3d electrons), but also by RE–Fe interaction which originates from 3d–4f spin coupling; this interaction triggers changes in magnetic properties [25,26]. The effect of Y-, Eu- or Gd-for Fe substitution has been studied by Sun et al. [27]. it was found that the substitutions of all rare earth ions (Y, Eu and Gd) greatly increase the electrical resistivity and decrease relative loss factor (RLF). Substitution of Nd3 þ and its influence on magnetic properties was reported by Shinde et al. [28] observing a decrease in the Curie temperature and increase in the saturation magnetization with increasing Nd3 þ content. Harish et al. [29] successfully reported a dramatic change in the photocatalytic activity of the inert NiFe2O4 and highly solar active Nd substituted NiFe2O4. On the other hand, Bharathi et al. [30] reported the measurement of dielectric and magnetic properties of Gd- and Nd-doped nickel ferrites. It was found that the saturation magnetostriction does not change for NiOFe1.95Gd0.05O3 and NiOFe1.95Nd0.05O3; also, a small amount of NdFeO3 phase was identified in this Nd-substituted nickel ferrite. Sometimes, during the synthesis of such ferrites (iron perovskite), NdFeO3 can be synthesized as an additional product. The crystal structure of NdFeO3 has been described as orthorhombic with the Pbnm space group. From the magnetic viewpoint, NdFeO3 shows an antiferromagnetic ordering of the Fe3 þ magnetic moments with the Néel temperature TN ¼ 760 K [31]. In this work, we report on changes of structural and magnetic properties of NiFe2O4 doped with various level of Nd. The substitution-induced changes were monitored by X-ray powder diffraction (XRD), Furrier Transform Infrared spectroscopy (FTIR), magnetization measurements, and 57Fe Mössbauer spectroscopy. From the experimental data, the position of Nd ions in the spinel structure of NiFe2O4 was investigated. 2. Materials and experimental methods All materials were prepared from stoichiometric amounts of high-purity NiO, Fe2O3, and Nd2O3 (Sigma-Aldrich499.99%), also were synthesized with various dope ranges (0.2, 0.3, 0.4,

0.5, and 5.0% (mol) by solid-state reaction. For undoped material, the Fe2O3 and NiO were weighed with precision balance, and then mixed in the agate mortar. Finally, all mixed materials were heated in cylindrical oven at 1200 1C for 12 h and then were cooled down to room temperature. For Nd doped material, the same procedure was applied again at 1250 1C per material using Nd2O3 chemical. XRD measurements were carried using X'Pert PRO MPD diffractometer (PANalytical) in Bragg-Brentano geometry equipped with a Co X-ray tube (iron filtered CoKα radiation: λ¼ 0.178901 nm), fast X'Celerator detector and programmable divergence and diffracted beam antiscatter slits. All materials were prepared on zero-background Si slide and measured in 2θ range 5–1051 (resolution 0.0171 2θ). Commercial standards SRM640 (Si) and SRM660 (LaB6) from NIST (National Institute of Standards and Technologies) were used for line position and instrumental line broadening evaluation. High Score plus software (PANalytical) in conjunction with PDF4þ and ICSD databases were used for crystalline phase identification and Rietveld refinement, respectively. The morphology and grain size of the prepared powders were monitored with a scanning electron microscopy (Philips XL-30 S FEG, SEM) and chemical composition was assessed by Energy Dispersive X-Ray (EDX) analysis. FTIR spectra of the materials were measured at room temperature using an Agilent Cary 660 FTIR-ATR system. All measurements of the materials were reported in the spectral range from 400 to 1500 cm  1. The superconducting quantum interference device (SQUID, MPMS XL-7, Quantum Design, USA) was used for magnetization measurements. Hysteresis loops were recorded at a temperature of 300 K and in externally magnetic fields ranging from  5 to þ 5 T. The 57Fe Mössbauer spectra of the studied materials were measured at room temperature employing a MS2007 Mössbauer spectrometer based on virtual instrumentation technique [32,33], operating at a constant acceleration mode and equipped with a 57Co(Rh) source. The acquired Mössbauer spectra were processed (i.e., noise filtering and fitting) using the MossWinn software program [34]. The isomer shift values are referred to α-Fe foil sample at room temperature. 3. Results and discussion 3.1. XRD and SEM In the XRD diffraction patterns, the NiFe2O4 phase (PDF4þ number: 01-078-6781) was identified for the materials with all Nd concentrations. For the material with 0.5% Nd concentration in the reaction mixture the additional peaks were observed and the analysis showed that they correspond to NdFeO3 (PDF-4þ number: 01–082-2421) (see Fig. 1 and Table 1). From the XRD analysis, it turned out that the Nd atoms are substituted into the spinel structure of NiFe2O4, and NdFeO3 phase formed when the Nd content exceeded a percolation limit, i.e., limit when the crystal structure of NiFe2O4 cannot accommodate more Nd atoms.

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High resolution SEM images of NiFe2O4 materials are shown in Fig. 2. The uniform nature of the grains is shown in some agglomeration. All the images show well-defined crystals with different shapes. The linear intercept method was used for the calculation of average grain size. It was observed that the average grain size decreased from 3.3 to 1.8 μm with increasing Nd concentrations in the material. The similar results have been reported [28,35]. The backscatter images indicate the presence of grains of different phases in the Nd doped samples. Their number was maximum in the sample doped by 5% Nd and therefore we ascribe these grains to NdFeO3 phase which was identified by XRD and Mössbauer spectroscopy. However, this second phase grains appear in all samples except for the undoped sample in contrary to XRD and Mössbauer spectroscopy results. The pictures show that the grain volumes of NdFeO3 phase are substantially smaller than those of ferrite. It could be an explanation that the finer coherent volumes of NdFeO3 do not show clear XRD diffraction lines.

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v1 is caused by the stretching vibrations of tetrahedral metal– oxygen bond, and the absorption band v2 is caused by the stretching vibrations of octahedral metal–oxygen bond [37]. It is seen from Fig. 3, the high frequency band  600 cm  1 peak is the stretching vibration of tetrahedral metal oxygen bond between Fe3 þ and O2  that commonly features in the ferrites [38]. The other specific frequency bands were not observed due to the low resolution of the spectrometer. As it is seen in Fig. 3, the Fe–O stretching vibration has a little shifting to lower wavenumber, and there is difference between host and doped materials. Similarly shifting in the band positions is reported in the literature [28,39]. Doped materials have bigger/smaller wavenumber value than host material at  600 cm  1 (v1). This shift is due to the disorder occurring between Fe3 þ and O2  bond kind of substituting Nd3 þ atoms [28]. The force constants of doped and undoped NiFe2O4 materials was calculated by using the following classic equation, k ¼ 3:51 U1022 ϑ2 m; where, ϑ, wavenumber, and m, reduced mass. The force constants and dope rates (mol) are shown in Fig. 4. According to dope rate of Nd, the force constants have increasing values according to dope rate of Nd except for 5% mol dope rate. In general, the force constants of NiFe2O4 materials doped with Nd show bigger value than host material. Nd-O vibration can be caused an increase of force constant in the tetrahedral site. Also 5% mol doped material has lowest force constant. This can be due to the high dope rate to the host material and the occurrence of a new phase (NdFeO3) in the octahedral site. Also, the NdFeO3 phase can split spectrum in the v2 frequency band, and achieve small distortion of the spinel symmetry of the lattice [40].

3.2. FT-IR spectroscopy The infrared absorption spectra of materials are shown in Fig. 3. AB2O4 spinel type metals have four infrared active modes which are 650–550 Hz (v1), 525–390 Hz (v2), 380– 335 Hz (v3), and 300–200 Hz (v4) [36]. The absorption band

3.3. Magnetic measurement Hysteresis loops of all materials were measured at 300 K in external magnetic fields up to 7 5 T. Fig. 5 shows the hysteresis curves for NiFe2O4 with the different level of Nd3 þ doping. The derived parameters are listed in Table 2. The values of the saturation magnetization MS is decreasing with increasing in Nd3 þ content. This can be the consequence of the Nd3 þ - for - Ni2 þ substitution. The effective magnetic moment of Nd3 þ ( 3.62 μB) is higher than Ni2 þ ( 2.83 μB) [41,42] which can lead to a decrease in the magnetization. This

Fig. 1. The XRD patterns of undoped and Nd-doped NiFe2O4 materials.

Table 1 Quantification of phase content in measured materials (MCL - mean coherent length; a, b and c - lattice constants). Material

NiFe2O4

NdFeO3 a

MCL (nm)

a (nm)

Phase content (wt%)a

MCL (nm)

a; b; c (nm)

92.8

502

0.8340

7.2

227

100.0

440

0.8340

–

–

0.5585 0.7663 0.5453 –

Phase content (wt%) NiFe2O4:Nd (5%)

NiFe2O4:Nd (0.5%, 0.4%, 0.3%, 0.2%); NiFe2O4 undoped a

Quantification error of Rietveld's analysis is up to 1 wt%

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T%

0.5% 0.4% 0.3% 0.2% 5% Host

1300

1100

900

700

500

Wavenumber (1/cm) Fig. 3. FTIR spectroscopy of the ferrite materials.

Fig. 4. Force constants of all materials.

Fig. 2. The SEM of the undoped NiFe2O4 (a), and the Nd doped NiFe2O4 (b– f), left in backscatter electrons and right in secondary electrons. The backscatter images indicate presence of two different phases' grains. The second phase (NdFeO3) is labeled by the red circles.

Fig. 5. Hysteresis loops of NiFe2O4 host and Nd doped materials at 300 K.

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Table 2 Values of the hysteresis parameters derived from room-temperature hysteresis loops of all the studied materials, where MS is saturation magnetization, HC is coercivity and MR is remanent magnetization. Material

MS (Am2/kg)

HC (mT)

MR (Am2/kg)

NiFe2O4 host NiFe2O4_Nd (0.2%) NiFe2O4_Nd (0.3%) NiFe2O4_Nd (0.4%) NiFe2O4_Nd (0.5%) NiFe2O4_Nd (5.0%)

51.75 48.09 49.66 48.68 44.10 36.88

0.84 0.73 0.50 0.34 0.48 0.14

1.27 1.15 0.64 0.50 0.45 0.11

Fig. 6. Details of hysteresis loops of NiFe2O4 host and Nd doped materials at 300 K.

is due enhancement of the octahedral sublattice magnetization at the expense of the tetrahedral sublattice magnetization involving evolution of vacancies due to the charge compensation during substitution. A decrease in the value of the saturation magnetization in the case of 5% Nd doped material could be explained by higher amount of NdFeO3 phase which is antiferromagnetic at room temperature [31]. A change in the slope of the magnetization vs. field (Fig. 6) with increasing Nd content can be taken as the result of some factors, i.e. change in magnetocrystalline anisotropy, particle size, etc. The observed changes in MS for the different content of Nd are in a good agreement with some published results. Bharathi et al. [43] have reported that MS of pure NiFe2O4 at 300 K (49.8 Am2/kg) decreased with partial substitution of Fe3 þ by Sm3 þ (42.5 Am2/kg) and Ho3 þ (45 Am2/kg). Zhao et al. [44] have shown a decrease in MS with the increasing Nd3 þ content in Ni0.7Mn0.3NdxFe2-xO4 ferrites. The Nd3 þ substitution alters the magnetic anisotropy in the NiFe2O4 due to the difference between the magneto-crystalline anisotropy (K) of Nd and Ni atoms in octahedral coordination (|K|E 1.5  105 J/m3 and 5.7  103 J/m3 for Nd3 þ and Ni2 þ , respectively [42,45,46]). The substitution influences the

Fig. 7. Room-temperature Mössbauer spectra of undoped NiFe2O4 and Nd doped materials and their deconvolution into individual spectral components.

coercivity Hc as well, which decreases with increasing Nd substitution level (Fig. 6, Table 2) similarly as in the previous study [44].

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Table 3 Values of the Mössbauer hyperfine parameters, derived from fitting of the Mössbauer spectra, recorded for all the studied material at room temperature, where δ is the isomer shift, ΔEQ is quadrupole splitting, Bhf is hyperfine magnetic field, and RA is the spectral area of individual spectral components. Material

Site (sextet)

δ (mm/s) 7 0.01

ΔEQ (mm/s)7 0.01

Bhf (T)70.3

RA (%)7 1

NiFe2O4_undoped

Octahedral Tetrahedral Octahedral Tetrahedral Octahedral Tetrahedral Octahedral Tetrahedral Octahedral Tetrahedral Octahedral Tetrahedral NdFeO3

0.36 0.26 0.36 0.26 0.36 0.26 0.36 0.26 0.36 0.26 0.35 0.25 0.42

0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.10

52.7 49.5 52.8 49.6 52.8 49.5 52.6 49.4 52.7 49.4 52.4 49.2 54.4

48 52 48 52 48 52 48 52 48 52 46 47 7

NiFe2O4:Nd (0.2%) NiFe2O4:Nd (0.3%) NiFe2O4:Nd (0.4%) NiFe2O4:Nd (0.5%) NiFe2O4:Nd (5.0%)

3.4. Mössbauer spectroscopy The Mössbauer spectra of the studied materials, recorded at room temperature, are shown in Fig. 7 and the values of the Mössbauer hyperfine parameters derived from spectrum fits are listed in Table 3. Mössbauer spectrum of the bulk NiFe2O4 can be fitted by two sextets. The first sextet represents iron ions in the tetrahedral positions and the second sextet iron ions in octahedral positions of the NiFe2O4 spinel structure. In an ideal stoichiometric structure, the numbers of iron ions in tetrahedral and octahedral positions are equal and, therefore, the intensity ratio of the corresponding sextets must be 1:1. Similar results were obtained by Lee et al. [47] and Sorrescu et al. [48]. These authors concluded that Ni2 þ ions prefer octahedral sublattice of magnetite and are distributed randomly over the crystal. At room temperature, these two sextets can be clearly distinguished due to different values of the isomer shift (δ) and hyperfine magnetic field (Bhf). Typically, the sextet with δ¼ 0.23 mm/s and Bhf ¼ 49.5 T is ascribed to iron ions in the tetrahedral positions and the sextet with δ¼ 0.35 mm/s and Bhf ¼ 52.5 T to the octahedral ones. It should be noted that the sextets may become overlapped when studying systems consisting of very fine NiFe2O4 nanoparticles ( o20 nm); in this case, an external magnetic field must be used to resolve the sextets [49]. Mössbauer spectrum of the NiFe2O4 was fitted by two sextets mentioned above with the intensity ratio close to 1:1, which implies an equal distribution of Fe3 þ ions over the tetrahedral and octahedral sites. In the spectra of the doped materials, no significant changes in the intensity ratios were observed. This can be explained in the way that Nd3 þ substituted Ni2 þ on octahedral sites (the ionic radius of Nd3 þ and Ni2 þ is 98 pm and 69 pm, respectively [50]). Indeed Nd3 þ cannot substitute Fe3 þ ions due to narrower volume of the tetrahedral position of iron ions ( 49 pm [50]). The substitution Nd3 þ for Ni2 þ must be compensated by introduction of vacancies carrying a negative charge. As the

spectral ratio between the two sextets remains almost identical, vacancies must be formed on the tetrahedral sites. A percolation limit for increasing concentration of Nd atoms and the formation of a new phase can be expected. It corresponds with the formation of NdFeO3 phase which was detected by XRD and confirmed by SEM. In the Mössbauer spectrum of the sample with 5% Nd the additional third sextet was analyzed with hyperfine parameters δ ¼ 0.42 mm/s, ΔEQ ¼ 0.10 mm/s, and Bhf ¼ 54.4 T. It was ascribed to Fe3 þ ions in the NdFeO3 phase in accordance to previous study [51]. However, in contrary to SEM the corresponding sextet of NdFeO3 was not identified in the spectra of the samples with lower Nd concentrations because of low intensity and strong overlapping with the sextet of NiFe2O4. 4. Conclusions The phase analysis of the samples of NiFe2O4 doped by various content of Nd showed that they consist of two different phases: Nd3 þ doped NiFe2O4 and NdFeO3. NdFeO3 phase occurred in the sample where 5% Nd doping was applied. These two phases were confirmed by SEM observations. From the Mössbauer spectra analysis, it can be concluded that Nd atoms (Nd3 þ ions) substitute Ni atoms (Ni2 þ ions) in the octahedral sites probably due to its ionic radius which is close to the limiting size of the octahedral sites. The occurrence of the NdFeO3 impurity phase reflects the limiting capacity of the NiFe2O4 lattice to incorporate Nd ions in its crystal structure. In other words, the concentration of Nd atoms is so high that it is not possible to substitute all Ni2 þ ions in the octahedral sites by Nd atoms available in the reaction system. Thus, the excess of Nd atoms favors the formation of the NdFeO3 phase. All NiFe2O4 materials have well-defined crystals with different shapes, the new NdFeO3 phase could be identified in the SEM images as well. The NdFeO3 phase occurrence increases with the ratio of Nd. The average grain size decreases with an increase of content of the rare earth atoms. A typical NiFe2O4 spinel type metal's infrared active modes are observed at 650–550 Hz (v1). The Fe–O stretching vibration is shifted

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due to dope rate of Nd atoms, and the force constants increase according to the dope rate of Nd. The force constant of the 5% mol doped NiFe2O4 material shows lower values in comparison with other material. It may be due to the saturated state, and a new phase has emerged in the structure. Because of the new phases split spectrum, the new phase bond between Nd and O is demonstrated at the v2 frequencies. Acknowledgments The authors acknowledge the support by the Operational Program Research and Development for Innovations – European Regional Development Fund (CZ.1.05/2.1.00/ 03.0058), Operational Program Education for Competitiveness – European Social Fund (CZ.1.07/2.3.00/20.0017, CZ.1.07/ 2.3.00/20.0170, CZ.1.07/2.3.00/20.0155, and CZ.1.07/2.3.00/ 20.0058) of the Ministry of Education, Youth and Sports of the Czech Republic, and internal IGA grant of Palacký University (IGA_PrF_2014017). References [1] V.A.M. Brabers,, Elsevier, 189–325, 1995. [2] B.D. Cullity, C.D. Graham, New Jersey: IEEE Press & Wiley, 2009. [3] A.S. Albuquerque, J.D. Ardisson, W.A.A. Macedo, J.L. López, R. Paniago, A.I.C. Persiano., J. Magn. Magn. Mater. 226 (2001) 1379–1381. [4] J. Cuda, I. Mousa, B. David, N. Pizúrová, J. Tuček, T. Žák, M. Mašláň, et al., Mössbauer spectroscopy in materials science, AIP Conf. Proc. 1489 (2012) 123–132. [5] G.R. Dube, V.S. Darshane., J. Mol. Catal. 79 (1993) 285–296. [6] V.P.M. Kurikka Shafi, Y. Koltypin, A. Gedanken, R. Prozorov, J. Balogh, J. Lendvai, et al., J. Phys. Chem. B 101 (1997) 6409–6414. [7] T.F. Marinca, I. Chicinas, O. Isnard, V. Pop, F. Popa, J. Alloy. Compd 509 (2011) 7931–7936. [8] T.B. Sankaam, G.S. Kumar, G. Prasad. India: BS publications, 2005. [9] H.T. Jeng, G.Y. Gou., J. Magn. Magn. Mater. 240 (2002) 436–438. [10] K. Maaz, S. Karim, A. Mumtaz, S.K. Hasanain, J. Liu, J.L. Duan., J. Magn. Magn. Mater. 321 (2008) 1838–1842. [11] L. Liu, H. Wang, Y. Yang, Z.M. Liu, H.F. Yang, G.L. Shen., Sens Actuators B 102 (2004) 148–154. [12] S. Balaji, R.K. Selvan, L.J. Berchmans, S. Angappanb, K. Subramanian, C.O. Augustin, Mater. Sci. Eng. B 119 (2005) 119–124. [13] A. Kale, S. Gubbala, R.D.K. Misra, J. Magn. Magn. Mater. 277 (2004) 350–358. [14] D. Li, T. Herricks, Y. Xia, Appl. Phys. Lett. 83 (2003) 4586–4588. [15] N.S. Gajbhiye, S. Prasad., Thermochim Acta 285 (1996) 325–336. [16] S.W. Lee, S. Bae, Y. Takemura, E. Yamashita, J. Kunisaki, S. Zurn, et al., IEEE Trans. Magn. 42 (2006) 2833–2835. [17] M. Stoia, P. Barvinschi, L.B. Tudoran, M. Barbu, M. Stefanescu, J. Therm. Anal. Calorim. 108 (2012) 1033–1039. [18] A.B. Nawale, N.S. Kanhea, K.R. Patil, S.V. Bhoraskar, V.L. Mathe, A.K Das., J Alloy. Compd. 509 (2011) 4404–4413.

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