- Email: [email protected]
ZnFe2O4 nanocomposites
Accepted Manuscript Research articles Effect of annealing on structural and magnetic properties of NiFe2O4/ZnFe2O4 nanocomposites C.N. Anumol, M. Chithra, M. Govindaraj Shalini, Subasa C. Sahoo PII: DOI: Reference:
S0304-8853(18)31854-7 https://doi.org/10.1016/j.jmmm.2018.08.036 MAGMA 64233
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
15 June 2018 7 August 2018 14 August 2018
Please cite this article as: C.N. Anumol, M. Chithra, M. Govindaraj Shalini, S.C. Sahoo, Effect of annealing on structural and magnetic properties of NiFe2O4/ZnFe2O4 nanocomposites, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.08.036
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of annealing on structural and magnetic properties of NiFe2O4/ZnFe2O4 nanocomposites
C.N. Anumol1, M. Chithra1, M. Govindaraj Shalini1, Subasa C. Sahoo1* 1Department
of Physics, Central University of Kerala, Riverside Transit Campus, P.O. Padnekkad, Kasaragod, Kerala - 671314, India
*Corresponding author; [email protected] (S.C.Sahoo)
Abstract: Nanocomposites of Ni-ferrite (NF) and Zn-ferrite (ZF) were prepared by mixing them at different ratio and were subsequently annealed at different temperatures. Structural studies showed the appearance of NiZn-ferrite along with the constituent ferrites in the nanocomposite samples with the increase in annealing temperature. Magnetization value increased and the coercivity showed a peak around 750 °C with the increase in annealing temperature. Both the magnetization and coercivity decreased with the increase in ZF concentration in these samples annealed at temperatures lower than 900°C. The highest magnetization value of 52 emu/g and the lowest coercivity of 40 Oe were observed at 300 K in the nanocomposite sample with NF:ZF = 3:2 and annealed at 900 °C. The observed magnetization value was higher than the expected values whereas the observed coercivity was lower than the expected values in the annealed samples. Grain growth, intergranular interactions, formation of NiZn-ferrite and cation distribution in the spinel structure explain the observed magnetic behaviour in these nanocomposites. Keywords: Ferrites, Sol-gel method, Nanocomposites, Magnetic properties
1
1. Introduction Magnetic nanocomposites of core-shell structures [1,2], bilayered thin films [3,4], and solid mixtures [5,6] of different materials are of great scientific interest to understand the coupling between the constituent materials and to tune the magnetic properties for their possible applications in technology, biomedical and environmental fields [7–11]. These composites are made by the combination of ferromagnetic, ferrimagnetic or antiferromagnetic material with any other material. Magnetic behaviour of the nanocomposites of bimagnetic systems depends on the grain size and their distribution, packing density, intergranular interactions and composition of these materials. Depending on the type and strength of magnetic interactions magnetic properties like exchange bias [12,13], exchange spring behaviour [14,15], enhanced remanence, coercivity and high energy product [16,17] are observed in these bimagnetic nanocomposites. Nanocomposites consisting of spinel ferrite have been studied for their possible applications in different fields. NiFe2O4/SiO2/graphene oxide, NiFe2O4/MnO2/ graphene, ZnFe2O4/graphene/TiO2, nanocomposites
were
BaFe12O19/ZnFe2O4/carbon studied
for
microwave
nanotubes,
device
BaFe12O19/NiFe2O4
applications
[18–22].
The
nanocomposites of NiFe2O4/MnO2 and Fe3O4/carbon have been reported for water purification by removal of heavy metal like Pb(II) and Cr(VI) respectively from waste water [23,24]. CoFe2O4/CoFe2 nanocomposites were studied by Prabhakaran et al. for magnetic refrigeration applications [25]. Wu et al. studied the influence of stochiometric ratio of SrFe12O19 to ZnFe2O4 on magnetic and photocatalytic properties of SrFe12O19/ZnFe2O4 nanocomposites [26]. Lorenz et al. reported exchange bias and magnetodielectric coupling in ZnFe2O4/BaTiO3 composite thin films [27]. Bimagnetic nanocomposites consisting of two spinel compounds have also been studied by many researchers. Song et al. reported that by varying the core and shell materials, and volume fraction of soft and hard phase one can 2
control the blocking temperature, coercivity and switching of moments in CoFe2O4/MnFe2O4 core/shell nanoparticles [2]. Oberdick et al. studied spin canting in Fe3O4/MnxFe3-xO4 core/shell nanoparticles [1]. Magnetic coupling in nanocrystalline CoFe2O4/ZnFe2O4 bilayered thin films was studied Sahoo et al. [4]. Thickness dependent exchange spring behaviour in epitaxial Fe3O4/CoFe2O4 magnetic bilayers was reported by Lavorato et al. [3]. In the present work we studied the effect of annealing on the magnetic properties of NiFe2O4/ZnFe2O4 nanocomposites. NiFe2O4 (NF) shows grain size dependent ferrimagnetic behaviour in nanoscale [28,29]. ZnFe2O4 (ZF) though antiferromagnetic in bulk shows ferrimagnetic behaviour depending on grain size and their distribution in the nanomaterials [30,31]. In the present study we mixed these two spinel ferrite nanoparticles and investigated their magnetic behaviour. Magnetic properties of the nanocomposite depend on the grain size, volume fraction of the constituent magnetic phases and effective exchange coupling between them. Grain sizes in these nanomaterials can be controlled by thermal treatment. So, we varied the concentration of constituent ferrites in the nanocomposite and annealed them at different temperature. Different synthesis methods like mechanical milling [30], sol gel [32], co-precipitation [33], hydrothermal [34], reverse micelle [35], conventional ceramic method [36], sono chemical method [37], greener synthesis [38], mechanochemical synthesis [39] are used to synthesize magnetic nanoparticles. In the present study we used sol-gel method to prepare the NF and ZF nanoparticles and mixed the as-prepared (asp) nanopowders physically to get the nanocomposites. Magnetic properties were studied after annealing at different temperatures.
2. Experiments NF and ZF nanoparticles were separately prepared by taking the stoichiometric ratio of AR grade salts of ferric nitrate nonahydrate [Fe(NO3)3.9H2O], nickel nitrate hexahydrate [Ni(NO3)2.6H2O] and zinc nitrate hexahydrate [Zn(NO3)2.6H2O]. Ethylene glycol (100ml) 3
was used as the solvent in each case. The detailed synthesis procedure has been discussed elsewhere [40]. The powder was collected at room temperature after autocombustion. The nanocomposites were prepared by mixing the asp NF and ZF nanopowders in different concentrations. The NF:ZF ratio was varied as 4:1, 3:2, 2:3 and 1:4 in the nanocomposite sample. The solid mixers were ground well by an agate mortar pestle and were annealed at different temperatures (TA); 350, 550,750 and 900 °C in air for 2 hr. The asp NF and ZF nanopowders were also annealed separately at 550 and 900 °C in air for 2 hr for comparative study. Structural properties were studied by a Rigaku Miniflex 600 X-ray diffractometer with Cu-Kα radiation (λ=1.5406 Å) in θ/2θ mode. Fourier transformed infrared (FTIR) spectra of the samples were recorded at room temperature by a Thermo-Scientific Nicolet iS5 spectrophotometer. Microstructures of the samples were studied by a JEOL JEM 2100 high resolution transmission electron microscope (HRTEM). Magnetic measurements were carried out at different temperatures ranging from 310 K to 50 K by a vibrating sample magnetometer (VSM) of Quantum Design Versa Lab Physical Property Measurement System by applying maximum magnetic field up to ±30 kOe.
3. Results and Discussion 3.1 Structural and microstructural properties Fig.1(a) shows the X-ray diffraction (XRD) patterns of the NF/ZF nanocomposite samples with NF:ZF = 3:2. As seen in Fig.1(a) all the peaks were indexed to spinel phase using standard JCPDS data of Ni-ferrite (PDF card No. 0742081) and Zn-ferrite (PDF Card no. 221012). Fig.1 (b) shows the zoomed view of the observed (311) peak in the XRD patterns. It is seen from the Fig.1(a) and (b) that, double peaks were observed side by side in these nanocomposites at particular peak positions. The peak appearing at the lower angle side corresponds to ZF and the peak at higher angle corresponds to NF as the lattice constant of 4
ZF is higher than that of NF. Up to TA = 750 °C, two overlapped peaks were observed in these samples. However, at TA= 900 °C, a broad single peak was observed. Similar XRD patterns were also observed for the other nanocomposite samples in the present study. Such type of XRD pattern was observed in Co-ferrite/Zn-ferrite bilayers when deposited at different substrate temperatures [4]. Lattice constants for the two ferrites were obtained by fitting double peaks to the observed (311) peaks and the values are presented in Table 1. Grain sizes of the respective ferrites in the nanocomposites were also calculated from the XRD data by using Scherrer’s formula [41] and are also listed in the Table 1. As seen in Table 1, lattice constant of NF slightly increases and for ZF it decreases with the increase in TA in the nanocomposite samples. Crystallite size of the individual ferrites increases with the increase in TA.
Fig. 1 XRD patterns of the NF/ZF nanocomposites with NF:ZF = 3:2 and (b) zoomed view of the (311) peaks.
5
Table 1 Lattice constant and grain size of NF and ZF in the NF/ZF nanocomposite samples with NF:ZF = 3:2. (Lattice constant of bulk NF = 8.34 Å and of bulk ZF = 8.44 Å)
TA (ºC) asp 350 550 750 900
Lattice constant (Å) NF 8.35 8.35 8.35 8.36 8.37
ZF 8.44 8.45 8.44 8.43 8.41
Grain size (nm) NF 17 19 22 28 38
ZF 25 21 26 28 33
Fig. 2 (a) and Fig.2 (b) show the FTIR spectra of the annealed (TA = 900 °C) nanocomposite samples with NF:ZF = 4:1 and 1:4 respectively. Four infrared active modes are observed in spinel ferrites. The high frequency absorption band 550 - 650 cm-1 and the low frequency absorption band 390 - 525 cm-1 correspond to the metal-oxygen vibrations at tetrahedral (A) and octahedral (B) sites respectively [40,42]. Some shoulder peaks were also observed in the FTIR spectra as seen in Fig. 2. Moreover the absorption band width became narrower and the band was asymmetric in the high frequency region with the increase in ZF concentration in these annealed samples. This may be due to the changed cation distribution in the different lattice sites in the mixed spinel ferrite formed at higher annealing temperature [32].
6
Fig. 2 FTIR spectra of the nanocomposite samples annealed at TA = 900 °C with (a) NF:ZF = 4:1 and (b) NF:ZF = 1:4. Fig. 3 shows HRTEM image, grain size distribution, selected area electron diffraction (SAED) pattern and d-spacing of the annealed (TA = 900 °C) nanocomposite sample with NF:ZF = 3:2. From the Fig. 3 (d-f), we clearly see the presence of grains corresponding to NF, ZF and the mixed spinel Ni0.4Zn0.6Fe2O4 (NZF) ferrite (PDF Card No 00-062-0437) in the annealed sample though only one broad peak was observed in the XRD pattern. This clearly shows that the mixed spinel ferrite was formed in the nanocomposite sample after annealing at high temperature.
7
Fig. 3 (a) TEM image, (b) grain size distribution (160 grains), (c) SAED pattern and (d-f) dspacing of the annealed (TA = 900 °C) nanocomposite sample with NF:ZF = 3:2.
3.2 Magnetic Properties Fig. 4 shows the magnetic hysteresis (M-H) loops of the asp and the annealed (TA = 900 °C) nanocomposite samples at 300 and 60 K. We see from the Fig. 4 that the loops do not saturate with the applied magnetic field. Moreover the non-saturation increases with the increase in ZF concentration in the nanocomposite and with the decrease in measurement temperature. So, the magnetization value was obtained by extrapolating the high field part of the M-H loop to the zero applied fields [28,40]. The spontaneous magnetization (MS) and the coercivity (HC) values obtained from the M-H loops are shown in Fig. 5.
8
Fig. 4 M-H loops at 300 and 60 K of the (a-d) as-prepared and the (e-h) annealed (TA = 900 °C) nanocomposite samples with different NF:ZF ratio.
As seen in Fig. 5(a), the MS value increases slowly up to TA = 750 °C for the nanocomposite samples except for the sample with NF:ZF = 1:4. For TA = 900 °C, there is a sharp increase in the MS value from that observed at TA = 750 °C (see Fig. 5(a) and Fig. 5(b)). The highest MS value of 52 emu/g was observed at 300 K for the nanocomposite sample with NF:ZF = 4:1 and 3:2. For the sample with NF:ZF = 1:4, the MS value slightly decreases initially and then increases with increasing TA. Moreover, the MS value decreases with the increase in ZF concentration in the nanocomposite samples. The MS values are enhanced at 60 K from those at 300 K for all the samples as seen in Fig.5(a) and (b). At 60 K the maximum MS value of 76 emu/g was observed for the sample annealed at TA = 900 °C with NF:ZF = 3:2. As seen in Fig. 5(c) HC gradually increases with increase in TA and shows a maximum around TA = 750 °C and decreases with further increase in TA. At 60 K the HC values are also enhanced from those observed at 300 K as seen in the case of MS values. Moreover, the peak in HC values at 60K is shifted to lower TA as seen in Fig. 5 (d). With the increase in ZF concentration the
9
variation of HC also shows decreasing trend in these nanocomposite samples. Interestingly the lowest HC of 40 Oe was observed for the sample for which the highest magnetization was observed at 300K (i.e. for the sample annealed at TA = 900 °C with NF:ZF = 3:2).
Fig. 5 Variation of MS at (a) 300 K (b) 60 K, and HC at (c) 300 K and (d) 60 K with increasing annealing temperature (TA). As seen in Fig. 1, two separate peaks were observed up to TA= 750 °C. So, it is inferred that most of the grains present in the nanocomposite are of individual ferrites. Grain size of the constituent ferrites also increased with the increase in TA as seen in the Table 1. In a separate study we observed increase of magnetization with the increase in grain size in NF. As mentioned earlier ZF shows ferrimagnetic behavior in nanomaterials depending on grain size [31]. So, the increase in MS value with the increase in TA is mainly due to the increase in grain size [32,43]. The stronger exchange interactions as well as magnetic dipolar interaction 10
also support the enhancement in the MS value. Moreover, with the increase in TA mixed spinel NiZn-ferrite is formed. Zn2+ prefers A-site; Ni2+ prefers B-site and Fe2+ is distributed in both the sites in the mixed spinel structure. Due to this changed cation distribution, magnetization is also enhanced. For the sample with higher ZF concentration, i.e. for NF:ZF = 1:4, the magnetic behaviour of the nanocomposite is dominated by the contribution from ZF. With the increase in TA and grain size ZF show antiferromagnetic behaviour [30–31]. So, the initial decrease in MS value is dominated by the antiferromagnetic behaviour of ZF and the increase in MS value is due to formation of mixed spinel ferrite. As ZF has lower magnetization compared to NF, the MS value decreases with the increase in ZF concentration in the NF/ZF nanocomposite samples. The variation of HC with the increase in TA may be understood as follows. With the increase in TA, grain size attains the critical single domain size at TA = 750 °C. After TA = 750 °C with the formation of grains with multidomain, HC decreases [32,43,44]. Moreover, the stronger intergranular interaction enhances the coercivity in the nanocomposites. With the formation of mixed spinel in the nanocomposite, anisotropy decreases in the sample. With the decrease in measurement temperature to 60 K, HC increases with the increase in anisotropy and decrease in thermal energy. As ZF has very small anisotropy and coercivity compared to those of NF, the HC decreases with the increase in ZF concentration in the NF/ZF nanocomposite. In order to further investigate the magnetic behaviour of the nanocomposites, we plotted the expected loop considering that there is no magnetic interaction between the two spinel compounds in the nanocomposite and compared it with the observed loop. The expected loop was obtained using the formula, 𝑀=
𝑀1𝑥1 + 𝑀2𝑥2 𝑥1 + 𝑥2
11
(1)
where M1, M2 are the observed magnetizations of NF, ZF respectively, M is the expected magnetization of NF/ZF nanocomposite, x1 and x2 are the individual weights of NF and ZF in the nanocomposite.Fig.6 shows the expected and the observed M-H loops of the nanocomposite sample with NF:ZF = 3:2 and annealed at TA = 550 °C. As seen in the Fig.6, the observed loops show single phase magnetic behaviour and are significantly different from the expected loops indicating that the two magnetic phases are well coupled. In our study we did not observe either the loop shifts or two step behaviour in the M-H loop which are generally observed in other bimagnetic systems [14,16]. The observed and expected MS and HC values of the asp and the annealed NF/ZF nanocomposite samples with the increase in ZF concentration are shown in Fig. 7. As seen in Fig. 7 both the expected and observed MS values decrease with the increase in ZF concentration both at 300 K and 60 K. In the case of asp samples shown in Fig. 7(a), the observed MS values are almost similar to the expected values. After annealing, the observed MS values are higher than the expected values and monotonically decrease with the increase in ZF concentration for the nanocomposite sample annealed at TA = 550 °C. The difference between the observed and the expected values also decreases with the increase in ZF concentration. However, for the sample annealed at TA = 900 °C, the observed MS value at 60 K increases with the increase in ZF concentration and a maximum MS value of 76 emu/g was observed for the sample with NF:ZF = 3:2. For higher ZF concentration, the MS value decreased and 30.6 emu/g was observed for the sample with NF:ZF = 1:4. Just like magnetization Hc also decreased with the increase in ZF concentration in these samples. Interestingly, unlike magnetization, the observed Hc is always lower than the expected Hc for the asp and the annealed nanocomposite samples as seen in Fig.7(d-f).
12
Fig.6 Expected and observed M-H loops of the NF/ZF nanocomposite sample with NF:ZF = 3:2 and annealed at TA = 550 °C at (A) 300 K and (C) 60 K, and the zoomed view of the respective loops near origin at (B) 300 K and (D) 60 K.
13
Fig. 7 Variation of the expected (solid symbol) and observed (open symbol) MS and HC values with ZF concentration for the (a) and (d) as-prepared, and the annealed; (b) and (e) TA = 550 °C, (c) and (f) TA = 900 °C nanocomposite samples.
Magnetic behaviour in the composite depends on the grain size distribution and anisotropies of the constituent phases as well as strength of intergranular interactions [45,46]. In the asprepared samples, two ferrite nanopowders were just mixed and the observed magnetization is the sum of the two individual magnetizations. With the increase in annealing temperature mixed spinel NiZn-ferrite was formed. Moreover grain size increased with the increase in TA in these nanocomposite samples. As mentioned above due to stronger intergranular interaction and the changed cation distribution in the mixed spinel structure the MS values are enhanced in the nanocomposite than the expected values. At TA = 900 °C, we observed a maximum MS for the sample with NF:ZF = 3:2 (i.e. 40% ZF in NF/ZF nanocomposite) and then it decreased for higher ZF concentration (see Fig. 7(c)). Such type of behaviour has been observed in mixed spinel NiZn-ferrite nanoparticles with the increase in Zn2+ concentration 14
[47]. With the formation of mixed spinel in the nanocomposite the magnetization increases as mentioned above. However, the decrease of MS value with the increase in ZF concentration in NF/ZF nanocomposites is not understood. The observed increase in MS value is understood on the basis of Nèel’s two sublattice model and collinear moments in NiZn-ferrite for lower Zn2+ concentration [32]. With the increase in Zn2+ concentration in the mixed spinel, as the Zn2+ in the A-site replaces equal amount of Fe3+ to the B-site, the moments in the B-site is no more collinear. The moments in the B-site are canted with respect to the net magnetization and the magnetic behaviour is explained by Yafet-Kittel model with three magnetic sublattices [32][48]. Considering the same canting angle for the two magnetic moments in the B-site, we have calculated Yafet-Kittel angle (αYK) using the equation; M = MB CosαYK - MA
(2)
where, αYK is the angle between magnetic moments in the B-sites to the applied field (H) direction, MA and MB are the magnetic moments of A- and B- sites respectively and M is the net magnetization. As seen in the Fig. 8, αYK increases with the increase in ZF concentration. Moreover, we can see that for the asp sample and the samples annealed at TA = 550 °C, the values of αYK angle are same. It shows that very less amount of mixed ferrite was formed after annealing at TA = 550 °C and is also supported by the observation of two distinct peaks in the XRD. After annealing at TA = 900 °C, most of the ferrite grains are of mixed NiZnferrite and αYK decreases as seen in Fig. 8. The canting angle also decreases at 60 K compared to that at 300 K, which explains the increase in MS value at 60 K.
15
Fig. 8 Variation of Yafet-Kittel angle with ZF concentration in the NF/ZF nanocomposites at (a) 300 K and (b) 60 K. Zero field cooled (ZFC) and field cooled (FC) magnetization versus temperature (MT) curves of nanocomposite samples annealed at TA = 900 °C in an applied field of 100 Oe are shown in the Fig.9. It is seen from the Fig. 9 that for all the samples magnetization decreases with increase in temperature in FC curve except for the sample with NF:ZF = 2:3. As seen in Fig. 9 (a), for the sample with NF:ZF = 4:1, magnetization of ZFC curve increases with the increase in temperature and a small kink was observed around 263K. For the nanocomposite samples with NF:ZF = 3:2 and NF:ZF = 2:3, maximum magnetization in ZFC curve, known as blocking temperature (TB), was observed at 160 K and 112 K respectively as seen in Fig.9(b) and (c). For the sample with NF:ZF = 1:4, the magnetization in both ZFC and FC curve decreases with increase in temperature. It is interesting to notice that position of maximum magnetization shifts to lower temperature as the ZF concentration increases in the NF/ZF nanocomposites.
16
Fig. 9 Temperature dependence of magnetization of nanocomposite samples annealed at TA= 900 °C with (a) NF:ZF = 4:1, (b) NF:ZF = 3:2, (c) NF:ZF = 2:3, (d) NF:ZF = 1:4.
4. Conclusions NF/ZF nanocomposites with different ratio of NF:ZF were prepared by mixing the constituent ferrites prepared by sol-gel method. Magnetic behaviour of these nanocomposite samples were studied after annealing. Magnetization value was found to decrease with the increase in ZF concentration. However, for the annealed samples observed MS values were higher than the expected values. The nanocomposite sample with NF:ZF = 3:2 annealed at TA = 900 °C showed the highest MS value of 52 and 76emu/g at 300 and 60 K respectively. The observed HC values were lower than the expected values in these nanocomposite samples and also decreased with the increase in ZF concentration. The observed magnetic behaviour can be understood on the basis of grain growth, intergranular interaction, formation of mixed spinel and cation distribution in these nanocomposite samples. 17
Acknowledgement Authors thank Department of Physics, National Institute of Technology Trichy, Tamilnadu, India for FTIR spectra of the samples. REFERENCE [1]
S.D. Oberdick, A. Abdelgawad, C. Moya, S. Mesbahi-Vasey, D. Kepaptsoglou, V.K. Lazarov, R.F.L. Evans, D. Meilak, E. Skoropata, J. Van Lierop, I. Hunt-Isaak, H. Pan, Y. Ijiri, K.L. Krycka, J.A. Borchers, S.A. Majetich, Spin canting across core/shell Fe3O4/MnxFe3-xO4 nanoparticles, Sci. Rep. 8 (2018) 3425. doi:10.1038/s41598-01821626-0.
[2]
Q. Song, Z.J. Zhang, Controlled synthesis and magnetic properties of bimagnetic spinel ferrite CoFe2O4 and MnFe2O4 nanocrystals with core-shell architecture, J. Am. Chem. Soc. 134 (2012) 10182–10190. doi:10.1021/ja302856z.
[3]
G. Lavorato, E. Winkler, B. Rivas-Murias, F. Rivadulla, Thickness dependence of exchange coupling in epitaxial Fe3O4/ CoFe2O4 soft/hard magnetic bilayers, Phys. Rev. B. 94 (2016) 054405. doi:10.1103/PhysRevB.94.054405.
[4]
S.C. Sahoo, N. Venkataramani, S. Prasad, M. Bohra, R. Krishnan, Magnetic properties of nanocrystalline CoFe2O4/ZnFe2O4 bilayers, J. Supercond. Nov. Magn. 25 (2012) 2653–2657. doi: 10.1007/s10948-011-1237-y
[5]
H. Nikmanesh, M. Moradi, P. Kameli, G.H. Bordbar, Effects of annealing temperature on exchange spring behavior of barium hexaferrite/nickel zinc ferrite nanocomposites, J. Electron. Mater. 46 (2017) 5933–5941. doi:10.1007/s11664-017-5576-8.
[6]
Z. Zheng, H. Zhang, J.Q. Xiao, Q. Yang, L. Jia, Low loss NiZn spinel ferrite – W-type hexaferrite composites from BaM addition for antenna applications, J. Phys. D. Appl. Phys. 47 (2014) 115001. doi:10.1088/0022-3727/47/11/115001. 18
[7]
L. V. Leonel, A. Righi, W.N. Mussel, J.B. Silva, N.D.S. Mohallem, Structural characterization of barium titanate-cobalt ferrite composite powders, Ceram. Int. 37 (2011) 1259–1264. doi:10.1016/j.ceramint.2011.01.017.
[8]
C. Feng, X. Liu, S.W. Or, S.L.Ho, Exchange coupling and microwave absorption in core/shell-structured hard/soft ferrite - based CoFe2O4/NiFe2O4 nanocapsules, AIP Adv. 7 (2017) 056403. doi:10.1063/1.4972805.
[9]
J.-H. Lee, J.-T. Jang, J.-S. Choi, S.H. Moon, S.-H. Noh, J.-W. Kim, J.-G. Kim, I.-S. Kim, K.I. Park, J. Cheon, Exchange-coupled magnetic nanoparticles for efficient heat induction., Nat. Nanotechnol. 6 (2011) 418 – 422. doi:10.1038/nnano.2011.95.
[10]
K. Kan-Dapaah, N. Rahbar, W. Soboyejo, Implantable magnetic nanocomposites for the localized treatment of breastcancer, J. Appl. Phys. 116 (2014) 233505. doi:10.1063/1.4903736.
[11]
T. Sun, Y.S. Zhang, B. Pang, D.C. Hyun, M. Yang, Y. Xia, Engineered nanoparticles for drug delivery in cancer therapy, Angew. Chemie - Int. Ed. 53 (2014) 2–47. doi:10.1002/anie.201403036.
[12]
E. Lage, C. Kirchhof, V. Hrkac, L. Kienle, R. Jahns, R. Knöchel, E. Quandt, D. Meyners, Exchange biasing of magnetoelectric composites, Nat. Mater. 11 (2012) 523–529. doi:10.1038/nmat3306.
[13]
P.K. Manna, S.M. Yusuf, Two interface effects: Exchange bias and magnetic proximity, Phys. Rep. 535 (2014) 61–99. doi:10.1016/j.physrep.2013.10.002.
[14]
A.
Poorbafrani,
H.
Salamati,
P.
Kameli,
Exchange
spring
behavior
in
Co0.6Zn0.4Fe2O4/SrFe10.5O16.75 nanocomposites, Ceram. Int. 41 (2015) 1603-1608. doi:10.1016/j.ceramint.2014.09.097. 19
[15]
D. Roy, C. Shivakumara, P.S. Anil Kumar, Observation of the exchange spring behavior in hard-soft-ferrite nanocomposite, J. Magn. Magn. Mater. 321 (2009) L11– L14. doi:10.1016/j.jmmm.2008.09.017.
[16]
B.K. Rai, L. Wang, S.R. Mishra, V. V Nguyen, J.P. Liu, Synthesis and magnetic properties of hard-soft SrFe10Al2O19/NiZnFe2O4 ferrite nanocomposites, J. Nanosci. Nanotechnol. 14 (2014) 5272–5277. doi:10.1166/jnn.2014.8836.
[17]
M.A. Radmanesh, S.A. Seyyed Ebrahimi, Synthesis and magnetic properties of hard/soft SrFe12O19/Ni0.7Zn0.3Fe2O4 nanocomposite magnets, J. Magn. Magn. Mater. 324 (2012) 3094–3098. doi:10.1016/j.jmmm.2012.05.008.
[18]
Y. Wang, W. Zhang, C. Luo, X. Wu, Q. Wang, W. Chen, J. Li, Synthesis, characterization and enhanced electromagnetic properties of NiFe2O4@SiO2-decorated reduced graphene oxide nanosheets, Ceram. Int. 42 (2016) 17374–17381. doi:10.1016/j.ceramint.2016.08.036.
[19]
Y. Wang, Y. Fu, X. Wu, W. Zhang, Q. Wang, J. Li, Synthesis of hierarchical coreshell NiFe2O4@MnO2 composite microspheres decorated graphene nanosheet for enhanced microwave absorption performance, Ceram. Int. 43 (2017) 11367–11375. doi:10.1016/j.ceramint.2017.05.344.
[20]
Y. Wang, H. Zhu, Y. Chen, X. Wu, W. Zhang, C. Luo, J. Li, Design of hollow ZnFe2O4 microspheres@graphene decorated with TiO2 nanosheets as a highperformance low frequency absorber, Mater. Chem. Phys. 202 (2017) 184–189. doi:10.1016/j.matchemphys.2017.09.036.
[21]
S. Tyagi, V.S. Pandey, H.B. Baskey, N. Tyagi, A. Garg, S. Goel, T.C. Shami, Radar absorption study of BaFe12O19/ZnFe2O4/CNTs nanocomposite, J. Alloys Compd. 731 20
(2018) 584–590. doi:10.1016/j.jallcom.2017.10.071. [22]
C. Pahwa, S. Mahadevan, S.B. Narang, P. Sharma, Structural, magnetic and microwave
properties
of
exchange
coupled
and
non-exchange
coupled
BaFe12O19/NiFe2O4 nanocomposites, J. Alloys Compd. 725 (2017) 1175–1181. doi:10.1016/j.jallcom.2017.07.220. [23]
B. Xiang, D. Ling, H. Lou, H. Gu, 3D hierarchical flower-like nickel ferrite/manganese dioxide toward lead (II) removal from aqueous water, J. Hazard. Mater. 325 (2017) 178–188. doi:10.1016/j.jhazmat.2016.11.011.
[24]
X. Xu, H. Zhang, C. Ma, H. Gu, H. Lou, S. Lyu, C. Liang, J. Kong, J. Gu, A superfast hexavalent chromium scavenger: Magnetic nanocarbon bridged nanomagnetite network with excellent recyclability, J. Hazard. Mater. 353 (2018) 166–172. doi:10.1016/j.jhazmat.2018.03.059.
[25]
T. Prabhakaran, R. V. Mangalaraja, J.C. Denardin, The structural, magnetic and magnetic entropy changes on CoFe2O4/CoFe2 composites for magnetic refrigeration application,J.Magn.Magn.Mater.444(2017) 97–306. doi:10.1016/j.jmmm.2017.08.008.
[26]
Q. Wu, Z. Yu, Y. Wu, Z. gao, H. Xie, The magnetic and photocatalytic properties of
nanocomposites SrFe12O19/ZnFe2O4, 465 (2018) 1-8 doi:10.1016/j.jmmm.2018.05.098. [27]
M. Lorenz, M. Ziese, G. Wagner, J. Lenzner, C. Kranert, K. Brachwitz, H. Hochmuth, P. Esquinazi, M. Grundmann, Exchange bias and magnetodielectric coupling effects in ZnFe2O4–BaTiO3 composite thin films, CrystEngComm. 14 (2012) 6477–6486. doi:10.1039/c2ce25505g.
[28]
C.N. Anumol, M. Chithra, S.C. Sahoo, Magnetic studies of nickel ferrite nanoparticles prepared
by
sol-gel
technique,
AIP 21
Conf.
Proc.
1728
(2016)
020623.
doi:10.1063/1.4946674. [29]
C. Dong, G. Wang, D. Guo, C. Jiang, D. Xue, Growth, structure, morphology, and magnetic properties of Ni ferrite films., Nanoscale Res. Lett. 8 (2013) 196. doi:10.1186/1556-276X-8-196.
[30]
C.N. Chinnasamy, A. Narayanasamy, N. Ponpandian, K. Chattopadhyay, H. Gu´erault, J.-M. Greneche, Magnetic properties of nanostructured ferrimagnetic zinc ferrite, J. Phys. Condens. Matter. 12 (2000) 7795–7805. doi:10.1088/0953-8984/12/35/314.
[31]
M. Bohra, S. Prasad, N. Venkataramani, S.C. Sahoo, N. Kumar, R. Krishnan, Low temperature magnetization studies of nanocrystalline zn-ferrite thin films, IEEE Trans. Magn. 49 (2013) 4249. doi:10.1109/TMAG.2013.2239969.
[32]
M. Chithra, C.N. Anumol, B. Sahu, S.C. Sahoo, Structural and magnetic properties of ZnXCo1−XFe2O4 nanoparticles: Nonsaturation of magnetization, J. Magn. Magn. Mater. 424 (2017) 174–184. doi:10.1016/j.jmmm.2016.10.064.
[33]
A. Thakur, P. Thakur, J.-H. Hsu, Magnetic behaviour of Ni0.4Zn0.6Co0.1Fe1.9O4 spinel nano-ferrite, J. Appl. Phys. 111 (2012) 07A305. doi:10.1063/1.3670606.
[34]
M. Stingaciu, H.L. Andersen, C. Granados-Miralles, A. Mamakhel, M. Christensen, Magnetism in CoFe2O4 nanoparticles produced at sub- and near-supercritical conditions
of
water,
CrystEngComm.
19
(2017)
3986–3996.
doi:10.1039/C7CE00666G. [35]
S. Thakur, S.C. Katyal, A. Gupta, V.R. Reddy, S.K. Sharma, M. Knobel, M. Singh, Nickel - zinc ferrite from reverse micelle process : Structural and magnetic properties , mossbauer spectroscopy characterization, J. Phys. Chem. C. 113 (2009) 20785–20794. doi:10.1021/jp9050287. 22
[36]
C.N.Chinnasamy, A.Narayanasamy, N.Ponpandian, Mixed spinel structure in nanocrystalline
NiFe2O4,
Phys.
Rev.
B.
63
(2001)
1814108.
doi:10.1103/PhysRevB.63.184108. [37]
A. Rezaei, G. Nabiyouni, D. Ghanbari, Photo-catalyst and magnetic investigation of BaFe12O19–ZnO nanoparticles and nanocomposites, J. Mater. Sci. Mater. Electron. 27 (2016) 11339–11352. doi:10.1007/s10854-016-5258-y.
[38]
V.C. Karade, P.P. Waifalkar, T.D. Dongle, S.C. Sahoo, P. Kollu, P.S. Patil, P.B. Patil, Greener synthesis of magnetite nanoparticles using green tea extract and their magnetic properties, Mater. Res. Express. 4 (2017) 096102. doi:10.1088/20531591/aa892f.
[39]
S. Vladimir, I. Bergmann, A. Feldhoff, P. Heitjans, F. Krumeich, D. Menzel, F.J. Litterst, S.J. Campbell, K.D. Becker, Nanocrystalline nickel ferrite , NiFe2O4 : Mechanosynthesis , nonequilibrium cation distribution , canted spin arrangement , and magnetic behavior, J. Phys. Chem. C. 111 (2007) 5026–5033. doi:10.1021/jp067620s.
[40]
M. Chithra, C.N. Anumol, B. Sahu, S.C. Sahoo, Exchange spring like magnetic behavior in cobalt ferrite nanoparticles, J. Magn. Magn. Mater. 401 (2016) 1–8. doi:10.1016/j.jmmm.2015.10.007.
[41]
B.D. Cullity, Elements of X-ray diffraction, Addison Wesley Publishing company, Massachusetts,USA, 1978.
[42]
M.K. Anupama, B. Rudraswamy, N. Dhananjaya, Investigation on impedance response and dielectric relaxation of Ni–Zn ferrites prepared by self-combustion technique, J. Alloys Compd. 706 (2017) 554–561. doi:10.1016/j.jallcom.2017.02.241.
[43]
Adriana S. Albuquerque, J. D.Ardisson, W. A.A.Macedo, M.C. M.Alves, Nanosized 23
powders of NiZn ferrite : Synthesis , structure , and magnetism, J. Appl. Phys. 87 (2000) 4352. doi:10.1063/1.373077. [44]
N.A. Spaldin, Magnetic materials : Fundamental and Applications, Cambridge University Press, New york, 2003.
[45]
E.F. Kneller, R. Hawig, The exchange-spring magnet: A new material principle for permanent magnets, IEEE Trans. Magn. 27 (1991) 3588. doi:10.1109/20.102931.
[46]
J.S. Jiang, E.E. Fullerton, C.H. Sowers, A. Inomata, S.D. Bader, A.J. Shapiroand, R. D.shull, V.S. Gornakov, V.I. Nikitenko, Spring magnet films, IEEE Trans. Magn. 35 (1999) 3229. doi:10.1109/20.800484.
[47]
D. V. Kurmude, C.M. Kale, P.S. Aghav, D.R. Shengule, K.M. Jadhav, Superparamagnetic behavior of zinc-substituted nickel ferrite nanoparticles and its effect on mossbauer and magnetic parameters, J. Supercond. Nov. Magn. 27 (2014) 1889–1897. doi:10.1007/s10948-014-2535-y.
[48]
Y. Yafet, C. Kittel, Antiferromagnetic Arrangements in Ferrites, Phys. Rev. 87 (1952) 290–294. doi:10.1103/PhysRev.87.290.
24
S0304-8853(18)31854-7 https://doi.org/10.1016/j.jmmm.2018.08.036 MAGMA 64233
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
15 June 2018 7 August 2018 14 August 2018
Please cite this article as: C.N. Anumol, M. Chithra, M. Govindaraj Shalini, S.C. Sahoo, Effect of annealing on structural and magnetic properties of NiFe2O4/ZnFe2O4 nanocomposites, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.08.036
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of annealing on structural and magnetic properties of NiFe2O4/ZnFe2O4 nanocomposites
C.N. Anumol1, M. Chithra1, M. Govindaraj Shalini1, Subasa C. Sahoo1* 1Department
of Physics, Central University of Kerala, Riverside Transit Campus, P.O. Padnekkad, Kasaragod, Kerala - 671314, India
*Corresponding author; [email protected] (S.C.Sahoo)
Abstract: Nanocomposites of Ni-ferrite (NF) and Zn-ferrite (ZF) were prepared by mixing them at different ratio and were subsequently annealed at different temperatures. Structural studies showed the appearance of NiZn-ferrite along with the constituent ferrites in the nanocomposite samples with the increase in annealing temperature. Magnetization value increased and the coercivity showed a peak around 750 °C with the increase in annealing temperature. Both the magnetization and coercivity decreased with the increase in ZF concentration in these samples annealed at temperatures lower than 900°C. The highest magnetization value of 52 emu/g and the lowest coercivity of 40 Oe were observed at 300 K in the nanocomposite sample with NF:ZF = 3:2 and annealed at 900 °C. The observed magnetization value was higher than the expected values whereas the observed coercivity was lower than the expected values in the annealed samples. Grain growth, intergranular interactions, formation of NiZn-ferrite and cation distribution in the spinel structure explain the observed magnetic behaviour in these nanocomposites. Keywords: Ferrites, Sol-gel method, Nanocomposites, Magnetic properties
1
1. Introduction Magnetic nanocomposites of core-shell structures [1,2], bilayered thin films [3,4], and solid mixtures [5,6] of different materials are of great scientific interest to understand the coupling between the constituent materials and to tune the magnetic properties for their possible applications in technology, biomedical and environmental fields [7–11]. These composites are made by the combination of ferromagnetic, ferrimagnetic or antiferromagnetic material with any other material. Magnetic behaviour of the nanocomposites of bimagnetic systems depends on the grain size and their distribution, packing density, intergranular interactions and composition of these materials. Depending on the type and strength of magnetic interactions magnetic properties like exchange bias [12,13], exchange spring behaviour [14,15], enhanced remanence, coercivity and high energy product [16,17] are observed in these bimagnetic nanocomposites. Nanocomposites consisting of spinel ferrite have been studied for their possible applications in different fields. NiFe2O4/SiO2/graphene oxide, NiFe2O4/MnO2/ graphene, ZnFe2O4/graphene/TiO2, nanocomposites
were
BaFe12O19/ZnFe2O4/carbon studied
for
microwave
nanotubes,
device
BaFe12O19/NiFe2O4
applications
[18–22].
The
nanocomposites of NiFe2O4/MnO2 and Fe3O4/carbon have been reported for water purification by removal of heavy metal like Pb(II) and Cr(VI) respectively from waste water [23,24]. CoFe2O4/CoFe2 nanocomposites were studied by Prabhakaran et al. for magnetic refrigeration applications [25]. Wu et al. studied the influence of stochiometric ratio of SrFe12O19 to ZnFe2O4 on magnetic and photocatalytic properties of SrFe12O19/ZnFe2O4 nanocomposites [26]. Lorenz et al. reported exchange bias and magnetodielectric coupling in ZnFe2O4/BaTiO3 composite thin films [27]. Bimagnetic nanocomposites consisting of two spinel compounds have also been studied by many researchers. Song et al. reported that by varying the core and shell materials, and volume fraction of soft and hard phase one can 2
control the blocking temperature, coercivity and switching of moments in CoFe2O4/MnFe2O4 core/shell nanoparticles [2]. Oberdick et al. studied spin canting in Fe3O4/MnxFe3-xO4 core/shell nanoparticles [1]. Magnetic coupling in nanocrystalline CoFe2O4/ZnFe2O4 bilayered thin films was studied Sahoo et al. [4]. Thickness dependent exchange spring behaviour in epitaxial Fe3O4/CoFe2O4 magnetic bilayers was reported by Lavorato et al. [3]. In the present work we studied the effect of annealing on the magnetic properties of NiFe2O4/ZnFe2O4 nanocomposites. NiFe2O4 (NF) shows grain size dependent ferrimagnetic behaviour in nanoscale [28,29]. ZnFe2O4 (ZF) though antiferromagnetic in bulk shows ferrimagnetic behaviour depending on grain size and their distribution in the nanomaterials [30,31]. In the present study we mixed these two spinel ferrite nanoparticles and investigated their magnetic behaviour. Magnetic properties of the nanocomposite depend on the grain size, volume fraction of the constituent magnetic phases and effective exchange coupling between them. Grain sizes in these nanomaterials can be controlled by thermal treatment. So, we varied the concentration of constituent ferrites in the nanocomposite and annealed them at different temperature. Different synthesis methods like mechanical milling [30], sol gel [32], co-precipitation [33], hydrothermal [34], reverse micelle [35], conventional ceramic method [36], sono chemical method [37], greener synthesis [38], mechanochemical synthesis [39] are used to synthesize magnetic nanoparticles. In the present study we used sol-gel method to prepare the NF and ZF nanoparticles and mixed the as-prepared (asp) nanopowders physically to get the nanocomposites. Magnetic properties were studied after annealing at different temperatures.
2. Experiments NF and ZF nanoparticles were separately prepared by taking the stoichiometric ratio of AR grade salts of ferric nitrate nonahydrate [Fe(NO3)3.9H2O], nickel nitrate hexahydrate [Ni(NO3)2.6H2O] and zinc nitrate hexahydrate [Zn(NO3)2.6H2O]. Ethylene glycol (100ml) 3
was used as the solvent in each case. The detailed synthesis procedure has been discussed elsewhere [40]. The powder was collected at room temperature after autocombustion. The nanocomposites were prepared by mixing the asp NF and ZF nanopowders in different concentrations. The NF:ZF ratio was varied as 4:1, 3:2, 2:3 and 1:4 in the nanocomposite sample. The solid mixers were ground well by an agate mortar pestle and were annealed at different temperatures (TA); 350, 550,750 and 900 °C in air for 2 hr. The asp NF and ZF nanopowders were also annealed separately at 550 and 900 °C in air for 2 hr for comparative study. Structural properties were studied by a Rigaku Miniflex 600 X-ray diffractometer with Cu-Kα radiation (λ=1.5406 Å) in θ/2θ mode. Fourier transformed infrared (FTIR) spectra of the samples were recorded at room temperature by a Thermo-Scientific Nicolet iS5 spectrophotometer. Microstructures of the samples were studied by a JEOL JEM 2100 high resolution transmission electron microscope (HRTEM). Magnetic measurements were carried out at different temperatures ranging from 310 K to 50 K by a vibrating sample magnetometer (VSM) of Quantum Design Versa Lab Physical Property Measurement System by applying maximum magnetic field up to ±30 kOe.
3. Results and Discussion 3.1 Structural and microstructural properties Fig.1(a) shows the X-ray diffraction (XRD) patterns of the NF/ZF nanocomposite samples with NF:ZF = 3:2. As seen in Fig.1(a) all the peaks were indexed to spinel phase using standard JCPDS data of Ni-ferrite (PDF card No. 0742081) and Zn-ferrite (PDF Card no. 221012). Fig.1 (b) shows the zoomed view of the observed (311) peak in the XRD patterns. It is seen from the Fig.1(a) and (b) that, double peaks were observed side by side in these nanocomposites at particular peak positions. The peak appearing at the lower angle side corresponds to ZF and the peak at higher angle corresponds to NF as the lattice constant of 4
ZF is higher than that of NF. Up to TA = 750 °C, two overlapped peaks were observed in these samples. However, at TA= 900 °C, a broad single peak was observed. Similar XRD patterns were also observed for the other nanocomposite samples in the present study. Such type of XRD pattern was observed in Co-ferrite/Zn-ferrite bilayers when deposited at different substrate temperatures [4]. Lattice constants for the two ferrites were obtained by fitting double peaks to the observed (311) peaks and the values are presented in Table 1. Grain sizes of the respective ferrites in the nanocomposites were also calculated from the XRD data by using Scherrer’s formula [41] and are also listed in the Table 1. As seen in Table 1, lattice constant of NF slightly increases and for ZF it decreases with the increase in TA in the nanocomposite samples. Crystallite size of the individual ferrites increases with the increase in TA.
Fig. 1 XRD patterns of the NF/ZF nanocomposites with NF:ZF = 3:2 and (b) zoomed view of the (311) peaks.
5
Table 1 Lattice constant and grain size of NF and ZF in the NF/ZF nanocomposite samples with NF:ZF = 3:2. (Lattice constant of bulk NF = 8.34 Å and of bulk ZF = 8.44 Å)
TA (ºC) asp 350 550 750 900
Lattice constant (Å) NF 8.35 8.35 8.35 8.36 8.37
ZF 8.44 8.45 8.44 8.43 8.41
Grain size (nm) NF 17 19 22 28 38
ZF 25 21 26 28 33
Fig. 2 (a) and Fig.2 (b) show the FTIR spectra of the annealed (TA = 900 °C) nanocomposite samples with NF:ZF = 4:1 and 1:4 respectively. Four infrared active modes are observed in spinel ferrites. The high frequency absorption band 550 - 650 cm-1 and the low frequency absorption band 390 - 525 cm-1 correspond to the metal-oxygen vibrations at tetrahedral (A) and octahedral (B) sites respectively [40,42]. Some shoulder peaks were also observed in the FTIR spectra as seen in Fig. 2. Moreover the absorption band width became narrower and the band was asymmetric in the high frequency region with the increase in ZF concentration in these annealed samples. This may be due to the changed cation distribution in the different lattice sites in the mixed spinel ferrite formed at higher annealing temperature [32].
6
Fig. 2 FTIR spectra of the nanocomposite samples annealed at TA = 900 °C with (a) NF:ZF = 4:1 and (b) NF:ZF = 1:4. Fig. 3 shows HRTEM image, grain size distribution, selected area electron diffraction (SAED) pattern and d-spacing of the annealed (TA = 900 °C) nanocomposite sample with NF:ZF = 3:2. From the Fig. 3 (d-f), we clearly see the presence of grains corresponding to NF, ZF and the mixed spinel Ni0.4Zn0.6Fe2O4 (NZF) ferrite (PDF Card No 00-062-0437) in the annealed sample though only one broad peak was observed in the XRD pattern. This clearly shows that the mixed spinel ferrite was formed in the nanocomposite sample after annealing at high temperature.
7
Fig. 3 (a) TEM image, (b) grain size distribution (160 grains), (c) SAED pattern and (d-f) dspacing of the annealed (TA = 900 °C) nanocomposite sample with NF:ZF = 3:2.
3.2 Magnetic Properties Fig. 4 shows the magnetic hysteresis (M-H) loops of the asp and the annealed (TA = 900 °C) nanocomposite samples at 300 and 60 K. We see from the Fig. 4 that the loops do not saturate with the applied magnetic field. Moreover the non-saturation increases with the increase in ZF concentration in the nanocomposite and with the decrease in measurement temperature. So, the magnetization value was obtained by extrapolating the high field part of the M-H loop to the zero applied fields [28,40]. The spontaneous magnetization (MS) and the coercivity (HC) values obtained from the M-H loops are shown in Fig. 5.
8
Fig. 4 M-H loops at 300 and 60 K of the (a-d) as-prepared and the (e-h) annealed (TA = 900 °C) nanocomposite samples with different NF:ZF ratio.
As seen in Fig. 5(a), the MS value increases slowly up to TA = 750 °C for the nanocomposite samples except for the sample with NF:ZF = 1:4. For TA = 900 °C, there is a sharp increase in the MS value from that observed at TA = 750 °C (see Fig. 5(a) and Fig. 5(b)). The highest MS value of 52 emu/g was observed at 300 K for the nanocomposite sample with NF:ZF = 4:1 and 3:2. For the sample with NF:ZF = 1:4, the MS value slightly decreases initially and then increases with increasing TA. Moreover, the MS value decreases with the increase in ZF concentration in the nanocomposite samples. The MS values are enhanced at 60 K from those at 300 K for all the samples as seen in Fig.5(a) and (b). At 60 K the maximum MS value of 76 emu/g was observed for the sample annealed at TA = 900 °C with NF:ZF = 3:2. As seen in Fig. 5(c) HC gradually increases with increase in TA and shows a maximum around TA = 750 °C and decreases with further increase in TA. At 60 K the HC values are also enhanced from those observed at 300 K as seen in the case of MS values. Moreover, the peak in HC values at 60K is shifted to lower TA as seen in Fig. 5 (d). With the increase in ZF concentration the
9
variation of HC also shows decreasing trend in these nanocomposite samples. Interestingly the lowest HC of 40 Oe was observed for the sample for which the highest magnetization was observed at 300K (i.e. for the sample annealed at TA = 900 °C with NF:ZF = 3:2).
Fig. 5 Variation of MS at (a) 300 K (b) 60 K, and HC at (c) 300 K and (d) 60 K with increasing annealing temperature (TA). As seen in Fig. 1, two separate peaks were observed up to TA= 750 °C. So, it is inferred that most of the grains present in the nanocomposite are of individual ferrites. Grain size of the constituent ferrites also increased with the increase in TA as seen in the Table 1. In a separate study we observed increase of magnetization with the increase in grain size in NF. As mentioned earlier ZF shows ferrimagnetic behavior in nanomaterials depending on grain size [31]. So, the increase in MS value with the increase in TA is mainly due to the increase in grain size [32,43]. The stronger exchange interactions as well as magnetic dipolar interaction 10
also support the enhancement in the MS value. Moreover, with the increase in TA mixed spinel NiZn-ferrite is formed. Zn2+ prefers A-site; Ni2+ prefers B-site and Fe2+ is distributed in both the sites in the mixed spinel structure. Due to this changed cation distribution, magnetization is also enhanced. For the sample with higher ZF concentration, i.e. for NF:ZF = 1:4, the magnetic behaviour of the nanocomposite is dominated by the contribution from ZF. With the increase in TA and grain size ZF show antiferromagnetic behaviour [30–31]. So, the initial decrease in MS value is dominated by the antiferromagnetic behaviour of ZF and the increase in MS value is due to formation of mixed spinel ferrite. As ZF has lower magnetization compared to NF, the MS value decreases with the increase in ZF concentration in the NF/ZF nanocomposite samples. The variation of HC with the increase in TA may be understood as follows. With the increase in TA, grain size attains the critical single domain size at TA = 750 °C. After TA = 750 °C with the formation of grains with multidomain, HC decreases [32,43,44]. Moreover, the stronger intergranular interaction enhances the coercivity in the nanocomposites. With the formation of mixed spinel in the nanocomposite, anisotropy decreases in the sample. With the decrease in measurement temperature to 60 K, HC increases with the increase in anisotropy and decrease in thermal energy. As ZF has very small anisotropy and coercivity compared to those of NF, the HC decreases with the increase in ZF concentration in the NF/ZF nanocomposite. In order to further investigate the magnetic behaviour of the nanocomposites, we plotted the expected loop considering that there is no magnetic interaction between the two spinel compounds in the nanocomposite and compared it with the observed loop. The expected loop was obtained using the formula, 𝑀=
𝑀1𝑥1 + 𝑀2𝑥2 𝑥1 + 𝑥2
11
(1)
where M1, M2 are the observed magnetizations of NF, ZF respectively, M is the expected magnetization of NF/ZF nanocomposite, x1 and x2 are the individual weights of NF and ZF in the nanocomposite.Fig.6 shows the expected and the observed M-H loops of the nanocomposite sample with NF:ZF = 3:2 and annealed at TA = 550 °C. As seen in the Fig.6, the observed loops show single phase magnetic behaviour and are significantly different from the expected loops indicating that the two magnetic phases are well coupled. In our study we did not observe either the loop shifts or two step behaviour in the M-H loop which are generally observed in other bimagnetic systems [14,16]. The observed and expected MS and HC values of the asp and the annealed NF/ZF nanocomposite samples with the increase in ZF concentration are shown in Fig. 7. As seen in Fig. 7 both the expected and observed MS values decrease with the increase in ZF concentration both at 300 K and 60 K. In the case of asp samples shown in Fig. 7(a), the observed MS values are almost similar to the expected values. After annealing, the observed MS values are higher than the expected values and monotonically decrease with the increase in ZF concentration for the nanocomposite sample annealed at TA = 550 °C. The difference between the observed and the expected values also decreases with the increase in ZF concentration. However, for the sample annealed at TA = 900 °C, the observed MS value at 60 K increases with the increase in ZF concentration and a maximum MS value of 76 emu/g was observed for the sample with NF:ZF = 3:2. For higher ZF concentration, the MS value decreased and 30.6 emu/g was observed for the sample with NF:ZF = 1:4. Just like magnetization Hc also decreased with the increase in ZF concentration in these samples. Interestingly, unlike magnetization, the observed Hc is always lower than the expected Hc for the asp and the annealed nanocomposite samples as seen in Fig.7(d-f).
12
Fig.6 Expected and observed M-H loops of the NF/ZF nanocomposite sample with NF:ZF = 3:2 and annealed at TA = 550 °C at (A) 300 K and (C) 60 K, and the zoomed view of the respective loops near origin at (B) 300 K and (D) 60 K.
13
Fig. 7 Variation of the expected (solid symbol) and observed (open symbol) MS and HC values with ZF concentration for the (a) and (d) as-prepared, and the annealed; (b) and (e) TA = 550 °C, (c) and (f) TA = 900 °C nanocomposite samples.
Magnetic behaviour in the composite depends on the grain size distribution and anisotropies of the constituent phases as well as strength of intergranular interactions [45,46]. In the asprepared samples, two ferrite nanopowders were just mixed and the observed magnetization is the sum of the two individual magnetizations. With the increase in annealing temperature mixed spinel NiZn-ferrite was formed. Moreover grain size increased with the increase in TA in these nanocomposite samples. As mentioned above due to stronger intergranular interaction and the changed cation distribution in the mixed spinel structure the MS values are enhanced in the nanocomposite than the expected values. At TA = 900 °C, we observed a maximum MS for the sample with NF:ZF = 3:2 (i.e. 40% ZF in NF/ZF nanocomposite) and then it decreased for higher ZF concentration (see Fig. 7(c)). Such type of behaviour has been observed in mixed spinel NiZn-ferrite nanoparticles with the increase in Zn2+ concentration 14
[47]. With the formation of mixed spinel in the nanocomposite the magnetization increases as mentioned above. However, the decrease of MS value with the increase in ZF concentration in NF/ZF nanocomposites is not understood. The observed increase in MS value is understood on the basis of Nèel’s two sublattice model and collinear moments in NiZn-ferrite for lower Zn2+ concentration [32]. With the increase in Zn2+ concentration in the mixed spinel, as the Zn2+ in the A-site replaces equal amount of Fe3+ to the B-site, the moments in the B-site is no more collinear. The moments in the B-site are canted with respect to the net magnetization and the magnetic behaviour is explained by Yafet-Kittel model with three magnetic sublattices [32][48]. Considering the same canting angle for the two magnetic moments in the B-site, we have calculated Yafet-Kittel angle (αYK) using the equation; M = MB CosαYK - MA
(2)
where, αYK is the angle between magnetic moments in the B-sites to the applied field (H) direction, MA and MB are the magnetic moments of A- and B- sites respectively and M is the net magnetization. As seen in the Fig. 8, αYK increases with the increase in ZF concentration. Moreover, we can see that for the asp sample and the samples annealed at TA = 550 °C, the values of αYK angle are same. It shows that very less amount of mixed ferrite was formed after annealing at TA = 550 °C and is also supported by the observation of two distinct peaks in the XRD. After annealing at TA = 900 °C, most of the ferrite grains are of mixed NiZnferrite and αYK decreases as seen in Fig. 8. The canting angle also decreases at 60 K compared to that at 300 K, which explains the increase in MS value at 60 K.
15
Fig. 8 Variation of Yafet-Kittel angle with ZF concentration in the NF/ZF nanocomposites at (a) 300 K and (b) 60 K. Zero field cooled (ZFC) and field cooled (FC) magnetization versus temperature (MT) curves of nanocomposite samples annealed at TA = 900 °C in an applied field of 100 Oe are shown in the Fig.9. It is seen from the Fig. 9 that for all the samples magnetization decreases with increase in temperature in FC curve except for the sample with NF:ZF = 2:3. As seen in Fig. 9 (a), for the sample with NF:ZF = 4:1, magnetization of ZFC curve increases with the increase in temperature and a small kink was observed around 263K. For the nanocomposite samples with NF:ZF = 3:2 and NF:ZF = 2:3, maximum magnetization in ZFC curve, known as blocking temperature (TB), was observed at 160 K and 112 K respectively as seen in Fig.9(b) and (c). For the sample with NF:ZF = 1:4, the magnetization in both ZFC and FC curve decreases with increase in temperature. It is interesting to notice that position of maximum magnetization shifts to lower temperature as the ZF concentration increases in the NF/ZF nanocomposites.
16
Fig. 9 Temperature dependence of magnetization of nanocomposite samples annealed at TA= 900 °C with (a) NF:ZF = 4:1, (b) NF:ZF = 3:2, (c) NF:ZF = 2:3, (d) NF:ZF = 1:4.
4. Conclusions NF/ZF nanocomposites with different ratio of NF:ZF were prepared by mixing the constituent ferrites prepared by sol-gel method. Magnetic behaviour of these nanocomposite samples were studied after annealing. Magnetization value was found to decrease with the increase in ZF concentration. However, for the annealed samples observed MS values were higher than the expected values. The nanocomposite sample with NF:ZF = 3:2 annealed at TA = 900 °C showed the highest MS value of 52 and 76emu/g at 300 and 60 K respectively. The observed HC values were lower than the expected values in these nanocomposite samples and also decreased with the increase in ZF concentration. The observed magnetic behaviour can be understood on the basis of grain growth, intergranular interaction, formation of mixed spinel and cation distribution in these nanocomposite samples. 17
Acknowledgement Authors thank Department of Physics, National Institute of Technology Trichy, Tamilnadu, India for FTIR spectra of the samples. REFERENCE [1]
S.D. Oberdick, A. Abdelgawad, C. Moya, S. Mesbahi-Vasey, D. Kepaptsoglou, V.K. Lazarov, R.F.L. Evans, D. Meilak, E. Skoropata, J. Van Lierop, I. Hunt-Isaak, H. Pan, Y. Ijiri, K.L. Krycka, J.A. Borchers, S.A. Majetich, Spin canting across core/shell Fe3O4/MnxFe3-xO4 nanoparticles, Sci. Rep. 8 (2018) 3425. doi:10.1038/s41598-01821626-0.
[2]
Q. Song, Z.J. Zhang, Controlled synthesis and magnetic properties of bimagnetic spinel ferrite CoFe2O4 and MnFe2O4 nanocrystals with core-shell architecture, J. Am. Chem. Soc. 134 (2012) 10182–10190. doi:10.1021/ja302856z.
[3]
G. Lavorato, E. Winkler, B. Rivas-Murias, F. Rivadulla, Thickness dependence of exchange coupling in epitaxial Fe3O4/ CoFe2O4 soft/hard magnetic bilayers, Phys. Rev. B. 94 (2016) 054405. doi:10.1103/PhysRevB.94.054405.
[4]
S.C. Sahoo, N. Venkataramani, S. Prasad, M. Bohra, R. Krishnan, Magnetic properties of nanocrystalline CoFe2O4/ZnFe2O4 bilayers, J. Supercond. Nov. Magn. 25 (2012) 2653–2657. doi: 10.1007/s10948-011-1237-y
[5]
H. Nikmanesh, M. Moradi, P. Kameli, G.H. Bordbar, Effects of annealing temperature on exchange spring behavior of barium hexaferrite/nickel zinc ferrite nanocomposites, J. Electron. Mater. 46 (2017) 5933–5941. doi:10.1007/s11664-017-5576-8.
[6]
Z. Zheng, H. Zhang, J.Q. Xiao, Q. Yang, L. Jia, Low loss NiZn spinel ferrite – W-type hexaferrite composites from BaM addition for antenna applications, J. Phys. D. Appl. Phys. 47 (2014) 115001. doi:10.1088/0022-3727/47/11/115001. 18
[7]
L. V. Leonel, A. Righi, W.N. Mussel, J.B. Silva, N.D.S. Mohallem, Structural characterization of barium titanate-cobalt ferrite composite powders, Ceram. Int. 37 (2011) 1259–1264. doi:10.1016/j.ceramint.2011.01.017.
[8]
C. Feng, X. Liu, S.W. Or, S.L.Ho, Exchange coupling and microwave absorption in core/shell-structured hard/soft ferrite - based CoFe2O4/NiFe2O4 nanocapsules, AIP Adv. 7 (2017) 056403. doi:10.1063/1.4972805.
[9]
J.-H. Lee, J.-T. Jang, J.-S. Choi, S.H. Moon, S.-H. Noh, J.-W. Kim, J.-G. Kim, I.-S. Kim, K.I. Park, J. Cheon, Exchange-coupled magnetic nanoparticles for efficient heat induction., Nat. Nanotechnol. 6 (2011) 418 – 422. doi:10.1038/nnano.2011.95.
[10]
K. Kan-Dapaah, N. Rahbar, W. Soboyejo, Implantable magnetic nanocomposites for the localized treatment of breastcancer, J. Appl. Phys. 116 (2014) 233505. doi:10.1063/1.4903736.
[11]
T. Sun, Y.S. Zhang, B. Pang, D.C. Hyun, M. Yang, Y. Xia, Engineered nanoparticles for drug delivery in cancer therapy, Angew. Chemie - Int. Ed. 53 (2014) 2–47. doi:10.1002/anie.201403036.
[12]
E. Lage, C. Kirchhof, V. Hrkac, L. Kienle, R. Jahns, R. Knöchel, E. Quandt, D. Meyners, Exchange biasing of magnetoelectric composites, Nat. Mater. 11 (2012) 523–529. doi:10.1038/nmat3306.
[13]
P.K. Manna, S.M. Yusuf, Two interface effects: Exchange bias and magnetic proximity, Phys. Rep. 535 (2014) 61–99. doi:10.1016/j.physrep.2013.10.002.
[14]
A.
Poorbafrani,
H.
Salamati,
P.
Kameli,
Exchange
spring
behavior
in
Co0.6Zn0.4Fe2O4/SrFe10.5O16.75 nanocomposites, Ceram. Int. 41 (2015) 1603-1608. doi:10.1016/j.ceramint.2014.09.097. 19
[15]
D. Roy, C. Shivakumara, P.S. Anil Kumar, Observation of the exchange spring behavior in hard-soft-ferrite nanocomposite, J. Magn. Magn. Mater. 321 (2009) L11– L14. doi:10.1016/j.jmmm.2008.09.017.
[16]
B.K. Rai, L. Wang, S.R. Mishra, V. V Nguyen, J.P. Liu, Synthesis and magnetic properties of hard-soft SrFe10Al2O19/NiZnFe2O4 ferrite nanocomposites, J. Nanosci. Nanotechnol. 14 (2014) 5272–5277. doi:10.1166/jnn.2014.8836.
[17]
M.A. Radmanesh, S.A. Seyyed Ebrahimi, Synthesis and magnetic properties of hard/soft SrFe12O19/Ni0.7Zn0.3Fe2O4 nanocomposite magnets, J. Magn. Magn. Mater. 324 (2012) 3094–3098. doi:10.1016/j.jmmm.2012.05.008.
[18]
Y. Wang, W. Zhang, C. Luo, X. Wu, Q. Wang, W. Chen, J. Li, Synthesis, characterization and enhanced electromagnetic properties of NiFe2O4@SiO2-decorated reduced graphene oxide nanosheets, Ceram. Int. 42 (2016) 17374–17381. doi:10.1016/j.ceramint.2016.08.036.
[19]
Y. Wang, Y. Fu, X. Wu, W. Zhang, Q. Wang, J. Li, Synthesis of hierarchical coreshell NiFe2O4@MnO2 composite microspheres decorated graphene nanosheet for enhanced microwave absorption performance, Ceram. Int. 43 (2017) 11367–11375. doi:10.1016/j.ceramint.2017.05.344.
[20]
Y. Wang, H. Zhu, Y. Chen, X. Wu, W. Zhang, C. Luo, J. Li, Design of hollow ZnFe2O4 microspheres@graphene decorated with TiO2 nanosheets as a highperformance low frequency absorber, Mater. Chem. Phys. 202 (2017) 184–189. doi:10.1016/j.matchemphys.2017.09.036.
[21]
S. Tyagi, V.S. Pandey, H.B. Baskey, N. Tyagi, A. Garg, S. Goel, T.C. Shami, Radar absorption study of BaFe12O19/ZnFe2O4/CNTs nanocomposite, J. Alloys Compd. 731 20
(2018) 584–590. doi:10.1016/j.jallcom.2017.10.071. [22]
C. Pahwa, S. Mahadevan, S.B. Narang, P. Sharma, Structural, magnetic and microwave
properties
of
exchange
coupled
and
non-exchange
coupled
BaFe12O19/NiFe2O4 nanocomposites, J. Alloys Compd. 725 (2017) 1175–1181. doi:10.1016/j.jallcom.2017.07.220. [23]
B. Xiang, D. Ling, H. Lou, H. Gu, 3D hierarchical flower-like nickel ferrite/manganese dioxide toward lead (II) removal from aqueous water, J. Hazard. Mater. 325 (2017) 178–188. doi:10.1016/j.jhazmat.2016.11.011.
[24]
X. Xu, H. Zhang, C. Ma, H. Gu, H. Lou, S. Lyu, C. Liang, J. Kong, J. Gu, A superfast hexavalent chromium scavenger: Magnetic nanocarbon bridged nanomagnetite network with excellent recyclability, J. Hazard. Mater. 353 (2018) 166–172. doi:10.1016/j.jhazmat.2018.03.059.
[25]
T. Prabhakaran, R. V. Mangalaraja, J.C. Denardin, The structural, magnetic and magnetic entropy changes on CoFe2O4/CoFe2 composites for magnetic refrigeration application,J.Magn.Magn.Mater.444(2017) 97–306. doi:10.1016/j.jmmm.2017.08.008.
[26]
Q. Wu, Z. Yu, Y. Wu, Z. gao, H. Xie, The magnetic and photocatalytic properties of
nanocomposites SrFe12O19/ZnFe2O4, 465 (2018) 1-8 doi:10.1016/j.jmmm.2018.05.098. [27]
M. Lorenz, M. Ziese, G. Wagner, J. Lenzner, C. Kranert, K. Brachwitz, H. Hochmuth, P. Esquinazi, M. Grundmann, Exchange bias and magnetodielectric coupling effects in ZnFe2O4–BaTiO3 composite thin films, CrystEngComm. 14 (2012) 6477–6486. doi:10.1039/c2ce25505g.
[28]
C.N. Anumol, M. Chithra, S.C. Sahoo, Magnetic studies of nickel ferrite nanoparticles prepared
by
sol-gel
technique,
AIP 21
Conf.
Proc.
1728
(2016)
020623.
doi:10.1063/1.4946674. [29]
C. Dong, G. Wang, D. Guo, C. Jiang, D. Xue, Growth, structure, morphology, and magnetic properties of Ni ferrite films., Nanoscale Res. Lett. 8 (2013) 196. doi:10.1186/1556-276X-8-196.
[30]
C.N. Chinnasamy, A. Narayanasamy, N. Ponpandian, K. Chattopadhyay, H. Gu´erault, J.-M. Greneche, Magnetic properties of nanostructured ferrimagnetic zinc ferrite, J. Phys. Condens. Matter. 12 (2000) 7795–7805. doi:10.1088/0953-8984/12/35/314.
[31]
M. Bohra, S. Prasad, N. Venkataramani, S.C. Sahoo, N. Kumar, R. Krishnan, Low temperature magnetization studies of nanocrystalline zn-ferrite thin films, IEEE Trans. Magn. 49 (2013) 4249. doi:10.1109/TMAG.2013.2239969.
[32]
M. Chithra, C.N. Anumol, B. Sahu, S.C. Sahoo, Structural and magnetic properties of ZnXCo1−XFe2O4 nanoparticles: Nonsaturation of magnetization, J. Magn. Magn. Mater. 424 (2017) 174–184. doi:10.1016/j.jmmm.2016.10.064.
[33]
A. Thakur, P. Thakur, J.-H. Hsu, Magnetic behaviour of Ni0.4Zn0.6Co0.1Fe1.9O4 spinel nano-ferrite, J. Appl. Phys. 111 (2012) 07A305. doi:10.1063/1.3670606.
[34]
M. Stingaciu, H.L. Andersen, C. Granados-Miralles, A. Mamakhel, M. Christensen, Magnetism in CoFe2O4 nanoparticles produced at sub- and near-supercritical conditions
of
water,
CrystEngComm.
19
(2017)
3986–3996.
doi:10.1039/C7CE00666G. [35]
S. Thakur, S.C. Katyal, A. Gupta, V.R. Reddy, S.K. Sharma, M. Knobel, M. Singh, Nickel - zinc ferrite from reverse micelle process : Structural and magnetic properties , mossbauer spectroscopy characterization, J. Phys. Chem. C. 113 (2009) 20785–20794. doi:10.1021/jp9050287. 22
[36]
C.N.Chinnasamy, A.Narayanasamy, N.Ponpandian, Mixed spinel structure in nanocrystalline
NiFe2O4,
Phys.
Rev.
B.
63
(2001)
1814108.
doi:10.1103/PhysRevB.63.184108. [37]
A. Rezaei, G. Nabiyouni, D. Ghanbari, Photo-catalyst and magnetic investigation of BaFe12O19–ZnO nanoparticles and nanocomposites, J. Mater. Sci. Mater. Electron. 27 (2016) 11339–11352. doi:10.1007/s10854-016-5258-y.
[38]
V.C. Karade, P.P. Waifalkar, T.D. Dongle, S.C. Sahoo, P. Kollu, P.S. Patil, P.B. Patil, Greener synthesis of magnetite nanoparticles using green tea extract and their magnetic properties, Mater. Res. Express. 4 (2017) 096102. doi:10.1088/20531591/aa892f.
[39]
S. Vladimir, I. Bergmann, A. Feldhoff, P. Heitjans, F. Krumeich, D. Menzel, F.J. Litterst, S.J. Campbell, K.D. Becker, Nanocrystalline nickel ferrite , NiFe2O4 : Mechanosynthesis , nonequilibrium cation distribution , canted spin arrangement , and magnetic behavior, J. Phys. Chem. C. 111 (2007) 5026–5033. doi:10.1021/jp067620s.
[40]
M. Chithra, C.N. Anumol, B. Sahu, S.C. Sahoo, Exchange spring like magnetic behavior in cobalt ferrite nanoparticles, J. Magn. Magn. Mater. 401 (2016) 1–8. doi:10.1016/j.jmmm.2015.10.007.
[41]
B.D. Cullity, Elements of X-ray diffraction, Addison Wesley Publishing company, Massachusetts,USA, 1978.
[42]
M.K. Anupama, B. Rudraswamy, N. Dhananjaya, Investigation on impedance response and dielectric relaxation of Ni–Zn ferrites prepared by self-combustion technique, J. Alloys Compd. 706 (2017) 554–561. doi:10.1016/j.jallcom.2017.02.241.
[43]
Adriana S. Albuquerque, J. D.Ardisson, W. A.A.Macedo, M.C. M.Alves, Nanosized 23
powders of NiZn ferrite : Synthesis , structure , and magnetism, J. Appl. Phys. 87 (2000) 4352. doi:10.1063/1.373077. [44]
N.A. Spaldin, Magnetic materials : Fundamental and Applications, Cambridge University Press, New york, 2003.
[45]
E.F. Kneller, R. Hawig, The exchange-spring magnet: A new material principle for permanent magnets, IEEE Trans. Magn. 27 (1991) 3588. doi:10.1109/20.102931.
[46]
J.S. Jiang, E.E. Fullerton, C.H. Sowers, A. Inomata, S.D. Bader, A.J. Shapiroand, R. D.shull, V.S. Gornakov, V.I. Nikitenko, Spring magnet films, IEEE Trans. Magn. 35 (1999) 3229. doi:10.1109/20.800484.
[47]
D. V. Kurmude, C.M. Kale, P.S. Aghav, D.R. Shengule, K.M. Jadhav, Superparamagnetic behavior of zinc-substituted nickel ferrite nanoparticles and its effect on mossbauer and magnetic parameters, J. Supercond. Nov. Magn. 27 (2014) 1889–1897. doi:10.1007/s10948-014-2535-y.
[48]
Y. Yafet, C. Kittel, Antiferromagnetic Arrangements in Ferrites, Phys. Rev. 87 (1952) 290–294. doi:10.1103/PhysRev.87.290.
24