Magnetic and electrical properties of Co1−x Cax Fe2O4 nanoparticles synthesized by the auto combustion method

Journal of Alloys and Compounds 542 (2012) 192–198

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Magnetic and electrical properties of Co1x Cax Fe2O4 nanoparticles synthesized by the auto combustion method S.A. Saafan a,⇑, S.T. Assar b, S.F. Mansour c a

Physics Department, Faculty of Science, Tanta University, Tanta, Egypt Engineering Physics and Mathematics Department, Faculty of Engineering, Tanta University, Tanta, Egypt c Physics Department, Faculty of Science, Zagazig University, Zagazig, Egypt b

a r t i c l e

i n f o

Article history: Received 20 January 2012 Received in revised form 7 July 2012 Accepted 9 July 2012 Available online 20 July 2012 Keywords: Ferrites Nanoparticles Saturation magnetization AC conductivity Dielectric constant Loss factor

a b s t r a c t Nano-sized particles of Co1x Cax Fe2O4 (x = 0.0, 0.01, 0.03, 0.05, 0.07 and 0.09) have been prepared by using the citrate–nitrate auto combustion method. X-Ray diffraction analysis has ensured the formation of the desired ferrites and has been used also to determine some of their structural properties. The particle size of only three samples has been checked out by using both transmission electron microscope (TEM) and particle size analyzer (PSA). Magnetic measurements have been performed by using vibrating sample magnetometer (VSM) at room temperature. The reduction of saturation magnetization of the nano-structured samples in comparison to bulk samples in literature and the effect of Ca addition on saturation magnetization and coercivity of all the samples are discussed. The AC conductivity and dielectric properties of these spinel ferrite nanoparticles have been investigated too as function of frequency at room temperature by using a broadband dielectric spectrometer. The AC conductivity of all the samples has been found to increase with increasing Ca substitution. This behavior is interpreted according to a suggested cationic distribution in agreement with literature and in consistency with the dielectric and loss factor results. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction During the last few decades, ferrites have achieved a primary position of economic and engineering importance within the family of magnetic materials because of their excellent physical properties. Practically all TV sets have ferrite cores in the fly back transformers, while portable radios make use of a pencil of ferrite as an antenna core. Long distance carrier telephone circuits are employing ferrite cores in high quality filter coils and transformers [1]. Also, it is well known that most of the physical and chemical properties of ferrites depend strongly on their particle size, shape and composition [2]. Among the spinels, cobalt ferrite (CoFe2O4) is a candidate of particular interest due to its high saturation magnetization, high coercivity, strong anisotropy and excellent chemical stability [3]. Moreover, nano-scale materials are nowadays of great interest due to their fascinating size dependent optical, electronic, magnetic, thermal, mechanical and chemical properties such that they may have properties extremely different from their bulk counterparts. Therefore, recent studies on mixed cobalt ferrite nano-samples showed interesting properties as a result of inserting additives ⇑ Corresponding author. Tel.: +20 1006619326. E-mail address: [email protected] (S.A. Saafan). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.07.050

to the original Co ferrite [3–5]. As far as the authors know, Co–Ca mixed ferrites in nano-scale particle size have not been studied yet, where only two studies of the properties of these materials in their bulk form had been reported [6,7]. The authors believe that studying the properties of Co–Ca nanoparticles may reveal the possibility of potential uses of these compounds such as microwave absorption, medical and sensing applications. The chemical properties of spinel nanoparticles are greatly affected by the synthesis route. For this reason, various methods have been reported in the literature for the preparation of these nanoscale spinel particles such as ceramic method [8], sol–gel [9] and co-precipitation [10]. Considering ferrites as dielectrics originates from their microstructure. The dielectric properties of ferrites are dependent upon several factors including the method of preparation, sintering time and temperature, chemical composition, type and quantity of additives [11,12]. Dielectric properties of ferrites are often stable at high frequencies; this can be useful for relevant applications. The study of dielectric properties may provide material designers by valuable information for potential applications. Therefore, in this article, some structural and magnetic investigations of nano-sized particles of Co–Ca ferrites are reported in addition to their frequency and composition dependence of the AC electrical conductivity (rAC), the dielectric constant (e0 ), and the loss tangent (tan d).

S.A. Saafan et al. / Journal of Alloys and Compounds 542 (2012) 192–198 2. Experimental Nano-sized particles of Co1x Cax Fe2O4 (x = 0.0, 0.01, 0.03, 0.05, 0.07 and 0.09) were prepared using the citrate-nitrate auto combustion method [13]. According to this method; the proper amounts of Co(NO3)26H2O, Ca(NO3)24H2O and Fe(NO3)39H2O) were dissolved in distilled water under constant stirring and heated to about 90 °C for 1 h. The solution was added to citric acid maintaining the molar ratio between the metal nitrates and the citric acid as 1:1. This mixed solution was then stirred for 2 h to achieve good homogeneity. Ammonia was added drop-wise to make the pH = 7. The precursor mixture was then heated to allow evaporation then combustion to obtain finally a product in the form of gray ash powder containing all the cations homogeneously mixed together at the atomic level. The product has been characterized by X-ray diffractometer model BRUKER D8 with CuKa radiation (k = 1.5418 Å) at room temperature to ensure the formation of the desired ferrites. The crystallite size of the samples (estimated from the line width of the (3 1 1) peaks), the lattice constant (a), the measured density (qm), the X-ray density (qx), the specific surface area (S), and the porosity (P) were calculated using the wellknown mathematical relations [14,15]. Moreover, the particle size of only three samples have been checked out by using both transmission electron microscope (TEM) model (JEOL JEM-1230) and particle size analyzer (N5 Submicron Particle Size Analyzer- BECKMAN COULTER). The hysteresis and magnetization measurements were performed using vibrating sample magnetometer (VSM); Model LAKE SHORE 7410 with maximum applied field 20 kOe at room temperature. Dielectric properties and AC conductivity have been investigated at room temperature as functions of frequency by using a broadband dielectric spectrometer (Novocontrol GmbH, Germany).

3. Results and discussion 3.1. XRD analysis The X-ray diffraction patterns of calcium-substituted cobalt ferrites having the compositions Co1x Cax Fe2O4 (x = 0.0, 0.01, 0.03, 0.05, 0.07 and 0.09) are shown in Fig. 1. All patterns show obviously the main diffraction lines corresponding to the cubic spinel structure of the desired ferrites (ICDD Card No.: 3-864). Table 1 shows some structural properties including the variation of the lattice parameter (a), with the calcium content (x). It may be noticed that the lattice parameter (a) is almost constant with increasing

193

Ca2+ content up to x = 0.05 thereafter a slight increase occurs with increasing (x); this behavior may be due to the small amount of Ca substitution up to x = 0.05, after which that slight increase occurs at x = 0.07 and 0.09 when the larger radius of Ca2+ (1.12 Å) than that of Co2+ (0.82 Å) [16] begins to be effective. Also, the porosity (P), the measured density (qm), and the X-ray density (qx) are found to be almost constant with increasing Ca2+ ion content because of the small amount of Ca substitution too. An estimation of the crystallite size calculated from the maximum intensity peak at the plane 3 1 1 is displayed in Table 1. It can be obviously seen that the crystallite size decreases with Ca addition ranging from 60.4 nm at x = 0–40.3 nm at x = 0.09. The Ca2+ ion substitution seems to prevent the crystal growth in agreement with literature [17] which may indicate an exothermic effect of introducing Ca to the lattice [18]. The specific surface area (S) increases with increasing Ca substitution as expected because it is inversely proportional to the crystallite size. 3.2. TEM and PSA analysis The particle sizes of three of the investigated samples (x = 0.0, 0.03, 0.07) were checked out by using TEM and PSA to be sure of the nanoscale of the prepared samples i.e. the particle size is less than 100 nm. Fig. 2 shows the particle size distribution of the selected samples obtained at two angles of incidence of the laser beam (10.9° and 15.4°) in the particle size analyzer (PSA). It implies that the samples have a variety of particle sizes in nano scale with maximum intensity at certain sizes shown in Table 2. The average particle sizes of the samples estimated from PSA and TEM are displayed in Table 2. The TEM images shown in Fig. 3 ensure the nanoscale nature of the particles. Some of the particles tend to agglomerate, may be due to the existence of permanent magnetic moment of the particles which is proportional to their volume [19,20]. The values of the crystallite sizes calculated from the XRD patterns and the average particle sizes estimated from the TEM images are all observed in the particle size distribution detected by the PSA. In other words, the particle size distribution detected by the PSA enhances the calculated particle sizes from both the XRD patterns and the TEM images. 3.3. Magnetic characterization

Fig. 1. The XRD patterns of the investigated nano samples of the compositions Co1xCaxFe2O4.

Magnetic characterization of the investigated nanoparticles was performed at room temperature (300 K) with maximum applied field up to 20 kG. Fig. 4 shows the hysteresis loops of the Ca substituted Co-ferrite samples. The values of saturation magnetization Ms, remnant magnetization Mr, and coercivity Hc are included in Table 3. In general, the values of the saturation magnetization of the investigated nanoparticles are lower than the reported values of the Co–Ca ferrite bulk samples prepared by ceramic method [7]. This can be explained on the basis of core-shell model which reveals that the finite size effect of the nanoparticles leads to non-collinearity or canting of spins on their surface [21]. Also, it is observed that while the coercivity decreases monotonically with increasing Ca concentration, the magnetization and remnant magnetization don’t show linear variation with it. The magnetization ranges from 58.726 to 64.03 emu/g with maximum values at x = 0.01 and x = 0.07. The remnant magnetization ranges from 31.29 to 32.204 emu/g with maximum values at x = 0.01 and x = 0.07 also. This may be interpreted as follows: It is well known that the magnetization is a result of simultaneous influence of several factors such as density, anisotropy, grain size, cation distribution and A–B exchange interaction [22]. Also, substituted cations can play an important role in determining the magnetization [23]. In the present work, the composition dependence of the magnetization can be explained on both the basis of the site preference

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Table 1 Some structural properties of Co1xCaxFe2O4 (x = 0.0–0.09) nanoparticles. Ca content x

Crystallite size (nm)

Lattice constant a (Å)

Density (qm) (g/cm3)

X-ray density (qx) (g/cm3)

Porosity (P) %

Specific surface area (S) (m2/g)

0.00 0.01 0.03 0.05 0.07 0.09

60.4 56 52.5 49.2 48.3 40.3

8.365 8.369 8.363 8.362 8.381 8.382

2.654 2.611 2.648 2.654 2.588 2.610

5.325 5.313 5.316 5.309 5.264 5.254

50.16 50.86 50.18 50.00 50.83 50.33

37.43 41.04 43.15 45.95 48.00 57.05

Fig. 2. The particle size distribution of the Co1xCaxFe2O4 samples of x = 0.0, 0.03, and 0.07 obtained from PSA.

of cations to occupy either tetrahedral or octahedral sites, and noncollinear nature of moments in the B-site. It was reported that Co2+ ions have preference to occupy octahedral B-site [24] and Ca2+ ions too [25]. Therefore, we may suggest as a first trial of interpretation the cationic distribution of the investigated samples as follows: (Fe1)[Co1xCaxFe1]; where parenthesis and square brackets represent tetrahedral A-site and octahedral B-site, respectively. Ca2+ ions as being nonmagnetic don’t contribute to the magnetization. Since the magnetic moment per ion for Co2+, and Fe3+ are 3 and 5 lB respectively, therefore, according to Néel’s model, the net expected theoretical magnetic moment in Bohr magneton should be:

mth ¼ mB  mA ¼ 3  3x

ð1Þ

Hence, the saturation magnetization is expected to decrease as x increases. The experimental magnetic moment per formula unit (mexp) expressed in Bohr magneton was calculated by using the following relation [26].

mexp ¼

MW  MS 5585

ð2Þ

where, MW is the molecular weight of the sample and MS is the saturation magnetization in emu/g. The values of the theoretical and

S.A. Saafan et al. / Journal of Alloys and Compounds 542 (2012) 192–198 Table 2 Particle size distribution and the average particle sizes of Co1xCaxFe2O4 samples estimated by PSA and TEM images. Ca content x

Particle size of max. intensity at each angle (nm)

Percentage of each size in each sample (%)

Average particle size calculated from PSA data (nm)

Average particle size estimated from TEM images (nm)

0.00

32.1 60

59.9 40.1

34.3

77.9

0.03

25.4 35.5

55.5 44.5

29.9

48.0

0.07

27.8 42.2

56.2 43.8

34.1

32.16

experimental magnetic moment per formula unit are shown in Table 3 and displayed in Fig. 5 as a function of Ca substitution (x). The theoretical magnetic moment ranges from 2.73 to 3, while

195

the experimental magnetic moment ranges from 2.32 to 2.67. These values are in good agreement with each other enhancing the suggested cationic distribution. It is worth mentioning that, while the theoretical magnetic moment decreases continuously, the experimental magnetic moment has slightly higher values at x = 0.01 and 0.07 coinciding with the behavior of the measured saturation magnetization. This may be explained according to the probability that Co2+ ions may sometimes enter both A-site and B-site [27]. So that we may now present a second suggestion of cationic distribution upon introducing Ca; (CoyFe1y)[Co1xyCaxFe1+y]; y is a small fraction that may vary from sample to another. Its effect appears in x = 0.01 and 0.07 samples more significantly where it seems that the Ca substitution induces a relatively more intensive migration of Co ions to the A-site causing improvement in both saturation magnetization and experimental magnetic moment. Moreover, the values of the experimental magnetic moment are slightly lower than the values of the theoretical magnetic moment. This indicates a possibility of non-collinear spin arrangement due to small canting angles of the spins in the B-sites with respect to the

Fig. 3. The TEM images of Co1xCaxFe2O4 samples of (a) x = 0.0, (b) x = 0.03 and (c) x = 0.07.

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80 x = 0.00 x = 0.01

60

x = 0.03 x = 0.05

40

Magnetization M, emu/g

x = 0.07 x = 0.09

20

0 -3.E+04

-2.E+04

-1.E+04

0.E+00

1.E+04

2.E+04

3.E+04

-20

-40

-60

-80

Applied field H, Gauss Fig. 4. The hysteresis loops of the nanoparticles of Co1xCaxFe2O4 obtained by using vibrating sample magnetometer VSM.

Table 3 Saturation magnetization, remnant magnetization, coercivity, magnetic moments, and Y–K angle of Co1xCaxFe2O4. Ca content x

Saturation magnetization (Ms) emu/g

Remnant magnetization (Mr) emu/g

Coercivity (Hc) G

mth Theoretical magnetization lB

mexp experimental magnetic moment lB

h°Y–K Yafet–Kittel angle

0.00 0.01 0.03 0.05 0.07 0.09

58.726 62.505 61.179 55.531 64.030 59.556

31.290 32.683 31.422 28.244 32.204 29.304

1742.8 1519.5 1471.5 1463.3 1407.3 1299.9

3 2.97 2.91 2.85 2.79 2.73

2.47 2.62 2.56 2.32 2.67 2.48

20.97 17.04 17.11 21.17 10.07 14.61

spins in A-sites [28]. The values of the Yafet–Kittel canting angles of the magnetic moments were calculated by the following equation [29] and they are tabulated in Table 3:

mexp ¼ M B ðcoshYK Þ  MA :

ð3Þ

It is worth mentioning that both values of the magnetic moments either theoretical or experimental are lower than the reported values of bulk samples [7]. This may be due to the fact that nanoferrites are characterized, as mentioned before, by canted spins or spin glass-like layer at the surface (shell) of their particles. This arises due to the larger fraction of surface to volume atoms in small particles. These canted spins basically reduce the saturation magnetization in the nanoparticles [30]. The coercivity of the prepared samples decreases with Ca2+ addition. It is well known that the coercivity is related to the magnetocrystalline anisotropy constant K1 and the saturation magnetization MS by the relation [31]:

Hc 1

K1 Ms

In the present work, the reduction of the coercivity may be occurred due to the decrease in the anisotropy constant K1 as a consequence of decreasing the contribution of Co2+ ions by the addition of nonmagnetic Ca2+ ions. 3.4. conductivity and dielectric constant Spinel ferrites are suitable candidates for manufacturing microwave absorbing materials because of their high specific resistance [32]. The spinel ferrites have been utilized most frequently as electromagnetic radiation absorbing materials in various forms [33]. Also, many other applications depend on the dielectric properties of the ferrites. Materials with electrically conducting regions separated by non-conducting regions exhibit the so called Maxwell Wagner AC conductivity i.e. increasing conductivity with increasing frequency [34] as shown in Fig. 6 which displays the AC conductivity (r0 AC) of the investigated samples at room temperature. It is observed that r0 AC of the sample of x = 0.0 has the lowest values so that it is obvious that introducing Ca ions to the sample increases the conductivity significantly. This enhances the above

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mth theoretical magnetic moment

1.E+08

mexp experimental magnetic moment

1.E+07

x = 0.00 x = 0.01

1.E+06

x = 0.03 x = 0.05

1.E+05

x = 0.07 x = 0.09

3 2.5 2

ε'

magnetic moment / molecule (μB)

3.5

1.E+04

1.5 1.E+03

1 1.E+02

0.5 1.E+01

0 0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

1.E+00 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

x, Ca concentration

f , Hz

3.0E-05

2.5E-05

σ'ac, S/cm

2.0E-05

x = 0.00 x = 0.01 x = 0.03 x = 0.05 x = 0.07 x = 0.09

1.5E-05

16

0.8

x = 0.01 x = 0.03 x = 0.05 x = 0.07 x = 0.09 x = 0.00

14 12

0.7

0.6

10

0.5

8

0.4

6

0.3

4

0.2

2

0.1

0 1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

tan δ1, x = 0.00

mentioned cationic distribution (CoyFe1y)[Co1xyCaxFe1+y]; where upon introducing Ca a partial migration of Co ions to A-sites and a consequent transfer of equivalent Fe ions to the B-sites increases the probability of electron hopping between Fe3+ M Fe2+ ions which is the basic conduction mechanism always suggested in ferrites [35,36]. The Fig. 7. shows the variation of the dielectric constant (e0 ) with frequency at room temperature for all the investigated samples, the dielectric constant decreases with increasing frequency because at low frequencies the charges that have been accumulated at the borders of the conducting regions cause significant interfacial polarization and consequently e0 increases. At higher frequencies the polarization cannot follow the varying field and e0 decreases. Therefore, as the frequency increases, e0 decreases. Also, remembering that the main contributor to the polarization in ferrites- (the interfacial polarization)- has a mechanism similar to the conduction process; where it results originally from electron hopping between Fe3+ M Fe2+ ions existing simultaneously in the B–sites [37,38]. Therefore, it is expected that the dielectric constant (e0 ) should behave like the conductivity because of having the same mechanism of charge transfer. This is obviously observed in the composition dependence of both e0 and r0 AC enhancing again the above suggested cationic distribution.

Fig. 7. The dielectric constant of the nanoparticles of Co1xCaxFe2O4 as a function of frequency at room temperature.

tan δ

Fig. 5. The theoretical and experimental magnetic moment per formula in Bohr magneton for the nanoparticles of Co1xCaxFe2O4.

0 1.E+08

f , Hz Fig. 8. The loss tangent of the nanoparticles of Co1xCaxFe2O4 as a function of frequency at room temperature.

The Fig. 8. displays the loss tangent (tan d) as a function of frequency at room temperature. Only for the sample x = 0.0, the y-axis is on the right hand side having a different scale than that of the other samples because of the great difference between the values. The loss tangent tan d is known to represent the losses in the sample since it is equal to the ratio of the resistive current to the capacitive current. The lowest values of tan d of the sample of x = 0.0 enhances the above discussion interpreting the lowest conductivity observed in the same sample. Moreover, about the maxima observed in all curves, it is known that these maxima occur when the hopping frequency coincides with the applied frequency [39].

4. Conclusions

1.0E-05

5.0E-06

5.0E-09 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

f , Hz Fig. 6. The AC conductivity of the nanoparticles of Co1xCaxFe2O4 as a function of frequency at room temperature.

Ca2+ ion substitution into Co ferrite seems to limit the crystal growth in agreement with literature. The samples prepared by using the citrate–nitrate auto combustion method have a variety of particle sizes in nanoscale with maximum intensity at certain sizes. The values of the crystallite sizes calculated from the XRD patterns and the average particle sizes estimated from the TEM images are all observed in the particle size distribution detected by the PSA. So it can be concluded that using the three techniques

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to check the particle sizes gives a realistic picture without contradictions. The values of the saturation magnetization of the investigated nanoparticles are lower than the values of the Co–Ca ferrite bulk samples reported in literature and prepared by the conventional ceramic method. It is suggested that upon introducing Ca the cationic distribution of the investigated samples becomes (CoyFe1y) [Co1xy Cax Fe1+y]; where y is a small fraction that may vary from sample to another. According to this suggestion both magnetic properties and AC electrical properties could be interpreted. Finally, it could be obviously observed that upon introducing Ca the magnetization and the coercivity decrease whereas the AC conductivity and the dielectric constant increase. These observations should be taken into consideration while choosing these investigated materials for specific practical applications. Acknowledgements The authors are deeply grateful to Professor Dr. Abdelhadi Kassiba head of Physics department in Maine University, Le Mans, France for performing the AC measurements in his lab and to Professor Dr. Nicolas Errien for helping the authors in recording the AC measurements. References [1] M. Hashim, Alimuddin, ShalendraKumar, SikanderAli, B.H. Koob, H. Chungc, R. Kumar, J. Alloys Compds. 511 (2012) 107–114. [2] A.B. Salunkhe, V.M. Khot, M.R. Phadatare, S.H. Pawar, J. Alloys Compds. 514 (2012) 91–96. [3] P. Kumar, S.K. Sharma, M. Knobel, M. Singh, J. Alloys Compds. 508 (2010) 115– 118. [4] M. Hashim, Alimuddin, ShalendraKumar, B.H. Koob, Sagar E. Shirsath, E.M. Mohammed, JyotiShahe, R.K. Kotnalae, H.K. Choi, H. Chungf, RaviKumar, J. Alloys Compds. 518 (2012) 11–18. [5] KavitaVerma, AshwiniKumar, DineshVarshney, J. Alloys Compds. 526 (2012) 91–97. [6] G.J. Baldha, R.V. Upadhyay, R.G. Kulkarni, Mat. Res. Bull. 21 (1986) 1051–1055. [7] R.V. Upadhyay, G.J. Baldha, R.G. Kulkarni, J. Magn. Magn. Mater. 61 (1986) 109– 113. [8] D.W. Johnson, B.B. Ghate, F.Y. Wang, Advances in Ceramics, 15, American Ceramic Society, Columbus, OH, 1985. p. 27. [9] S. Suder, B.K. Srivastava, A. Krisnamurty, Ind. J. pure Appl. Phys. 42 (2004) 366.

[10] S.S. Hayraetyan, H.G. Khachatryan, Micropror. Mesopror. 72 (2004) 105. [11] H.M. Zaki, J. Phys. B Cond. Matter 363 (2005) 232. [12] B. Kaur, M. Bhat, F. Licci, R. Kumar, K.K. Bamzai, P.N. Kotru, J. Mater. Chem. Phys. 103 (2007) 255. [13] I. Soibama, S. Phanjoubam, C. Prakash, J. Alloys Compds. 475 (2009) 328–331. [14] S. Son, M. Taheri, E. Carpenter, V.G. Harris, M.E. McHenry, J. Appl. Phys. 91 (10) (2002) 7589–7591. [15] N. Chu, X. Wang, Y. Liu, H. Jin, Q. Wu, L. Li, Z. Wang, H. Ge, N. Chu, J. Alloys Compds. 470 (2009) 438–442. [16] F.F.Y. Wang, Treatise Mater. Sci. Technol. 2 (1973) 280. [17] H. Hirazawa, S. Kusamoto, H. Aonoa, T. Naohara, K. Mori, Y. Hattori, T. Maehara, Y. Watanabe, J. Alloys Compds. 461 (2008) 467–473. [18] S.A. Saafan, S.T. Assar, B.M. Moharram, M.K. ElNimr, J. Magn. Magn. Mater. 322 (2010) 628–632. [19] M. Javad, N. Isfahani, M. Myndyk, D. Menzel, A. Feldhoff, J. Amighain, V. Šepelák, J. Magn. Magn. Mater. 321 (2009) 152–156. [20] K. Maaz, s. Karim, A. Mumtaz, S.K. Hasanain, J. Liu, J.L. Duan, J. Magn. Magn. Mater. 321 (2009) 1838. [21] P. Priyadharsini, A. Pradeep, P. SambasivaRao, G. Chandrasekaran, Mater. Chem. Phys. 116 (2009) 207–213. [22] N. Singh, A. Agarwal, S. Sanghi, P. Singh, J. Alloys Compds. 509 (2011) 7543– 7548. [23] G.O. White, C.E. Patton, J. Magn. Magn. Mater. 9 (1978) 299–317. [24] K. Maaz, W. Khalid, A. Mumtaz, S.K. Hasanain, J. Liu, J.L. Duan, Phys. E. 41 (2009) 593–599. [25] S.N. Dolia, P.K. Sharma, M.S. Dhawan, S. Kumar, A.S. Prasad, A. Samariya, S.P. Pareek, R.K. Singhal, K. Asokan, Y.T. Xing, M. Alzamora, E. Saitovitach, J. Appl. Surf. Sci. 258 (2012) 4207–4211. [26] P.A. Shaikh, R.C. Kambale, A.V. Raoa, Y.D. Kolekar, J. Alloys Compds. 492 (2010) 590–596. [27] M. Maisnama, S. Phanjoubama, H.N.K. Sarmaa, Ch. Prakashb, L.R. Devib, O.P. Thakurb, J. Phys. B 370 (2005) 1–5. [28] M.A. Gabal, Y.M. Al Angari, S.S. Al-Juaid, J. Alloys Compds. 492 (2010) 411–415. [29] D.S. Birajdar, D.R. Mane, S.S. More, V.B. Kawade, K.M. Jadhav, Mater. Lett. 59 (2005) 2981–2985. [30] K. Maaz, S. Karim, A. Mashiatullah, J. Liu, M.D. Hou, Y.M. Sun, J.L. Duan, H.J. Yao, D. Mo, Y.F. Chen, Phys. B. 404 (2009) 3947–3951. [31] J. Xiang, X. Shen, F. Song, M. Liu, J. Solid State Chem. 183 (2010) 1239–1244. [32] M.H. Pardavi, J. Magn. Magn. Mater. 215–216 (2000) 171. [33] M. Buc´ko, K. Haberko, J. Eur. Ceram. Soc. 27 (2007) 723. [34] J.C. Dyre, T.B. Schroder, Rev. Modern Phys 72 (3) (2000) 873. [35] KhalidMujasamBatoo, ShalendraKumar, ChanGyuLee, limuddin, Current App. Phys. 9 (2009) 1072–1078. [36] MuhammadJavedIqbal, MahRukhSiddiquah, J. Magn. Magn. Mater. 320 (2008) 845–850. [37] A.N. Patil, M.G. Patil, K.K. Patankar, V.L. Mathe, R.P. Mahajan, S.A. Patil, Bull. Mater. Sci. 23 (5) (2000) 447–452. [38] S.A. Mazen, H.M. Zaki, S.F. Mansour, J. Pure Appl. Phys. ISSN No. 0973-1776 3 (1) (2007) 40–48. [39] H. Ismael, M.K. E1 Nimr, A.M. Abou El Ata, M.A. El Hiti, M.A. Ahmed, A.A. Murakhowski, J. Magn. Magn. Mater. 150 (1995) 403–408.