Magnetic and MÃ¶ssbauer Studies of Bare and Encapsulated

Article pubs.acs.org/JPCC

Magnetic and Mössbauer Studies of Bare and Encapsulated Nanoparticles of [(Co0.2Mn0.3Zn0.5Fe2O4)(1‑x) (ZnO/PVA)x (x = 0 and 0.30)] S. Mukherjee,† K. Mukhopadhyay,† S. Sutradhar,† S. Pati,‡ A. K. Deb,§ D. Das,‡ and P. K. Chakrabarti*,† †

Solid State Research Laboratory, Department of Physics, Burdwan University, Burdwan 713 104, West Bengal, India UGC-DAE Consortium for Scientific Research, III/LB-8, Kolkata 700 092 § Department of Physics, Raiganj College (University College), Uttar Dinajpur, West Bengal 733 134, India ‡

ABSTRACT: Nanoparticles of Co substituted Mn−Zn-ferrite (Co0.2Mn0.3Zn0.5Fe2O4) were prepared by coprecipitation method. Co substitution in Mn−Zn-ferrite was considered to obtain the superparamagnetic (SPM) behavior at room temperature (RT) in bigger-sized particles as well as to enhance the saturation magnetization. To modulate the dipolar interaction, magnetic nanoparticles were coated with nonmagnetic matrices of ZnO and polyvinyl alcohol. The formation of the mixed spinel phase of different heat-treated samples was confirmed by X-ray diffractograms (XRD). The sizes of nanocrystallites obtained from the Debye−Scherrer formula are in the range of 10−51 nm. The average sizes of nanoparticles and their distribution, morphology, crystallographic phase, etc., of some selected samples were determined from the results of high-resolution transmission electron microscopy (HRTEM). The average particle sizes and their distributions of all the samples were obtained from the dynamic light scattering measurement. Hysteresis loops, magnetization versus field curve, zero-field cooled, and field-cooled magnetization versus temperature curves of some samples were recorded by superconducting quantum interference device (SQUID) magnetometer. The magneto-crystalline anisotropy, average particle size, and its distribution, etc., of the bare and coated samples were calculated from the analysis of the static magnetic data and the results are in good agreement with those obtained from XRD and HRTEM observations. Dynamic hysteresis loops at different frequencies and Mössbauer spectra of the samples were recorded at RT that indicate the presence of SPM relaxation of the nanoparticles.

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INTRODUCTION During the past few years, nanoparticles of mixed spinel ferrites have been investigated exhaustively for the interesting deviation of their physical properties from those of their bulk counterparts. Various novel properties, viz., superparamagnetic (SPM) relaxation, high magnetic moment, very low hysteresis loss, have been observed in the nanoparticle state, which are not yet found in their bulk counterparts.1−9 Due to this interesting departure in physical properties, these systems have enormous potential for applications in various electromagnetic devices,10−20 viz., high-density data storage, ferrofluid technology, sensor technology, catalysis, magnetically guided drug delivery, and magnetic resonance imaging. Besides these, the magnetic behavior of nanoparticles can be modulated by encapsulating them using various nonmagnetic matrices.2,4 This coating not only modulates the magnetic behavior by changing the super exchange or dipolar interaction, but also protects them from various effects of the environment. This coating also provides a matrix for binding the particles and at the same time prevents grain growth, agglomerations, etc. Normally, the novel magnetic property is associated with isolated nanoparticles, but for practical applications, it is necessary to use aggregation of nanoparticles without agglomeration. Though it is not possible © XXXX American Chemical Society

to overcome the agglomeration completely, encapsulation of nanoparticles by nonmagnetic host2,4,21,22 reduces this to an appreciable amount. In the family of mixed spinel systems, Mn−Zn-ferrite is an important soft magnetic material, which has high potential for applications in electromagnetic devices. Its permeability, saturation magnetization, etc., are considerably high and its coercive field is quite low. To increase the magnetocrystalline anisotropy, which is necessary to get the SPM behavior at room temperature (RT) in bigger-sized particles and also to enhance the saturation magnetization of the nanocrystalline Mn−Zn-ferrite, the present paper considers the preparation and characterization of nanoparticles of Cosubstituted Mn−Zn-ferrite (Co0.2Mn0.3Zn0.5Fe2O4, CMZF). The samples were prepared by the simple coprecipitation method and the ultrasonication technique was employed to reduce the distribution of sizes and agglomeration of nanoparticles. To modulate the magnetic interaction and also to protect them from various environmental effects, the bare nanoparticles of CMZF were encapsulated by the nonmagnetic Received: November 26, 2012 Revised: May 18, 2013

A

dx.doi.org/10.1021/jp311620u | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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designated as CMZF-PVA. For the preparation of CMZF-PVA, we have used the same method as considered in our earlier paper.4 In this method PVA was dissolved in boiled water and the required amount of CMZF 1 was slowly added to it under boiling condition. The aqueous solution of PVA dispersed with particles of CMZF 1 was boiled for about 4 h to get the dried sample of CMZF-PVA. The X-ray diffractograms (XRD) of all the samples were taken in an Xpert Pro Phillips XRD unit with Co-radiation (λ = 0.178 897 nm) in the range of 2θ from 20° to 80°. The highresolution transmission electron microscopy (HRTEM) images were observed in JEOL JEM 2100 HRTEM, Japan with resolution = 1.9 Å. The average particle size and distribution of all the samples were also determined by dynamic light scattering (DLS) setup [Model: DLS-nano ZS, Zetasizer, Nanoseries, Malvern Instruments]. Digital hysteresis loops at different frequencies were observed by using a digital hysteresis loop tracer supplied by Metis Instruments and Equipments NV, Belgium. Mössbauer effect measurements were carried out by using a PC based spectrometer with 1024 channels MCA card operating in the constant acceleration mode. All measurements were carried out in transmission geometry using a 10 mCi 57Co source in Rh matrix. The details of our Mössbauer effect measurements were given in our earlier papers.7,8,18,23 Static magnetic measurements were carried out in Superconducting Quantum Interference Device (SQUID) Magnetometer.

matrices of ZnO and polyvinyl alcohol (PVA), respectively. Rietveld analysis of the observed X-ray diffraction patterns was carried out to confirm the desired crystallographic phase of all the samples. High resolution transmission electron microscopy (HRTEM) of some selected samples and dynamic light scattering measurements (DLS) of all the samples were also carried out to know the various features of nanoparticles, viz., crystallographic phase, particle sizes, distribution of particle sizes, coating of nanoparticles, nanocrystallite fringe pattern. Static and dynamic magnetic measurements of some samples were carried out and for the first time the magnetonanocrystalline anisotropy, particle size and its distribution, etc., of the bare and nanocomposite samples were evaluated from the detailed analysis of the static magnetic data. To confirm the onset of SPM relaxation of the nanoparticles and to estimate different hyperfine parameters, Mössbauer effect measurements of all the samples were also carried out at RT.

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EXPERIMENTAL SECTION Mixed spinel samples of CMZF were prepared by the standard coprecipitation method. Iron nitrate nonahydrate [Fe(NO3)2·9H2O, 0.8 M], zinc nitrate [Zn(NO3)2, 0.2M], cobalt nitrate hexahydrate [Co(NO3)2·6H2O, 0.08 M], and manganese acetate [C4H6MnO4·4H2O, 0.12M] were used as starting materials. A complete homogeneous solution of all the four starting materials was made by triple distilled water. A few drops of hydrochloric acid were added to obtain a clear solution. The stoichiometric ratio of Co, Mn, Zn, and Fe was taken as 0.2:0.3:0.5:2. Dilute NaOH solution was added dropwise to the salt solution at 80 °C and the final pH was kept at 10. During the slow addition of NaOH, the ultrasonication was continued and the ultrasonic vibrations were applied during the whole course of preparation, viz., homogenization of salt solution, coprecipitation, and digestion of the coprecipitation. This ultrasonication technique applied in the coprecipitation method normally reduces the distribution of the particle sizes.4,21 To obtain a neutral pH condition as well as to remove the extra ions, the coprecipitated particles were filtered and washed several times by using triple distilled water. Finally, the precipitated particles were dried at RT. The dried samples were annealed at 100, 400, 600, 800, and 1000 °C for 6 h, which are named as CMZF 1, CMZF 2, CMZF 3, CMZF 4, and CMZF 5. To coat the magnetic nanoparticles of CMZF by the matrix of ZnO, we have used the sample of CMZF 1 and this coated sample is designated as CMZF-ZnO. For the preparation of the nanocomposite sample of CMZF-ZnO [(Co0.2Mn0.3Zn0.5Fe2O4)(1‑x) (ZnO)x (x = 0.30)], zinc acetate salt [0.026 (M)] was used as precursor material for ZnO and NaOH was used for its precipitation. The required amount of zinc acetate salt was dissolved in the triple distilled water and the required amount of nanoparticles of CMZF 1 was dispersed homogeneously in Zn-acetate solution by ultrasonication. NaOH solution was added dropwise to the homogeneous mixture of Zn-acetate solution and the particles of CMZF 1. The final pH of the solution was kept at 7.5. For complete precipitation and digestion, sonication was also continued for an hour after the completion of NaOH addition. The precipitate was washed several times by decantation for neutralization as well as to remove the extra ions. To obtain the desired phase of coated particles of CMZF-ZnO, the coprecipitated particles along with the particles of CMZF were dried and annealed at 600 °C for 6 h. The nanoparticles of CMZF 1 were coated by the matrix of PVA and this sample is

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RESULTS, ANALYSIS, AND DISCUSSION XRD and TEM Analyses. X-ray diffraction patterns (Figures 1 and 2) confirmed the formation of the desired crystallographic phase of all the samples of CMZF. Though the

Figure 1. X-ray diffraction patterns with Rietveld analysis of different samples (a) CMZF 1, (b) CMZF 2, (c) CMZF 3, (d) CMZF 4, and (e) CMZF 5. B



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and

with

sample of CMZF 1 is annealed at low temperature (∼100 °C), in this case nanocrystalline mixed spinel phase has also been formed (Figure 1a). No extra peak was found in the case of CMZF 1 and CMZF 2 (annealed at 100 and 400 °C), i.e., pure nanocrystalline spinel phase was obtained in these samples. However, in the case of CMZF 3, 4, and 5 obtained by annealing at 600, 800, and 1000 °C, we have observed the presence of the spinel phase (CMZF) along with a minor phase of α-Fe2O3. All the peaks were assigned using JCPDS file No. 74−2400 for the spinel ferrite phase and JCPDS file No. 89− 0599 for α- Fe2O3 phase. The indexing of two phases (spinel and α-Fe2O3) is shown in Figure 1. To confirm the desired crystallographic phase and also to estimate the values of lattice parameters, Rietveld analysis of the XRD patterns of all the samples waas carried out. The whole pattern fitting was carried out with the help of software MAUD.24,25 Initial simulation of the diffraction pattern was started with the spinel phase having space group Fd3̅m. The fitted patterns of the bare samples and the difference plots are shown in Figure 1; Figure 2 represents those of coated samples. The accuracy of profile fitting is judged by the goodness of fit (GOF) together with the reliability parameters (Rexp, Rw, RB) which are defined as

RB =

(1)

∑ |Iko − Ikc| ∑ Iko

Rw R exp

(3)

⎡N − P⎤ ⎥ R exp = ⎢ ⎣ ∑ wIi io2 ⎦

(4)

Here Iio and Iic are the observed and calculated intensities at the ith step, Ik represents the intensities assigned to the kth Bragg reflection at the end of the refinement cycles; wi = (1/Iio) is the weight factor. The quantitative amount of the impurity phases of α-Fe2O3 in CMZF 3, 4, and 5 were also estimated from the Rietveld analysis. The formation of this impurity phase was also observed in our previous work22 and in other works.26,27 The results of Rietveld refinements of all the samples are summarized in Table 1. The coated sample of CMZF-ZnO was annealed two times: first at 100 °C for 6 h before encapsulation and, finally, at 600 °C for 6 h after encapsulation. Thus, there would be an impurity phase of α-Fe2O3 in the sample of CMZF-ZnO, as obtained in the bare sample of CMZF 3, which is also annealed at 600 °C. But from the Rietveld analysis, it was found that there was no extra peak of α-Fe2O3 in the XRD pattern of CMZF-ZnO. Thus, the tendency of the formation of the impurity phase was suppressed by the coating with nonmagnetic matrix of ZnO. This observation also confirmed that the nanoparticles of CMZF were well coated by the nonmagnetic matrix of ZnO. Thus, the present method successfully encapsulated the nanoparticles of CMZF in the matrix of ZnO. This was achieved due to the fact that the coating with ZnO was carried out during the coprecipitation of Zn-acetate in presence of the nanoparticles of CMZF. The good encapsulation of nanoparticles of CMZF 1 by ZnO matrix in the present method is also established by the TEM observation discussed in the next section. In the XRD pattern (Figure 2), we did not detect any crystalline phase corresponding to ZnO in the coated sample of CMZF-ZnO. Thus, the coating matrix of ZnO may be in the amorphous phase. However, this conclusion cannot be confirmed, as the positions of major peaks of ZnO are very close to those of CMZF and, consequently, there is always a chance of overlapping of peaks of both the crystalline phase of CMZF and ZnO. Here, it is also noted that in the selected area electron diffraction patterns (SAED) of CMZF-ZnO, we have observed some rings corresponding to the crystalline phases of CMZF and ZnO, which is discussed in the next section. Actually, in the nanoparticle state, the peaks are normally broadened and because of this all the peaks corresponding to ZnO and CMZF are also broadened, which will increase the probability of overlap of the closely spaced peaks. For example, two major peak positions of ZnO [(101) and (002)] are

Figure 2. X-ray diffraction patterns with Rietveld analysis of coated samples of CMZF-PVA and CMZF-ZnO.

⎡ ∑ w (I − I )2 ⎤1/2 i io ic ⎥ Rw = ⎢ ∑ wIi io2 ⎦ ⎣

GOF =

(2)

Table 1. Results of Rietveld Analysis of All the Samples Annealed at Different Temperatures wt % samples CMZF 1 CMZF 2 CMZF 3 CMZF 4 CMZF 5 CMZF-ZnO CMZF-PVA

annealed temp. (°C) 100 400 600 800 1000 600 100

spinel 100 100 84.0 (0.8) 80.1 (0.8) 90.8 (0.5) 100 100

cell (å) α-Fe2O3 ----16.0 (0.8) 19.9 (0.8) 9.2 (0.5) -----

spinel 8.400 8.417 8.425 8.438 8.458 8.418 8.428

(9 (9 (8 (3 (1 (8 (1 C

× × × × × × ×

−4

10 ) 10−4) 10−4) 10−4) 10−4) 10−4) 10−3)

α-Fe2O3

Rw (%)

Rb (%)

GoF

----5.036 (8× 10−4) 13.748(3 × 10−3) 5.037 (6× 10−4) 13.739 (2× 10−3) 5.041 (2× 10−3) 13.742 (6× 10−3) -----

4.601 4.814 4.817 4.828 4.874 2.624 4.730

3.622 3.857 3.832 3.821 3.876 2.083 3.726

1.027 1.069 1.057 1.019 1.052 1.036 0.999



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42.357° (100%) and 40.192° (44%) (JCPDS file No. 36−1451) and these positions are within the observed range of 100% peak [(311)] of CMZF (39.903° to 42.812°). This overlap has also been observed in the case of other planes of ZnO, viz., (110), (103), (004), and (203) with the planes [(511), (440), (533), and (731)] of CMZF. Besides this, the intensity of peaks of ZnO may be very low compared to those of CMZF, and due to this fact the peaks of the former were not yet detected together with those of CMZF. This observation was found in our previous work28 where we could not detect the peaks of ZnO in the nanocomposite state of {(ZnO)0.55 (Eu2O3)0.45} together with those of Eu2O3. Besides this, no impurity peak was found in the other coated sample of CMZF-PVA. This is also expected, as in this case the nanoparticles of CMZF 1, annealed at 100 °C, were coated in the boiling condition of aqueous solution of PVA and no further heat treatment was given after coating. The average crystallite sizes of all the samples were calculated from the line-broadening of the (311) peak of the XRD pattern using the well-known Debye−Scherrer equation.22 The average crystallite size of all the samples lies in the range of 9−51 nm. Their nanocrystallite sizes obtained from XRD, average particle sizes estimated from TEM, and DLS measurements of all the samples are shown in Table 2. Crystallite sizes of CMZF

Figure 3. Variation of lattice parameter including standard deviation as a function of annealing temperature of all the samples of CMZF.

annealing temperature, etc.21 We have also estimated the degree of inversion from the values of saturation magnetization discussed in the next section. Some selected samples were studied by HRTEM and the corresponding images of the samples, viz., CMZF 1, CMZFZnO, and CMZF-PVA, are shown in Figure 4. Some representative micrographs are displayed in Figure 4a-f. The nanoparticles of each sample are well dispersed, i.e., they are not yet agglomerated (Figure 4a-f). The overall view of the micrographs of CMZF 1 (Figure 4a), CMZF-ZnO (Figure 4b), and CMZF-PVA (Figure 4c) confirmed that the nanoparticles are more or less of uniform and spherical sizes. To display the coating or encapsulation of CMZF nanoparticles in nonmagnetic matrices of ZnO and PVA, three other micrographs are shown in Figure 4d,e,f. Figure 4d,e shows the encapsulation of CMZF nanoparticles annealed at 600 °C in ZnO matrix, and from these figures it is seen that the nanoparticles are well encapsulated in the matrix of ZnO. Figure 4e shows a single nanoparticle coated uniformly by the matrix of ZnO. Figure 4f displays the encapsulated nanoparticles of CMZF annealed at 100 °C in the matrix of PVA. From Figure 4d,e,f, it is confirmed that in the coated samples, the nanoparticles of CMZF are well coated by the matrix of ZnO compared to that of PVA. In our previous investigations,21 we have also observed that the nanoparticles of Co0.5Zn0.4Cu0.1Fe2O4 were well coated by the matrix of Al2O3 compared to that by PVA. In all the cases of coating with ZnO or Al2O3, the coprecipitation of the salts of Zn or Al was carried out in the presence of ferrite nanoparticles, and thus for the coating with nonmagnetic oxides, this method is very effective for uniform coating. From the HRTEM observations, structural information has also been obtained from the selected area electron diffraction (SAED) patterns of the samples of CMZF 1, CMZF-ZnO, and CMZF-PVA and the corresponding patterns are shown in Figure 4g,h,i, respectively. The rings in the SAED patterns which are the representatives of lattice planes are assigned and these are in good agreement with the present mixed spinel system. The planes corresponding to CMZF and ZnO are also detected and assigned in the SAED pattern of CMZF-ZnO. To get the distribution of sizes of nanoparticles and hence to draw the histogram in each case, we have measured the size of 300 nanoparticles selected from the different observed micrographs of the samples. The distributions of particle sizes of the samples of CMZF 1, CMZF-PVA, and CMZF-ZnO, obtained from their measured average diameters, are displayed

Table 2. Average Crystallite and Particles Sizes of All the Samples

samples CMZF 1 CMZF 2 CMZF 3 CMZF 4 CMZF 5 CMZFZnO CMZFPVA

annealing temperature (in °C) 100 400 600 800 1000 600 100

average crystallite size with error (nm) obtained from XRD

average particle size with error (nm) obtained from TEM

± ± ± ± ± ±

11.2 ± 0.2

10.0 13.7 16.7 36.1 51.3 11.0

0.5 0.5 0.5 0.5 0.5 0.5

11.4 ± 0.2

8.8 ± 0.5

15 ± 0.2

average particle size with error (nm) obtained from DLS 5.7 15.5 18.5 33.7 53.1 10.2

± ± ± ± ± ±

0.5 0.5 0.5 0.5 0.5 0.5

17.6 ± 0.5

sample annealed at 600 °C and CMZF-ZnO are respectively 16.7 (±0.2) and 11.0 (±0.2) nm. The average crystallite size of CMZF 3 (annealed at 600 °C) is larger than the coated sample of CMZF-ZnO (Table 1). Thus the growth rate of the nanocrystallite of CMZF in CMZF-ZnO during the annealing was prevented by the coating matrix of ZnO. The variation of lattice parameter extracted from the Rietveld refinements as a function of annealing temperature is shown in Figure 3 and the value of lattice parameter steadily increases with the increase of annealing temperature. This sort of variation is quite likely due to the redistribution of the divalent zinc cations between A- and B-sites of the nanocrystalline spinel lattice.23 In general, Zncations prefer the tetrahedral site in the ferrite lattice, but this is not strictly applicable in the nanoparticle state. In the ZnFeO4 substituted nanoparticle system of mixed spinel ferrite, the degree of inversion may occur due to redistribution of Zn2+ cation between tetrahedral and octahedral sites. In the present mixed spinel system, the ionic radii of Zn2+, Mn2+, and Co2+ are respectively 0.88, 0.89, and 0.84 Å. The variation of lattice parameters is most likely due to this redistribution of cations with different ionic radii. The degree of cationic inversion in the nanoparticle system depends on the method of preparation, D



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Figure 4. Results of HRTEM observations: micrographs of (a) CMZF 1, (b), (d), and (e) CMZF-ZnO, (c) and (f) CMZF-PVA; SAED patterns of (g) CMZF 1, (h) CMZF-ZnO, (i) CMZF-PVA (* represents the SAED patterns of ZnO); histograms of (j) CMZF 1, (k) CMZF-ZnO, (l) CMZFPVA; fringe patterns of (m) CMZF 1, (n) CMZF-ZnO, and (o) CMZF-PVA.

in the histograms of Figure 4j,k,l. The distribution of particle sizes is much narrower in the case of coated samples of CMZF 1 and CMZF-ZnO. From the displayed histograms (Figure

4j,k,l), it is seen that the overall distribution of particle sizes is also reduced by the ultrasonication technique applied in the coprecipitation method. This reduction in the distribution of E



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particle sizes has also been observed in our previous work,4,21 where we reported that ultrasonication promoted the monodispersity in the ferrite sample. However, the distribution of sizes of nanoparticles in CMZF-PVA is a bit large (Figure 4) compared to those of CMZF 1 and CMZF-ZnO. This is due to the fact that the nanoparticles of CMZF were coated with PVA under boiling condition in water. The boiling of water together with CMZF nanoparticles was maintained during the whole course of preparation of CMZF-PVA and the boiling condition was maintained until the drying state of the CMZF-PVA was achieved. During this time we could not apply ultrasonication, and because of this, the particle sizes may be largely distributed. This observation also substantiated the fact that the ultrasonication promoted the monodispersity of the nanoparticles. The average diameters of the maximum number of particles present in the samples of CMZF 1 and CMZF-ZnO are between 7 and 15 nm. However, for the sample of CMZF-PVA, the average diameters of the maximum number of particles are in the range of 10−21 nm. The average particle sizes of the CMZF 1 and CMZF-ZnO obtained from the micrographs are, respectively, ∼11.1 (±0.2) and 11.4 (±0.2) nm, and for CMZFPVA, the average particle size is 15.0 (±0.2) nm. The estimated values of particle sizes obtained from different micrographs agree well with those extracted from XRD patterns. During the HRTEM observations, single crystalline lattice fringes of the samples of CMZF 1, CMZF-ZnO, and CMZFPVA were recorded. All the lattice fringes displayed in Figure 4m,n,o correspond to a group of atomic planes within the nanoparticle, indicating that the nanoparticle of each sample is single crystallite with no defect. The distance between two adjacent planes is calculated and its value is ∼0.286 nm (Figure 4m), which corresponds to the (311) plane of the spinel ferrite sample of CMZF 1, and for CMZF-ZnO, the corresponding value is ∼0.307 nm (Figure 4n), which corresponds to the (111) plane of CMZF. For the sample of CMZF-PVA, the value of the distance between two consecutive fringes is ∼0.498 nm (Figure 3o), which corresponds to the (111) plane of CMZF in CMZF-PVA. The average particle sizes estimated from the DLS measurements are shown in Table 2. The distributions of particle sizes of all the samples obtained from the DLS measurements are depicted in Figure 5. The average particle sizes obtained from the TEM micrographs are in good agreement with those obtained from DLS study. The average particle size of CMZF 3 is greatly reduced in the case of CMZF-ZnO sample compared to that of the CMZF 3 (Table 2), which is in good agreement with that obtained from the TEM study. Thus the reduction of the growth of the nanocrystallite sample of CMZF 3 by the encapsulation of the matrix of ZnO has been confirmed by XRD, TEM, and DLS observations. The distribution of particle sizes of all the samples measured from the DLS observation is depicted in Figure 5. It is evident that the distributions of particle sizes are also greatly reduced due to the effect of ultrasonication (Figure 5). Magnetic Properties. dc Magnetic Properties. Zero Field Cooled (ZFC) and Field Cooled (FC) magnetizations of some samples were measured at different temperatures from 300 K down to 2 K and the corresponding thermal variations are shown in Figure 6. The FC magnetization curve in the range of 300−2 K was recorded after cooling the sample in presence of an applied field of ∼39.81 × 103 A/m (500 Oe). A relatively high field of 500 Oe was applied to get an appreciable difference between the FC and ZFC magnetizations, which will

Figure 5. Histograms obtained from DLS measurements of all the samples of (a) CMZF 1, (b) CMZF 2, (c) CMZF 3, (d) CMZF 4, and (e) CMZF 5 and (f) CMZF-PVA and (g) CMZF-ZnO.

Figure 6. FC and ZFC magnetization curves of (a) CMZF 2, (b) CMZF 4, and (c) CMZF 5.

minimize the error in the determination of particle size, as the difference was utilized to evaluate the average particle size of the samples, discussed in the next section. The FC and ZFC curves of CMZF 2 and 4 (annealed at 400, 800 °C) are bifurcated below RT, but no such bifurcation was observed in CMZF 5 (annealed at 1000 °C). The FC and ZFC magnetization curves are normally bifurcated due to the presence of SPM relaxation of the nanoparticles and this fact confirmed that most of the nanoparticles of CMZF 5 are in the ferrimagnetic state at 300 K. Thus, this observation also confirmed that the blocking temperature of CMZF 5 is above 300 K. This observation is also in good agreement with the Mössbauer pattern of the sample of CMZF 5 and this is discussed in the next section. In general, the blocking temperature of a monodisperse magnetic nanoparticle system is defined as the temperature below which the SPM relaxation is arrested, i.e., blocked. However, for a magnetic nanoparticle system with size distribution, there is no single temperature below which the phenomenon of blocking would be achieved; F



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Table 3. Blocking Temperatures (Tp and TB), Magnetocrystalline Anisotropy, and Particle Sizes of CMZF 2 and 4 Obtained from the Analysis of Magnetic Data Ka × 10‑5 and errors (erg/cm3)

particle sizes (nm) estimated from blocking temperature with different frequency factors f 0 (s−1) and estimated errors

samples

TA (°C)

TP (K)

TB (K)

from fitting

from TB

109

1010

1011

1012

1013

CMZF 2 CMZF 4

400 800

189.7 246.2

300 >300

66.2 ± 0.02 13.5 ± 0.06

10.6 ± 0.3 0.58 ± 0.3

8.4 ± 0.1 32.5 ± 0.2

12.7 ± 0.02 33.3 ± 0.02

13.0 ± 0.04 34.4 ± 0.03

13.3 ± 0.04 35.2 ± 0.04

13.7 ± 0.03 36.0 ± 0.05

loops mentioned in the next section. All the values of ΔM were successfully fitted by eq 6 and one representative fitted curve for CMZF 2 along with the observed data is shown in Figure 6. The average blocking temperatures of the samples (CMZF 2 and 4) were estimated from the analysis of this fitting. Using the parameters obtained from the fitting of eq 6, the distribution of blocking temperatures has also been calculated using eq 5, and this distribution is shown in the inset of Figure

rather the blocking temperature will also be distributed according to the size distribution. For monodisperse particles, the maximum point in the ZFC curve and the bifurcation point of FC and ZFC curves are very closely spaced.29 In these monodisperse particles, normally a single temperature is defined to mark the blocking condition which is the maximum point in the ZFC curve, but in the case of distributed particles, normally two temperatures (TB and TP) are defined30−34 to describe the phenomenon of blocking condition. In such a system with a size distribution, the temperature TB is regarded as the highest temperature at which the ZFC and FC magnetizations are bifurcated corresponding to the larger group of particles in the system. The temperature Tp is defined as the temperature where the ZFC magnetization shows a peak in its thermal variation. Actually, the ZFC magnetization decreases below Tp due to the smaller group of particles in the system and TP is lower than TB. In the case of CMZF 4 (annealed at 800 °C), TB is very close to 300 K, but in the case CMZF 2 (annealed at 400 °C), TB is lower than 300 K. We have found that TP is closer to TB for CMZF 4 (Figure 6b) than that of the CMZF 2 (Figure 6a). This may be due to lower distribution of sizes in the case of CMZF 4 compared to that of CMZF 2. The extracted values of TB and TP are listed in Table 3. To estimate the average particle size and magnetic anisotropy of CMZF 2 and 4, we have computed the difference of FC and ZFC magnetizations and from these data the blocking temperature and the particle size distribution were computed. The distributions of blocking temperatures in both samples of CMZF 2 and 4 are considered as log-normal type and are expressed below:35 f (TB) =

⎧ [ln(T /T )]2 ⎫ 1 B B0 ⎬ exp⎨− 2π TBλB ⎩ 2λB2 ⎭

Figure 7. Variation of ΔM = MFC − MZFC with T of CMZF 4. Symbol (■): experimental. Solid line: theoretically fitted curve.

7. The average diameter of the nanoparticles was estimated from the relation26 ⎡ 6kBTB0 λ 2 /6 ⎤1/3 B ⟨DTB⟩ = ⎢ e ln(f0 τ )⎥ ⎣ πK a ⎦

(5)

The symbols have their usual meanings.35 If f(TB) dTB represent the fraction of particles having blocking temperatures lying in the interval TB to TB+dTB, the difference between ZFC and FC curves is given by36 ΔM =

⎡ ln(T /TB0) 29MS2H ⎧ λ ⎤⎫ ⎨1 − erf⎢ − B ⎥⎬ 6K a ⎩ 2 λB 2 ⎦⎭ ⎣ ⎪

⎪

⎪

⎪

Here we have assumed that ⟨DTB⟩ = TB0 e and 1 = f0 e−KaV / kT (8) τ where TB0, B, and Ka are the parameters obtained from the fitting of eq 6 and f 0 is the characteristic relaxation frequency, ranging typically from 109 to 1013 s−1 for SPM nanoparticle. The estimated values of anisotropies of CMZF 1 and 2 are high (Table 3) compared to the available values of the bulk systems.28 Though the stoichiometery and the exact elemental composition are not similar to the present case, but from the reported values of the bulk systems,37 it is evident that the anisotropy of the present system is quite high. This high value of anisotropy in each case mainly arises from the various contributions coming from shape, sizes, and surface in the anisotropy of the nanoparticles. By estimation of average particle size from different frequency factors, it is seen that the particle sizes obtained from the relatively higher values of frequency factors are more appropriate as far as the crystallite/ particle size obtained from XRD and DLS measurements are

(6)

Here, we consider TB = TB0 exp( 2 λBx)

and

x=

ln(TB/TB0) 2 λB

The error function is introduced as

∫T

∞

(7) λB2/2

2

e−t dt = ( π /2) erfc(T )

where erfc(T) = 1 − erf(T). The observed values of ΔM estimated from FC and ZFC curves were fitted by varying the parameters of TB0, Ka, and λB in eq 6, and in this fitting MS was taken from our observed M-H G

λ2B



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concerned. Thus the frequency factors on the order of 1012 to 1013 may be appropriate for the description of the SPM relaxation of the present systems. The magnetization as a function of applied field (M-H loops) of some selected samples, CMZF 1, 2, and 5 and CMZF-ZnO, recorded at various temperatures, 300, 200, 5, and 2 K, are shown in Figures 8, 9, and 10. The magnetization of each

Figure 10. Hysteresis loops recorded at 2 and 5 K of different samples of (a) CMZF-ZnO (b) CMZF 1, (c) CMZF 2, and (d) CMZF 5.

errors = ±0.1) emu/g, respectively. The saturation magnetization of CMZF-ZnO has been reduced compare to those of bare samples of CMZF and this is due to the nonmagnetic fraction of ZnO. If we calculate the saturation magnetization for the magnetic part, i.e., for the CMZF nanoparticles, then it would be ∼48.0 ± 0.1 emu/g, which is higher than for the sample annealed at 400 °C but lower than the sample annealed at 1000 °C. The M−H loops measured at 200 K of CMZF-1, 2, 5, and CMZF-ZnO are shown in Figure 9. All these magnetization loops are saturated with higher value of magnetization compared to those recorded at RT. This increase of magnetization may be attributed to various factors, viz., increase of ferrimagnetic fraction with the lowering of temperature, lowering of nanocrystalline anisotropy, etc. The values of saturation magnetization of the samples of CMZF 1, 2, and 5 are, respectively, 66.5 ± 0.1 and 80.5 ± 0.1 emu/g. Figure 10 displays the M-H loops of CMZF-ZnO and CMZF 1, 2, and 5 recorded at 5 and 2 K. Here also, the saturation magnetization of all the samples has been increased from the above-mentioned values recorded at 300 and 200 K. The values of maximum saturation magnetization of the samples of CMZFZnO and CMZF (annealed at 100, 400, 1000 °C) are, respectively, 75.0 ± 0.1, 91.0 ± 0.1, 91.7 ± 0.1, and 124.2 ± 0.1 emu/g. The saturation magnetizations are considerably high with low value of coercive field. For example the values of the coercive field of CMZF 1, 2, 4, and 5 extracted from the M-H loops recorded at RT are, respectively, 5.6, 16.7, 13.3, and 12.5 (with errors = ±0.1) Oe. Apart from these facts, the M-H loops of the samples are square shaped, and consequently the saturation to remanance ratio will not be large compared to unity. For example, the ratio of saturation and remanance magnetizations extracted from loops recorded at RT of CMZF 1, 2, 4, and 5 are, respectively, 6.8, 4.5, 5.7, and 2.6. Thus the high value of saturation magnetization, low value of HC, and low value of saturation to remanence ratio confirmed that the present samples would be suitable for applications in soft magnetic devices. Thus the substitutions of Co2+ ions in the Mn−Zn-ferrite can modulate the shape of the hysteresis loop from flat-type to square-type. In the case of CMZF 2 and 4, SPM behavior has been found at temperatures which are very close to RT. These relatively high values of blocking temperatures observed in samples with relatively large particle sizes have been achieved due to the increase of magnetocrysl-

Figure 8. Hysteresis loops recorded at 300 K of different samples of (a) CMZF 1, (b) CMZF 2, (c) CMZF 4, and (d) CMZF 5.

Figure 9. Hysteresis loops recorded at 200 K (a) CMZF 2, (b) CMZF 4, and (b) CMZF 5.

sample increases with the increase of applied magnetic field. In most of these samples the magnetization curves observed at RT are nearly saturated in the region of the applied field of 1−2 T. But in the samples with lower particle/crystallite sizes, the magnetization increases slowly with the increase of the applied field. For example, the values of magnetizations of CMZF 1 and 2 measured with the applied field of 1, 2, 3, 4, and 5 T are respectively 45.9, 48.7, 50.2, 50.3, and 51.2 (with errors = ±0.1) emu/g for CMZF 1 and 38.7, 41.5, 44.2, 45.9, and 46.8 emu/g for CMZF 2. The maximum magnetization of CMZF 2 (at 5 T) is quite high and in the case of CMZF 1, where the average crystallite/particle size is low, the corresponding value is also high. The saturation magnetization of the samples of CMZF 1, 2, and 5 and CMZF-ZnO are 47.6, 52.7, 57.5, and 32.9 (with H



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is also reflected in these loops. The coercivity of the loops increases with the increase of particle size and also, for a given particle size, the coerceive field increases with the increase of frequencies. Thus the fraction of ferrimagnetic nanoparticles increases compared to SPM particles with the increase of average particle size of the samples. The low coercive field observed in the samples with sizes up to 15.5 nm (obtained from DLS measurement) may be due to the presence of a small number of ferrimagnetic particles. Hysteresis loops of all the samples were also observed at different frequencies and some representative loops are displayed in Figure 12. For a given particle size, the hysteresis loss slowly increases with the increase of frequency. On the other hand, for a given frequency the magnetic hysteresis loss increases with the increase of particle size. Low loss of sample is due to the small value of coercive field of the samples. Also, the coercive field increases with the increase of frequency. This fact is due to the reduction of the measurement time window with the increase of frequency. For a nanoparticle system with uniaxial anisotropy, the energy barrier, which separates the two easy directions of magnetization, may be smaller than the thermal energy even at RT, which leads to spontaneous fluctuation of the magnetization direction having relaxation time given by

line anisotropy, and this increase was obtained due to the substitution of Co2+ ions in Mn−Zn-ferrite. In the case of inverse spinel ferrite, Fe3+ ions are equally distributed in tetrahedral (A) and octahedral (B) sites and all the divalent cations (M2+) are in the B-site. In case of normal spinel structure, like ZnFe2O4 in the bulk state, divalent Zn2+ cations are in the A-site and all Fe3+ ions are in the B-site. But in the nanocrystalline state of ZnFe 2 O 4 , this normal distributions of Fe3+ and Zn2+ cations are not yet maintained; rather a fraction of Zn2+ cations replaces the Fe3+ cations in the B-site, which eventually go to the A-site, and this is called the degree of inversion. Thus, the degree of inversion mainly arises due to the redistribution of Zn2+ and Fe3+ cations.37 The value of the degree of inversion or the occupancy of Zn2+ cations in the B-site estimated from the saturation magnetization are 0.196, 0.162, and 0.159 for CMZF 5, 2, and 1, respectively. Also, in such a system, the spin canting effect normally occurs due to redistribution of Zn2+ and Fe3+cations between A- and B-sites.38 When the concentration of Fe3+ ions in the A-site is diluted by diamagnetic cations of Zn2+, the exchange interactions are weakened. Due to this, the B spins are no longer held rigidly parallel to the remaining A spins. This decrease in B sub lattice moment, interpreted as spin departure from colinearity, is known as the spin canting effect. We have calculated the canting angle using the relation39 ηB = MB cos αYK − MA where αYK is called Yafet−Kittel angle, MA and MB are magnetic moments of A- and B-site, respectively, and ηB is the experimental magnetic moment in Bohr magnetons. The estimated angles are 11.5°, 12.35°, and 17.6° (with errors ±0.1°) for CMZF 5, 2, and 1, respectively. ac Magnetic Properties. The dynamic hysteresis loops of all the samples were recorded at RT and the maximum ac magnetic field applied for this measurement was ∼60 kA/m. To investigate the presence of SPM and/or ferrimagnetic particles, we have recorded the loops of the samples using different time windows with frequency range of 10−1000 Hz. The development of the ac hysteresis loops observed at 50 Hz as a function of particle sizes of all the samples is shown in Figure 11. The shape of the ac hysteresis loops recorded at 50 Hz is not square; rather, these loops are flat. This sort of deviation in the shape from those recorded in the static mode of SQUID measurements is attributed to the fact that the ac loops are not yet saturated due to the low value of the ac field. The SPM nature

⎛ KV ⎞ τ = τ0exp⎜ ⎟ ⎝ kBT ⎠

(9)

where τo (10−9−10−10 s) is the inverse of the natural frequency of the gyro-magnetic precession, K is the anisotropy energy constant, V is the volume of the particle, kB is the Boltzmann constant, and T is the temperature in K. As we increase the frequency, the value of τ, the measurement time window, decreases. For this the difference of relaxation time and measurement time window decreases. The particles which behave SPM at a given frequency, such as 10 Hz, may behave as ferrimagnetically ordered particles at higher frequency, such as 100 Hz. Thus once we increase the frequency, the number of SPM particles decreases and at the same time the number of ferrimagnetic particles increases. Due to this fact ac hysteresis loops become wider and wider with the increase of frequency. Mö ssbauer Analysis. Mössbauer spectroscopy is a powerful technique to investigate the SPM relaxation of magnetic nanoparticles. Figure 13 displays Mössbauer spectra of different samples, viz., CMZF 1, 2, 3, 4, and 5, CMZF-ZnO, and CMZFPVA. In the case of CMZF 1 (Figure 13a), a broad quadrupole doublet was observed. Thus all the particles of CMZF 1 are in the SPM state. For the sample annealed at 400 °C, CMZF 2 (Figure 13b), a very broad doublet sitting over a partially collapsed sextet pattern is seen. The doublet corresponds to very small particles in the sample undergoing SPM relaxation at RT, whereas the partially collapsed sextet corresponds to bigger-sized particles with relaxation time comparable to the characteristic Larmor precession frequency of the Mössbauer nucleus (∼10−8 s). For the samples of CMZF annealed at 600 and 800 °C, CMZF 3, 4 (Figure 13c,d), a mixture of doublet and sextet patterns were observed. For the sample annealed at 1000 °C, CMZF 5 (Figure 13e), a relaxed sextet with a very weak and broad doublet pattern is seen. The weak doublet pattern indicates the presence of a very small fraction of SPM particles and the relaxed sextet confirmed the presence of a major fraction of bigger-sized ferrimagnetic particles. This is also expected as the sample is annealed at a higher temperature of 1000 °C. This observation is in good agreement with the dc

Figure 11. Dynamic hysteresis loops observed at 50 Hz: (a) CMZF 1, (b) CMZF 2, (c) CMZF 3, (d) CMZF 4, and (e) CMZF 5. I



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Figure 12. Dynamic hysteresis loops observed at different frequencies of (a) CMZF 1 and (b) CMZF 5.

The particle may behave either as SPM or ferrimagnetically, depending on the critical size, nanocrystalline anisotropy, measuring temperature, and time window. Mössbauer spectra of the samples of CMZF coated with PVA and ZnO are shown in Figure 13f,g, respectively. In the PVA coated sample, a doublet and a partially collapsed sextet are observed corresponding to the mixture of SPM and ferrimagnetic particles. It may be noted that the Mössbauer pattern of CMZF-PVA is quite different from that of CMZF 1 though the sample CMZF-PVA was derived from the sample CMZF 1 after PVA coating. This is most likely due to the long time of heating of CMZF 1 in the aqueous solution of PVA at boiling condition. In contrast, the magnetic order is completely absent (no signature of a sextet) in the sample CMZF-ZnO and all the particles are SPM, due to which a clean quadrupole doublet pattern was found. This observation also confirmed that the dipolar interaction has been fully restricted in the sample of CMZF-ZnO, though it is annealed at 100 °C for 6 h before coating and 600 °C for 6 h after coating. However, the dipolar interaction among the CMZF particles is partially present in CMZF-PVA, though it is annealed at 100 °C before coating and the coating was done under the boiling condition of the aqueous solution of PVA. This observation also confirmed that, as far as the dipolar interaction is concerned, nanoparticles of CMZF are not well-protected/coated in CMZF-PVA compared to that of ZnO. The comparatively poor coating in the case of PVA is quite likely due to its different preparation condition (mentioned in the Experimental section) as well as the nature of the coating material. The blocking temperature corresponding to the Mössbauer time window is the temperature at which the fraction of SPM particles and those of ferrimagnetic particles are equal, i.e., the intensities corresponding to doublet and sextet patterns are equal. Thus Mössbauer spectra of the samples confirmed that

Figure 13. Mössbauer spectra of the different samples (a) CMZF 1, (b) CMZF 2, (c) CMZF 3, (d) CMZF 4, (e) CMZF 5, (f) CMZFPVA, and (g) CMZF-ZnO.

magnetization measurements (FC and ZFC curves of CMZF 5), where it was found that most of the particles are ferrimagnetic at RT. Normally for a pattern where a sextet and a doublet coexist, the intensity of the sextet increases at the cost of the intensity of the doublet with the increase of average particle size, i.e., with the increase of annealing temperature. This generally happens in the case of systems containing both types of SPM and ferrimagnetic particles. Once we increase the annealing temperature, the fraction of SPM particles responsible for the doublet pattern decreases and consequently the fraction of ferrimagnetic particles with sextet pattern increases. J



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Table 4. Isomer Shift, Quadrupole Splitting, and Hyperfine Field of All the Samples Extracted from the Analysis of Mössbauer Spectra samples

annealing temperature (in °C)

IS (mm s‑1) (±0.02)

spectrum fitted with

0.36 0.34 0.30 0.34 0.43 0.38 0.44 0.26

QS (mm s‑1) (±0.02)

CMZF 1 CMZF 2

100 400

distribution of quadruple splitting One doublet and distribution of hyperfine fields

CMZF 3

600

one doublet and one sextet

CMZF 4

800

one doublet and one sextet

CMZF 5

1000

CMZFZnO CMZFPVA

600

one doublet

0.34

0.50

600

one doublet and distribution of hyperfine field

0.25

-

one doublet and distribution of hyperfine field

■

(D) (S) (D) (S) (D) (S)

0.23 0.60 0.07 0.47 0.19 0.60 0.18 -

(D) (S) (D) (S) (D) (S)

Hint (T) (±0.2) 32.2 51.7 51.7 32.1 29.9

relative area of doublet (%) (±1) 100 19 70 66 14 100 19

CONCLUSION The sono-chemical technique was applied during the whole course of preparation of CMZF nanoparticles by the coprecipitation method, and interestingly, the distribution of particle sizes was drastically reduced by this sono-chemical technique. Besides this, the tendency of agglomeration of ferrite nanoparticles was also reduced due to the application of ultrasonic vibration in the present method. The growth of nanoparticles was restricted at higher annealing temperature by the coating matrix of ZnO in CMZF-ZnO. The formation of the impurity phase of α-Fe2O3 in the bare sample of CMZF (annealed at 600 °C) was completely arrested by the coating layer in the nanocomposite sample of CMZF-ZnO annealed at 600 °C. These findings further confirmed that the nanoparticles of CMZF are well encapsulated by the matrix of ZnO in the present method of preparation where the precursor salt of the coating material (here zinc acetate) was precipitated in the presence of ferrite nanoparticles. Thus, as far as the method of coating is concerned, the present technique is more effective compared to other techniques.18,21 Results of HRTEM observations also confirmed that the nanoparticles of CMZFZnO are well coated and also the extracted results from HRTEM observations are in good agreement with those extracted from the XRD patterns. The results of dynamic and static magnetic measurements indicate that some samples (CMZF 2,3,4, CMZF-PVA) are in the mixed state of SPM and ferrimagnetic particles and the fraction of ferrimagnetic particles decrease with the increase of particle size. However, the samples of CMZF 1 and CMZF-ZnO are in the pure state of SPM particles, where the sample of CMZF-5 is in a ferrimagnetic state. These findings are in agreement with those obtained from the Mössbauer effect measurements. Analysis of magnetic data revealed that the anisotropy of the samples of CMZF (2 and 4) is quite high. Due to this enhancement of anisotropy, the onset of SPM relaxation has been found in the bigger-sized particles at RT. This was achieved by the incorporation of Co ions in the Mn−Zn ferrite. Both the samples of CMZF 3 and CMZF-ZnO were annealed at 600 °C, but the Mössbauer spectrum of CMZF-ZnO sample gives a clear doublet pattern, whereas the Mössbauer spectrum of CMZF 3 gives a mixture of doublet and sextet patterns. This doublet pattern confirmed the restricted growth of particles which is achieved by the uniform coating of CMZF particles in CMZF-ZnO. The high value of saturation magnetization, low

only two samples (CMZF 1 and CMZF-ZnO) are in the pure SPM state and the rest of the samples are in the mixed state of SPM and ferrimagnetic particles. The results of Mössbauer spectra of all the samples are also in good agreement with the results of magnetic measurements. The increase of the area of the sextet pattern at the cost of the doublet area is due to the increase of particle size that is similar to the increase of the area of the ac hysteresis loops of the samples with increasing particle size. Mössbauer spectrum of the sample of CMZF 1 has been fitted using a distribution of quadrupole splitting using the NORMOS40 fitting program. The sample CMZF-ZnO is fitted with a discrete quadrupole doublet using a least-squares fitting program LGFIT241 and the samples CMZF 3 and CMZF 4 were fitted with a discrete doublet and a discrete sextet with broad lines using the same fitting program. The samples CMZF 2, CMZF-PVA, and CMZF 5 were fitted with a discrete doublet and a distribution of hyperfine fields using the NORMOS.40 The extracted hyperfine parameters including the values of isomer shift of all the samples (IS, with reference to α-Fe) and quadrupole splitting/shift (for doublets, QS represents quadrupole splitting, and for sextet, QS represents quadrupole shift) are shown in Table 4. The isomer shift, quadrupole splitting, and hyperfine field for the “distribution” fitting scheme are the average values. The values of isomer shift (IS) confirmed the presence of Fe3+ valence state of iron in the samples whereas the QS values are indicative of the presence of substantial electric field gradient (EFG) around the 57Fe probe nuclei. The hyperfine field obtained from fitting of the weak sextets of CMZF 3 and 4 are the same (∼51.7 T) and this value is quite close to that of α-Fe2O3 (51.5 T). Thus the weak sextets in CMZF 3 and 4 may be due to nanocrystallites of α-Fe2O3, which are not in the SPM state. The discrete sextets observed for these samples could not be fitted with typical two sextets of a crystalline ferrite which indicates that most of the CMZF particles in these samples are in the SPM state giving a doublet absorption pattern. The sextet of CMZF 5 is highly relaxed showing broad absorption lines that may have masked the sharp lines of α-Fe2O3 as detected by XRD analysis. The average value of hyperfine field extracted from the fitting of the relaxed sextet of CMZF 5 is ∼32.1 T, which suggests that the majority of the sextets in the distribution of hyperfine fields are due to CMZF particles. K



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value of coercive field, along with the good encapsulation of the nanoparticles (for coated samples) would be quite interesting for soft magnetic device applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Tel: 0091 342 2657800, FAX: 0091 342 2634200. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors wish to acknowledge the financial support obtained from the DST, Govt. of India (Project file No. SR/S2/CMP/ 0058-2010) and UGC, Govt. of India in the CAS Program. Authors are also thankful to Dr. A. Saha of UGC-DAE CSR, III/LB-8, Kolkata for providing the DLS measurement facility.

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