Journal of Magnetism and Magnetic Materials 392 (2015) 101–106

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Journal of Magnetism and Magnetic Materials

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Investigation of structural and magnetic properties of co-precipitatedMn–Ni ferrite nanoparticles in the presence of α-Fe2O3 phase

B.V. Tirupanyama, Ch. Srinivas b, S.S. Meena c, S.M. Yusuf c, A. Satish Kumar d, D.L. Sastry e,n,V. Seshubai f

a Department of Physics, Government Arts College (Autonomous), Rajahmundry 533401, Indiab Department of Physics, Sasi Institute of Technology and Engineering, Tadepalligudem 534101, Indiac Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, Indiad Department of Physics, Rajiv Gandhi University of Knowledge Technologies, Nuzvid 521201, Indiae Department of Physics, Andhra University, Visakhapatnam 530003, Indiaf Department of Physics, University of Hyderabad, Hyderabad 500046, India

a r t i c l e i n f o

Article history:Received 24 March 2015Received in revised form27 April 2015Accepted 8 May 2015Available online 9 May 2015

Keywords:XRDFE-SEMVSMMössbauer spectroscopyFerritesNanoparticles

x.doi.org/10.1016/j.jmmm.2015.05.01053/& 2015 Elsevier B.V. All rights reserved.

esponding author.ail addresses: srinivas.chintoju75@gmail.comy@rediffmail.com (D.L. Sastry).

a b s t r a c t

A systematic study on structural and magnetic properties of co-precipitated MnxNi1�xFe2O4 (x¼0.5, 0.6,0.7) ferrite nanoparticles annealed at 800 °C was carried out using XRD, FE-SEM, VSM and MÖSSBAUERtechniques. Anti-ferromagnetic α-Fe2O3 phase was observed along with the magnetic spinel phase in theXRD patterns. It is observed that both lattice parameter and crystallite size of spinel phase increase withincrease in concentration of Mn2þ along with the amount of α-Fe2O3 phase. The saturation magneti-zation (Ms) decreases while coercivity (Hc) increases with increase of Mn2þ ion concentration. Möss-bauer spectra indicate that iron ions present in A and B sites are in the Fe3þ state and Fe2þ is absent. Theresults are interpreted in terms of observed anti-ferromagnetic α-Fe2O3 phase, core–shell interactionsand cation redistribution.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

Spinel ferrites which are an important group of magnetic ma-terials have several applications ranging from millimeter waveintegrated circuitry to transformer cores and magnetic recordingheads [1–3]. These materials are revisited at nanoscale to under-stand the involvement of various interactions which give rise todifferent properties at nanoscale. In spinel ferrites the structuraland magnetic properties are strongly dependent on cation dis-tribution and method of preparation. Spinel ferrite nanoparticlesare generally prepared using various methods such as sol–gel [4],reverse micelle method [5], ultrasound irradiation [6], hydro-thermal method, etc. [7]. Among all these methods a co-pre-cipitation method is widely used for preparation of ferrites due toits overriding advantages such as ease of preparation, compositionflexibility, homogeneity, etc. [8].

As a promising material for various applications many re-searchers focused on the structural and magnetic properties of

(Ch. Srinivas),

Mn–Ni ferrite which is regarded as a mixed ferrite of MnFe2O4 andNiFe2O4. Mn-ferrite has partially inverse spinel structure in which80% of Mn2þ has strong tendency to occupy tetrahedral (A) siteswhile the remaining 20% occupies octahedral (B) sites in the formof Mn3þ [9] and Fe3þ ions which are distributed between A and Bsites. NiFe2O4 ferrite has inverse spinel structure in which Ni2þ

ions occupy octahedral (B) sites and Fe3þ ions occupy both A and Bsites equally [10]. In nanoform however the cation distribution isgenerally different from that observed in the bulk form [11]. In themixed ferrite when Mn2þ ions are substituted for Ni2þ ions, Ni2þ

ions are expected to occupy octahedral (B) sites while Mn2þ israndomly distributed between tetrahedral (A) and octahedral(B) sites. Ferrites containing manganese have a tendency to formanti-ferromagnetic α-Fe2O3 phase when annealed at temperatureshigher than 200 °C in air atmosphere [12]. This anti-ferromagneticα-Fe2O3 phase is not observed in as prepared samples [13] orsamples annealed at much higher temperatures like 1200 °C [14].To explain this interesting behavior it has been proposed thatsome Mn2þ in B- sites is reduced to Mn3þ along with some phaseseparation of α-Fe2O3 [15]. However it is not clear why this anti-ferromagnetic α-Fe2O3 phase is forming at low sintering tem-peratures and disappearing at higher sintering temperatures [14].

B.V. Tirupanyam et al. / Journal of Magnetism and Magnetic Materials 392 (2015) 101–106102

Besides the location of α-Fe2O3 phase in the compound containingboth ferrite and anti-ferromagnetic phases is not clear. In literaturethe formation of Fe2þ is proposed in the Mn–Ni mixed ferritesamples to account for the existence of Mn3þ in B sites [15,16] butMössbauer evidence for Fe2þ seems to be lacking. The purpose ofthe present studies is not only to study the structural and mag-netic properties of Mn–Ni ferrite nanoparticles in the presence ofα-Fe2O3 phase but also to investigate for the presence of Fe2þ ionsand also to understand the location of α-Fe2O3 phase.

In the present paper the results obtained in the studies on co-precipitated MnxNi1�xFe2O4 (x¼0.5, 0.6, 0.7) ferrite nanoparticlesannealed at 800 °C are reported. X-ray diffraction (XRD), fieldemission scanning electron microscopy (FE-SEM), vibrating scan-ning magnetometer (VSM) and Mössbauer spectroscopy techni-ques were employed for structural and magnetic characterizations.

The structural and magnetic properties are discussed andcompared with earlier reports. Our Mӧssbauer studies do not giveany evidence for the formation of Fe2þ in the present Mn–Niferrite samples. These results are interpreted in terms of the ob-served anti-ferromagnetic α-Fe2O3 phase, cation redistributionand possible core–shell interactions in these systems. Our studiesindicate that the anti-ferromagnetic α-Fe2O3 phase is more likelyto exist in the shell of the ferrite nanoparticles.

Fig. 1. XRD patterns of MnxNi1�xFe2O4 (x¼0.5, 0.6, 0.7) annealed at 800 °C. *Peaksare due to α-Fe2O3.

2. Experimental methods

2.1. Synthesis

High purity manganese chloride (MnCl2), nickel chloride(NiCl2 �6H2O), ferric chloride (FeCl3 �6H2O) were taken as startingmaterials for the preparation of MnxNi1�xFe2O4 (x¼0.5, 0.6, 0.7)spinel ferrites by the co-precipitation method [17]. An appropriateamount of each material in stoichiometric ratio is dissolved se-parately in a suitable quantity of deionized water to make 0.5 Msolutions. The cationic solutions were mixed thoroughly using amagnetic stirrer for complete dissolution and heated to 60 °C. ANaOH solution of 0.4 M concentration was prepared and heated to60 °C and quickly transferred into the hot cationic solution whilemaintaining the stirring and heating till complete precipitationoccurred. Heating of the precipitate in its alkaline condition iscontinued at a soaking temperature of 100 °C for 1 h. Stirring isfurther continued for 12 h for complete aging. The precipitatedparticles were washed several times and were dried at 80 °C for48 h. The ferrite powders have been gently pressed into pellets ofuniform diameter of 1.5 cm and varying thickness from 2 mm to3 mm under a pressure of 5 MPa. The pellets were heat treated at800 °C in air for 2 h and were ground into fine powder in an agatemortar. The powder was used for XRD, FE-SEM, VSM, and Möss-bauer spectroscopic studies.

2.2. Measurements and characterization

A Philiip's X'pert–PRO X-ray powder diffractometer was em-ployed to obtain the X-ray diffraction patterns of the samplesusing CuKα (1.5406 Å) radiation.

A Carl Zeiss Ultra 55 model field emission scanning electronmicroscope (FE-SEM) was employed for obtaining micrographs.The working distance for most of the samples was in the range 4–10 mm.

In the present studies the room temperature magnetizationmeasurements (hysteresis loop) were carried out employing aLakeshore vibrating sample magnetometer (VSM) 718 model.

Mössbauer spectra were recorded at room temperature (300 K)using a Nucleonix Mössbauer spectrometer which operated inconstant acceleration mode (triangular wave) in transmission

geometry employing a Co-57 in Rh matrix of strength 50 mCi. Thecalibration of the velocity scale is done by using α-Fe metal foil.The outer line width of calibration spectra is 0.30 mm/s. Möss-bauer spectra were fitted by a least square fit (MOSFIT) programassuming Lorentzian line shapes. The isomer shifts were measuredrelative to α-Fe metal foil.

3. Results and discussion

3.1. XRD studies

The X-ray diffractograms of MnxNi1�xFe2O4 (x¼0.5, 0.6, 0.7)annealed at 800 °C in air are given in Fig. 1 and the calculatedvalues of crystallite sizes and lattice parameters are given in Ta-ble 1. The XRD patterns show all the characteristic lines of thespinel structure. The prominent (hkl) planes ((111), (220), (311),(222), (400), (422), (511), (440), etc.) are indexed and latticeparameter ‘a’was obtained using Bragg's diffraction condition [18].

n d2 .sin 1λ θ= ( )

Where n¼1 and d a

h k l2 2 2=

+ +The average crystallite sizes are calculated from most intense

(311) peak using Debye–Scherrer's equation [19].

D0.89

cos 2λ

β θ=

( )

Here λ is the X-ray wavelength used and β is full width at halfmaximum (FWHM) intensity taking into account of instrumentalbroadening.

From XRD patterns it is clear that all experimental ferrite peaksmatch with those reported in earlier studies [20] and also withthose given in JCPDS (75-0894) data. This confirms the formationof cubic spinel phase. Besides the presence of some additionalpeaks can also be detected. The intensity of these peaks increasesas concentration of Mn2þ ions increases. These peaks were iden-tified due to anti-ferromagnetic α-Fe2O3 phase from a comparisonwith JCPDS (87-1166) data. The present results indicate that for agiven sintering temperature the intensity of anti-ferromagneticα-Fe2O3 phase goes on increasing as the concentration of Mn2þ isincreasing, which is in conformity with the studies reported onferrites like Mn–Zn [15] or Mn–Ni [21] which contain Mn2þ ions.In the present studies we have not found any detectable amounts

Table 1Lattice parameter (a), crystallite sizes (D) from XRD and FE-SEM for Mn–Ni ferriteannealed at 800 °C.

Sample a (Ǻ) (70.002) Crystallite size D (nm) (70.1)

XRD FE-SEM

Mn0.5Ni0.5Fe2O4 8.321 12.8 14.6Mn0.6Ni0.4Fe2O4 8.325 15.6 18.5Mn0.7Ni0.3Fe2O4 8.349 18.2 20.9

B.V. Tirupanyam et al. / Journal of Magnetism and Magnetic Materials 392 (2015) 101–106 103

of orthoferrite MnFeO3 which was reported to be present (0.3%) insol–gel prepared Mn–Ni ferrites annealed at 900 °C [22]. Thepresent results indicate that as Mn2þ ion concentration increaseslattice parameter ‘a’ and crystallite size ‘D’ increase which is inconformity with the earlier reports on Mn-containing ferrites [23,24]. The values of lattice parameter are close to the earlier re-ported values for Mn–Ni ferrites and vary according to Vegard'slaw [25] as shown in Fig. 2. Akther et al. [26] also observed a si-milar variation in Mn-substitued Ni–Zn nanoferrites. The increasein lattice parameter may be due to the replacement of smallerNi2þ (0.69 Å) ions by the larger Mn2þ (0.82 Å) ions which causesthe lattice expansion. Similar observations were reported in earlierliterature when bigger ions are substituted in place of smaller ionsin different ferrite systems [27, 28].

3.2. FE-SEM studies

The FE-SEM micrographs of MnxNi1�xFe2O4 (x¼0.5, 0.6, 0.7)annealed at 800 °C in air are given in Fig. 3. The micrographs re-veal that particles of slightly different sizes are distributed in thesamples and average particle size increases with increase in con-centration of Mn2þ . The average particle sizes are found to be14.6 nm, 18.5 nm, 20.9 nm and are in good agreement with thevalues of crystallite sizes obtained from XRD. Shobana et al. [29],Airimioaei et al. [22] report a decrease in particle size as Mn2þ ionconcentration is increasing in Mn–Ni ferrites. However there arealso reports which indicate that the particle size increases asMn2þ ion concentration increases in Mn–Ni ferrites [25] and Mn–Zn ferrites [30]. In the present studies it is observed that theparticle size increases as Mn2þ ion concentration increases. Theaverage particle size observed in the present studies is smallerthan the particle size reported in the literature for Mn–Ni ferritesof similar composition. For a comparison, for MnxNi1�xFe2O4

(x¼0.5) composition the reported particle sizes are 40 nm (at900 °C anealing temperature)[22], 19 nm (as prepared) [29], 30 nm

Fig. 2. Variation of lattice parameter (a) and crystallite size (D) with concentrationof Mn2þfor Mn–Ni ferrite annealed at 800 °C.

Fig. 3. FE-SEM micrographs of (a) Mn0.5Ni0.5Fe2O4, (b) Mn0.6Ni0.4Fe2O4 and (c)Mn0.7Ni0.3Fe2O4 annealed at 800 °C in air.

(at 750 °C annealing temperature) [31], 22 nm (as prepared) [32]when compared to 14.6 nm (at 800 °C annealing temperature)obtained from FE-SEM studies in the present case.

3.3. VSM studies

The hysteresis (M–H) loops recorded at 300 K using VSM areshown in Fig. 4. Saturation magnetization (Ms), remnant magne-tization (Mr) and coercive field (Hc) calculated from the M–H loops

Fig. 4. Room temperature M–H loops of MnxNi1�xFe2O4 (x¼0.5, 0.6, 0.7) annealedat 800 °C in air.

Table 2Saturation magnetization (Ms), remnant magnetization (Mr) and coercivity (Hc)values obtained from VSM studies for Mn–Ni ferrite annealed at 800 °C.

Sample Ms (emu/g) Mr (emu/g) Hc (Oe)

Mn0.5Ni0.5Fe2O4 41.39 1.52 36.27Mn0.6Ni0.4Fe2O4 34.64 1.74 43.60Mn0.7Ni0.3Fe2O4 27.38 3.39 82.83

B.V. Tirupanyam et al. / Journal of Magnetism and Magnetic Materials 392 (2015) 101–106104

are given in Table 2. The obtained magnetization values are greaterthan the magnetization of Mn-ferrite (Ms¼7.41 emu/g) and Ni-ferrite (Ms¼21.59 emu/g) prepared in nanoform by the co-pre-cipitation method [32] and less than the values obtained for bulksamples prepared by a ceramic method [33]. It is observed that thesaturation magnetization decreases with an increase of Mn2þ ionconcentration. This observation is in accordance with the earlierreports of studies on Mn–Ni ferrites. Shobana et al. [29] who ob-served a decrease in particle size with an increase in Mn2þ ionconcentration attributed the decrease of magnetization to thedecrease in particle size. Iyer et al. [34] made similar observationin the studies on Cd2þ substituted Mn–Ni ferrite. Arimioaei et al.[22] attributed the decrease of Ms with increase of Mn2þ ionconcentration to the formation of anti-ferromagnetic secondaryphases like α-Fe2O3 besides a decrease in particle size. In thepresent studies since an increase in particle size was observedwith the increase of Mn2þ ion concentration, the decrease in Ms

can be attributed to either an increase in anti-ferromagneticα-Fe2O3 phase or cation redistribution in tetrahedral (A) and oc-tahedral (B) sites. In the present studies the observation of

Table 3Mössbauer parameters of MnxNi1�xFe2O4 annealed at 800 °C in air. Hyperfine field (Hhf

Concentration (x) Iron site Hhf (T) Δ (mm/s) (70.001)

0.5 Sextet1 51.2 �0.19Set2(A) 47.2 0.01Set3(B) 42.0 �0.02

0.6 Sextet1 51.1 �0.23Set2(A) 46.9 0.00Set3(B 43.6 0.05

0.7 Sextet1 51.0 �0.22Set2(A) 46.7 0.02Set3(B 43.3 �0.04

increase in Hc with increase of Mn2þ ion concentration is in ac-cordance with the observation of Arimioaei et al. [22] but incontrast with the observation of Shobana et al. [29] who reporteda decrease in Hc with the increase of Mn2þ ion concentration.

As particle size reduces to nano-level the variation of coercivityis governed by [35]

HK DM A 3

cs

4 6=

( )

Where K is magnetocrystalline anisotropy, D is particle size, A isexchange energy constant and Ms is saturated magnetization.Therefore the increase in coercivity is expected due to decrease inMs or increase in K or increase in D. The present particle sizes canbe supposed to be greater than Dp (below which the particlesexhibit superparamagnetism) and smaller than Ds (below whichthe particles exhibit single domain nature). The particles can beconsidered to be single domain as coercivity increases as D in-creases as can be seen from Tables 1 and 2. From Fig. 4 the M–Hhysteresis loops can be considered to reach saturation whenmagnetic field reaches 2 kOe. The single domain nanoparticlesare generally supposed to contain an ordered (crystalline) ferritecore and a shell which contains disordered spin glass like structurewhich does not show saturation behavior [36]. The observation ofsaturation behavior in the present single domain particles can takeplace if the ferrite core is surrounded by anti-ferromagnetic shell.Since the anti-ferromagnetic α-Fe2O3 phase is known to disappearat higher annealing temperatures like 1200 °C [14], we proposethat the anti-ferromagnetic α-Fe2O3 phase exists in the shell of thenanoparticles. An increase in this phase can be considered to causea decrease in the amount of ferrite core giving rise to a decreasedin the value of Ms which is apparently reached at an appliedmagnetic field of 2 kOe.

3.4. Mössbauer studies

Mӧssbauer spectra of MnxNi1�xFe2O4 (x¼0.5, 0.6, 0.7) an-nealed in air at 800 °C are given in Fig. 5. All results are relative toα-Fe metal foil and are given in Table 3. The spectra exhibit threesets of fully resolved sextets. The sextet having hyperfine field (Hhf)around 51 T is assigned to anti-ferromagnetic α-Fe2O3 phase (he-matite) basing on earlier report for this compound [37]. The othertwo sextets are assigned to Fe3þ at tetrahedral (A) and octahedral(B) sites from the nature of their hyperfine field values (hyperfinefield at tetrahedral sites is greater than the hyperfine field at oc-tahedral sites) [38, 39]. The hyperfine fields (Hhf) at A and B sitesdo not seem to vary much with increase in concentration of Mn2þ .This may be regarded as an evidence that the surface effects/orcore–shell interactions are negligible in Mn–Ni ferrite nano-particles in the present case [40]. It can also be seen from Table 3that as Mn2þ ion concentration increases the area under Möss-bauer spectral lines due to anti-ferromagnetic α-Fe2O3 phasealso increases which indicates increase in the concentration of

), quodupole shift (Δ), isomer shift (δ), line width (Γ) and area under the peak (%).

δ (mm/s) (70.002) Γ (mm/s) (70.002) Area (%)

0.36 0.22 10.30.28 0.33 38.20.33 1.04 51.50.38 0.22 25.40.28 0.43 29.90.32 1.04 44.70.38 0.23 38.00.29 0.33 31.90.36 1.04 30.1

Table 4Fe3þ cation distribution from Mӧssbauer studies of MnxNi1�xFe2O4 annealed at800 °C in air as represented in Eq. (4).

Concentration (x) A-site (p) B-site (q) α-Fe2O3 (2m)

0.5 0.76 1.03 0.210.6 0.6 0.9 0.50.7 0.64 0.6 0.76

Fig. 5. Room temperature Mӧssbauer spectra of MnxNi1�xFe2O4 (x¼0.5, 0.6, 0.7)annealed at 800 °C in air.

B.V. Tirupanyam et al. / Journal of Magnetism and Magnetic Materials 392 (2015) 101–106 105

anti-ferromagnetic α-Fe2O3 phase which is in accordance with theXRD data. The decrease in saturation magnetization as Mn2þ ionconcentration increases can be attributed to increased formationof this antiferromagnetic phase. The quadrupole splitting averagesto zero (Δ�0) representing cubic symmetry around Mössbauernucleus. The isomer shift (δ) for tetrahedral (A) sites is less thanthat of octahedral (B) sites, because of higher covalence i.e. largeroverlapping of Fe3þ–O2� ions at A-sites when compared to B-sites[41, 42]. The isomer shift values does not vary with concentrationof Mn2þ , which indicates that there is no change in s-electroncharge distribution around the iron nucleus at A or B sites withincrease in concentration of Mn2þ [43]. The observed values ofisomer shift are less than 0.5 mm/s, indicating the presence of onlyFe3þ and ruling out the detectable presence of Fe2þ ions [44] inthese systems. From the ratio of Mössbauer spectral line in-tensities (i.e. ratio of the areas) of lines arising due to Fe3þ ions atA or B-sites and in α-Fe2O3, the relative amounts of Fe3þ at all thethree sites can be calculated and are given in Table 4. From thenature of their ionic radii Ni2þ (0.69 Å) is expected to be in octa-hedral sites, whereas Mn2þ (0.82 Å) is expected to be pre-dominantly in tetrahedral sites. There is an evidence that someamount of Mn2þ can be in octahedral sites in the form of Mn3þ

(0.66 Å) [45] while Fe3þ (0.64 Å) is distributed between A and Bsites. It is well known that the cation distribution in nanoform notonly depends on particle size [46] but also on the method ofpreparation [47].

Ghazanfar et al. [48] showed by neutron diffraction that inMn1�xFe2�xO4 30% of octahedral sites are occupied by Mn3þ withthe formation of some Fe2þ also in B-sites for charge compensa-tion. Gimines et al. [15] in their studies on MnxZn1�xFe2O4 pro-posed that manganese ion oxidation state in B-sites will be Mn2þ

when the samples are annealed in air resulting in the formation ofα-Fe2O3 phase and Mn3þ when the samples are annealed in N2

atmosphere resulting in the absence of α-Fe2O3 phase. In the latercase they also proposed that some Fe3þ in B-sites will be reducedto Fe2þ for charge compensation. In the present studies sinceMössbauer spectra do not give any evidence of Fe2þ we proposethat manganese in A-sites and B-sites will be in Mn2þ state withthe following molecular formula for the ferrite

⎡⎣ ⎤⎦Mn Fe Ni Mn Fe O m Fe O 4x z x z2 3

p2

12 3

q 4 3m 2 3)( + α − ( )+

−+ +

−+ +

−

with the condition pþqþ2m¼2 in order to maintain the stoi-chiometry of the starting materials. The estimated Fe3þ cationdistribution is given in Table 4.

Even though the particle size is small (�14–21 nm) super-paramagnetism is absent in the present ferrite systems as can beseen from well resolved sextet patterns in the Mössbauer spectra.This indicates that the blocking temperatures (TB) are higher thanroom temperature (300 K) for these materials. The value of TBbelow which thermal activation cannot overcome magnetic ani-sotropy energy is given by

TKV

k25 5B

B=

( )Where V is volume of the particles and kB is Boltzmann's constant.

We can expect large values for magnetocrystalline anisotropy

constant K since V volume of the nanoparticles is quite small. TheK values calculated following the Brown's equation given by

HK

M2

6c

S=

( )

are quite small and give TB values much lower than room tem-perature and hence Brown's equation is not applicable in thepresent case.

In ferrites the resultant magnetization is given as |MB�MA|. Itcan be seen from Table 4 that as Mn2þ ion concentration increasesthe difference between Fe3þ ion concentrations at A and B sitesdecreases indicating that magnetization contribution pre-dominantly comes from Mn2þ and Ni2þ magnetic ions occupyingA and B sites.

4. Conclusions

From the present studies on co-precipitated MnxNi1�xFe2O4

(x¼0.5, 0.6, 0.7) ferrite nanoparticles annealed at 800 °C the fol-lowing conclusions can be drawn. Since the samples are annealedin air, influence of manganese causes formation of anti-ferro-magnetic α-Fe2O3 which increases with the increase in con-centration of Mn2þ . The lattice parameter as well as particle sizeincreases as concentration of Mn2þ increases. The decrease inmagnetization is attributed to the formation of anti-ferromagneticα-Fe2O3 phase or cation redistribution in A and B sites. TheMössbauer studies indicate that Fe2þ ion formation is absent andit is proposed that manganese in B-sites is also in Mn2þstate. Thecore–shell interactions are supposed to be negligible with α-Fe2O3

B.V. Tirupanyam et al. / Journal of Magnetism and Magnetic Materials 392 (2015) 101–106106

anti-ferromagnetic phase occurring in the shell region in thepresent ferrite nanoparticles.

Acknowledgment

We are very thankful to BARC, Mumbai for providing Mӧss-bauer measurements and thankful to SAIF, IIT Madras, for pro-viding VSM measurements.

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