XRD, magnetic and Mössbauer spectral studies of nano size aluminum substituted cobalt ferrites...

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Journal of Magnetism and Magnetic Materials 306 (2006) 233–240

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XRD, magnetic and Mossbauer spectral studies of nano size aluminumsubstituted cobalt ferrites (CoAlxFe2�xO4)

Sonal Singhala, S.K. Barthwalb, Kailash Chandrac,�

aDepartment of Chemistry, Punjab University, Chandigarh, IndiabIndian Institute of Technology-Roorkee, Roorkee 247 667, India

cInstitute Instrumentation Centre, Indian Institute of Technology-Roorkee, Roorkee 247 667, India

Received 5 December 2005; received in revised form 4 March 2006

Available online 30 March 2006

Abstract

Aluminum substituted cobalt ferrites having particle size �10 nm were prepared using aerosol route, which increases with annealing

temperature. The unit cell parameter ‘a’ and saturation magnetization decreases linearly with the increase of aluminum concentration.

This is due to the smaller ionic radius and the diamagnetic nature of the Al3+, respectively. Room temperature Mossbauer spectra of the

samples annealed at 1200 1C show broad sextets, which were fitted with different sextets, indicating different local environment of both

tetrahedrally and octahedrally coordinated iron cation. Further, hyperfine field of the samples decreases with the increase of Al3+

concentration due to the fact that the Al3+ diamagnetic species reduce the magnetic interactions. Cations distribution indicated a

increase in Fe3+(oct.)/Fe3+(tet.) ratio on increasing the Al3+ concentration.

r 2006 Elsevier B.V. All rights reserved.

Keywords: Nano particles; Saturation magnetization; Coercivity; X-ray diffraction; Mossbauer spectra; Cation distribution

1. Introduction

Cobalt ferrites have been regarded as one of thecompetitive candidates for high density magnetic recordingmedia because of its moderate saturation magnetization,high coercivity, mechanical hardness and chemical stability[1,2]. Aluminum substituted cobalt ferrite is a soft ferritehaving low magnetic coercivity and high resistivity. Thehigh electrical resistivity and good magnetic propertiesmakes this ferrite an excellent core material for powertransformer in electronic and telecommunication applica-tion [3,4]. The addition of foreign cations of differentvalence states can lead to a variety of tetrahedral andoctahedral sites. These changes have been of specialinterest in the control of electrical properties of ferrites[5,6].

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/j.jmmm.2006.03.023

onding author. Tel.: +911332 285250, +91 1332 276119;

32 273560.

ddresses: [email protected] (S. Singhal),

iitr.ernet.in (K. Chandra).

Mane et al. [7] prepared cobalt aluminum chromiumferrites (CoAlxCrxFe2�2xO4) with varying ‘x’ by doublesintering technique. It was concluded that, magnetizationmeasurements exhibit non-colinear ferrimagnetic structurefor x40.3. Hanh et al. [8] synthesized 4 nm particles ofcobalt ferrite by the forced hydrolysis method, andreported the saturation magnetization and coercivityvalues at 5K as 75 emu/g and 10.3 kOe, respectively.Nakatsuka et al. [9] calculated cation distribution andbond lengths in CoAl2O4 using Rietvald analysis andfound that CoAl2O4 is largely inverse spinel, local bondlength on A-site abnormally long and repulsion between B-site cations considerably large. Chae et al. [10] prepared Alas well as Ti doped CoFe2O4 by sol–gel method andobserved that the coercivity is less while saturationmagnetization is more in Al0.2CoFe1.8O4 (870Oe and72.1 emu/g), in respect of Ti0.2Co1.2Fe1.6O4 (1.56 kOe and62.6 emu/g).The present work deals with the synthesis of nano

particles of aluminum substituted cobalt ferrite (CoAlxFe2�xO4 with x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2 and 1.4) viaaerosol route. Aerosol technology is the key process for

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ARTICLE IN PRESSS. Singhal et al. / Journal of Magnetism and Magnetic Materials 306 (2006) 233–240234

large scale production of nano structured materials. Usingthis method particle size, degree of agglomeration,chemical homogeneity can be controlled easily [11,12].This work is an attempt to investigate the cationdistribution and magnetic properties of CoAlxFe2�xO4

(x ¼ 021:4) through XRD, magnetic measurements andMossbauer spectroscopy.

2. Experimental

Nano particles of aluminum substituted cobalt ferriteswere prepared via aerosol route using a setup described inour earlier paper [13]. The desired proportions of cobalt,aluminum and iron nitrates were weighed and dissolved inwater to prepare 5� 10�2M solutions. Air pressure,sample uptake and furnace temperature were maintainedat 40 psi, 3–4ml/min and �650 1C, respectively, duringpreparation. The ferrite powder was deposited on thePTFE coated pan.

The elemental analysis was carried out on an electronprobe micro analyzer (EPMA) (JEOL, 8600M) and atomicabsorption spectrophotometer (AAS) (GBC, Avanta),while the particle morphology was examined undertransmission electron microscope (TEM) (Philips,EM400). The X-ray diffraction (XRD) studies were carriedon X-ray spectrometer (Bruker AXS, D8 Advance) withFeKa radiation and magnetic measurements were made ona vibrating sample magnetometer (VSM) (155, PAR).Mossbauer spectra were recorded on a constant accelera-tion transducer driven Mossbauer spectrometer using57Co(Rh) source of 25mCi initial activity. The spectro-meter was calibrated using a natural iron foil as well asrecrystallized sodium nitroprusside dihydrate (SNP) asstandards.

3. Results and discussion

Elemental analytical data for Co, Al and Fe in all the as-obtained samples by EPMA and AAS were consistentwithin 2%. Column one of Table 1 shows the formulaobtained through this data.

The TEM micrographs were recorded for all the samplesas described in our earlier papers [13,14]. A typicalmicrograph of CoAl0.58Fe1.42O4 is shown in Fig. 1(a),

Table 1

Lattice parameters derived from X-ray diffraction pattern and saturation mag

Ferrites composition Lattice parameter

a (A)

Volume

CoFe2O4 8.3834 589.19

CoAl0.18Fe1.82O4 8.3659 585.52

CoAl0.37Fe1.63O4 8.3451 581.16

CoAl0.58Fe1.42O4 8.3231 576.59

CoAl0.74Fe1.26O4 8.2969 571.15

CoAl0.95Fe1.05O4 8.2675 565.09

CoAl1.18Fe0.82O4 8.2379 559.05

CoAl1.36Fe0.64O4 8.2194 555.29

show particles of size �10 nm. Selected area electrondiffraction (SAED) pattern of the particle shown as insetof Fig. 1(a) suggests the amorphous nature of the sample.The micrograph for sample annealed at �1200 1C is shownin Fig. 1(b) indicating the increase of particle size withannealing temperature. Such an increase has also beenobserved by various workers [15,16]. It is widely believedthat the net decrease in free energy of solid–solid andsolid–vapor interface provides the necessary driving forcefor particle growth during annealing process [17]. SAED ofthe annealed particle shown in the inset of Fig. 1(b) showsthe crystallinity in the larger size particles, which was later,confirmed by XRD.The powder X-ray diffractographs were recorded for all

the as-obtained samples and those annealed at varioustemperatures. Typical X-ray diffractographs of CoAl0.95Fe1.05O4, as-obtained and after annealing at differenttemperatures are given in Fig. 2. The diffraction patternof the as-obtained samples reaffirms the amorphous natureof the samples. Peaks start appearing and the lines becomesharp as the annealing temperature increases, which can beattributed to the grain growth at higher temperatures asseen in TEM. The crystallite size was calculated from themost intense peak (3 1 1) using Sherrer equation [18]. It isseen that the particle size increases linearly from �20 nm to�92 nm as the annealing temperatures are raised from 400to 1200 1C.Fig. 3 represents X-ray powder diffraction patterns of all

the ferrite compositions annealed at 1200 1C. The latticeparameters were calculated using Powley as well as Le Bailrefinement methods (built in TOPAS V2.1 of BRUKERAXS) and are listed in Table 1. All the samples are foundto be face centered cubic with Fd-3m space group. It isobserved that lattice parameter ‘a’ decreases linearly withincrease in aluminum concentration (Fig. 4). This decreaseis expected in view of the fact that ionic radius of Al3+

(0.50 A) is lower than that of Fe3+ (0.64 A). The decreasein ‘a’ and shift of peak towards higher angle with theincreasing aluminum concentration (Figs. 3 and 4) showsthat Al atoms have entered substitutionally incorporatedinto the spinel structure [19].X-ray density dx was calculated using the formula [20]

dx ¼ 8M=Na3 where M is the molecular weight of thesample, N is Avogadro’s number and a the lattice

netization of the ferrites after annealing at 1200 1C

(A3) X-ray density

(g/cm3)

Saturation magnetization

(emu/g)

5.29 84.5

5.19 70.5

5.10 62.8

5.01 48.9

4.93 40.6

4.84 26.0

4.75 17.0

4.65 8.5

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Fig. 1. TEM micrograph of CoAl0.37Fe1.63O4 (a) as-obtained, and (b) after annealing at 1200 1C. Insets in (a) and (b) are the diffraction patterns.

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

2600

2800

27 30 40 50 60 70 80 90 100 110 120

Rel

ativ

e In

tens

ity

(a)

(b)

(c)

(d)

(e)

(f)

Angle (2θ)

(311

)

(220

)

(400

) (440

)

(511

)

(422

)

Fig. 2. X-ray diffraction pattern of CoAl0.95Fe1.05O4 (a) as-obtained and after annealing at (b) 400, (c) 600, (d) 800, (e) 1000 and (f) 1200 1C.

S. Singhal et al. / Journal of Magnetism and Magnetic Materials 306 (2006) 233–240 235

parameter and tabulated in Table 1. From Fig. 4, it can beseen that X-ray density decreases linearly with the increaseof aluminum concentration. The variation of latticeparameter and X-ray density are in conformity with thatthe aluminum atom is lighter than the cobalt atom, whilethe size of Al3+ ion is smaller than that of the Fe3+ ions.

The intensities of (2 2 0), (4 2 2) and (4 0 0) planes aremostly sensitive to cations on tetrahedral and octahedralsites [21,22]. Therefore, the intensities of these reflectionswere used to estimate the cation distribution using the

formula suggested by Buerger [23]

Ihkl ¼ F hklj j2PLp,

where notations have their usual meanings. It is wellknown [24] that the intensity ratio of planes I(220)/I(400),I(220)/I(440) and I(400)/I(440) are considered to be sensitive tothe cation distribution, therefore, these ratios are given inTable 2 and used in estimating cation distribution. Theionic configuration based on the site preference energyvalue for individual cation suggested that Co2+, Al3+ and

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Rel

ativ

e in

tens

ity

0

2000

4000

26 40 60 80 100 120

x=0x=0.18

x=0.37

x=0.58

x=0.74x=0.95

x=1.18x=1.36

(440

)

(511

)

(422

)(400

)

(311

)

(220

)

Angle (2θ)

Fig. 3. X-ray diffraction pattern of CoAlxFe2�xO4 after annealing at 1200 1C.

8.20

8.22

8.24

8.26

8.28

8.30

8.32

8.34

8.36

8.38

8.40

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6Aluminum concentration

Lat

tice

par

amet

er (

A)

4.6

4.7

4.8

4.9

5.0

5.1

5.2

5.3

5.4

X-r

ay d

ensi

ty (

g/c

m3 )

Lattice parameter

X-ray Density

Fig. 4. Variation of (a) lattice parameter and (b) density with the

aluminum concentration.

S. Singhal et al. / Journal of Magnetism and Magnetic Materials 306 (2006) 233–240236

Fe3+ ions can occupy both A- and B-sites [25]. Finally, thecation distribution is estimated for the best fit X-rayintensity ratio and listed in Table 2, which clearly showsthat the Al3+ ions enters into A- and B-sites in the �2:3ratio.

Magnetic hysteresis loops at room temperature wererecorded for all the as-obtained as well as annealedsamples. Typical loops for as-obtained CoAl0.58Fe1.42O4

and after annealing at 600, 800 and 1200 1C are shown inFig. 5. Fig. 6 indicates that Ms increases with annealingtemperature due to increase in particle size [26]. Thesaturation magnetization for all the ferrites after annealingat 1200 1C are listed in Table 1 indicates that Ms decreases

with the increase of aluminum concentration. This may beattributed to the weakening of exchange interactions due tonon-magnetic Al3+ ions. Fig. 7 shows the variation of thecoercivity with average grain size for some typical ferrites.It is evident from the graph that as the grain size increases,the value of coercivity (Hc) reaches a maximum value andthen decreases. This variation of Hc with grain size can beexplained on the basis of domain structure, criticaldiameter and the anisotropy of the crystal [27–29].In the single domain region as the grain size decreases

the coercivity decreases because of the thermal effects. Thecoercivity Hc in the single domain region is expresses asHc ¼ g� h=D2 where g and h are constants. In the multidomain region the variation of coercivity with grain sizecan be expressed as in [27] Hc ¼ aþ b=D where; ‘a’ and ‘b’are constants and ‘D’ is the diameter of the particle.Hence in the multidomain region the coercivitydecreases as the particle diameter increases. A decreasein coercivity (Fig. 7) with increase in aluminum concentra-tion may be attributed to the decrease in anisotropyfield, which in turn decreases the domain wall energy[30,31].The Mossbauer spectra recorded at 300K after anneal-

ing at 1200 1C for x ¼ 0 and 0.18 exhibit two normalzeeman split sextets due to the A-site Fe3+ ions and otherdue to B-site Fe3+, which indicates ferrimagnetic behaviorof the samples. A typical Mossbauer spectrum ofCoAl0.18Fe1.82O4 is given in Fig. 8. The two sextets aredue to the presence of different magnetic neighborsaffecting the Fe atoms distinctly at these sites. In theMossbauer spectra of the samples CoAl0.37Fe1.63O4 andCoAl0.58Fe1.42O4 best fit could be obtained by employingfour sextets in the Mossbauer spectra. This indicates the

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-50

-40

-30

-20

-10

0

10

20

30

40

50

-10000 -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 10000Magnetic Field (G)

Sat

ura

tio

n M

agn

etiz

atio

n (

emu

/g)

1200800600As Obtained

(a)

(b)

(c)

(d)

Fig. 5. Hysteresis loops for the CoAl0.58Fe1.42O4 (a) as-obtained and after annealing at (b) 600, (c) 800 and (d) 1200 1C.

Table 2

Cation distribution data calculated from X-ray diffraction pattern of the ferrites after annealing at 1200 1C

Ferrites composition I(220)/I(400) I(220)/I(440) I(400)/I(440) Cation distribution

Experimental Calculated Experimental Calculated Experimental Calculated

CoFe2O4 1.36 1.36 0.62 0.63 0.45 0.47 [Co0.40Fe0.60]A[Co0.60Fe1.40]

BO4

CoAl0.18Fe1.82O4 1.29 1.27 0.64 0.61 0.47 0.48 [Co0.40Al0.07Fe0.53]A[Co0.60Al0.11Fe1.29]

BO4


BO4


BO4


BO4


BO4


BO4


BO4


distributed local environment of Fe cation and broaderdistribution of interatomic spacing. One sextet wasattributed to tetrahedral site, while the others wereassigned to octahedral sites. A typical Mossbauer spectrumof CoAl0.58Fe1.42O4 is given in Fig. 8.

In the samples of CoAl0.74Fe1.26O4 and CoAl0.95Fe1.05O4

a doublet is also observed with the magnetic sextets may bedue to the presence of magnetically isolated Fe3+ ions,which are not participating in the long range magneticordering due to the presence of A- and B-sites non-magnetic nearest neighbor [32]. A typical Mossbauerspectrum of CoAl0.74Fe1.26O4 is given in Fig. 8. However,samples CoAl1.18Fe0.82O4 and CoAl1.36Fe0.64O4 showparamagnetic doublets as given in Fig. 8; which may beattributed to the weakening of magnetic coupling becauseof increase in non-magnetic ions. It is evident from the

fitted data in Table 3 for all the samples that isomer shiftsshow very little change with Al3+ substitution indicatingthat se� charge distribution of Fe3+ is not much influencedby aluminum substitution.The variation in the mean hyperfine field acting on A-

and B-sites 57Fe nuclei (HA and HB) with aluminumconcentration is shown in Fig. 9. It is evident that there is amonotonic decrease in the internal hyperfine field valueswith increasing aluminum substitution. This happens,because the replacement of Fe3+ by Al3+ influences theinternal hyperfine field of the nearest Fe3+ sites throughsuper transferred hyperfine fields. It was also observed thatthe HB decreases with larger rate than HA which suggeststhat larger fraction of Al3+ ion goes to B-site.The variation of HB and HA was attributed to spin

canting due to the presence of Jahn–Teller cation, Al3+

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0

100

200

300

400

500

600

700

800

900

1000

0 20 40 60 80 100Particle Size (nm)

Coe

rciv

ity (

G)

(a)

(b)

(c)

Fig. 7. Variation of coercivity with particle size for (a) CoFe2O4, (b)

CoAl0.37Fe1.63O4 and (c) CoAl0.58Fe1.42O4.

0

10

20

30

40

50

60

70

80

90

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6Aluminum concentration

Sat

ura

tio

n m

agn

etiz

atio

n (

emu

/g)

1200800600400

Fig. 6. Variation of saturation magnetization with the aluminum

concentration after annealing at different temperatures.

Fig. 8. Typical Mossbauer spectra of some ferrites after annealing at

1200 1C.

S. Singhal et al. / Journal of Magnetism and Magnetic Materials 306 (2006) 233–240238

which give rise to Yafet–Kittle angle and therefore weakensthe JAB super exchange interaction on account of strengththe JBB type of the extent that JBB compete JAB andbecome comparable [33].

The cation distribution was also calculated using theintensity ratio of the sextets corresponding to thetetrahedral and octahedral sites. From the data it is evidentthat the ratio of Fe3+(oct.)/Fe3+(tet.) changes as the Al3+

concentration increases. The change Fe3+(oct.)/Fe3+(tet.)increases from 2.33 to 4.00 on increasing the aluminum

concentration. Also the cation distribution was calculatedusing this data as shown in Table 3. The cation distributionobtained from the X-ray intensity data agrees fairly wellwith the cation distribution estimated from Mossbauerdata.

4. Conclusions

Aluminum substituted cobalt ferrites have been preparedsuccessfully with particle size �10 nm by aerosol route.

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Table 3

Mossbauer parameters of the ferrites after annealing at 1200 1C

Ferrite composition Isomer shift (mm/s) Hyperfine field (kOe) Fe3+(oct.)/Fe3+(tet.) Cation distribution

CoFe2O4 0.48 516 (T) 2.33 [Co0.40Fe0.60]A[Co0.60Fe1.40]

BO4

0.40 490 (O)

CoAl0.18Fe1.82O4 0.39 485 (T) 2.46 [Co0.40Al0.07Fe0.53]A[Co0.60Al0.11Fe1.29]

BO4

0.43 464 (O)


BO4

0.43 457 (O)

0.41 433 (O)

0.45 404 (O)


BO4

0.42 426 (O)

0.42 400 (O)

0.42 364 (O)


BO4

0.41 380 (O)

0.40 350 (O)

0.44 242 (O)

0.43 DEQ ¼ 1:67mms�1

CoAl0.95Fe1.05O4 0.34 380 4.00 [Co0.40Al0.36Fe0.24]A[Co0.60Al0.59Fe0.81]

BO4

0.40 330

0.42 299

0.43 230

0.45 DEQ ¼ 1:32mms�1

CoAl1.18Fe0.82O4 0.41 DEQ ¼ 1:30mms�1 –

CoAl1.36Fe0.64O4 0.41 DEQ ¼ 1:30mms�1 –

0

100

200

300

400

500

600

0 0.2 0.4 0.6 0.8 1 1.2Aluminum Concentration

Hyp

erfin

e F

ield

TetrahedralOctahedral

Fig. 9. Variation of the hyperfine field with the aluminum concentration

of the annealed samples.


Lattice parameter and saturation magnetization decreaseswith the increase of aluminum concentration. Mossbauerspectra suggest that sample annealed at 1200 1C havedistributed local environment of Fe-cation. The cationdistribution obtained from the X-ray intensity datamatches very well with the cation distribution estimatedfrom Mossbauer data.

Acknowledgment

Grateful thanks are due to the Department of Scienceand Technology, New Delhi, for providing the grant underfast track proposal for young scientist to S. S.

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XRD, magnetic and Mössbauer spectral studies of nano size aluminum substituted cobalt ferrites...

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