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Ceramics International 40 (2014) 6487–6493www.elsevier.com/locate/ceramint

Structural and magnetic properties of Pr–Ni substitutedCa0.5Ba0.5Fe12O19 hexa-ferrite nanoparticles

Hasan M. Khana,b,n, M.U. Islama,n, Yongbing Xub,c, Muhammad Naeem Ashiqd, Irshad Alia,M. Asif Iqbala,e, Muhammad Ishaquea

aDepartment of Physics, Bahauddin Zakariya University, Multan 60800, PakistanbDepartment of Electronics, University of York, York, YO10 5DD, UK

cNanjing-York International Centre of spintronics and Nano-Engineering, Nanjing University, Nanjing 210093, ChinadInstitute of Chemical Sciences, Bahauddin Zakariya University, Multan, Pakistan

eCollege of E&ME, National University of Science and Technology, Islamabad, Pakistan

Received 15 June 2013; received in revised form 4 November 2013; accepted 19 November 2013Available online 28 November 2013

Abstract

Effect of Pr–Ni substitution on structural and magnetic properties of Ca0.5Ba0.5�xPrxNiyFe12�yO19 (x¼0.00–0.10 and y¼0.00–1.00) preparedby the sol–gel auto combustion method were investigated. The XRD analysis confirmed the single phase M-type hexa-ferrite structure. The latticeparameters were found to increase as Pr–Ni content increases, which is attributed to the ionic size of the implicated cations. The Pr–Ni seems tobe completely soluble in the lattice. Transmission electron microscopy reveals that the grain size decreases with increase of Pr–Ni substitution.The coercivity and remanent magnetization ranges from 1511 to 1925 (Oe) and 21.4 to 26.5 (emu/g), respectively. The coercivity values of all thesamples fall in the range of M-type hexa-ferrites.& 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: C. Magnetic properties; Sol–gel; M-type; Hexaferrite; SEM

1. Introduction

Hexa-ferrites continue to be very attractive materials fortechnological applications due to their unique electrical andmagnetic properties. The M-type hexa-ferrites MFe12O19

(M¼Ba, Sr or Pb) are important ferromagnetic oxides due totheir high intrinsic coercivity, fairly large crystalline aniso-tropy, high electrical resistivity and low cost. These propertiesmake them potential candidate for their use in permanentmagnets, recording media and microwave devices. For manyapplications, permanent magnets are the best choice because itprovides a constant field without continuous expenditure ofelectric power and without generation of heat [1–4].

e front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All ri10.1016/j.ceramint.2013.11.101

g authors.sses: muislampk@yahoo.com,oo.com (M.U. Islam).

Moreover, the low cost of materials and easy manufacturinghave greatly enhanced its production and commercial effec-tiveness. The magnetic properties of BaFe12O19 stronglydepend on the mean grain size and morphology of the powderssynthesized [5,6]. Combination of rare earth (RE) elementswith divalent ion can drastically affect the electromagneticproperties of hexa-ferrites [7,8]. It has been reported that Cahas positive effects in obtaining the fine particles and theuniform incorporation of Ca was also observed by using sol–gel route [9,10].Many methods have been used for the synthesis of M-type

hexaferrites such as co precipitation, micro emulsion, sol–geland hydrothermal methods. The sol–gel auto-combustiontechnique was used for the synthesis of samples due to itsmany advantages such as energy efficient, short reaction time,ultra fine powders of nano-size, easy operation, low annealingor calcinations temperature, better particle size distribution,excellent homogeneity and more probability of formation of

ghts reserved.

H.M. Khan et al. / Ceramics International 40 (2014) 6487–64936488

single domain. The sol–gel auto-combustion technique isalso more useful to prepare the hexa-ferrites powdershaving suitable coercivity for recording applications [11,12].The substitution of rare earth cations at Fe or M (M=Ba, Sr, Caor Pb) sites is an effective method to vary the physical,magnetic and electrical properties of strontium hexa-ferrites.To the best of our knowledge very few systematic studies havebeen performed on Ca0.5Ba0.5Fe12O19 hexa-ferrites [13,14].Iqbal et al. [15] synthesized Ce–Ni doped strontium–bariumM-type hexaferrites by the co-precipitation method andobserved that substitution of Ce–Ni ions enhances the magne-tization. Lechevallier et al. [16,17] reported the hyperfineparameters of La–Zn/La–Co and Pr–Co substituted strontiumhexaferrites and investigated the crystallographic position ofthe substituted cations. It was observed that doped cations hadpositively affected the hyperfine parameters of the hexaferritesystem. Keeping in view the above facts we substituted Pr–Niand the effects of Pr–Ni substitution on Ca0.5Ba0.5Fe12O19

have been reported and discussed thoroughly.In the present work, the sol–gel method has been used for

the fabrication of calcium barium hexa-ferrites. The effectof Pr–Ni substitution on the magnetic properties such assaturation magnetization, remanence and coercivity has beendiscussed.

2. Experimental details

2.1. Sample preparation

M-type hexa-ferrites with chemical composition Ca0.5Ba0.5�x

PrxNiyFe12�yO19 (x=0.00–0.10 and y=0.00–1.00) were synthe-sized by the sol–gel auto-combustion technique [14]. The chemicalused for the synthesis of required materials were citric acid, ironchloride (FeCl3 � 6H2O), calcium chloride (CaCl2), barium nitrateBa(No3)2, nickel nitrate Ni(NO3)2 and praseodymium oxide(Pr2O3). These chemicals were of analytical grade and were usedas such without further treatment. The stoichiometric amounts ofstarting materials were dissolved in 100 ml deionized water. Thestock solutions of other salts were prepared accordingly. As Pr2O3

is insoluble in water, therefore it was first dissolved in nitric acid at80 1C and poured into the prepared solution. To maintain the pHvalue at 7, an aqueous ammonia solution (at a ratio of 40:60) wasadded drop wise, while stirring at 60 1C. Then the sols wereallowed to become gel at 60 1C. The gel was then burnt intopowder at 200 1C. The powder was ground in an agate mortar andpestle for 20 min. Finally, the homogenized powder was thensintered at 1100 1C for 8 h.

2.2. Characterization

Fourier transform infrared spectra were taken on FTIRspectrophotometer BioRad Merlin FTS 3000MX. The mag-netic properties were calculated form hysteresis loops whichwere measured by using the vibrating sample magnetometer(VSM) model LakeShore, new 7400 series USA, was used tomeasure M–H loops at room temperature. X-ray diffractionanalysis was performed to analyze the phase of the materials

by using a Shimadzu XD-5A X-ray diffractometer equippedwith CuKα radiation (λ¼1.5406 Å). The crystallite size wasdetermined by the Scherer's formula [18].

D¼ Kλ

β Cos θð1Þ

Where K is a shape constant, λ is the wavelength, β is the linebroadening at FWHM and θ is the Bragg's angle.The values of X-ray density (ρX-ray), bulk density (ρm),

porosity (P) and the unit cell volumes Vcell were calculated bythe following equations [19]:

1

d2¼ 4ðh2þhkþk2Þ

3a2þ l2

c2ð2Þ

Vcell ¼ 0:8666a2 ð3Þ

P¼ 1�ρmρX�ray

ð4Þ

ρm ¼ m

πhr2ð5Þ

ρX�ray ¼2M

NAV cellð6Þ

where ‘M’ is the molar mass, a and c are the lattice constants,‘m’ is the mass of pellets, ‘r’ is the radius of the pellets, ‘NA’ isthe Avogadro's number and ‘Vcell’ is the unit cell volume.The SEM and TEM analyses were performed to study the

surface morphology, particles size and aggregation of grains. Theanalysis of the samples were investigated using a TEM:JEOLJapan 2011 (with LaB6 filament). Samples were prepared bysonicating the powder particles in the ethanol for 1 h. A fewdrops of the suspension were put on to carbon-coated copperTEM grids before allowing them to dry in air. SEM analyses ofthe samples were performed using FEI SIRION SEM operating at20 kV and observed the microstructure of the sintered specimens.The energy dispersive X-ray spectrum (EDXS) was carried outon Horriba MESA-500 spectrometer.

3. Results and discussions

3.1. FTIR spectroscopy

Fig. 1 shows FTIR profile of the three sintered samples;Ca0.5Ba0.5�xPrxNiyFe12�yO19 (x¼0.02, 0.06, and 0.10;y¼0.20, 0.60, and 1.00) hexa-ferrite materials. The dominantabsorption bands on 516, 528 and 532 cm�1 were observedrespectively. All these bands were identified as metal oxygenstretching vibrations in hexa-ferrite which indicate that thehexagonal phase has been formed [20]. The shoulder on peaksabove 1000 cm�1 corresponds to the bending vibration of Fe–O–H [21].

3.2. X-ray diffraction analysis

Fig. 2shows the indexed XRD patterns of the Ca0.5Ba0.5�xPrxNiyFe12�yO19 (x¼0.00–0.10 and y¼0.00–1.00)

4000 3500 3000 2500 2000 1500 1000 500

0

10

20

30

40

50

60

70

80

x=0.02 x=0.06 x=0.10

Tran

smitt

ance

%

Wave Number (cm-1)

Fig.1. FTIR analysis of Ca0.5Ba0.5�xPrxNiyFe12�yO19, x¼0.02, 0.06, 0.10 andy¼0.20, 0.60,1.00.

226208

X=0.00006220 110008

110221

114201

203301205

201

206214 300

281264

218 30020 13

11 14 214

X=0.02

X=0.04

X=0.06

X=0.08

20 30 40 50 60 70

X=0.10

Inte

nsity

(arb

. uni

ts)

2Theta(Degrees)

Fig. 2. XRD analysis of Ca0.5Ba0.5�xPrxNiyFe12�yO19, x¼0.00–0.10 andy¼0.00–1.00.

0.0 0.2 0.4 0.6 0.8 1.0

0.00 0.02 0.04 0.06 0.08 0.105.50

5.55

5.60

5.65

5.70

5.75

5.80

5.85

5.90

5.95

6.00

Ni.Contents

a( ) C( )

Pr. Contents

a()

23.0

23.2

23.4

23.6

23.8

24.0

24.2

24.4

C(

)

Fig. 3. Variation of lattice parameters for Ca0.5Ba0.5�xPrxNiyFe12�yO19,x¼0.00–0.10 and y¼0.00–1.00.

H.M. Khan et al. / Ceramics International 40 (2014) 6487–6493 6489

hexa-ferrites. All the peaks identified, match well with the M-type ICDD card no. 00-051-1879 which indicate the formationof single magnetoplumbite phase. The lattice parameters werecalculated by using the XRD data. The substitution of rareearth element indicates an increase in the lattice constants[22,23]. The crystallite size calculated from the Scherer'sformula was found in the range of 17–24 nm which is lessthan 50 nm. Gleiter [24] proposed that materials with anaverage grain size in the range of 1–50 nm have their physicalproperties enhanced from that of their bulk micron-sizedcounterparts because of the large volume fraction of atomsthat occupies the grain boundary area.

Fig. 3 shows the variation of lattice parameters as a functionof Pr–Ni content. The lattice constants of Ca0.5Ba0.5�xPrxNiyFe12�yO19 hexa-ferrites vary between a¼5.73–5.95 Å andc¼23.19–23.81 Å, which are slightly different than that ofBaFe12O19. The slight change in the lattice constant is due tothe difference between the ionic radius of Pr3þ (0.99 Å) and

Fe3þ (0.645 Å) [18]. Hence the variation of lattice parameters andgrain size also indicate that Pr3þ has entered into the lattice of thehexa-ferrites. It has been reported in the literature earlier that the ‘c/a’ ratio can be used to quantify the structure of the hexa-ferrites[25]. However the range of lattice parameters in the presentsamples falls in the expected range from 3.98 to 4.05 andmanifested the M-type hexa-ferrites.

3.3. Magnetic measurements

The magnetic properties of hexa-ferrites depend on both itsmicrostructure and distribution of cations at different sites.According to the Gorter model [26] of the magnetic structure,these ferrimagnets have five different sublattices, two antipar-allel (4f1 and 4f2) and three parallel (12k, 2a, and 2b) whichare coupled by super exchange interactions through the O2

�

ions. In order to have a high remanence (Mr) in the magnets,the powder material should be sintered so as to have a highdensity; whereas a high coercivity is obtained by keeping thegrain size small.Fig. 4 shows the M–H loops for Ca0.5Ba0.5�xPrxNiy

Fe12�yO19 (x¼0.00–0.10 and y¼0.00–1.00) measured on VSMat a maximum applied field of 10000 Oe at room temperature. Theeffect of different doping amounts of Pr–Ni on the magneticproperties of the calcium barium ferrite powders are shown inTable 1. The trend of the magnetic properties of the samples inFig. 5 shows that coercivity (HC) rapidly falls from 1635 Oe to1277 Oe with substitution of dopants at x¼0.02 and y¼0.20,which is due to the replacement of Fe3þ ions at 4f2 site. It hasbeen reported that 4f2 and 2b sites contribute towards largeanisotropy field and hence the coercivity [27]. At higher substitu-tion levels at x¼0.10 and y¼1.00, the value of HC increases to itsmaximum value of 2025 Oe due to the presence of Fe2þ ions at 2asite which positively contributes for enhancement of the magneto-crystalline anisotropy. Similar variation is also observed by Wanget. al in La substituted hexagonal ferrite [28,29]. It has beeninferred that large pinning fields exist at grain boundaries which are

H.M. Khan et al. / Ceramics International 40 (2014) 6487–64936490

very simple in structure and their origin must be sought in relationto perturbations in the magnetic properties on a very fine scale.There must be some degree of strain or disorder at the interfacebetween the lattices of adjoining crystallites. A discrete-momentcalculation shows that domain walls will be impeded by the grainboundaries with pinning fields equal to the coercivity, if theexchange coupling across them is reduced to some 20% of itsnormal value. The role of grain boundaries is discussed in relationto the coercivity mechanisms generally [30,31]. The longitudinalmagnetic recording media requires high enough coercivity (Hc) of600 Oe while perpendicular magnetic recording media coercivity(Hc) requires above 1200 Oe [32]. It is also reported that the ferritematerials having low coercivity are beneficial for magneticrecording applications such as in hard disks, floppy disks andvideo tapes etc. [33]. In the present study, the coercivity ofthe synthesized samples lie in the range of 1277–2025 Oe. Thisindicates that the concentration of PrNi can be used to tune the

Table 1Magnetic properties, lattice parameters, porosity, particle size, grain size for Ca0.5B

Contents of (PrNi) Y¼0.0 X¼0.02 XX¼0.0 Y¼0.20 Y

Magnetization 37.08 40.2Ms (emu/g)Remanence 23.6 25.7Mr (emu/g)Mr/Ms 0.63 0.63Coercivity 1645 1577 1Hc (Oe)a (Å) 5.73 5.81c (Å) 23.19 23.27Ratio c/a 4.04 4.00Vcell 456.21 453.62D (nm)7(0.01) 17.21 22.81P (%) 26 42D (nm), SEM 48D (nm), TEM 19

-10000 -5000 0 5000 10000

-50

-40

-30

-20

-10

0

10

20

30

40

50 x=0.00,y=0.00 x=0.02,y=0.20 x=0.02,y=0.40 x=0.06,y=0.60 x=0.08,y=0.80 x=0.10,y=1.00

M (e

mu/

g)

H (Oe)

Fig. 4. VSM analysis of Ca0.5Ba0.5�xPrxNiyFe12�yO19, x¼0.00–0.10 andy¼0.00–1.00.

coercivity of the material and the variation of these magneticparameters make these substituted ferrites suitable for data proces-sing and magnetic recording devices. Fig. 6 shows the saturationmagnetization(Ms) and the remanent magnetization (Mr) whichincrease to their maximum up to 36.76 and 20.95 respectively atx¼0.04 concentration of the rare earth element Pr3þ , after whichthere is a decrease in Mr and Ms values for all the samples thathave been observed. The magnetic behavior of the samples maydepend upon the following factors. The ionic radiiof Praseodymium Pr3þ , Iron Fe3þ and Ba2þ , Ni2þ are notcomparable, so this difference in ionic radii introduces local strainwhich may cause the disorder and modifications of local electronicstates. Various rare earth concentrations have different influence onthe super exchange interactions between magnetic cations andhence, on the magnetization [34]. As reported in the literature [35]the rare earth element has slow diffusion rate and possesses lowsolubility in the M-type structure, so the dissolution of the rare

a0.5�xPrxNiyFe12�yO19 hexa-ferrites (x¼0.00–0.10).

¼0.04 X¼0.06 X¼0.08 X¼0.10¼0.40 Y¼0.60 Y¼0.80 Y¼1.0

41.7 35.8 38.2 37.8

26.5 21.4 23.9 22.9

0.63 0.59 0.62 0.60511 1925 1786 1805

5.87 5.91 5.86 5.9523.31 23.58 23.67 23.813.97 3.98 4.03 4.00

445.15 451.47 449.34 452.1312.16 14.19 21.27 24.1247 28 27 21

32 2715 23

-0.02 0.00 0.02 0.04 0.06 0.08 0.10 0.121000

1200

1400

1600

1800

2000

2200

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2Zn. Contents

Hc(

Oe)

Pr. Contents

Fig. 5. Variation of coercivity of Ca0.5Ba0.5�xPrxNiyFe12�yO19, x¼0.00–0.10and y¼0.00–1.00.

0.00 0.02 0.04 0.06 0.08 0.1010

20

30

40

50

600.0 0.2 0.4 0.6 0.8 1.0

Ni. Contents

Ms&

Mr(

emu/

g)

Pr. Contents

Ms

Mr

Fig. 6. Variation in Ms and Mr of Ca0.5Ba0.5�xPrxNiyFe12�yO19, x¼0.00–0.10and y¼0.00–1.00.

Fig. 7. SEM analysis of Ca0.5Ba0.48Pr0.02Ni0.20Fe11.8O19 i�e x¼0.02 andy¼0.20.

Fig. 8. SEM analysis of Ca0.5Ba0.44Pr0.06Ni0.60Fe11.4O19 i�e x¼0.06 andy¼0.60.

Fig. 9. SEM analysis of Ca0.5Ba0.4Pr0.10Ni1.00Fe11O19 i�e x¼0.10 andy¼1.00.

Fig.10. TEM analysis of Ca0.5Ba0.48Pr0.02Ni0.20Fe11.8O19 i�e x¼0.02 andy¼0.20.

H.M. Khan et al. / Ceramics International 40 (2014) 6487–6493 6491

earth ions into the hexagonal structure will result into reduction ofthe total magnetic moment.

3.4. Scanning and transmission electron microscopy

SEM was used to analyze the surface morphology and shape ofthe particles produced by the sol–gel technique, while TEM wasused for the estimation of nanocrystalline nature of the synthesizedcalcium barium ferrite and aggregation of the particles. SEM andTEM analyses of the three selected samples with compositionsCa0.5Ba0.48Pr0.02Ni0.20Fe11.8O19, Ca0.5Ba0.44Pr0.06Ni0.60Fe11.4O19

and Ca0.5Ba0.4Pr0.10Ni1.00Fe11O19 were performed.Figs.7–9 show the SEM micrographs of the selected

samples; Ca0.5Ba0.48Pr0.02Ni0.20Fe11.8O19, Ca0.5Ba0.44Pr0.06-Ni0.60Fe11.4O19 and Ca0.5Ba0.4Pr0.10Zn1.00Fe11O19, respectively.It is clear from the figures that the particles have well definedhexagonal shape and also have clear boundaries. The SEMimages of grain size reveal different ranges of 38–73, 32–58and 18–43 nm for the Ca0.5Ba0.48Pr0.02Ni0.20Fe11.8O19, Ca0.5Ba0.44Pr0.06Ni0.60Fe11.4O19, and Ca0.5Ba0.4Pr0.10Ni1.00Fe11O19

Fig. 11. TEM analysis of Ca0.5Ba0.44Pr0.06Ni0.60Fe11.4O19 (x¼0.06 andy¼0.60).

Fig. 12. TEM analysis of Ca0.5Ba0.4Pr0.10Ni1.00Fe11O19 (x¼0.10 andy¼1.00).

Table 2EDXS analysis of Ca0.5Ba0.5�xPrxNiyFe12�yO19 (x¼0.02, 0.06, and 1.00)powders.

Sr. no. Composition Observed weight (%)

Ni Fe Ba Ca Pr Total

1 Ca0.5Ba0.48Pr0.02Ni0.20Fe11.8O19 2.30 77.7 4.41 7.50 8.06 1002 Ca0.5Ba0.44Pr0.06Ni0.60Fe11.4O19 4.75 71.09 4.33 7.43 12.29 1003 Ca0.5Ba0.4Pr0.10Ni1.00Fe11O19 5.46 66.84 4.21 7.47 15.83 100

H.M. Khan et al. / Ceramics International 40 (2014) 6487–64936492

samples respectively. The particle sizes for the samplessynthesized in the present study are much smaller comparedto those of the reported earlier 151–200, 70–100 and 60–300 nm for the M-type hexa-ferrites. Figs.10–12 show theTEM analysis of Ca0.5Ba0.48Pr0.02Ni0.20Fe11.8O19, Ca0.5Ba0.44Pr0.06Ni0.60Fe11.4O19, and Ca0.5Ba0.4Pr0.10Ni1.00Fe11O19 sam-ples and the basic powder morphology of the smallest visibleultrafine particles that might be identified with the crystallites orwith their aggregates. The surface of the grid was covered withhexagonal ferrite nano-particles as shown in the images. Thevalues of particle size calculated by TEM were found to be17–24, 15–22 and 13–20 nm, which is in accordance with theresults obtained from the Scherrer's formula of XRD [36–40].The synthesized samples can therefore be used for applicationin high density magnetic recording media as these devicesrequire the size below 50 nm to obtain the suitable signal tonoise ratio. The elemental composition of the synthesizedsamples was obtained by energy dispersive X-ray spectroscopyanalysis. However, it is indisputable that stoichiometricamounts of Ba2þ and Fe3þ decreases while Pr3þand Ni2þ

increases, which signifies that Pr3þ is being substituted byBa2þ site and Ni2þ is being substituted by Fe site (Table 2).

4. Conclusions

All the Pr–Ni substituted Ca0.5Ba0.5Fe12O19 samples exhibita single magnetoplumbite phase with slightly increasing valuesof lattice parameters a and c. Coercivity can be tuned at lowersubstitution level, while increasing the saturation magnetiza-tion at the same time. The magnetization behavior has beencharacterized by keeping in view the preferential site occu-pancy of sub-lattice sites of substituted cations, magneticdilution and spin canting. The crystallite sizes were found inthe range 13–24 and 15–27 nm calculated by TEM andScherrer's equation, respectively. These particle sizes are smallenough to obtain a suitable signal to noise ratio in the magneticrecording media. The substitution of Pr–Ni dopants furtherimproves the efficiency of the Ca–Ba hexa-ferrite material inmicrowave devices, magnetic recording media and inductors.

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