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Journal of Magnetism and Magnetic Materials 320 (2008) 881–886

www.elsevier.com/locate/jmmm

Synthesis, physical, magnetic and electrical properties of Al–Gasubstituted co-precipitated nanocrystalline strontium hexaferrite

Muhammad Javed Iqbala,�, Muhammad Naeem Ashiqa,Pablo Hernandez-Gomezb, Jose Maria Munozb

aDepartment of Chemistry, Quaid-i-Azam University, Islamabad 45320, PakistanbDpto. Electricidad y Electronica, Universidad de Valladolid, 47071 Valladolid, Spain

Received 29 June 2007; received in revised form 29 August 2007

Available online 17 September 2007

Abstract

A series of Al–Ga substituted strontium hexaferrite materials of nominal composition SrAlxGaxFe12–2xO19 (x ¼ 0.0–0.8) have been

synthesized by the co-precipitation method. The XRD analysis confirms the single magnetoplumbite phase and various parameters such

as lattice constants (c and a), cell volume (V), crystallite size (D) and X-ray density (rx-ray) have also been calculated from the XRD data.

The crystallite size is found in the range of 30–62 nm. The elemental composition is determined by EDX analysis. DC electrical resistivity

is measured within the temperature range of 300–675K. It is observed that the resistivity increases with the Al–Ga content. The

activation energy (Ea) and drift mobility (md) are also calculated from the resistivity data. The dielectric constant (��) and dielectric loss

(tan d) have been measured in the frequency range of 80Hz–1MHz. The magnetic properties such as saturation magnetization (Ms),

remanence (Mr) and coercivity (Hc) are calculated from the hysteresis loops. Values of ��, tan d, Ms, Mr and Hc are found to decrease with

increase in Al–Ga concentration.

r 2007 Elsevier B.V. All rights reserved.

Keywords: Strontium hexaferrite; Coercivity; DC electrical resistivity; Al–Ga substitution; Saturation magnetization

1. Introduction

M-type hexaferrites BaFe12O19 (BaM) and SrFe12O19

(SrM) with magnetoplumbite structure have extensiveapplications as materials for permanent magnets, high-density recording media, telecommunication, magneto-optical and microwave devices [1,2]. The M-type hexagonalstructure consists of five distinct Fe crystallographic sites,i.e. three octahedral (2a, 12k and 4f2), one tetrahedral (4f1)and one trigonal bipyramid (2b) [3]. The electrical,dielectric and magnetic properties of hexaferrites dependupon the method of preparation, composition and thedistribution of the substituted cations at the five crystal-lographic sites.

As a consequence, a large number of synthetic techni-ques have been developed to control the particle size, shape

- see front matter r 2007 Elsevier B.V. All rights reserved.

/j.jmmm.2007.09.005

onding author. Tel.: +9251 90642143; fax: +92 51 90642241.

ddress: mjiqauchem@yahoo.com (M.J. Iqbal).

and the properties of the materials [4–9]. The solid statemethod [4] normally requires sintering temperature of1200 1C in addition to a period of several hours forobtaining an ultrafine and monodispersed particles. Someof the other low temperature methods [5,6] have compli-cated synthesis procedures and sometimes may even bemore expensive as compared to the co-precipitationmethod. The samples prepared by the co-precipitationmethod have shown better properties as compared tosol–gel combustion method as reported earlier [10]. Thecrystallite size (D) of 30–62 nm by the co-precipitationmethod has been reported in this work which is muchsmaller than 4.69 mm for the barium hexaferrites alsoprepared by co-precipitation method [11].The substitution of cations at Fe or Sr sites is an effective

method to vary the physical, magnetic and electricalproperties of strontium hexaferrites. In general, the ironoxides are magnetic semiconductors but divalent ortetravalent cations doped at iron site may be able to

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Table 1

Crystallite size, lattice constants, cell volume, X-ray density, bulk density,

porosity, transition temperatures, activation energy, dielectric loss factor,

dielectric constant and elemental composition at different AlxGax contents

(x) of SrAlxGaxFe12–2xO19

Parameters x ¼ 0.0 x ¼ 0.2 x ¼ 0.4 x ¼ 0.6 x ¼ 0.8

Crystallite sizes (D) nm 30 60 62 50 47

Lattice constant (c) A 23.06 23.05 23.03 22.95 22.91

Lattice constant (a) A 5.87 5.87 5.86 5.86 5.87

Cell volume (v) A3 688.10 688.10 684.86 682.48 683.63

Bulk density (rm) g cm�3 2.36 2.43 2.48 2.46 2.51

X-ray density (rx-ray) g cm�3 5.13 5.10 5.11 5.12 5.09

Porosity (P) % 0.54 0.42 0.40 0.42 0.41

Transition temperature (TM–S) K – 398 408 423 468

Activation energy (Ea) eV 0.491 0.71 0.79 0.83 0.94

Dielectric constant (��) at 600 kHz 220 212 196 179 146

Dielectric constant (��) at1000 kHz

183 178 164 143 129

Loss factor (tan d) at 600 kHz 1.09 0.99 0.74 0.65 0.32

Loss factor (tan d) at 1000 kHz 0.98 0.87 0.62 0.55 0.26

Fe mol 11.93 11.66 11.25 10.88 10.46

Sr mol 1.10 1.09 1.22 1.21 1.20

Al mol – 0.14 0.27 0.51 0.68

Ga mol – 0.12 0.28 0.43 0.61

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 881–886882

partially replace Fe3+and hence are likely to induce thep- or n-type conduction [12]. In the present study, however,both Al and Ga dopants are trivalent, with ionic radiicomparable to that of Fe3+, hence they would be expectedto completely replace the Fe3+ ions without making it n- orp-type. The selective occupation by non-magnetic ions,i.e. Al and Ga, at different crystallographic sites is alsoresponsible for altering the exchange interactions betweenFe3+ ions. The aim for the substitutions of Al–Ga is toimprove the electrical resistivity, decrease the dielectricconstant, dielectric loss factor and the drift mobility andalso to study magnetic properties of these substitutedstrontium hexaferrites.

In this paper, we report the structural changes (cellvolume, X-ray and bulk densities, crystallite size and cellconstants) that may occur due to substitution of Al–Gaand also the effect on the electrical (DC electricalresistivity, drift mobility, activation energy and transitiontemperature), dielectric (dielectric constant and dielectricloss factor) and magnetic (saturation magnetization,coercivity and remanence) properties of the materialssynthesized by the co-precipitation method.

2. Materials and method

The chemicals used in the synthesis of samples areFe(NO3)3 � 9H2O (Sigma-Aldrich, 98%), Sr(NO3)2 (Fluka,99%), Al(NO3)3 � 9H2O (Merck, 95%), Ga(NO3)3 (Aldrich,99.9%) and NaOH (Fluka, 97%). The strontium hexafer-rite samples with nominal composition SrAlxGaxFe12–2x

O19 (where x ¼ 0.0–0.8) are prepared by a chemicalco-precipitation method [7] keeping the molar ratio(Fe/Sr ¼ 11). The metallic salt solutions of requiredmolarities are prepared in distilled water and mixed in abeaker. This solution mixture is heated up to 70 1C withcontinuous stirring on a hot plate. The 2M solution ofNaOH has been used as precipitating agent. When thetemperature reached 70 1C the 2M NaOH is dropped toform the precipitate and the pH of the solution is kept12.5–13. All the samples are stirred for 3 hr in order tocontrol the crystallite size and homogeneity of the samples.The precipitates are washed with distilled water, dried at100 1C in an oven and annealed at an optimizedtemperature of 920 1C for 1 h in a temperature-pro-grammed tube furnace at a heating rate of 5 1Cmin�1.

The XRD analysis is performed to confirm the phaseformation of hexaferrites by Philips X’Pert PRO 3040/60diffractometer which uses CuKa as a radiation source. Theelemental composition is determined by energy dispersiveX-ray fluorescence (ED-XRF) (Horiba MESA-500) analy-sis. The DC electrical resistivity is measured by a two-pointprobe method [13]. As the resistance of the samples is veryhigh in the order of 108O cm, the potential differenceapplied is also high (in the order of 100V) and the samplesare in the form of thin disks so the four-point probemethod which is suitable for metallic (low resistance) orsuperconducting samples is not recommended for our

samples. Also, the effect of wire and contact resistancesseems to be negligible in case of two-point probe method.On the other hand, the sample geometry also makesdifficult to design a suitable four-point connection sampleholder. The use of the four-point method is also notrecommended at temperatures higher than �450K due toinstability of silver paste used for the connections. Thedielectric measurements are carried out at room tempera-ture in a frequency range 80Hz–1MHz using inductancecapacitance resistance (LCR) meter bridge (Wayne KerrLCR 4275). The DC electrical resistivity as well as thedielectric properties has been carried out with samplepellets of 13mm diameter and 2.0mm thickness. Thehysteresis loops of the samples are determined by thestandard AC induction method [14] where the magneticfield H and the magnetic induction B are both measuredthrough two coils placed near the sample. The values of thesaturation magnetization (Ms), remanence (Mr) andcoercivity (Hc) are calculated from the loops. The driftmobility (md) is calculated from the DC electrical resistivityr data using the following equations:

md ¼1

ner, (1)

where e is the charge on electron, r is the resistivity and n isthe concentration of charge carriers which can becalculated by the following equation:

n ¼NArmB

M, (2)

where NA is the Avogadro’s number, rm is the measureddensity of sample given in Table 1, B is the number of ironatoms in the chemical formula of the materials and M is themolar mass of the sample.

ARTICLE IN PRESSM.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 881–886 883

3. Results and discussion

3.1. Structural parameters

The indexed XRD patterns of the Al–Ga substitutedstrontium hexaferrite samples synthesized by the co-precipitation method are shown in Fig. 1. The peaksmatch perfectly with the standard M-type hexaferrite

Fig. 1. The indexed XRD patterns of SrAlxGaxFe12–2xO19 strontium

hexaferrite (a) x ¼ 0.0, (b) x ¼ 0.2, (c) x ¼ 0.4, (d) x ¼ 0.6 and (e) x ¼ 0.8.

pattern (ISCD 00-051-1879), indicating a single magneto-plumbite phase. Different parameters such as latticeconstants (a and c), cell volume (V), X-ray density (rx-ray)and crystallite size (D) are calculated from the XRD datausing well-known relations [7] and their values are given inTable 1. The lattice constant a remains constant but thevalues of c decrease with the Al–Ga content. This is due tothe smaller ionic radii of Al3+ (0.53 A) and Ga3+ (0.62 A)compared with that of Fe3+ (0.64 A). The crystallitesize (D) calculated by Scherrer’s formula is found in arange 30–62 nm which is much smaller as compared to4.69 mm for the strontium hexaferrite synthesized by theco-precipitation method [11]. The value of the X-raydensity (rx-ray) is greater than that of the bulk density(rm) indicating presence of pores in the material. Theporosity (P) is calculated using the following equation:

P ¼ 1�rm

rx�ray, (3)

where rm is the bulk density and rx-ray is the X-ray densityof the samples. The porosity of the substituted samples islower than that of the pure strontium hexaferrite, as shownin Table 1. This indicates that the doped elements may havecaused some densification of the strontium hexaferritematrix.The metallic contents of the synthesized materials has

been calculated by the EDX analysis as shown in Table 1.The amount of Al and Ga substituted in the synthesizedmaterials is found to be less than the actual amount addedto the system. The co-precipitation method involvesannealing at 920 1C where conversion of hydroxides tothe oxides takes place. Since the melting points of Al(660.37 1C) and Ga (29.79 1C) are much smaller than theannealing temperature some of the Al and Ga may haveevaporated resulting in a loss in the contents of theseelements. Although the oxides of Al and Ga have highmelting points (2054 and 1900 1C, respectively) yet such apossibility cannot be ruled out. The molarities of Sr and Feare almost in agreement with the nominal composition.The molar ratio of strontium is kept slightly higher due tolower solubility of strontium in water as compared to thatof the iron.

3.2. DC electrical resistivity

DC electrical resistivity (r) and the drift mobility (md)have been calculated as described in detail elsewhere [7].Figs. 2 and 3 show the effect of temperature on bothresistivity and the drift mobility. It is notable that theresistivity increases with the temperature at the beginning,reaches maxima at a specific temperature, called thetransition temperature TM–S, but it then decreases withfurther increase in the temperature. Such resistivity–temperature behavior has also been reported for Zr–Cusubstituted strontium hexaferrite [7] and Al–Cr substitutedspinel ferrite [15]. The value of TM–S increases withincrease in Al–Ga content as shown in Table 1. Such

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0

20

40

60

80

100

120

140

160

ρ (o

hm

cm

) 10

8

0

0.2

0.4

0.6

0.8

275

Temperature / K

375 475 575 675

Fig. 2. DC electrical resistivity (r) of the synthesized samples as a function

of temperature.

0

500

1000

1500

2000

2500

275 375 475 575 675

μ d (

cm

2 V

-1 s

ec

-1)

10

-13

0

0.2

0.4

0.6

0.8

Temperature / K

Fig. 3. Drift mobility (md) as a function of temperature for Al–Ga

substituted strontium hexaferrite.

0

5

10

15

20

25

30

0 0.2

ρ (o

hm

cm

) 1

08

0

2

4

6

8

10

12

14

16

18

μ d (

cm

2 V

-1 s

ec

-1)

10

-13μd

Al-Ga Content

0.4 0.6 0.8

ρ

Fig. 4. Plots of drift mobility (md) and DC electrical resistivity (r) at roomtemperature as a function of Al–Ga content.

0

10

20

30

40

50

60

70

80

6 7 8 9 10 11 12 13 14

� +

10

3

0

0.2

0.4

0.6

0.8

ln (F / Hz)

Fig. 5. Plots of dielectric constant (��) versus frequency (F) of Al–Ga

substitutes strontium hexaferrites.

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 881–886884

resistivity–temperature behavior may be attributed to theoccurrence of phase transition, cation migration, cation re-ordering, the presence of impurities and the magneto-transport effect. In our case, however, there is no realpossibility of any phase transition at such low temperatures(398–468K) and the XRD data do not show any impurityphases in the samples. The cation migration is also not so fastat such low temperature to give rise to the observed behaviorbut cannot be excluded. The other possibility therefore is thespin canting [15] because with substitution and temperaturevariation the spin canting angle changes which may beresponsible for this striking peak in the resistivity. The driftmobility decreases with temperature in the ToTM–S regiondue to increase in resistivity of the sample but increases withthe increase in temperature in T4TM–S region because thecharge carrier mobility increases from one site to other butbelow transition temperature the mobility of charge carriersdecreases due to increase in DC resistivity.

The variation of the room temperature resistivity and thedrift mobility as a function of Al–Ga content is shown inFig. 4. The resistivity increases while the drift mobilitydecreases with an increase in the Al–Ga content. Theconduction in ferrites is considered to occur due to hopping

of electrons between F2+ and Fe3+ at octahedral sites. Ithas been reported that Al and Ga predominantly occupy12K octahedral site [16,17]. Due to the replacement ofiron by aluminum and gallium from the octahedral sites,the number of Fe3+ ions and hence the hopping ofelectrons decreases. Therefore, the resistivity increasesfrom 2.45� 108 to 25.96� 108O cm and the drift mobilitydecreases from 15.93� 10�13 to 1.39� 10�13 cm2V�1 s�1.The decrease in drift mobility is due to an increase in DCresistivity consequently the hopping of charge carriers fromone site to the other site is reduced. The energy ofactivation is calculated using the well-known Arrhenius-type equation [13]. The values of activation energy increasewith the Al–Ga content as shown in Table 1. The increasein the activation energy due to increase in the resistivitywith Al–Ga content is expected because the sample withhigh resistivity value has high value of activation energyand vice versa.

3.3. Dielectric parameters

Figs. 5 and 6 show the effect of frequency on thedielectric constant (��) and the dielectric loss factor (tan d) of

ARTICLE IN PRESS

0

1

2

3

4

5

6

7

8

9

6 7 8 9 10 11 12 13 14

tan δ

0

0.2

0.4

0.6

0.8

ln (F / Hz)

Fig. 6. Dielectric loss factor (tan d) as a function of frequency (F) for

Al–Ga substituted strontium hexaferrite.

-100

-80

-60

-40

-20

0

20

40

60

80

100

-100 0 100 200 300 400

H / KA m-1

M /

KA

m-1

920

10201120

1170

-400 -300 -200

Fig. 7. Hysteresis loops of SrFe12O19 at different temperatures.

10

30

50

70

900 1200

Ms &

Mr / K

A m

-1

40

60

80

100

120

140

Hc /

KA

m-1

MsMrHc

Annealing Temp. / °C

1000 1100

Fig. 8. Effect of annealing temperature on the saturation magnetization

(Ms), remanence (Mr) and coercivity (Hc).

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 881–886 885

the synthesized samples. The decrease in the values of bothtan d and �� with the frequency is a normal behavior forferrites and can be explained on the basis of chargepolarization. The charge polarization is a result of thepresence of higher conductivity phase (grains) in theinsulating matrix (grain boundaries) of a dielectric, causinglocalized accumulation of charge under the influence ofelectric field. The reduction in tan d and �� occurs since thefrequency of hopping of electron exchange between Fe2+

and Fe3+ is far from the frequency of the alternatingelectric field.

The variation of tan d and �� as a function of Al–Gacontents at frequencies 600 and 1000 kHz is shown inTable 1. It is evident that values of both the parametersdecrease with increase in the Al–Ga contents. Iwauchi [18]pointed out that there is a strong relationship between theconduction mechanism and the polarization mechanism(dielectric constant). The space charge polarization occursdue to the electron displacement when electric field isapplied to the sample. The subsequent charge build up atthe insulating grain boundary acts as a major contributorto the dielectric constant in ferrites. Therefore, increasingthe number of Fe2+ ions at octahedral site the space chargepolarization is expected to be enhanced due to the ease ofelectron transfer between Fe2+ and Fe3+ and as a resultthe dielectric constant would have higher values. Theincrease in DC electrical resistivity with Al–Ga substitutioncauses obstruction to the flow of the carriers reducing thebuild-up of space charge polarization due to replacementof iron ions from octahedral site. The tan d and �� thereforewould be expected to decrease by increasing Al–Gaconcentration as it has also been proved experimentally.

3.4. Magnetic parameters

Fig. 7 shows the hysteresis loops for SrFe12O19 atdifferent annealing temperature and various magneticparameters such as saturation magnetization (Ms), rema-nence (Mr) and coercivity (Hc). Fig. 8 represents the effectof annealing temperature on the saturation magnetization

(Ms), coercivity (Hc) and remanence (Mr) of the purestrontium hexaferrite. It is clear that the Ms, Mr and Hc

continuously increase with temperature increment and themaximum value in each case is obtained at 1020 1Cfollowed by their continuous decrease with temperature.The lowering of Ms, Mr and Hc values at high temperaturemay be a consequence of larger particle size formed as aresult of their coalescence with each other. It is well knownthat particle size has a significant effect on the magneticproperties of the magnetic materials. When the particles aresmaller than the critical single domain size they are mainlyin single domain. On the contrary, when the particle sizebecomes bigger than the critical value most of them wouldexist in multi-domain. With an increase in the annealingtemperature the particle size also increases towards thecritical single domain size. As a result, the coercivity,remanence and saturation magnetization increase even-tually reaching a maximum value at single domain sizebecause of the coherent rotation of spins. As the particlesbecome larger than the single domain size at highertemperature, the values of Hc, Mr and Ms begin todecrease.The values of the saturation magnetization, coercivity and

remanence are calculated using the hysteresis loops of theAl-Ga substituted samples as shown in Fig. 9. Fig. 10represents the variation of Ms, Mr and Hc as a function of

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-80

-60

-40

-20

0

20

40

60

80

-400 -300 -200 -100 0 100 200 300 400

M /

KA

m-1

0.2

0.4

0.60.8

H / KA m-1

Fig. 9. Hysteresis loops for the Al–Ga substituted strontium hexaferrite

annealed at 1020 1C.

10

30

50

70

90

0

Al-Ga content

Ms &

Mr / K

A m

-1

60

80

100

120

140

Hc / K

A m

-1Ms

Mr

Hc

0.2 0.4 0.6 0.8

Fig. 10. Saturation magnetization (Ms), remanence (Mr) and coercivity

(Hc) as a function of Al–Ga content.

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 881–886886

the Al–Ga concentration. Clearly, the Ms and Hc valuesdecrease with the addition of Al–Ga contents. The decreasein the values of Ms and Mr is due to the fact that both Aland Ga are non-magnetic and they occupy the 12K sitewhich has the spin-up direction. The total magnetic momentin M-type hexaferrite is 20mB. The substitution of non-magnetic ions occupying the spin-up site reduces the totalmagnetic moment and as a result the saturation magnetiza-tion decreases. The coercivity of the samples decreasescontinuously with the substitution of Al–Ga contents due tothe likelihood of decrease in the values of anisotropyconstant. These results are in agreement with the literaturefor the Al-substituted barium hexaferrite [19,20].

4. Conclusion

The co-precipitation is a simple method to obtainingnanocrystallite and single phase Al–Ga substituted stron-tium hexaferrite. The XRD analysis assures the singlemagnetoplumbite phase. The crystallite size of the synthe-sized samples ranges between 30 and 62 nm. The roomtemperature DC electrical resistivity and activation energy

increase while the dielectric constant and dielectric lossfactor decrease with the increase in Al–Ga concentration.The improvement in the electrical resistivity and decreasein both the dielectric loss factor and the dielectric constantmake the synthesized materials suitable for the applicationin microwave devices because a high resistivity material isrequired for the applications in microwave devices. Themaximum values of magnetic parameters, i.e. saturationmagnetization, remanence and coercivity, are obtained forthe sample annealed at 1020 1C. The saturation magnetiza-tion, remanence and coercivity decrease with the increase inAl–Ga content and it has been explained on the basis of thesite occupation of the substitutions.

Acknowledgment

The authors are thankful to the higher educationcommission (HEC) of Pakistan for proving the financialsupport for this work under indigenous and IRSIPScholarship scheme.

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