A

Votcb©

P

K

1

admnsflftT

twcur

0d

Materials Chemistry and Physics 103 (2007) 255–263

Dielectric characteristics of substituted M-type strontium hexaferritecrystals and their modifications on swift heavy ion irradiation

Balwinder Kaur a, Monita Bhat a, F. Licci b, Ravi Kumar c, K.K. Bamzai a, P.N. Kotru a,∗a Crystal Growth & Materials Research Laboratory, Department of Physics & Electronics, University of Jammu, Jammu 180006, India

b Instituto MASPEC-CNR, Via Chiavari 18/A, 43100 Parma, Italyc Nuclear Science Centre, New Delhi 110067, India

Received 30 December 2004; received in revised form 12 December 2006; accepted 19 February 2007

bstract

Dielectric behaviour of flux grown substituted crystals bearing composition SrGaxInyFe12−(x+y)O19 (x = 5, 7, 9; y = 0.8, 1.3, 1.0) is reported.ariation of dielectric constant (ε/), dielectric loss (tan δ) and ac conductivity (σac) with temperature in the range 30–500 ◦C under the frequencyf the applied ac field in the range 1–104 kHz for all the varied compositions are analyzed and explained. Swift heavy ion irradiation effects onhese parameters are investigated. On irradiation the crystals show changes in the values of dielectric constant (ε/), dielectric loss (tan δ) and ac

onductivity (σac). The frequency variation of ε/ and tan δ is explained by Koop’s phenomenological theory and hopping frequency of electronsetween Fe2+ and Fe3+ on octahedral sites for both irradiated and unirradiated crystals. 2007 Published by Elsevier B.V.

ACS: 61.72.Ji; 61.80.Jh; 71.38.+i; 72.20.Fr; 77.22.Jp; 77.22.Gm

aferrit

bradfdNaasbnctt

eywords: Defects; Irradiation; Dielectric behaviour; Polaron interaction; Hex

. Introduction

Hexagonal hard ferrites such as SrFe12O19 and BaFe12O19re considered to be the most particulate media for perpen-icular recording due to their chemical, morphological andagnetic characteristics such as mm-devices, master tapes, mag-

etic heads and several others [1]. The iron ions in M-typetrontium hexaferrite are distributed among five lattice sites dif-erent in coordination and spin orientation as is given in theiterature and shown here in Table 1a [2,3]. The cell parametersor both unsubstituted SrFe12O19 and Ga–In substituted stron-ium hexaferrites as reported in the literature [4–6] is given inable 1b.

The authors have reported [6] that the irradiation leadso amorphization in pure and substituted M-type hexaferrites,hich decreases the value of microhardness and increases the

rack length in the samples. The defect characterization ofnsubstituted and Ga/In substituted hexaferrites have also beeneported [7–9].

∗ Corresponding author. Tel.: +91 191 2450939; fax: +91 191 2453079.E-mail address: pn kotru@yahoo.com (P.N. Kotru).

itstsfk

254-0584/$ – see front matter © 2007 Published by Elsevier B.V.oi:10.1016/j.matchemphys.2007.02.070

e

The dielectric properties of polycrystalline ferrites dependoth on chemical composition as well as the process of prepa-ation [10]. Studies on the effect of temperature, compositionnd frequency on the dielectric behaviour and ac electrical con-uctivity gives information about conduction phenomenon inerrites based on localized electric charge carriers [11]. Theependence of dielectric properties of Li–Mg–Zn ferrite [12],i–Zn ferrite (where 0 ≤ x ≤ 1) [13], Ba–Ni–Zn ferrites [14]

nd Mg–Zn ferrites [15] on frequency, composition and temper-ture are also reported in the literature. Iwauchi [16] explained atrong correlation between conduction mechanism and dielectricehaviour of ferrites. Iron oxides, in general, are termed as mag-etic semiconductors because doping by divalent or tetravalentations may replace Fe3+, thus inducing p- or n-type conduc-ion [17–20]. In the present case, Ga and In cations used arerivalent cations and their ionic radii are also comparable to theonic radius of Fe3+ ion, so Ga and In will completely substitutehe Fe3+ ions without making it n- or p-type. The Ga/In sub-tituted strontium hexaferrites are, therefore, insulators at room

emperature. In the present study flux grown crystals of Ga–Inubstituted hexaferrites of varied composition are consideredor the study of dielectric characteristics. To the best of authorsnowledge, the work is reported for the first time.

256 B. Kaur et al. / Materials Chemistry an

Table 1aThe five iron sites in SrFe12O19, their coordination, point symmetry, number ofFe ions per formula, spin orientation and block situation

Site Coordination Pointsymmetry

No. of Fe ionsper formula

Spin Block

12k Octahedral m 6 Up S–R4f1 Tetrahedral 3m 2 Down S4f2 Octahedral 3m 2 Down R2a Octahedral 3m 1 Up S2b Trigonal or

bipyramidal6m2 1 Up R

Table 1bData shows number of Fe atoms substituted, their cell parameters correspondingto different compositions

Sample Number of Fe atomssubstituted

Cellparameter,a (A)

Cellparameter,c (A)

Ga In

SrFe12O19 0 0 5.833 23.05SrGa5In0.8Fe6.2O19 5 0.8 5.884 22.93SS

etotTftnwodaaDw(Aww

2bv

hBhtOs5pos1DPaXa

meiLsttfl

2

yi1c[

aAn(wco

uaTp

TMfl

S

SSSS

rGa7In1.3Fe3.7O19 7 1.3 5.888 22.88rGa9In1Fe2O19 9 1.0 5.924 22.98

Swift heavy ion irradiation of magnetic insulators has beenxtensively studied over the last twenty years. It has been shownhat amorphous latent tracks are produced above a thresholdf the electronic stopping power Se = (−dE/dx)e, correspondingo a large density of ionization and electronic excitations [21].he swift heavy ion (SHI) irradiation in magnetic oxides and

errites has been of great interest to understand the damage struc-ures and modifications of their physical properties [22–26]. Theature of damage depends upon the electronic energy loss. It isell known that SHI irradiation produces stress/strain field inxide materials [27–31]. During the irradiation, damage is pro-uced in the near surface region of the substrate leading to stressnd amorphization in structures causing cracking, delamination,nomalous diffusion of dopants and void formation [32,33].amage formation due to elastic (atomic) collisions can be fairlyell predicted by TRIM (transport of ion in materials)/SRIM

the stopping and range of ions in matter) [34] calculations.ccording to Cazaux [35] and Costantini et al. [36], irradiationith ions or electron implantation create space charge effects,hich eventually may lead to breakdown in dielectric materials.

Irradiation effects induced by 3.1 GeV Xenon ions and

.3 GeV Molybdenum ions in BaFe12O19 and SrFe12O19 haveeen investigated by means of mossbauer spectroscopy andibrating sample magnetometry. Magnetic investigation of

dprim

able 2ean electronic stopping power (Se), mean nuclear stopping power (Sn), and mean

uence of 1 × 1013 ions cm−2

ample Ion E (MeV) Se (eV A−1

rFe12O19 Li3+ 50 18.66rGa5In0.8Fe6.2O19 Li3+ 50 19.75rGa7In1.3Fe3.7O19 Li3+ 50 20.26rGa9In1Fe2O19 Li3+ 50 20.02

d Physics 103 (2007) 255–263

eavy ion irradiation effects leads to the conclusion thataFe12O19 appears to be slightly more sensitive to high energyeavy ion irradiation than SrFe12O19 [37]. It has also been shownhat irradiation of the Strontium ferrite powders with 85 MeVxygen ions modifies the magnetic properties of the micron-

ized powders, but not of nano-sized powders [38]. Effect of0 MeV Li3+ ion irradiation on In3+ substituted Mg–Mn ferriteroduces defects and compressive strain in the lattice structuref ferrites. It is reported that in these materials, the dielectric con-tant decreases with irradiation [39]. The dielectric properties of90 MeV Ag ion irradiated ferrite thin films is also reported byogra et al. [40]. The dielectric behaviour of neutron irradiatedZT and PLZT—6 thin ferroelectric films [41] and dielectricnd transport properties of magnetic insulator irradiated withe and Pb ions in the GeV ranges [19] and MeV range [23] has

lso been reported.The dielectric properties of pure SrFe12O19 crystals and their

odifications on SHI irradiation have already been reportedlsewhere [42]. To the best of author’s knowledge, no works reported on SHI induced modifications caused by 50 MeVi3+ ions on the dielectric characteristics of single crystals ofubstituted strontium hexaferrite. In the present paper, dielec-ric characteristics of pristine Ga–In substituted hexaferrites andheir modifications on irradiation with 50 MeV Li3+ ion at auence value of 1 × 1013 ions cm−2 are reported.

. Experimental techniques

Single crystals of composition SrGaxInyFe12−(x+y)O19 (x = 0, 5, 7, 9;= 0, 0.8, 1.3, 1.0) were grown by flux growth technique on slow cool-

ng (5–7.8 ◦C h−1) of the supersaturated high temperature solution (soaked at350 ◦C for 24 h) in platinum crucibles using 70% molar concentration of ferriteomposition (SrCO3, Ga2O3, In2O3, Fe2O3) and 30% flux (Bi2O3 or Na2CO3)5].

Gallium–indium substituted single crystals of Sr-hexaferrite were irradiatedt room temperature with 50 MeV Li3+ ion beam delivered by 15 UD Pelletronccelerator at a fluence of 1 × 1013 ions cm−2. The electronic energy loss, theuclear energy loss and range of 50 MeV Li3+ ions were calculated using TRIMtransport of ions in materials) calculations and the thickness of the samplesere maintained higher than the mean projected range calculated with TRIM

alculations. Moreover, the mean electronic stopping power (Se) was three ordersf magnitude higher than the mean nuclear stopping power (Sn) (see Table 2).

The selected smooth (0 0 0 1) cleavage planes of appropriate thickness weresed for the present study. The irregular shaped samples were thinned to suit-ble thickness by mechanical polishing, using abrasive powder of suitable mesh.he electrodes were made on both sides of the single crystals by using silveraint, which then acts as a capacitor with the material as dielectric of suitable

imensions. The sample was mounted in a specially designed two terminal sam-le holder and the dielectric measurements were carried out in the frequencyange 1–104 kHz from room temperature to 500 ◦C by using (LF-4192A Model)mpedance analyzer manufactured by Hewlett–Packard (USA) and further auto-

ated by using a computer software HP VEE 4.0 for data recording, storage and

projected range (Rp) in pure and Ga–In substituted strontium hexaferrite at a

) Sn (eV A−1) Rp (�m) Thickness (�m)

0.0102 159.80 2150.0108 151.83 3550.0110 148.41 2600.0109 150.37 335

istry and Physics 103 (2007) 255–263 257

ad

ε

ww

σ

wl

t

ε

3

3c

ca1ii

Fat

B. Kaur et al. / Materials Chem

nalysis [43]. This instrument directly gives the values of capacitance (C) andielectric loss (tan δ). Dielectric constant (ε/) is calculated by using the relation:

/ = Ct

ε0A(1)

here C is the capacitance in F, t the thickness in m, ε0 the absolute permittivityhose value is 8.854 × 10−12 (F m−1) and A is the area in m2.

The ac conductivity is calculated by using the following relation:

ac = 2πνε0ε/ tan δ (2)

here ν is the frequency of the applied electric field and tan δ is the dielectricoss measured directly from the instrument.

The imaginary value of dielectric constant (ε//) is calculated using the rela-ion:

// = ε/ tan δ (3)

. Results and discussion

.1. Frequency and temperature dependence of dielectriconstant (ε/)

Figs. 1 and 2 shows the frequency dependence of dielectriconstant (ε/) at various temperatures for pristine and irradi-

ted substituted Sr-hexaferrite crystals in the frequency range–104 kHz. A strong dispersion occurs in both the samples show-ng a clear decrease of the low frequency dielectric constant. Its clearly evident from these figures that the dielectric constant

ig. 1. Variation of real dielectric constant with frequency for unirradiated (UIR)nd irradiated (IR) SrGa5In0.8Fe6.2O19 from room temperature (RT) to higheremperatures. Inset shows for higher temperatures.

Ftt

i1boaGptpIsAε

tε

TLr

C

SSS

ig. 2. Frequency dependence of real dielectric constant at different tempera-ures for unirradiated and irradiated SrGa7In1.3Fe3.7O19. Inset shows for higheremperatures.

nitially decreases rapidly with increase in the frequency up to00 kHz but thereafter remains fairly constant for all the samplesefore and after irradiation. The low frequency (1 kHz) valuesf dielectric constant as already reported elsewhere for pristinend irradiated unsubstituted SrFe12O19 [42] and Ga5In0.8 anda7In1.3 substituted strontium hexaferrite crystals at room tem-erature under discussion here are given in Table 3. It is clear thathere is a decrease in the dielectric constant on substitution ofristine strontium hexaferrite by Ga and In. It is because, Ga andn substitutes in the lattice preferably by replacing the octahedralites originally occupied by Fe as reported for other ferrites [12].

fter irradiation there is increase in the low frequency value of

/ for both unsubstituted SrFe12O19 [42] and Ga5In0.8 substi-uted hexaferrites whereas there is a decrease in the value of/ for Ga7In1.3 substituted strontium hexaferrite crystals. This

able 3ow frequency (1 kHz) values of ε/ for unsubstituted and substituted crystals at

oom temperature (30 ◦C)

omposition Values of dielectric constants

Before irradiation (UIR) After irradiation (IR)

rFe12O19a 11482.94 12564.90

rGa5In0.8Fe6.2O19 436.31 1397rGa7In1.3Fe3.7O19 91.70 73.90

a As given in the paper published elsewhere [42].

2 try and Physics 103 (2007) 255–263

dle

(

itwspasfa4losratseoinpdpstdtastFbbiTaF

Fi

dpdfifbtributions of grains and grain boundaries [18]. This behaviourwas explained by Maxwell–Wagner interfacial type polarization,which is in agreement with Koop’s phenomenological theory[45,57,58]. On the contrary, almost flat spectra are observed for

58 B. Kaur et al. / Materials Chemis

ecrease in the value of dielectric constant could be due to fol-owing reasons [39,40,44–49] as has also been argued by Dograt al. [40] for other ferrite films:

(a) On irradiation, SrGa7In1.3Fe3.7O19 crystals show decreasein the value of dielectric constant. It is well known that themaximum polarization occurs only if maximum number offerrous ions are involved in the exchange phenomenon. Theobserved decrease in the value of dielectric constant in acomposition where Fe content is least, may be attributedto decrease in the number of ferrous sites to a much lowervalue that may be available for polarization.

b) On irradiation, the flow of space charge carriers is obstructedby the defects generated by the irradiation and thus thebuild up of space charge polarization is impeded. This pos-sibility may be related to space charge polarization at themetal/insulator interface as reported by Costantini et al. [19].

The authors are inclined to suggest that both the possibil-ties may be applicable in the present case. The observationhat the decrease in ε/ occurs only in particular compositionshere Fe replacement is more (as it happens in case of Ga7In1.3

ubstituted Sr-hexaferrite) and is not applicable to all the com-ositions of Ga and In substituted Sr-hexaferrite, supports ourrgument. Similar behaviour is observed in some ferrites andame explanation has been held applicable [50–52]. Li is knownor its affinity towards iron at octahedral sites [53]. Irradiationlso affects more effectively Fe3+ site 4f1 (tetrahedral) besidesf2 (octahedral site) than the 12k sites [37]. It is, therefore,ogical to suggest that irradiation may lead to the reductionf Fe3+ ions, both at the octahedral as well as at tetrahedralites, thus reducing the dielectric constant (ε/). Iwauchi [16]eported a strong correlation between conduction mechanismnd dielectric behaviour of ferrites starting with the conjec-ure that the mechanism of polarization process in ferrites isimilar to that of conduction process [54]. They observed thelectronic exchange between Fe2+ ↔ Fe3+. It is well known thatctahedral sites of a hexagonal ferrite play a dominant rolen the phenomenon of electrical conductivity. Thus, it is theumber of ferrous ion concentration on octahedral sites thatlay a dominant role in the process of conduction as well asielectric polarization. After irradiation, the amorphization thusroduce amorphous tracks in the magnetic insulator leading touppression of atomic and ferrimagnetic long range order withhe result that the site distribution in the short range order getsifferent from that of the crystal as is reported in the litera-ure [19,55]. Further, the literature [19,55] also suggests thats a result of amorphization due to irradiation, the tetrahedralites occupied by Fe3+ get preferentially eliminated with respecto those at the octahedral sites and there is appearance of newe3+ sites in the amorphous phases with a coordination num-er of five. The nature of the induced damage is also examinedy the assumption of a transformation of Fe3+ ions into Fe2+

ons in tetrahedral site after low energy ion bombardment [56].hat the amorphization occurs on irradiation of Strontium hex-ferrite is supplemented by X-ray diffraction patterns shown inig. 3.

Fo

ig. 3. Plot showing single crystal X-ray diffraction patterns before and afterrradiation of Sr-hexaferrite.

A strong dispersion occurs in all the cases with a clearecrease of the low frequency dielectric constant at each tem-erature. Koops [45] gave the phenomenological theory for theielectric dispersion in ferrites at lower frequencies and was therst to study frequency dependence of dielectric behaviour oferrites. Such type of low frequency dispersion was interpretedy a two layer Maxwell–Wagner model to simulate the con-

ig. 4. Temperature dependence of real dielectric constant at various frequenciesf the applied ac field for unirradiated and irradiated SrGa5In0.8Fe6.2O19.

istry

ttasoa

htfTfbatoemttttito

Fc

ststohitbat

3l

oTsdielectric loss at different frequencies is shown as an inset toFigs. 6 and 7. In case of unsubstituted [42] and Ga5In0.8 substi-tuted Sr-hexaferrite, the variation of tan δ with frequency shows

B. Kaur et al. / Materials Chem

he undoped crystals, i.e., dielectric constant is 22 for single crys-als of M-type barium hexaferrite [59]. The authors, therefore,re inclined to suggest that the low frequency dielectric disper-ion in the single crystals under study comes from the relaxationf the space charge polarization at the metal/insulator interfaces reported in case of YIG and M-type BaFe12O19 [19].

The temperature variation of ε/ on Ga–In substituted Sr-exaferrite is shown in Figs. 4 and 5. From the plots, it is clearhat the dielectric constant (ε/) increases with temperature at lowrequencies but remains almost constant at higher frequencies.he temperature dependence of ε/ and tan δ at some selected

requencies have been reported for cobalt and zinc substitutedarium hexaferrite ceramics [60]. They also observed that both ε/

nd tan δ increase on increasing temperature. According to themhe electrical conductivity for semiconductor ferrites increasesn increasing the temperature due to increase in the number oflectric charge carriers and their drift mobilities which are ther-ally activated. Since dielectric polarization in ferrites is similar

o that for electric conduction [54,61], the dielectric polariza-ion (ε/ and tan δ) increases on increasing the temperature. Inhe case of present studies on single crystals, increase of ε/ andan δ (as described in the following section) occurs with increase

n temperature for frequencies <100 kHz. Increase of ε/ withemperature has been reported for other ceramic compositionsf ferrites [12,16]. According to Koops [45], the dielectric con-

ig. 5. Variation of real dielectric constant with temperature at various frequen-ies for unirradiated and irradiated SrGa7In1.3Fe3.7O19.

c

Ftta

and Physics 103 (2007) 255–263 259

tant being inversely proportional to the square root of resistivity,he increase in ε/ with temperature is expected. Low frequencyhifts of dielectric constant spectra with decrease in tempera-ure as reported in other ferrite compositions [19] has also beenbserved in the single crystals of Ga–In substituted strontiumexaferrite as shown in Fig. 1. As Ga–In (Ga9In1) substitutionn strontium hexaferrite increases, i.e., containing least Fe ion,he values of capacitance were very low at low frequencies andecome negative with increase in frequency for both unirradi-ted and irradiated crystals, thus making it difficult to calculatehe value of dielectric constant

.2. Frequency and temperature dependence of dielectricoss (tan δ)

The loss tangent (tan δ) is defined as the ratio of the lossr resistive current to the charging current in the sample.he frequency variation of tan δ for Ga5In0.8 and Ga7In1.3 ishown in Figs. 6 and 7 whereas the temperature dependence of

usp like behaviour, the value of which increases with tempera-

ig. 6. Plot showing frequency dependence of dielectric loss at differentemperatures for unirradiated and irradiated SrGa5In0.8Fe6.2O19. Inset showsemperature dependence of dielectric loss at different frequencies for unirradi-ted and irradiated SrGa5In0.8Fe6.2O19.

260 B. Kaur et al. / Materials Chemistry an

Fig. 7. Frequency dependence of dielectric loss at different temperatures forudS

tatfaqttfsd[piSoo(ltl

i

Eii

3c

Fw[tiipiafnblthe frequency increases, the mean displacement of the chargecarriers is reduced and the conductivity after reaching a certainfrequency, νc, follows the law:

σac(ω) ∼ (ω)s with 0 ≤ s ≤ 1

nirradiated and irradiated SrGa7In1.3Fe3.7O19. Inset shows temperature depen-ence of dielectric loss at different frequencies for unirradiated and irradiatedrGa7In1.3Fe3.7O19.

ure. As already said above, according to Iwauchi [16], there isstrong correlation between conduction mechanism and dielec-

ric behaviour of ferrites. The conduction mechanism in n-typeerrites is due to hopping of electrons between Fe2+ and Fe3+

nd when the hopping frequency is nearly equal to that of the fre-uency of externally applied electric field, a maximum of lossangent may be observed. This type of frequency variation ofan δ (Figs. 6 and 7) has also been reported for other ceramicerrites [45]. It appears that this explanation holds true in theubstituted hexaferrite crystals under study here. The maximumispersion is observed in case of unsubstituted Sr-hexaferrite42] in comparison to Ga5In0.8 substituted Sr-hexaferrite. Theeak shifts toward lower frequencies as the substitution of Fencreases and there is no peak in case of Ga7In1.3 substitutedr-hexaferrite indicating that there is less number of Fe ions onctahedral sites to take part in the process of conduction. More-ver such peaks might appear either at very low frequencies<1 kHz) or at very high frequencies (>10 MHz) which is theimit of the instrument used in this experiment. With the rise in

emperature, tan δ goes on increasing and the peak shifts towardsower frequencies as the temperature decreases.

Knowing dielectric loss, one can calculate the values of imag-nary dielectric constant using formula ε// = ε/ tan δ (as given in

Fu

d Physics 103 (2007) 255–263

q. (3)). The behaviour of imaginary dielectric constant is sim-lar to the real dielectric constant both for pristine as well asrradiated samples.

.3. Frequency and temperature dependence of aconductivity

The frequency dependence of ac conductivity is shown inigs. 8 and 9. Frequency dependence of σac has been explainedith the help of Maxwell–Wagner (MW) two-layer model

18,62]. According to this model the hopping frequency of elec-ron between Fe2+ and Fe3+ ions at the metal/insulator interfaces less at lower frequencies. As the frequency of the applied fields increased, the conduction mechanism becomes more active byromoting the hopping of charge carriers between Fe2+ and Fe3+

ons, thereby increasing the hopping frequency. They observedgradual increase in conductivity with frequency but at higher

requencies, the frequency of hopping between the ions couldot follow the applied field frequency but lagged behind it. It haseen reported that frequency dependence of ac conductivity isarge in ferrimagnetics or in lower temperature region [11]. As

ig. 8. Variation of ac conductivity with frequency at various temperatures fornirradiated and irradiated SrGa5In0.8Fe6.2O19.

B. Kaur et al. / Materials Chemistry and Physics 103 (2007) 255–263 261

Fu

ch

sipiGitbsta1alw3t0tsqvp

Table 4Values of s calculated from the νs dependence of ac conductivity

Temperature (◦C) Pristine Irradiated

(a) SrGa5In0.8Fe6.2O19

28 0.091 0.094100 0.135 0.098200 0.486 0.512300 0.301 0.335

(b) SrGa7In1.3Fe3.7O19

250 0.086 0.10433

hbfrig

l[55]. The hopping conduction between localized gap states [70]originating from the five coordinated iron sites might offer anexplanation of the dependence of νs on σac for ν > νm of thetype exhibited by SrGa5In0.8Fe6.2O19 as shown in Fig. 8. The

ig. 9. Frequency dependence of ac conductivity at various temperatures fornirradiated and irradiated SrGa7In1.3Fe3.7O19.

haracterizing hopping conduction. The critical frequency νcas been found to be dependent on temperature.

In case of pure strontium hexaferrite [42], σac increases verylowly up to 10 kHz and after 10 kHz, it increases rapidly withncrease in frequency at each temperature, thus confirming theolaron hopping [63–65] due to presence of large number of Fe3+

ons at the octahedral as well as at tetrahedral sites. In case ofa5In0.8 substituted Sr-hexaferrite (Fig. 8), this rate of increase

n ac conductivity becomes less in comparison to the unsubsti-uted hexaferrite crystals [42]. This confirms the lesser hoppingetween Fe3+ ions due to their replacement from the octahedralites by Ga/In in this particular case. In this case, the σac at roomemperature increases very slowly and obeys νs dependence forc conductivity with very small value of s (see Table 4). At00 ◦C the behaviour is similar to the room temperature but,t higher temperatures, i.e., 200 ◦C, the νs dependence of σacies above a critical frequency (νc), i.e., 2 kHz and up to 30 kHzith 0.48 ≤ s ≤ 0.51 for unirradiated and irradiated crystals. At00 ◦C the σac remains almost constant up to 10 kHz and abovehis frequency, it obeys νs dependence of σac up to 600 kHz with.30 ≤ s ≤ 0.33 for unirradiated and irradiated crystals. Afterhat it has the tendency to attain almost saturation. In Ga7In1.3

ubstituted Sr-hexaferrite (Fig. 9), the increase in σac with fre-uency is very slow and it obeys νs dependence of σac withery small values of s (see Table 4). This behaviour is similar inristine as well as irradiated samples.

Fa

00 0.083 0.0750 0.065 0.053

The dielectric loss peak in case of Ga5In0.8 substituted Sr-exaferrite could correspond to a polaron hopping mechanismetween Fe2+ and Fe3+ sites. This loss mechanism, at higherrequencies as seen in our data (Fig. 8) agrees with the reportedesults [19,66]. However, there is an extra contribution to thencrease of σac with frequency, like in a disordered or inhomo-eneous material as reported in the literature [19,62,67–69].

In case of amorphized YIG and amorphized Ba-hexaferrites,ocalized gap states originate as a consequence of irradiation

ig. 10. Graph of ln σac vs. T−1 corresponding to different frequencies of thepplied ac field for unirradiated and irradiated SrGa5In0.8Fe6.2O19.

262 B. Kaur et al. / Materials Chemistry an

Ft

nb[

fstddcSfts

TV

F

(

1

(

tTT

4

it

siqitoiilGrisbliifittrhcitrdpI

ig. 11. Plot showing ln σac vs. T−1 corresponding to different frequencies ofhe applied ac field for unirradiated and irradiated SrGa7In1.3Fe3.7O19.

on-Debye like behaviour as seen in Fig. 8 is reported to haveeen shown by several disordered and inhomogeneous systems62].

The measured ac conductivity at different frequencies as aunction of temperature for Ga–In substituted Sr-hexaferrite ishown in Figs. 10 and 11. For Ga5In0.8 substituted Sr-hexaferrite,he measured ac conductivity is almost temperature indepen-ent in the lower temperature region (T ≤ 100 ◦C) and stronglyependent at high temperatures (T > 100 ◦C) for higher frequen-ies as shown in Fig. 10. Whereas for Ga7In1.3 substituted

r-hexaferrite, the ac conductivity increases with increase inrequency (ν ≤ 10 kHz) but at higher frequencies and in lowemperature region (T < 350 ◦C), it almost attains saturation ashown in Fig. 11. The activation energies are calculated from

able 5alues of activation energies (eV) calculated from the plot of ln σac vs. T−1

requency (kHz) Pristine Irradiated

a) SrGa5In0.8Fe6.2O19

1 0.375 0.53410 0.470 0.461

100 0.472 0.467000 0.451 0.417

b) SrGa7In1.3Fe3.7O19 at 10 kHzPristine 0.737Irradiated 0.813

att

A

NUfwufuiNp

d Physics 103 (2007) 255–263

he slope of graph (Fig. 10 and 11) between ln σac versus−1. The calculated values of activation energies are given inable 5.

. Conclusions

From the results obtained on unirradiated and 50 MeV Li3+

on irradiated Ga–In substituted strontium hexaferrite crystals,he following conclusions can be drawn.

The crystals of SrGaxInyFe12−(x+y)O19 (x = 5, 7; y = 0.8, 1.3)how a decreasing trend in the dielectric constant with increasen frequency (≤100 kHz) which, however, saturates at higher fre-uencies irrespective of whether samples are irradiated or not. Its a normal behaviour of ferrites. The irradiated Ga7In1.3 substi-uted strontium hexaferrite crystals show a decrease in the valuef dielectric constant as compared to the unirradiated ones. Its suggested that irradiation may lead to the reduction of Fe3+

ons, both at the octahedral as well as at tetrahedral sites. Theow frequency dispersion occurring in all the compositions ofa–In substituted hexaferrite crystals is interpreted to be as a

esult of relaxation of the space-charge polarization at the block-ng metal/insulator contacts with the help of two-layer model asuggested by Koop’s phenomenological theory. The cusp likeehaviour observed in the frequency dependence of dielectricoss is explained in terms of Maxwell–Wagner interfacial polar-zation and hopping of charge carriers between Fe2+ and Fe3+

ons. The ac conductivity of unsubstituted and substituted hexa-errite crystals is dependent on frequency and temperature whilen case of the former, σac increases very slowly up to 10 kHz andhereafter increases rapidly with increase in frequency at eachemperature, which may be due to the polaron hopping as aesult of presence of large number of ferrous ions at the octa-edral and tetrahedral sites. For Ga5In0.8 substituted hexaferriterystals, the rate of increase in ac conductivity becomes lessn comparison with the unsubstituted one which is attributedo lesser hopping between ferrous ions because of reduced fer-ous ions at the octahedral sites obeying sub linear-power law (νs

ependence with 0 ≤ s ≤ 1) and also to two-site polaron hoppingrocess of charge transfer between ferrous (Fe2+ and Fe3+) ions.rradiation with swift heavy ions of substituted hexaferrites doesffect the values of ε/, tan δ and σac appreciably but the type ofheir dependence on frequency and temperature remains almosthe same.

cknowledgements

The authors are thankful to Nuclear Science Centre (NSC),ew Delhi for providing irradiation facility and funds underFUP Scheme no. 30312. One of the authors BK is thank-

ul to the NSC for awarding project fellowship. Part of thisork has received funding from DST, New Delhi for the projectnder SERC (Science) Scheme. The work reported in this paperorms a part of the PhD programme of one of the authors (BK)

nder the supervision of Dr. K.K. Bamzai. The correspond-ng author (The Emeritus Scientist) is thankful to the CSIR,ew Delhi for giving permission to guide the DST researchroject.

istry

R

[

[[

[[

[[[

[

[[

[

[[

[

[[

[

[

[

[

[

[[

[

[

[

[

[

[[

[

[

[

[

[[

[[

[

[

[

[[[

[

[[

[[[[[

[

[

[[

B. Kaur et al. / Materials Chem

eferences

[1] D. Speliotis, IEEE Trans. Magn. MAG 23 (1987) 25.[2] G. Albanese, M. Carbucicchio, A. Deriu, Phys. Stat. Sol. (a) 23 (1974) 351.[3] V. Adelskold, Ark. Kemi., Mineralog. Geol., Ser. A 12 (29) (1938) 1.[4] F. Licci, S. Rinaldi, T. Besagni, Mater. Lett. 1 (5) (1983) 163.[5] S. Rinaldi, F. Licci, IEEE Trans. Magn. MAG 20 (5) (1984) 1267.[6] B. Kaur, M. Bhat, F. Licci, R. Kumar, P.N. Kotru, K.K. Bamzai, Nucl.

Instrum. Meth. B 222 (2004) 175.[7] U. Raina, S. Bhat, P.N. Kotru, F. Licci, Mater. Chem. Phys. 39 (1994) 110.[8] U. Raina, S. Bhat, P.N. Kotru, P. Franzosi, F. Licci, Cryst. Res. Technol. 31

(6) (1996) 783.[9] U. Raina, S. Bhat, P.N. Kotru, F. Licci, J. Mater. Sci. 31 (1996) 3035.10] S.A. Patil, Studies on physical properties of CuxFe3−xO4 ferrites, PhD

thesis, Shivaji University, Kolhapur, 1981.11] M.A. El Hitti, J. Magn. Magn. Mater. 164 (1996) 187.12] A.M. Shaikh, S.S. Bellad, B.K. Chougle, J. Magn. Magn. Mater. 195 (1999)

384.13] A.M. Abdun, J. Magn. Magn. Mater. 192 (1999) 121.14] A.M.A. El Ata, M.A. Ahmed, M.A. El Hiti, M.K. El Nimr, J. Mater. Sci.

Lett. 18 (1999) 563.15] D. Ravinder, K. Latha, Mater. Lett. 41 (1999) 247.16] K. Iwauchi, Jpn. J. Appl. Phys. 10 (1971) 1520.17] E.J.W. Verweij, P.W. haaijman, F.C. Romeijn, G.W. Van Oosterhout, Philips

Res. Rep. 5 (1950) 173.18] R. Metselaar, P.K. Larsen, Physics of Magnetic Garnets, Enrico Fermi Inter-

national School 1977 (Societa Italiana di Fisica, Bologna 1978), p. 417;P.K. Larsen, R. Metselaar, Phys. Rev. 8 (1973) 2016.

19] J.M. Costantini, J.P. Salvetat, F. Brisard, J. Appl. Phys. 82 (10) (1997) 5063.20] Dr.-Ing. Carl Heck, Magnetic Materials and Their Applications, Butter-

worth, London, 1974, p. 224.21] M. Toulemonde, S. Bouffard, F. Studer, Nucl. Instrum. Meth. Phys. Res. B

91 (1994) 168.22] J.M. Costantini, F. Studer, J.C. Peuzin, J. Appl. Phys. 90 (2001) 126.23] J.M. Costantini, F. Brisard, J.L. Flament, D. Bourgault, L. Sinopoli, J.L.

Uzureau, Nucl. Instrum. Meth. B 59/60 (1991) 600.24] S. Meillon, F. Studer, M. Hervieu, H. Pascard, Nucl. Instrum. Meth. B 107

(1996) 363.25] F. Studer, M. Toulemonde, Nucl. Instrum. Meth. B 65 (1992) 560.26] S. Furono, N. Sasajima, K. Hojou, K. Izui, H. Ostu, T. Muromura, T. Matsui,

Nucl. Instrum. Meth. B 127/128 (1997) 181.27] B. Angadi, V.M. Jali, M.T. Lagare, N.S. Kini, A.M. Umarji, R. Kumar, S.K.

Arora, D. Kanjilal, Nucl. Instrum. Meth. B 187 (2002) 87.28] A. Deepthy, K.S.R.K. Rao, H.L. Bhat, R. Kumar, K. Asokan, J. Appl. Phys.

89 (2001) 6560.29] S.B. Ogale, K. Ghosh, J.Y. Gu, R. Shreekal, S.R. Shinde, M. Downes,

M. Rajeshwari, R.P. Sharma, R.L. Greene, T. Venkatesan, R. Ramesh, R.Bathe, S.I. Patil, R. Kumar, S.K. Arora, G.K. Mehta, J. Appl. Phys. 84(1998) 6255.

30] R. Kumar, S.K. Arora, D. Kanjilal, G.K. Mehta, R. Bathe, S.K. Date, S.R.Shinde, L.V. Saraf, S.B. Ogale, S.I. Patil, Rad. Eff. Def. Solids 147 (1999)187.

31] R. Bathe, S.K. Date, S.R. Shinde, L.V. Saraf, S.B. Ogale, S.I. Patil, R.

Kumar, S.K. Arora, G.K. Mehta, J. Appl. Phys. 83 (1998) 7174.

32] C.A. Volkert, J. Appl. Phys. 70 (1991) 3521.33] J.P. Singh, R. Singh, D. Kanjilal, N.C. Mehra, V. Ganesan, J. Appl. Phys.

87 (2000) 2742.34] J.F. Ziegler, J. Manoyan, Nucl. Instrum. Meth. B 35 (1988) 215.

[

[[[

and Physics 103 (2007) 255–263 263

35] J. Cazaux, in: R.A. Baragiola (Ed.), Ionization of Solids by Heavy Particles,Plenum, New York, 1993, p. 325.

36] J.M. Costantini, J.P. Salvetat, F. Brisard, M.P. Cals, F. Issac, J.P. Marque,Nucl. Instrum. Meth. Phys. Res. B 116 (1996) 496.

37] Ch. Houpert, N. Nguyen, F. Studer, D. Groult, M. Toulemonde, Nucl.Instrum. Meth. B 34 (1988) 228.

38] S.R. Shinde, A. Bhagwat, S.I. Patil, S.B. Ogale, G.K. Mehta, S.K. Date, G.Marest, J. Magn. Magn. Mater. 186 (1998) 342.

39] M. Singh, A. Dogra, R. Kumar, Nucl. Instrum. Meth. B 196 (2002) 315.40] A. Dogra, M. Singh, V.V. Siva Kumar, N. Kumar, R. Kumar, Nucl. Instrum.

Meth. B 212 (2003) 184.41] R. Bittner, K. Humer, H.W. Weber, M. Tyunina, L. Cakare, A. Stern-

berg, Proceedings of the 18th International Atomic Energy Agency FusionEnergy Conference, Sorento, Italy, October 4–10, 2000.

42] B. Kaur, M. Bhat, R. Tickoo, R. Kumar, F. Licci, K.K. Bamzai, P.N. Kotru,Ferroelectrics 323 (2005) 49.

43] R. Tickoo, K.K. Bamzai, R.P. Tandon, P.N. Kotru, J. Instrum. Soc. India33 (2) (2003) 117.

44] A. Dogra, M. Singh, R. Kumar, Nucl. Instrum. Meth. B 207 (2003)296.

45] C.G. Koops, Phys. Rev. 83 (1951) 121.46] J.R. Hook, H.E. Hall, Solid State Physics, 2nd ed., John Wiley, England,

1991.47] A.K. Jonscher, Nature 267 (1977) 673.48] A. Verma, T.C. Goel, R.G. Mendiratta, M.I. Alam, Mater. Sci. Eng. B 60

(1999) 156.49] A.K. Singh, T.C. Goel, R.G. Mendiratta, O.P. Thakur, C. Prakash, J. Appl.

Phys. 91 (10) (2002) 6626.50] C.B. Kolekar, P.N. Kamble, S.G. Kulkarni, A.S. Vaingankar, J. Mater. Sci.

30 (1995) 5784.51] A.V. Ramana Reddy, G. Ranga Mohan, D. Ravinder, B.S. Boyanov, J.

Mater. Sci. 34 (1999) 3169.52] D. Ravinder, K. Vijay Kumar, Bull. Mater. Sci. 24 (5) (2001) 505.53] M.A. Madare, S.V. Salvi, Indian J. Phys. 75A (4) (2001) 363.54] L.I. Rabinkin, Z.I. Novikova, Ferrites, Izv. Acad. Nauk, USSR, Minsk,

1960, p. 146.55] F. Studer, Ch. Houpert, M. Toulemonde, E. Dartyge, Solid State chem. 91

(1991) 238.56] H. Pascard, Phys. Rev. B 33 (1986) 7252.57] J.C. Maxwell, A Treatise on Electricity and Magnetism, vol. 2, Oxford

University Press, New York, 1954.58] K.W. Wagner, Am. Phys. 40 (1913) 817.59] K. Iwauchi, Y. Ikeda, Phys. Stat. Sol. A 93 (1986) 309.60] A.M. Abo El Ata, M.A. El Hiti, J. Phys. III 7 (1997) 883.61] N. Rezlescu, E. Rezlescu, Phys. Stat. Sol. (a) 23 (1974) 575.62] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsa Dielectrics, London,

1983.63] D. Alder, in: F. Seitz, D. Tunbull, H. Ehrenreich (Eds.), Solid State Physics,

vol. 21, Academic Press, NY, 1968, p. 1.64] J. Appel, in: F. Seitz, D. Tunbull, H. Ehrenreich (Eds.), Solid State Physics,

vol. 21, Academic Press, NY, 1968, p. 193.65] I.G. Austin, N.F. Mott, Adv. Phys. 18 (1969) 41.66] R.E. Fontana, D.J. Epstein, Mater. Res. Bull. 6 (1971) 959.

67] N.F. Mott, E.A. Davis, Electronic Processes in Noncrystalline Materials,

Clanrendon, Oxford, 1979.68] S. Summerfield, Philos. Mag. B 52 (1985) 9.69] D. Dyre, J. Appl. Phys. 64 (1988) 2456.70] D. Emin, Phys. Rev. Lett. 34 (1974) 471.