Journal of Magnetism and Magnetic Materials 345 (2013) 255–260

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Journal of Magnetism and Magnetic Materials

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Infrared spectroscopic and electron paramagnetic resonance studies onDy substituted magnesium ferrite

K.K. Bamzai a,n, Gurbinder Kour a, Balwinder Kaur a, Manju Arora b, R.P. Pant b

a Crystal Growth & Materials Research Laboratory, Department of Physics and Electronics, University of Jammu, Jammu, Indiab National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi, India

a r t i c l e i n f o

Article history:Received 12 March 2013Received in revised form21 June 2013Available online 6 July 2013

Keywords:Spinel ferriteFourier transform infraredElectron paramagnetic resonanceG-factor

53/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.jmmm.2013.07.002

esponding author. Tel.: +91 191 2450 939.ail address: kkbamz@yahoo.com (K.K. Bamzai

a b s t r a c t

Dysprosium substituted magnesium ferrite with composition MgDyxFe2�xO4 with 0.00≤x≤0.07 synthe-sized by the solid state reaction technique was subjected to Fourier transform infrared spectroscopy andelectron paramagnetic resonance studies. Infrared spectrum analysis were carried out to confirm thespinel phase formation and to ascertain the cation distribution in the ferrite phase. The absorptionspectra show two significant absorption bands between 400 and 1000 cm�1 which are attributed totetrahedral (A) and octahedral (B) sites of the spinel phase. The positions of bands were found to becomposition dependent. Splitting of bands as well as appearance of shoulders shows the presence of Fe2+

ions in the system. The force constants for tetrahedral and octahedral sites were calculated and found tovary with Dy3+ ions content. Electron paramagnetic resonance spectra of these samples exhibit broad,asymmetric resonance signal due to Fe3+/Dy3+ ions present in the host lattice. The spectra becomebroader with Dy3+ ions substitution in pure Mg-ferrite and this broadening is attributed to surface spindisorder (spin frustration) possibly coming from mainly antiferromagnetic interactions between theneighbouring spins in the magnetic grains. The weak superexchange interactions results in thebroadening of the resonance line width and large g-value as compared to the free electron value.

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1. Introduction

Magnetic spinel ferrites have captured the global market andgrabbed the attention of many researchers due to their fascinatingand exotic electromagnetic properties. The stability of thesematerials under various conditions has added a lot of significancefor their use in the technological industry. Considerable interesthave been developed in the study of such ferrite due to largediversity and the practical usefulness of their physical and chemi-cal properties including humidity-sensing, oxygen-sensing, photo-electrical and super paramagnetic as well as high temperatureceramic properties [1–3]. The structural, electrical and magneticproperties of such materials are sensitive to preparation conditionsuch as sintering temperature, time and type of additives. Additionof small amount of larger ions such as rare earth, can inducemodifications in both structural and magnetic properties, becauserare earth ion plays an important role in determining themagneto-crystalline anisotropy in 4f–3d intermetallic compounds[4,5]. Generally, the rare earth ion favours occurrence of secondaryphase, resulting in an increase of the electrical resistivity and bulkdensity. The electronic valencies of the rare earth ion are most

ll rights reserved.

).

important for compound formation and are most stable when theyare trivalent. Gd, Er and Lu are trivalent cations, whereas Ce, Tb isboth trivalent and tetravalent while Sm, Dy is divalent andtrivalent [6]. X-ray diffraction technique and infrared absorptionspectroscopy are the two non-destructive methods of character-ization which provides qualitative information regarding struc-tural details of crystalline materials [7,8]. X-ray providesinformation about the formation of the spinel system whereasinfrared helps in checking the formation and the structure of thespinel system and the presence of Fe2+ ions [9]. The absorption ofIR radiations (quantized) causes various bands in a molecule tostretch and bend with respect to one another. The absorptionbands from which the details regarding the functional groups andtheir linkages can be explored are found to be dependent onatomic mass, cation radius, cation anion bond distances, cationdistribution etc. [8]. It is well known that electron paramagneticresonance (EPR) is a powerful tool in investigation of magneticproperties and spin dynamics in solids. This technique makes useof the spin of the electron and its magnetic moment to revealinformation on the unpaired electron (spin) density distribution.

Analysis of infrared spectral have been carried out for severalferrites by Waldron [10] who reported two absorption bandswithin the wave numbers 800–200 cm�1, which could be respec-tively attributed to the tetrahedral and octahedral group com-plexes of the spinel structure. Many authors have dealt with the

K.K. Bamzai et al. / Journal of Magnetism and Magnetic Materials 345 (2013) 255–260256

investigation of IR absorption spectra of magnesium ferrite [10]substituted by various metal cations such as Co2+ [11], Li+ [12],Zn2+ [13] and Cr3+ [14]. Similarly, few reports on the influence ofrare earth elements like La3+ [5], Nd3+[8], Ce3+ [15], Sm3+ [5,16],and Gd3+ [17] are also available, but the information regarding thework on spectral and magnetic properties of magnesium ferritesubstituted by Dy3+ [18] is missing. On the other hand Pant et al.[17] studied the EPR spectra of Gd3+ substituted CoFe2O4. EPRstudies of nickel ferrite substituted with rare earths like Gd3+ [19]and Ce3+, Gd3+ [20] are also reported in the literature. Althoughthere are many studies on MgFe2O4 very scanty information isavailable on the electron paramagnetic resonance studies of Dy3+

substituted magnesium ferrite [21].The authors have already reported the synthesis, structural and

magnetic characteristics of Dy substituted magnesium ferrite [22].Therefore, the present work is focussed to study the influence ofDy3+ ion substitution instead of Fe3+ ions in Mg-ferrite as no suchreport is available on the effect of Dy3+ ion substitution on thestructure, IR absorption bands and EPR line width. Hence, byintroducing a relatively small amount of Dy2O3 instead of Fe2O3, amodification of both structure and magnetic properties can beobtained.

2. Experimental

The rare earth (Dy) substituted magnesium ferrite with com-position MgDyxFe2�xO4 (where x¼0.00, 0.01, 0.03, 0.05, and 0.07)were prepared by the solid state reaction technique. High puritygrade oxides of MgO, Fe2O3 and Dy2O3 have been used instoichiometric weight proportion. The solid state reaction wascarried out in muffle furnace maintained at 800 1C for 2 h followedby sintering at 1200 1C for 2 h with a heating rate of 4 1C/min asalready reported [22]. The crystalline phase, the structural andmagnetic properties have already been reported by our group [22].The grain size was found to decrease with increase in Dycomposition and has been explained. The X-ray diffraction pat-terns confirm the formation of spinel structure whereas magneticcharacteristic explains the soft ferromagnetic nature of the grownmaterials. Infra-red transmittance spectra were recorded on Per-kin-Elmer's IR spectrometer (Model no. 70768) using KBr disc in4000–400 cm�1 region at ambient temperature. Electron para-magnetic resonance spectra were recorded at ambient tempera-ture on X-band Bruker Biospin Make Model: A300 EPRspectrometer. DPPH (1,1-diphenyl 2-picryl hydrazyl) was used asa standard field marker for g-value and spin concentrationcalculations. The spectra were recorded in 0–8000 G range at23 db microwave power and 100 kHz modulation frequency.

3. Results and discussions

3.1. Spectroscopic characteristics

3.1.1. Fourier transform infrared spectroscopy (FT-IR)The relationship between structure and the electromagnetic

response of ferrites are useful in understanding their propertiesbecause the electrical and magnetic properties of these materialsdepend upon their precise configuration of the atoms or ions inthese structures. Infrared spectroscopy, a non-destructive techni-que, is useful for such investigations. The infrared spectra of puremagnesium ferrite (hereafter referred as MgF) and Dy substitutedmagnesium ferrite (hereafter referred as DyMgF) recorded in thefrequency range of 4000–400 cm�1 at room temperature isdepicted in Fig. 1(a–e). AB2O4 type transition metals are normaland inverse spinel oxides and have four infrared active modes viz.

ν1, ν2, ν3 and ν4 in the wave number range of 650–550 cm�1,525–390 cm�1, 380–335 cm�1 and 300–200 cm�1 respectively[23]. The occurrence of these four bands was based on grouptheoretical calculations employing space groups and point sym-metries both in the normal and inverse spinels [24]. From the Fig. 1(a–e), it is clear that high frequency bands (ν1) lies in the range585–618 cm�1 and the lower frequency bands (ν2) vary from 418to 437 cm�1. The formations of these bands confirm the presenceof ferrite group in the prepared material. In addition to thesevibrational bands, a broad signal due to water symmetric stretch-ing and antisymmetric stretching in the range of 3385–3448 cm�1

were observed except for 7% Dy substituted MgF. Another bandobserved due to bending mode of water molecules has beenobserved in the range of 1636–1654 cm�1 in these spectra. Thebroadness in these bands corresponds to the stretching vibrationsof the hydrogen-bonded OH groups [17]. The other two bandsobserved in AB2O4 type ferrites viz., ν3 which arises due to divalentmetal ion–oxygen complexes on B-sites and ν4 band whichdepends upon the mass of vibrating divalent tetrahedral cationappears generally below 400 cm�1 were not observed in thepresent study [24].

Infrared spectrum of pure and substituted magnesium ferrite asgiven in Fig. 1(a–e) shows absorption bands below 1000 cm�1,which is a common feature of all the ferrites. The bands arisefrom the lattice vibrations of the oxide ions against the cations.In Fig. 1(a), the first frequency band (ν1) is found at 617.4 cm�1 andsecond frequency band (ν2) is found at 418 cm�1. The difference inthe band positions of ν1 and ν2 is because of the difference in theFe3+–O2- distances for the octahedral and tetrahedral complexes.It is found that Fe– O distance of A-site (1.89 Å) is smaller than thatof the B-site (1.99 Å). This can be interpreted by the more covalentbonding of Fe3+ ions at the A-sites than B-sites [24]. Therefore, thetetrahedral stretching mode is observed at higher wave numberthan octahedral due to shorter bond length of Fe–O in tetrahedralsymmetry. These two absorption bands observed within this limitreveal the formation of single phase spinel structure having twosub-lattices, tetrahedral (A) site and octahedral [B] site. So, theabsorption band ν1 is caused by the stretching vibrations of thetetrahedral Fe3+–O2- bond and the absorption band ν2 is caused bythe Fe3+–O2- vibrations at octahedral site [10]. The presence oflong shoulder at ν1 for the tetrahedral (A) site gives an indicationof the presence of other ionic states in that site ( i.e. Mg2+ ion inA-site) [25]. It is reported [26] that due to the partially inversestructure of MgFe2O4, the metal ion distribution in MgFe2O4 isgiven as ( Mg0.1Fe0.9)A [Mg0.9Fe1.1]B. Hence, the proposed cationdistribution formula is justified using the FTIR spectrum.

From the Fig. 1(b–e), it has been observed that there is shift inthe band positions with the increasing concentration of Dy3+ ionin MgF. The vibrational band (ν1) shows constancy with theincrease in Dy3+ ion content due to its small ratio with a slightshift towards the lower frequency in case of Dy 7% with value584.9 cm�1. The shift in this band may be due to the chargeimbalance of A-sites as well as redistribution or inversion ofcations among A-and B-sites. The IR spectra also shows thepresence of shoulder besides the high frequency absorption band(ν1) which confirms the presence of Fe2+ ions at that particularsite. This is attributed to the Jahn–Teller distortions produced byFe2+ ions which cause local deformation in the lattice owing to thenon-cubic component of the crystal field potential and hence leadto the splitting of this band [24]. The shoulder is assigned fordivalent iron ion Fe2+–O2- bond vibration.

The vibration band (ν2) gets broadened, splitted as wellas shifted towards higher frequency with value varying from418–436.1 cm�1 with the addition of Dy3+ ions. It is well knownthat Dy3+ ion when substituted always resides on the octahedralsite. So, the shifting of absorption band ν2 towards higher

Fig. 1. Infrared spectra showing. (a) pure MgFe2O4 (MgF), (b) 1% Dy substituted MgF, (c) 3% Dy substituted MgF, (d) 5% Dy substituted MgF, and (e) 7% Dy substituted MgF.

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frequency which in turn increase the bond length at B-sitesuggests that the rare-earth ions occupy the B-site [5]. Moreover,the broadening of the ν2 band and the presence of another band inthis range (408–424 cm�1) with the addition of Dy3+ ion confirmsthe presence of Dy3+ ion in addition to Fe3+ ion at octahedral siteand these bands appear due to the formation of (Dy3+–O2-)complexes at B-sites [19]. This broadening of the ν2 band has alsobeen observed in rare-earth substituted MgFe2O4, which againsuggests the occupancy of rare-earth ions on the B-site [26]. Allthe Dy substituted samples show the sign of splitting of the ν2band. The splitting of octahedral (B-site) absorption band is due tothe presence of cations of different characters on the same site.Another reason for the splitting of ν2 band is the formation ofsecondary phase (DyFeO3) at B-site which becomes more promi-nent with the increase of Dy substitution. Thus, DyFeO3 phasecauses the splitting which in turn leads to the slight distortion ofthe cubic spinel symmetry of the lattice [27]. Local deformationsdue to Jahn–Teller effect [28] in Fe2+ ions lead to a non-cubic

component of the crystal field potential which can also beresponsible for the splitting of the band. Since such effect isobserved in our present case around ν1 as well as ν2 band,confirming the presence of Fe2+ ions in tetrahedral and octahedralsites [24]. In the light of above considerations, the cationsdistribution for the system MgDyxFe2�xO4 (x¼0.0, 0.01, 0.03,0.05, and 0.07) can be written as follows:

ðMg2þy�xFe3þ1�yþxÞA½Mg2þ1�yFe

3þ1þy�xDy

3þx �B

where ‘y’ is the measure of inversion which is equal to 0.1 forMgFe2O4 [25] and ‘x’ vary from 0.00 to 0.07. Table 1 gives thecompiled data for the wave number and their correspondingvibrations in all the compositions.

The ionic radius for the tetrahedral and octahedral sites iscalculated by using following equations [29]:

RT ¼ ðy�xÞRMg2þ þ ð1�yþ xÞRFe3þs

K.K. Bamzai et al. / Journal of Magnetism and Magnetic Materials 345 (2013) 255–260258

RO ¼ 12

ð1�yÞRMg2þ þ ð1þ y�xÞRFe3þ þ xRDy3þ

h i

where RT and RO are the mean ionic radii per molecule for thetetrahedral and octahedral site respectively; RMg2þ is the ionicradius of the Mg2+ ion; RFe3þ is the ionic radius of the Fe3+ ion;RDy3þ is the ionic radius of the Dy3+ ion.

The value of tetrahedral site radius (RT) and octahedral siteradius (RO) which depends on the assumption of cation distribu-tion is given in Table 2. It is seen that RO increases and RT decreaseswith increasing of the Dy3+ composition. The replacement of Fe3+

ions (0.64 Å) by Dy3+ ions (0.99 Å) decreases the concentration ofFe3+ ions at octahedral sites leading to increase of RO. It is seenfrom the proposed cation distribution formula that Dy3+ ionsentirely occupy the octahedral sites and force Fe3+ ions into thetetrahedral sites with the increase of Dy3+ ions. As the ionic radiiof Mg2+ ion (0.65 Å) is comparable to Fe3+ ion (0.64 Å) andmoreover a very small fraction of Mg2+ ions occupy the tetrahedral(A) sites, the tetrahedral site radius decreases slightly withincrease of the Dy3+ ions. Levine [30] has correlated the decreasein site radius to an increase in covalent character. Therefore, it isconcluded that the covalent character of the ferrites decreases assite radius increases. Thus, site radius exhibits a sensitive depen-dence on iono-covalency nature and composition [31]. This alsosupports the shifting of ν1 towards lower wave number which isattributed to the compression of Fe–O bonds, i.e. shifting of Fetowards oxygen ion whereas, shifting of ν2 to higher wave numberwith Dy3+ ion content signifies the stretching of Fe–O bond.

The force constants (FC) for the tetrahedral site (FCT) andoctahedral site (FCO) are calculated using the following relation[32]

FC¼4π2c2ν2m

where ‘c’ is the light velocity ∼2.99�1010 cm s�1; ‘ν’ is thevibration frequency of the tetrahedral and octahedral sites; ‘m’ isthe reduced mass for the Fe2+ ions and the O2� ions, it is foundequal ∼2.061�10�23 g.

Table 2 gives compiled data for the position of two different IRbands (ν1, ν2), tetrahedral site radii (RT), octahedral site radii (RO),force constant for tetrahedral site (FCT) and force constant for

Table 1Position of vibration bands showing stretching modes in case of MgDyxFe2-xO4

(where x¼0.00, 0.01, 0.03, 0.05, 0.07).

Band position (cm�1) Tentative assignment

MgFe2O4 Dy1% Dy3% Dy5% Dy7%

3448.1 3434.7 3447.9 3384.5 – (O–H) H2O stretching mode1654.2 1630.0 1636.3 1629.8 1636.2 (H–O–H) H2O bending mode617.4 616.8 617.1 616.8 584.9 (Fe–O) FeO4 stretching mode418.0 433.7 436.1 430.4 434.0 (Fe–O) FeO6 stretching mode

– 423.2 412.0 410.3 408.3 (Dy–O) stretching mode

Table 2Position of infrared absorption bands (ν1, ν2), tetrahedral site radii (RT), octahedral site rad(FCO) in case of MgDyxFe2�xO4 system (where x¼0.0, 0.01, 0.03, 0.05, and 0.07).

Content(x)

IR absorptionbands

Tetrahedral site radii,RT (Å)

Octahedral site radii,RO (Å)

Force co(dynes/c

ν1(cm�1)

ν2(cm�1)

0.00 617.4 418 0.641 0.644 2.78850.01 616.8 433.7 0.6409 0.6462 2.78300.03 617.1 436.1 0.6407 0.6497 2.78580.05 616.8 430.4 0.6405 0.6532 2.78300.07 584.9 434 0.6403 0.6576 2.5026

octahedral site (FCO) in case of pure MgF and Dy substituted MgF.Since vibrational frequency is proportional to the force constant,the shifting of ν1 towards lower frequency indicates that forceconstant for tetrahedral site ‘FCT’ decreases very slightly whileshowing constancy and this decrease suggests that bondingbetween the oxygen ions and metal ion at the A-sites reduces.On the contrary, addition of dysprosium at octahedral site leads toincrease of force constant for octahedral site ‘FCO’ resulting in thestrengthening of bond between oxygen ions (O2�) and metal ions(Fe3+, Dy3+) at the octahedral site. It is clear from Table 2 that FCT isgreater than FCO indicating that the band frequency associatedwith tetrahedral sites is higher than for octahedral sites.

Therefore, we can conclude that an IR spectrum gives informa-tion about the changes in the molecular structure of ferrite due tothe perturbation occurring in the (Fe3+–O2�) bond upon introdu-cing Dy3+ ion.

3.1.2. Electron paramagnetic resonance (EPR)Electron paramagnetic resonance (EPR) spectroscopy is a very

sensitive technique for determining the paramagnetic species, roleof magnetic dipolar interaction, anisotropy in solids and alsohomogeneity of the samples. EPR measurements are usuallycarried out by scanning the magnetic induction (B) at constantmicrowave frequency (f) (9.85 GHz) through the range of interest.The position of the EPR line depends on the ratio of B to f and theeffective gyromagnetic factor (g). This technique is not only usefulfor studying the microscopic magnetic behaviour of ferrite systembut also equally informative about the bulk behaviour in ferrites.The shape of the line width tells us about the loss in ferrites atmicrowave frequencies i.e., the broader the line width, the greateris the loss in the material.

Magnetic dipolar interactions and superexchange interactionsbetween the magnetic ions through oxygen ions are the twopredominant factors that determine the g-values and the reso-nance line width (ΔH). Strong superexchange interactions producea small line width and g-value, while strong dipolar interactionsgive a large resonance line width and g-value. The superexchangeinteractions generally increase when the distance between themagnetic ions and oxygen ions decreases and the correspondingbonding angles are close to 1801. EPR spectra of these sampleexhibits broad, asymmetric resonance signal due to Fe3+/Dy3+ ionspresent in the host lattice. Fig. 2 shows the comparison of all thefive compositions i.e., pure MgF and Dy MgF at room temperature.The spectra were analyzed using Lorentzian distribution functionto obtain the values of various parameters such as peak to peakline width (ΔHpp), g-value, spin concentration (Ns) and relaxationtime (τ2). The g-value is given by ΔE¼hν¼gμBHr, where “h” isPlanck's constant, “ν” is the microwave frequency, “μB” is Bohrmagnetron and “Hr” is the magnetic field at which resonanceoccurs. This ‘g’ is a function of the molecular motion, theparamagnetic properties and the symmetry of ions. The lande (g)

ii (RO), force constant for tetrahedral site (FCT) and force constant for octahedral site

nstant for tetrahedral site FCO�105

m)Force constant for octahedral site FCO�105

(dynes/cm)

1.27811.37601.39121.35511.3779

K.K. Bamzai et al. / Journal of Magnetism and Magnetic Materials 345 (2013) 255–260 259

factor equals 2.0023 for an isolated free electron, has differentvalues in the solid environment. It is known that there are tworelaxation processes in EPR system spin–lattice and spin–spin. Ifthe line width of the EPR is determined by spin–lattice relaxation,it will decrease rapidly as the temperature decreases. If involvedspins are alike, the magnetic exchange interaction causes a broad-ening of resonance lines as the temperature is increased and anarrowing of resonance lines is caused if the magnetic exchangetakes place between unlike spins.

The spin lattice relaxation process is characterized by a timeconstant, which is function of static magnetic field and depends onthe rate at which microwave energy can be absorbed and dis-sipated. The relaxation time is correlated with the line width ofEPR. The spin–spin relaxation process is the energy difference (ΔE)transferred to neighbouring electrons and the relaxation time (τ2)can be determined from the peak to peak line width according tothe relation given below [20].

1τ2

¼ πgμBΔHh

The spin–spin relaxation time (which arises from the influenceof one magnetic ion on another) limits the broadening of linewidth [33]. The spin concentration is calculated by the comparisonmethod using DPPH (1,1-diphenyl 2-picryl hydrazyl) as standardreference material. The values of various parameters such as g-value, peak to peak line width (ΔHpp), relaxation time (τ2), spinconcentration (Ns), resonance field (Hr) and signal intensity arecalculated and tabulated in Table 3.

It is clear from Fig. 2 and the Table 3 that the value of peak topeak line width (ΔHpp) slightly decreases in Dy1% composition,increases in Dy3% and Dy5% composition and then decreases again

Table 3Peak to peak line width (ΔHpp), g-value, resonance field (Hr), relaxation time (τ2), spin c0.03, 0.05, and 0.07).

Sample Peak to peak line width ΔHpp (G) g-value Resonance field Hr (G)

Pure 572.6 2.41041 2658.84Dy1% 451.6 2.1613 2854.34Dy3% 822.6 2.1535 2916.911Dy5% 1110.4 2.14099 2869.99Dy7% 844.6 2.16659 2854.34

0 2000 4000 6000 8000-600000

-400000

-200000

0

200000

400000

600000

800000

1000000

1200000

1400000

Dy1%

Dy7%

MgF

Dy5%

Inte

nsity

(a.u

.)

Magnetic Field (Gauss)

Pure MgF Dy1% Dy3% Dy5% Dy7%

Dy3%

Fig. 2. Comparison of signal intensity as a function of magnetic field for puremagnesium ferrite (MgF), 1% Dy substituted MgF, 3% Dy substituted MgF, 5% Dysubstituted MgF and 7% Dy substituted MgF.

for Dy7% composition. So, spectra is becoming broader with Dy3+

ions substitution in pure Mg-ferrite. This broadening in the linewidth can be attributed to surface spin disorder (spin frustration)possibly coming from mainly antiferromagnetic interactionsbetween the neighbouring spins in the magnetic grains. Thisfrustration might be partially connected to dipolar interactionsbetween the magnetic clusters [34]. These variations can also beattributed to the decrease or increase in the crystallinity with Dy3+

ions concentration because as the crystallinity of the sampleimproves, it reduces the distortion of polyhedra in the particles.The EPR data tend to yield ‘g’ values slightly larger than the freeelectron value in case of Dy3+ ions substituted Mg-ferrite. Theg-value is decreasing with Dy3+ ions substitution upto Dy5%composition and thereafter slightly increases in Dy7% substitutedMg-ferrite. The value of resonance field (Hr) increases with Dy3+

ions substitution and is maximum in Dy3% composition and thendecreases.

The spectrum of pure and Dy3+ ions substituted Mg-ferriteconsists of broad resonance peaks with line width ΔHpp rangingfrom ΔHpp¼573 G for pure Mg-ferrite to 1110 G for Dy5% sub-stituted Mg-ferrite and also higher g-values i.e., g¼2.41041 forpure Mg-ferrite to g¼2.14099 for Dy5% substituted Mg-ferrite. So,the higher values of ΔHpp and g-values in the Dy3+ ions substitutedcompositions indicates the substitution of Dy3+ ions replacing Fe3+

ions at octahedral as well as tetrahedral sites and strong magneticdipole interactions among these particles. Dy3+ ions preferentiallyenters the octahedral [B] site replacing Fe3+ ions from these sitesthereby decreasing superexchange interactions between Fe3+–O–Fe3+

ions at octahedral sites and increasing strong dipolar interactionsamong these particles. This can be explained as some of FeO6

octahedrons were distorted and some of them were transformedinto FeO4 tetrahedra. This means that Fe3+–O–Fe3+ and Fe3+–O–Dy3+

pairs have probably long bond lengths and large deviations ofbonding angles from 1801 which produce weak superexchangeinteractions among Fe3+–O–Fe3+ and Fe3+–O–Dy3+ pairs [17]. Theweak superexchange interactions result in the broadening of theresonance line width and large g-value as compared to free electrong-value e.g. 2.0023. Superexchange interactions should decrease dueto the defects introduced by the presence of Dy3+ ions. These studiesreveal the distortion in spinel lattice occur due to bigger sized Dy3+

ions substitution in the lattice which causes frustration in the latticeby enhancing or reducing crystallinity of the samples that leads to thevariation in EPR parameters.

4. Summary

Spectroscopic characterization of pure and dysprosium substi-tuted magnesium ferrite reveals the formation of two dominantFT-IR bands ν1 and ν2 which is attributed to the stretchingvibration of Fe3+–O2- in the tetrahedral and octahedral complexes.The positions of absorption bands are compositional dependentwhose dependence could be attributed to the variation in cationoxygen bond distances. The splitting of octahedral absorption

oncentration (Ns) and signal intensity (S.I.) for MgDyxFe2�xO4 (where x¼0.0, 0.01,

Relaxation time τ2 (s) Spin concentration (Ns) Signal intensity (S.I.)

9.518�10�15 5.83�10+15 6.527�10+9

1.345�10�14 7.28�10+15 8.159�10+9

7.415�10�15 8.0�10+14 8.959�10+8

5.525�10�15 7.53�10+15 8.437�10+9

7.178�10�15 5.02�10+15 5.629�10+9

K.K. Bamzai et al. / Journal of Magnetism and Magnetic Materials 345 (2013) 255–260260

band is due to presence of three kinds of cations at the B-site.Variation in the force constants of tetrahedral and octahedral sitessupport predicted cation distribution where dysprosium showsstrong occupancy to A-site and magnesium and iron ions distri-bute preferentially among A and B-sites. Electron paramagneticresonance spectra exhibits broad, asymmetric resonance signaldue to Fe3+/Dy3+ ions present in the host lattice. EPR spectrabecomes broader with Dy3+ ions substitution in pure Mg-ferrite.This broadening in the line width can be attributed to surface spindisorder (spin frustration) possibly coming from antiferromagneticinteractions between the neighbouring spins in the magneticgrains. This frustration might be partially connected to dipolarinteractions between the magnetic clusters. This can be explainedon the basis that some of FeO6 octahedron got distorted and someof them were transformed into FeO4 tetrahedra upon Dy3+ sub-stitution in the spinel structure. It means that Fe3+–O–Fe3+ andFe3+–O–Dy3+ pairs may have longer bond lengths and largedeviations of bond angles from 1801 which produces weak super-exchange interactions among Fe3+–O–Fe3+ and Fe3+–O–Dy3+ pairs.The weak superexchange interactions results in the broadening ofresonance line width and large g-value as compared to freeelectron g-value e.g. 2.0023.

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