Materials Chemistry and Physics 130 (2011) 513– 518
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Materials Chemistry and Physics
j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys
össbauer, magnetic and electric studies on mixed Rb–Zn ferrites prepared byolution combustion method
anik Gupta, Balwinder S. Randhawa ∗
epartment of Chemistry, UGC Sponsored-Centre for Advance Studies-I, Guru Nanak Dev University, Amritsar 143 001, India
r t i c l e i n f o
rticle history:eceived 14 December 2010eceived in revised form 8 June 2011ccepted 7 July 2011
a b s t r a c t
Single phase nanosized spinel ferrites with general formula Rb0.5−x/2ZnxMn0.05Fe2.45−x/2O4 (x = 0 → 0.5)were prepared by solution combustion route using ethylene glycol as a capping agent. The ferritesobtained have been characterized by powder XRD and Mössbauer spectroscopy. Their magnetic andelectric properties have been studied by employing Vibrating Sample Magnetometer (VSM), Curie
eywords:. Magnetic materials. Chemical synthesis. Mössbauer spectroscopy, D. Magneticnd electrical properties
temperature assembly and LCR meter. Mössbauer spectra display transition from ferrimagnetic to super-paramagnetic phase. The saturation magnetization (MS) initially exhibits an upward trend followed byregular decrease with increasing diamagnetic Zn content. Curie temperature also shows a downwardtrend with increasing Zn content. The resistivity of the doped samples decreases with temperaturesuggesting semiconductor behaviour of the ferrites. The dielectric constant (ε) and tangent loss (tan �)measured at room temperature as a function of frequency show the expected ferrite behaviour.
Magnetic ceramics belonging to mixed alkali metal ferritesnd extensive use in microwave components due to their attrac-ive electrical and magnetic properties [1–11]. The basic magneticnd electrical properties are mainly compositional dependent.herefore the molecular engineering of ferrite composition andmployment of appropriate process parameters play a significantole in tailoring the materials properties for a specific need. Lowermeability, wide range of saturation magnetisation, high resis-ivity and low losses even at microwave frequencies are some of theharacteristic properties of these ferrites. These vital parametersake them more versatile from technological point of view. It is,
herefore, desirable to investigate and understand the dependencef composition on magnetic/electric behaviour of these ferrites.everal investigations have been reported on the magnetic andlectric properties of substituted lithium, sodium and potassiumerrites [12–14]. In continuation to that work, we are reporting theynthesis of mixed Rb–Zn ferrites by solution combustion methodnd the effect of these substituents on structural, magnetic andlectrical properties like lattice constant, saturation magnetiza-
ion, Curie temperature, resistivity, dielectric and tangent loss as
function of their concentration. The results obtained have been
explained and understood on the basis of various models availablein the literature.
2. Experimental
Rubidium ferrites of varying compositions, i.e. Rb0.5−x/2ZnxMn0.05Fe2.45−x/2O4
were prepared by solution combustion method. Stoichiometric quantities of aque-ous solution of rubidium nitrate, zinc nitrate, ferric nitrate, manganese nitrate andethylene glycol were thoroughly mixed in a beaker. After vigorous stirring at 80 ◦Cfor 2 h, the reaction mixture was combusted in a muffle furnace at 600 ◦C for 30 minto obtain the desired product. Ethylene glycol used in this method acts as a fuel forthe combustion synthesis of ferrites. In these compositions x varies from 0 to 0.5 insteps of 0.1.
57Fe Mössbauer spectra were recorded on Wissel (Germany) Mössbauer spec-trometer. A 57Co (Rh) �-ray source was employed and the velocity scale wascalibrated relative to 57Fe in Rh matrix. Mössbauer spectral analysis software Win-Normos for Igor Pro has been used for the quantitative evaluation of the spectra.Isomer shift values are reported with respect to pure metallic iron absorber.
Saturation magnetization values were measured by using Vibrating SampleMagnetometer (Lake Shore’s new 7400 series). The electric properties (dielectricconstant and tangent loss) were measured with Agilent technologies, 8714ET pre-cision LCR meter. Pellets (15 mm diameter and 3 mm thickness) were prepared byusing 2% PVA as the binder and sintered at 1200 ◦C for 2 h. The Curie temperaturefor the Rb–Zn ferrite samples was determined by using a simple experimental setupbased on gravity effect in the laboratory. The ferrite specimen is made to attachitself to a bar magnet through a mild steel rod due to the magnetic attraction andcombination is suspended inside the furnace. A chromel–alumel thermocouple isattached to the sample holder to read the temperature of the specimen. As the tem-
perature of the system is increased, at a particular temperature the specimen losesits spontaneous magnetization and becomes paramagnetic.
For the dielectric measurements, Ag-paste was coated on the polished surfacesof the pellets to provide electrical contacts. Resistivity of the samples was measuredby using two probe Keithley high sensitive resistivity meter.
Fig. 1. (a–f) Mössbauer spectra for different compositions of Rb Znx
14 M. Gupta, B.S. Randhawa / Materials
. Results and discussion
.1. Mössbauer studies
Fig. 1a–f shows Mössbauer spectra for different compositions.oom temperature spectrum for the composition with x = 0 (Fig. 1a)xhibits two well resolved Zeeman sextets arising due to the Fe3+
ons present at both tetrahedral and octahedral sites (A and B sites)ndicated by red and blue lines respectively. The cationic distribu-ion of Fe3+ ion obtained (Table 1) for tetrahedral and octahedralites are nearly same in both sextets which may be attributed tonverse spinel structure of ferrite product. With the addition ofiamagnetic Zn2+, Mössbauer spectra show the presence of centralaramagnetic doublet superimposed on the sextets and its inten-ity goes on increasing with further increase in Zn2+ concentrations shown in Fig. 1b–f, also reflected in Fig. 2. The central doubletay be attributed to the Fe3+ ions which are magnetically iso-
ated and did not participate in long-range magnetic ordering dueo the presence of large number of nonmagnetic nearest neigh-ors (Zn2+ ions) thus there is a transition from ferrimagnetismo superparamagnetism with increasing diamagnetic Zn content.or diamagnetically substituted ferrites, the existence of a centraloublet superimposed on well-resolved magnetic sextets has beeneported for a number of systems [14–16]. In the present system,he central doublet arises due to the magnetically isolated Fe3+ ionsocated at the tetrahedral site and not because of any secondaryhase. As expected, the isomer shift for the octahedral site is slightlyreater than that of tetrahedral site. The isomer shift value at thewo sites is different because of the fact that Fe3+–O2− distances implying difference in covalency of Fe–O bond. The variation in
agnetic hyperfine field at A and B sites is induced by nonmagnetic
ubstitution. The value of quadrupole shift of the A and B mag-etic patterns is very small in all the samples indicating that the
ocal symmetry of the ferrites obtained is close to cubic. Mössbauerarameters for various compositions are listed in Table 1.
0.5−x/2
Mn0.05Fe2.45−x/2O4.
Fig. 2. Variation in paramagnetic character with Zn content.
M. Gupta, B.S. Randhawa / Materials Chemistry and Physics 130 (2011) 513– 518 515
Table 1Mössbauer parameters for various compositions of ‘x’ in Rb0.5−x/2ZnxMn0.05Fe2.45−x/2O4 recorded at 300 K.
Composition ıamm s−1 �mm s−1 Line widthmm s−1 Magnetic hyperfine fieldT Distribution of Fe3+ ions%
a w.r.t. pure metallic iron absorber, oct: octahedral site, tet: tetrahedral site, C.D.:
.2. Magnetic studies
The magnetic studies of the samples were performed withibrating Sample Magnetometer and all the samples show a hys-
ersis loop, common ferrimagnetic behaviour of the ferrites. Theystersis loop traced at room temperature for all the compositionsith x varies from 0 to 0.5 are displayed in Fig. 3a. Fig. 3b shows
he variation of saturation magnetization as a function of Zn con-ent. It has been observed that saturation magnetization increasesnitially up to a certain level of substitution and then follows theeverse trend. The observed variation can be explained on the basis
f exchange interactions [17]. The saturation magnetization can bealculated as
S = MB − MA
00.20.10.015
20
25
30
35
40a
Ms(
emu/
g)
Zn content
-6
b
Fig. 3. (a) Hysteresis loops obtained for various ferrite compositions. (b)
– 21.24 (C.D.)
al doublet.
where MB is the net magnetic moment of ions on B site and MA isthe net magnetic moment of ions on A site. It is well known thatif a diamagnetic substituent ion occupies the A site in the spinel,it initially increases the magnetization up to a certain level of sub-stitution and then follows a downward trend. Since non-magneticZn2+ ion has a strong affinity for A site, its substitution reduces themagnetization of A sub lattice (MA) and thereby increases the netMS value. It reaches a maximum at x = 0.1, and afterwards reversesits course (Table 2). The fall in magnetization on addition of Znbeyond this limit can be attributed to the fact that A-sublattice isso much diluted that the A–B interaction becomes weaker than
the B–B interaction. This disturbs the parallel allignment of spinmagnetic moments on B-site, thus paving way to the canted spins[18].
0.50.4.3
6000400020000-2000-4000000
-40
-30
-20
-10
0
10
20
30
40
Mom
ent/M
ass (
emu/
g)
Field (G)
X=0 X=0.1 X=0.2 X=0.3 X=0.4 X=0.5
Variation of saturation magnetization with increasing Zn content.
516 M. Gupta, B.S. Randhawa / Materials Chemistry and Physics 130 (2011) 513– 518
Table 2Variation of saturation magnetization, average particle size, Curie’s temperature and DC resistivity with composition ‘x’.
Composition (x) Saturation magnetization, MS (emu g−1) Average particle/grain size (XRD) Curie’s temperature (◦C) Resistivity (Ohm cm)
0 22.195 8.1 437 4.26 × 105
0.1 35.463 13.3 370 5.6 × 105
0.2 32.832 10.8 298 7.2 × 105
6
oftetelhttfwid
3
sZe[lddoeompatRuc
similar trend was reported by several workers [24,25] and hasestablished a strong relation between conduction mechanism anddielectric behaviour of ferrites. The decrease in dielectric value israpid at lower frequency and slower at higher frequency which
0.3 28.646 12.6
0.4 25.497 13.5
0.5 21.843 13.5
The variation in Curie temperature (TC) with the substitutionf non-magnetic Zn2+ ion content (x) in the basic compositionalormula has been studied. Fig. 4 shows a regular decrease in Curieemperature with increase in Zn2+ ion content (x) which can bexplained on the basis of exchange interactions [17]. An increasehe concentration of diamagnetic Zn2+ ions weakens the inter-sitexchange interaction due to reduction in the number of magneticinkages and consequently a fall in Curie temperature. Similar trendas already been reported for different systems [19–21]. The Curieemperatures for various compositions are listed in Table 2. Sincehe magnitude of Curie temperatures for Rb–Zn ferrites has beenound to be higher than that of respective Na–Zn and K–Zn ferritesith same composition [14], mixed rubidium ferrites may be des-
gnated as the better option for application in magnetic/microwaveevices.
.3. DC electric resistivity
The DC electric resistivity of the ferrite nanoparticles has beentudied at room temperature. Fig. 5 shows a regular increase withn content. The increase in resistivity with Zn content can bexplained on the basis of Verwey mechanism of electron hopping22]. According to this model, ferrites form closed packed oxygenattice with metal ions located at tetrahedral (A-site) and octahe-ral (B-site) sites and since A–B distance is greater than the B–Bistance therefore dominant mode of conduction due to hoppingf Fe2+ and Fe3+ occurs at B site. It has been concluded that electronxchange between Fe2+ and Fe3+ ions results in local displacementf charges which is responsible for the polarization in ferrites. Theagnitude depends upon concentration of Fe2+ and Fe3+ ion pairs
resent on the B site. The evaporation of metal ion during sinteringt higher temperature is mainly responsible for the excess concen-
ration of Fe2+ ions in substituted alkali metal ferrites [23]. Whenb+ ion is removed from a region in the crystal and assuming thatnit cell retains its full complement of 32 O2− ions, the positiveharge of the departed Rb+ must be compensated. This is accom-
0.50.40.30.20.10.0
100
150
200
250
300
350
400
450
Tem
pera
ture
ºC
Zn Content
Fig. 4. Variation of Curie temperature with increasing Zn content.
245 2.1 × 10180 3.2 × 106
105 7.70 × 106
plished by diffusion of Fe3+ ion from the surface as the O2− ion onthe surface is removed. However, charge neutrality is maintainedby the concurrent reduction of that ion from Fe3+ to Fe2+. There-fore, a partial substitution of the Fe3+ ion on the octahedral site byother ions is expected to result in a change in electric propertiesof the ferrite materials. The higher value of DC resistivity obtainedmay be contributed to nanosized ferrite particles obtained by solu-tion combustion method. Samples with smaller particles consist ofmore number of grain boundries which act as barriers to the flowof electrons. Another advantage for small size is that it helps inreducing Fe2+ ion concentration as oxygen moves faster in smallgrains keeping iron in Fe3+ state. The temperature dependence ofDC resistivity was also studied in the temperature range 308–398 Kas displayed in Fig. 6, shows an almost linear decrease in resistiv-ity with temperature suggesting semiconductor behaviour of theferrite materials.
3.4. Dielectric and tangent loss studies
Dielectric and tangent loss studies were carried out using Agi-lent technologies, 8714ET precision LCR meter. Figs. 7 and 8 showvariation of dielectric constant of ferrite samples with frequencyand it is observed that the value of dielectric constant decreases reg-ularly with increase in the frequency. It is clear from the Figs. 5 and 7that the variation of DC resistivity and dielectric constant as a func-tion of Zn content are having opposite trend with each other. A
0.50.40.30.20.10.0
1x106
2x106
3x106
4x106
5x106
6x106
7x106
8x106
Res
istiv
ity (o
hm-c
m)
Zn content
Fig. 5. Variation of DC resistivity with Zn content.
M. Gupta, B.S. Randhawa / Materials Chemistry and Physics 130 (2011) 513– 518 517
Fig. 6. Temperature dependence of DC electrical resistivity of ferrite samples.
Fig. 7. Variation of dielectric constant with frequency (x = 0.1).
Fig. 8. Variation of dielectric constant with frequency (x = 0.5).
Fig. 9. Variation of tangent loss (�) with frequency (x = 0.1).
Fig. 10. Variation of tangent loss (�) with frequency (x = 0.5).
is a common feature of ferrimagnetic behaviour [26,27]. A moredielectric dispersion is observed at lower frequency range and itremains almost independent of applied external field at high fre-quency domain. The dielectric dispersion at low frequency is dueto Maxwell–Wagner type interfacial polarization, well in agree-ment with Koop’s phenomenological theory [22]. Ferrite sampleswith heterogenous structure can be imagined as systems consist-ing of high conductive grains separated by highly resistive thingrain boundaries. It causes localized charge accumulation underapplied electric field which results in interfacial polarization. Athigher frequencies, the electron exchange between Fe2+ and Fe3+
ions cannot follow the alternating field, thus causing a decreasein the contribution of interfacial polarization to dielectric constantand results in a decrease in the dielectric constant at higher fre-quencies. Figs. 9 and 10 show an initial increase in the value oftangent loss (ı) to attain a maxima followed by a regular decreasewith frequency. Such peak behaviour occurs when jump frequencyof electron exchange between Fe2+ and Fe3+ becomes equal to theapplied field [28,29].
4. Conclusion
A series of substituted rubidium ferrites with compositionRb0.5−x/2ZnxMn0.05Fe2.45−x/2O4 (x = 0 → 0.5) were prepared by solu-
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18 M. Gupta, B.S. Randhawa / Materials
ion combustion method. The effect of substitution on magneticnd electrical properties has been studied and explained in theight of various models. Mössbauer results show a transition fromerrimagnetism to superparamagnetism with increasing diamag-etic Zn content. An initial increase in the magnitude of saturationagnetization with increasing Zn content up to x = 0.1 has been
xplained on the basis of Neel’s two sublattices model, while decrease in the saturation magnetization for x > 0.1 has beenescribed on account of Yafet and Kittel spin canted structure. Theurie temperature has been found to decrease with increasing Znontent (x) and is explained on the basis of exchange ion interac-ions. A study of DC resistivity of the samples at room temperaturehows that resistivity increases with zinc content, whereas the tem-erature dependence measurements for DC resistivity display aegular decrease with temperature. The value of dielectric constantas been found to decrease with increase in frequency. Tangent
oss (ı) studies show an initial increase in the value to attain maxima followed by a regular decrease with frequency. Sucheak behaviour occurs when jump frequency of electron exchangeetween Fe2+ and Fe3+ becomes equal to the applied field. Signif-
cant magnitude of Curie temperature, saturation magnetization,esistivity and low dielectric constant value for Rb–Zn mixed fer-ites make these materials suitable for the magnetic/microwavepplications.
cknowledgement
The financial support provided by CSIR, New Delhi is highlycknowledged.
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