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Materials Chemistry and Physics 128 (2011) 489–494

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

tructural and transport properties of stoichiometric Mn2+-doped magnetite:e3−xMnxO4

inesh Varshneya,b,∗, Arvind Yogia

School of Physics, Vigyan Bhavan, Devi Ahilya University, Khandwa Road Campus, Indore-452001, IndiaSchool of Physics, USIC Bhavan, Devi Ahilya University, Khandwa Road Campus, Indore-452001, India

r t i c l e i n f o

rticle history:eceived 3 November 2010eceived in revised form 5 March 2011ccepted 20 March 2011

ACS:4.25.Fy7.80.dk1.05.Qr1.05.cp1.30.+h

a b s t r a c t

The polycrystalline samples of Fe3−xMnxO4 (0.10 ≤ x ≤ 0.50) were prepared by a solid-state route reac-tion method. X-ray diffraction pattern shows that Mn2+ doped magnetites are in single phase and possesscubic inverse spinel structure. The resistivity measurements (10 < T < 300 K) for x = 0.0 and 0.01 confirmsthe first order phase transition at the Verwey transition TV = 123 K and 117 K, respectively. No first orderphase transition was evidenced for Fe3−xMnxO4 (0.10 ≤ x ≤ 0.50). Small polaron model has been used tofit the semiconducting resistivity behavior and the activation energy εa, for samples x = 0.10 and 0.50is about 72.41 meV and 77.39 meV, respectively. The Raman spectra of Fe3−xMnxO4 at room tempera-ture reveal five phonons modes for Fe3−xMnxO4 (0.01 ≤ x ≤ 0.50) as expected for the magnetite (Fe3O4).Increased Mn2+ doping at Fe site leads to a gradual changes in phonon modes. The Raman active modefor Fe3−xMnxO4 (x = 0.50) at ∼=641.5 cm−1 is shifted as compared to parent Fe3O4 at ∼=669.7 cm−1, infer-

eywords:. Magnetic materials. Chemical synthesis. Mössbauer spectroscopy-ray diffraction. Electrical properties

ring that Mn+2 ions are located mostly on the octahedral sites. The laser power is fixed to 5 mW causesthe bands to broaden and to undergo a small shift to lower wave numbers as well as increase in the fullwidth half maxima for A1g phonon mode with the enhancement of Mn2+ doping. Mössbauer spectroscopyprobes the site preference of the substitutions and their effect on the hyperfine magnetic fields confirmsthat Mn+2 ions are located mostly on the octahedral sites of the Fe3−xMnxO4 spinel structure.

© 2011 Elsevier B.V. All rights reserved.

agnetic properties

. Introduction

Magnetites have been extensively studied due to a competitivelectronic, lattice, and magnetic degrees of freedom as well the firstrder phase transition. At ambient pressure, the Verwey transitionv of pure or near-stoichiometric magnetite is on the first orderransition. This transition occurs at about Tv ∼ 123 K, with changesn crystal structure, latent heat, and the dc conductivity decrease intwo-order-of-magnitude [1]. Furthermore, the oxygen deficiencyr cation doping at Fe site may reduce the first order phase transi-ion temperature, may cause the transition to become higher order

nd may suppress it completely.

The doping of cations (X = Cr, Co, Ni, Cu, Zn, and Ti) are suc-essfully substituted at Fe site with a motivation that the cationistribution between tetrahedral (A) and octahedral (B) sites in

∗ Corresponding author at: School of Physics, Vigyan Bhavan, Devi Ahilya Univer-ity, Khandwa Road Campus, Indore-452001, India. Tel.: +91 731 2467028;ax: +91 731 2465689.

E-mail addresses: vdinesh33@rediffmail.com, vdinesh33@gmail.comD. Varshney).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.03.040

Fe3O4 may cause changes in the transport properties (i.e. electri-cal resistivity). The doped structure is represented as (XıFe1−ı)A

[X1−ıFe1+ı]BO4, with ı as the inversion parameter. The charge den-sity changes with different dopings such as with Zn, Ti, and Al. Theseare nonmagnetic dopants that enter different lattice positions: Zn2+

– tetrahedral, Ti4+ – octahedral, and Al3+ – (tetrahedral as well asoctahedral). Moreover, for very small substitution for Fe lowers thetransition temperature and the nature of the Verwey transition ischanged from first order to the continuous one [2].

Furthermore, in ZnFe2O4 magnetites, the electrical resistivity ofZn doped magnetites ZnFe2O4 reveals that the under-doped sam-ples x ≤ 0.035 show a first order Verwey transition, for x = 0.035,the second order transformation is barely detectable and for higherdoping (0.13 ≤ x ≤ 0.29) the electrical resistivity show only a gentlevariation [3]. The cations as Cu are known to enter at octahedral sitein the resultant spinel lattice; Cu substitution for Fe in Fe3−xCuxO4[0.20 ≤ x ≤ 1.0] no Verwey transition was documented [4,5]. As far

as the cations as Cr, Co, Ni, Cu, and Ag doped magnetites; no sys-tematic efforts have been made to address the electrical resistivityas functions of doping.

Raman spectroscopy is an impressive technique to probe thevibrational and structural properties of materials. Raman mea-

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90 D. Varshney, A. Yogi / Materials Ch

urements of parent magnetites have identified five Raman activeodes at room temperature: A1g(669 cm−1), Eg(410 cm−1), and

T2g {ω(T2g1) = 193 cm−1, ω(T2g

2) = 540 cm−1, ω(T2g3) = 300 cm−1}

6]. While, the unpolarized Raman measurements on polycrys-alline Fe3O4 have identified six Raman bands, an additional A1gand [7]. The Raman and reflectivity measurements on naturalingle crystals of Fe3O4 above and below Verwey transition (Tv)ocuments only four Raman modes [8]. Furthermore, the Ramanpectrum of the substituted magnetites as Fe2ZnO4 reveals fouraman active modes at room temperature. As compared to thearent Fe3O4, the doped Fe3−x ZnxO4 (x = 0.015), infers the disap-earance of weakest T2g mode at 193 cm−1 [6]. We admit that noystematic effort has been made on other Mn, Cr, Co, and Ag dopedagnetites.Mössbauer spectroscopy is a powerful probe not only to inves-

igate the magnetic phases but also to identify the structuralspects. The hyperfine interactions between the 57Fe nuclei and thelectrons surrounding them are provided through the hyperfinearameters: isomer shift (ı), quadrupole splitting (�), and mag-etic hyperfine field (Bhf). The isomer shift parameter ı is mainlyelated to the oxidation state of the Fe ion, � describes the sym-etry of the charge distribution around the Fe nucleus or site

istortion, and Bhf provides information about magnetic propertiesf the compound. Doped magnetites seem to be an ideal illustrationo identify the Fe distribution at tetrahedral (A) and octahedral (B)ites [9].

The hyperfine parameters are unaffected for small Cr substitu-ion in magnetite, however larger quantities indicate the featuresf the FeCr2O4 spinel [10]. The magnetic ion (Co) substituted mag-etites Fe3−xCoxO4 have been studied by Mössbauer spectroscopyt low and high temperatures with low doping concentrationsx ≤ 0.04). The hyperfine parameters obtained from the transmis-ion spectra reveals that the largest hyperfine field arises fromhe tetrahedral ferric ions [11]. The Mössbauer spectrum for otheration substitutions infers that Cr, Co, Ni, and Zn were distributedniformly over the system while Mn, Cd and Cu have the tendencyo accumulate on the particles surface [12–14].

It is thus noticed that parent Fe3O4 magnetite, the first ordererwey transition is revealed from resistivity, and the active modesarticipating in the process of conduction as well structural infor-ations are obtained from Raman spectroscopy. The above is

acking for cation as Mn substituted magnetites (for lower andigher doping concentrations) and provides the motivation ofhe present investigations. The Mössbauer spectroscopic measure-

ents [12] have been successfully made for lower doping samplesnd no effort have been made for higher and completely substitutedagnetite. This motivates us to study the Mössbauer spectroscopy

o identify the oxidation state of Fe3+ as well to seek whether Mnnters at tetrahedral or octahedral site in the resultant spinel struc-ure.

We thus aimed at studying the changes produced in thetructural, spectroscopic, magnetic and transport properties ofagnetites upon divalent ions Mn2+ for trivalent Fe3+ substi-

ution in octahedral (B) spinel site: Fe3−xMnxO4 (0.0 ≤ x ≤ 0.50)repared by conventional solid-state ceramic route and charac-erized by X-ray diffraction, resistivity, Raman, and Mössbauerpectroscopy.

. Experimental techniques

.1. Synthesis of samples

The polycrystalline samples with the composition Fe3−xMnxO4 (x = 0, 0.01, 0.10nd 0.50) samples were prepared by the solid-state ceramic route technique asescribed earlier [5]. Stoichiometric amounts for high purity (Merck-with 99.99%)erric oxide (Fe2O3), manganese oxide (MnO2) and iron oxide (FeO), were mixed inn agate-mortar. The mixture was then milled for 5–6 h. Then the resultant pow-ers of all the samples were calcined at 850 ◦C for 24 h and cooled slowly at a rate of

y and Physics 128 (2011) 489–494

2 ◦C min−1 to room temperature. In order to obtain better homogeneity of sampleswe repeated this processes for 950 and 1050 ◦C temperatures. There after calcina-tions, for adjusting stoichiometric properties of the prepared samples we performoxygen annealing at temperatures (950, 1050 ◦C) for 24 h with 3–4 h intermedi-ate grindings. The annealed powder was again milled for 1 h and pelletized intothe form of circular discs of 15 mm diameter and then the pellets were finally sin-tered and annealed at 1050 ◦C in oxygen atmosphere for 24 h and slowly cooled at arate of 2 ◦C min−1 to room temperature. This procedure yielded near-stoichiometricproperties for all the prepared samples. X-ray diffraction confirmed the spinel-likestructure of all the samples. Transport measurements detect drop of conductiv-ity at Tv = 123 K for parent Fe3O4. These measurements indicating that preparedsamples are near stoichiometric [1]. The samples were characterized by means ofX-ray diffraction (XRD), resistivity and room temperature Raman and Mössbauerspectroscopy.

2.2. Characterization

2.2.1. X-ray diffraction (XRD)The X-ray diffraction (XRD) measurements were carried out with Cu K˛ radia-

tion using a Rigaku powder diffractometer equipped with a rotating anode scanning(0.01 step in 2�) over the angular range 10–80◦ at room temperature generating X-ray by 40 kV and 100 mA power settings. Monochromatic X-rays of � = 1.5406 A K˛1

line from a Cu target were made to fall on the prepared samples. The diffractionpattern was obtained by varying the scattering angle 2� from 10◦ to 80◦ in step sizeof 0.01◦ .

2.2.2. Electrical resistivityThe dc electrical resistivity measurements of the sample have been done from

10 to 300 K using four-probe method. Indium contacts are made on the polishedsurface of the sample. The sample is in the form of a rectangular rod. The samplesizes of Fe3−xMnxO4 (x = 0, 0.01, 0.10 and 0.50) are 0.746, 0.730, 0.754, and 0.756 mm,respectively. The SI temperature controller, Schlumberger multimeter and Advan-test current source is used for the measurement. A vacuum of the order of 10−5 mbarwas maintained in the chamber with the help of turbo molecular pump assembly.The setup uses silicon diode as the sensor.

2.2.3. Raman spectroscopyThe Raman equipment was a Jobin-Yovn Horiba Labram (System HR800) con-

sisting of a single spectrograph (0.25 m focal length) containing a holographicgrating filter (1800 grooves mm−1), and a Peltier-cooled CCD detector (1024 × 256pixels of 26 �m). The spectra were excited with 632.8 nm radiation (1.95 eV) froma 19 mW air-cooled He–Ne laser (Max Laser power: 19 mW) and the laser beamwas focused on the sample by a 50× lens to give a spot size of 1 �m; the resolutionwas better than 2 cm−1. The laser power was always kept on 5 mW at the sample,to avoid sample degradation, except in the laser power dependence experiments.After each spectrum had been recorded, a careful visual inspection was performedusing white light illumination on the microscope stage in order to detect any changethat could have been caused by the laser.

2.2.4. Mössbauer Spectroscopy57Fe Mössbauer measurements were performed in transmission mode with a

57Co radioactive source in constant acceleration mode using a standard PC basedMössbauer spectrometer equipped with a Weissel velocity drive. Velocity calibra-tion of the spectrometer was done with a natural iron absorber at room temperature.High magnetic field 57Fe Mössbauer measurements were carried out using a Janissuperconducting magnet. For the high magnetic field measurements, the externalfield was applied parallel to the �-rays (i.e. longitudinal geometry). Absorbers werecarefully prepared to optimize both a good compactness and an adequate heat trans-mission. Analysis of the spectra was performed using the Normos [15] least squaresfitting programs. Doublets and sextets of Lorentzian lines were used, doublets in theparamagnetic region and sextets in the magnetic region. The spectra were calibratedreferring to ˛-Fe.

3. Results and discussion

The X-ray diffraction patterns of Fe3−xMnxO4 (0.0 ≤ x ≤ 0.50)have been collected on the surfaces of the disks with a Rigakudiffractometer using Cu K˛ radiation. Fig. 1 illustrates the represen-tative � – 2� scans of XRD for parent and Mn2+ as cation substitutedMnxFe3−xO4 and could be indexed as that of cubic system with facecentered lattice. Compared with the standard diffraction patterns

of Joint committee for powder diffraction (JCPDS) file No. 85-1436,and the indexed peaks are consistent with the Joint committee forpowder diffraction (JCPD) data [16]. In the diffraction pattern forx = 0, 0.01, and 0.10 samples, very small impurity peak has beendetected in between (2 2 0) and (3 1 1) peaks. This peak does not

D. Varshney, A. Yogi / Materials Chemistry and Physics 128 (2011) 489–494 491

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Fe2.50Mn0.50O4Fitted data

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ig. 1. The X-ray diffraction patterns for Fe3−xMnxO4 (x = 0, 0.01, 0.10, and 0.50)agnetites. The impurity peak in the diffraction pattern (for x = 0, 0.01, and 0.10

amples) between (2 2 0) and (3 1 1) are indicated by arrows.

een in the diffraction pattern for x = 0.50 sample. In the diffractionattern (Fig. 1), the arrow indicates the impurity peak.

The Fe3O4 and Mn substituted Fe3O4 magnetites shows thetrongest Bragg peak to occur at 2� ≈ 35.7◦ corresponding to (3 1 1)eflections. The calculated lattice parameter as illustrated in Table 1f parent magnetite Fe3O4 is estimated as 8.379 A, which is close tohe experimental bulk lattice parameter (8.393 A) [17]. The cations Mn2+ doping enhances the lattice parameter and unit cell vol-me as Mn2+ concentration (x) increases. The increase of the latticearameters with the increased Mn2+ as cation doping concentrationis attributed to the difference in the ionic radius of Fe3+(0.64 A)

nd Mn2+(0.80 A) ions [5].The dc electrical resistivity measurements of the magnetites

e3−xMnxO4 (x = 0.0, 0.01, 0.10 and 0.50) have been done from 10o 300 K using four-probe method. Fig. 2 illustrates the resistivityehavior of the parent Fe3O4 and substituted Mn2+ ion in mag-etite Fe2.99Mn0.01O4. In parent magnetite, the resistivity behaviorxhibits a discontinuity (first order phase transition) at about 123 Konsistent with the Verwey transition temperature Tv (the slopef the plot log10 � versus 1000/T is maximum) [1]. The resistiv-ty behavior of magnetite Fe2.99Mn0.01O4 show a phase transitiont about 117 K. The suppression in Verwey transition temperatureTv) as a cation-substituted magnetites are sensitive to the dopingoncentration. Similar features are reported earlier for cation-ubstituted magnetites such as Fe2CuO4 and Fe2ZnO4 magnetites3–5].

Fig. 3 illustrates the resistivity behavior of Fe3−xMnxO4 for= 0.10 and x = 0.50 samples, respectively. The resistivity shows

emi-conducting behavior and no Verwey transition is documentedor higher doped Mn2+ ion in Fe3−xMnxO4 (x = 0.10 and 0.50). The

agnetite (Fe3O4) shows the conduction mechanism is mainly dueo the electron exchange between ferrous (Fe2+) and ferric (Fe3+)

able 1rystal structure parameters of Mn substituted Fe3−xMnxO4.

Samples Lattice parameter a (A) Unit cell volume (A3)

Fe3O4 8.379 588.270Fe2.99Mn0.01O4 8.388 590.167Fe2.90Mn0.10O4 8.396 591.858Fe2.50Mn0.50O4 8.402 593.127

T (K)

Fig. 2. Variation of resistivity of Mn-ferrites with temperature, plotted as log10 �versus T for Fe3−xMnxO4 (x = 0, 0.01) magnetites.

ions on the octahedral (B) sites Fe2+ − e− ⇔ Fe3+ [18]. The Mn2+ iondoped in Fe3−xMnxO4 (x = 0.01 and 0.50) have considerably higherresistivities in the range 10 K < T < 300 K as compared to the par-ent and the doped x = 1% magnetites. We end up by stating thatthe transport properties of the cation-substituted magnetites aresensitive to the doping concentration.

We have analyzed the resistivity data of Fe3−xMnxO4 (x = 0.10and x = 0.50) magnetites using adiabatic small polaron conduction(SPC) model. SPC Model does not fit the experimental data of Fe3O4and Fe2.99Mn0.01O4 samples, which do not show the semi conduct-ing behavior. While into that Fe2.90Mn0.10O4 and Fe2.50Mn0.50O4samples show the semi conducting behavior, which is well fittedby SPC Model. The most rapid motion of a small polaron occurswhen the carrier hops each time the configuration of vibratingatoms in an adjacent site coincides with that in the occupied site.

300250200150100-1

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T (K)

Fig. 3. Variation of resistivity of Mn-ferrites with temperature, plotted as log10 �versus T for Fe3−xMnxO4 (x = 0.10, 0.50) magnetites. The solid line shows fitting tothe equation � = �pT exp(EP/kBT).

4 emistry and Physics 128 (2011) 489–494

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92 D. Varshney, A. Yogi / Materials Ch

s the activation energy. The resistivity in terms of SPC model isefined as: � = �0T exp(εa/kBT), with εa is the activation energy,B is the Boltzmann constant, T is the temperature and �0 is theesidual resistivity. The observed resistivity data is fitted using thearameters (�0 and εa) as shown in Fig. 3. The activation energyf Fe3−xMnxO4 for x = 0.10 and x = 0.50 samples are εa = 72.41 meVnd εa = 77.39 meV, respectively.

The parent Fe3O4 shows the conduction process is due to thelectron exchange between ferrous (Fe2+) and ferric (Fe3+) ions onhe octahedral (B) sites while into that Mn2+ doped Fe3O4 shows theonduction mechanism is due to the electron exchange betweenn2+ and Fe3+ octahedral (B) sites [4,19]. Henceforth, no phase

ransition was found for higher doped Mn2+ ion in Fe3−xMnxO4x = 0.10 and 0.50). This is also true for single crystal samples, theesistivity measurements on magnetite single crystals Fe3−xMxO4oped with M = Ni, Co, Mg, Al, Ga, and Ti shows that the Verweyemperature shift as function of the doping concentration (x) [20].

Doped magnetites with Al, Ni, Co, or Mg yield a transitionemperature shift towards lower temperature from 123 K [20].he partial or complete replacement of Fe3+ ions by Mn2+ ionss evidenced from the changes in the electrical resistivity due toharge localization on the octahedral Fe sites. The viable conduc-ion mechanism is the electron hopping between two adjacentctahedral sites (B-sites) in the spinel lattice and a transitionetween Fe2+ ⇔ Fe3+ ions or Mn2+ ⇔ Mn3+ might take place. Thenhe replacement of Fe3+ ions by Mn2+ ions from periodical order-ng of Fe3+ and Fe2+ ions in the crystallographic lattice resultse3+ + Mn2+ ⇔ Fe2+ + Mn3+ [4,19].

The Raman measurements in the range 150–800 cm−1 of theamples Fe3−xMnxO4 (x = 0, 0.01, 0.10 and 0.50) has been made atoom temperature. Fig. 4 illustrates the Raman spectrum of thearent Fe3O4 and substituted Mn2+ ion in magnetite Fe3−xMnxO4.or parent Fe3O4 magnetite five Raman active modes are iden-ified as: A1g = 669.7, Eg = 543.7, T2g(1) = 433.5, T2g(2) = 307.4 and2g(3) = 188.2 cm−1 according to group theory assignment andonsistent with the previous reports [6–8]. For Fe2.99Mn0.01O4ample, weak phonon features with a minor shift in the samerequency range are observed. Fig. 5 illustrates the Raman spec-rum of Fe3−xMnxO4 for x = 0.10 and x = 0.50 samples, respectively.he observed data were fitted to a sum of appropriate number oforentzians and a second-degree polynomial baseline. Here we useorentzians fitting for weak phonon frequency modes (T2g

(1) and2g

(3)) which are illustrated in the inset of Figs. 4 and 5. The Ramanpectroscopy reveals all the five-phonon modes (one A1g, one Eg

nd three T2g modes) as documented in Table 2.Furthermore, Fig. 5 illustrates that for x = 0.10 sample, the

honon modes are further weakened, while to that for x = 0.50, allhe five Raman active modes are more sharp and pronounced withleft side Raman shift. The changes observed in Raman shift for

e3−xMnxO4 (x = 0.50) shows a crossover from a first order tran-

ition to a higher order transition. Thus Fe3−xMnxO4 (x = 0, 0.01,.10 and 0.50) magnetites document a Raman-active phonon at70 cm−1 (please see Figs. 4 and 5) confirming the cubic inversepinel structure of magnetites. We note that similar features are

Wavenumber ( cm-1)

Fig. 5. Raman shift as a function of wave number for Fe3−xMnxO4 (x = 0.10, 0.50)magnetites. The solid line in inset of the figure shows fitting of Lorentzians to theexperimental data.

able 2aman active mode, Raman shift () and full widths at half maximum FWHM ( ) for A1g mode of Fe3−xMnxO4 (x = 0, 0.01, 0.10 and 0.50).

Raman active mode Fe3O4 [Raman shift (cm−1)] Fe3−xMnxO4 [Raman shift (cm−1)]

x = 0.01 x = 0.10 x = 0.50

A1g 669.7 668.2 665.4 641.5T2g(2) 543.7 539.7 536.0 495.5Eg 433.5 429.8 428.1 407.8T2g(3) 307.4 307.2 306.8 296.2T2g(1) 188.2 190.4 190.6 191.5A1g mode FWHM ( /cm−1) 36.46 38.17 42.40 68.62

D. Varshney, A. Yogi / Materials Chemistry and Physics 128 (2011) 489–494 493

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ig. 6. The full width half maxima (FWHM) and Raman shift as a function ofavenumber (ω) for strong characteristic modes corresponding to A1g mode for

e3−xMnxO4 (x = 0, 0.01, 0.10, and 0.50) magnetites.

bserved for the room temperature Raman spectra of Fe3−xZnxO46].

For parent Fe3O4, A1g mode is observed at 669.7 cm−1 and withnhanced Mn doping concentration, the A1g mode shifts towardsower wave numbers. It is observed that for Fe2.5Mn0.5O4 the1g mode is Raman shifted and observed at about 641.5 cm−1.urthermore, the A1g mode’s intensity of Fe2.5Mn0.5O4 decreasesrastically, which may be due to the strong electron–phonon inter-ction. The full width half maxima (FWHM) as well as Raman shifts a function of wavenumber (ω) for strong characteristic modesorresponding to A1g is illustrated in Fig. 6. The full width halfaxima (FWHM) of A1g modes are significantly increased with

nhanced Mn2+ doping concentration (x) in Fe3−xMnxO4 and isttributed to the lattice expansion due to the replacement of Mn ate site in Fe3O4. The replacement of Fe3+ ions by Mn2+ ions leads toperiodical ordering of Fe3+ and Fe2+ ions in the octahedral crys-

allographic site of Fe3O4. The higher ionic radius of Mn2+(0.80 A)eplaced by lower ionic radius of Fe3+(0.64 A) results in an increasen full width half maxima (FWHM).

Fig. 7 illustrates the room temperature 57Fe Mössbauerpectra of the parent Fe3O4 and Mn2+ substituted magnetitee2.99Mn0.01O4. The Mössbauer spectrum is analyzed by consid-ring two (three) symmetric Lorentzian shaped sextets model,orresponding to tetrahedral (A) and octahedral (B) magnetic sub-attices for parent (doped) magnetites. Fig. 8 illustrates the roomemperature 57Fe Mössbauer spectra of Fe3−xMnxO4 for x = 0.10 and= 0.50 samples, respectively. The room temperature Mössbauerpectra are fitted with NORMOS-SITE program and all sampleshow magnetic ordering. The obtained value of chi-2 is minimums possible and NORMOS-SITE program fitted well to the experi-ental data and are illustrated in Figs. 7 and 8, respectively. The

7Fe fitted Mössbauer parameters are listed in Table 3. We notehat the calculated hyperfine parameters are with respect to naturalron (Fe) are consistent with earlier reported data [12]. The room-emperature Mössbauer spectra of Mn2+-substituted magnetitesxhibit the additional presence of a quadrupole-split doublet, withhe hyperfine parameters typical of super-paramagnetic magnetitearticles consistent with the earlier experimental data [5,9].

The Mn2+ as cation-substituted Fe3−xMnxO4, the peak with highyperfine field values is related to the Fe involved in the elec-

ron delocalization process, without the presence of Mn as nearesteighbors. The second peak corresponds to those Fe atoms sur-ounded by some Mn neighbors. The B site doped magnetites showshat charge distributions become broader and occupy more area as

Fig. 7. Room temperature Mössbauer spectra of Fe3−xMnxO4 (x = 0, 0.01) magnetites.The full lines are the A- and B-site Lorentzian-shaped sextets and their sum fitted tothe experimental data.

compared to the A site doped magnetites. Thus with the increasedMn2+ amount, the relative area decreases and is an indication of thefact that Mn2+ ions in Fe3−xMnxO4 have entered preferentially intothe B sites. The observed isomer shift values from room tempera-ture Mössbauer data clearly show the presence of Ferric (Fe3+) stateonly and are consistent with the previously reported data [11,14].

In passing, we may refer to the work of Sorescu and researcherswho observed an additional appearance of a sextet for Co2+ andNi2+ substitutions. The Co2+ and Ni2+ doping led to the with thehyperfine field of 49.0 T, which indicates a more pronounced Fe3+

character than the average Fe(B) line in the pure magnetite spec-

Velocity [mm/sec]

Fig. 8. Room temperature Mössbauer spectra of Fe3−xMnxO4 (x = 0.10, 0.50) mag-netites. The full lines are the A- and B-site Lorentzian-shaped sextets and their sumfitted to the experimental data.

494 D. Varshney, A. Yogi / Materials Chemistry and Physics 128 (2011) 489–494

Table 3Mössbauer parameters derived from the spectra using the two (three) symmetric Lorentzian shaped sextets model for parent (doped) magnetites.

Fe3−xMnxO4 ı (mm s−1) �EQ (mm s−1) Hhf (T) Relative area (%) Spectrum Assignment of site

x = 0 0.26 −0.35 49.54 36.3 6 A (Fe3+)0.59 −0.16 47.13 63.7 6 B (Fe3+, Fe2+)

x = 0.01 0.26 −0.39 51.92 42.4 2 A (Fe3+)0.67 −0.14 48.10 37.6 6 B (Fe3+, Fe2+)0.60 −0.18 46.30 20.0 6 B (Fe3+, Fe2+, Mn2+)

x = 0.10 0.25 −0.11 52.08 42.8 2 A (Fe3+)0.66 −0.22 49.00 33.6 6 B (Fe3+, Fe2+)0.56 −0.29 45.08 23.6 6 B (Fe3+, Fe2+, Mn2+)

x = 0.50 0.25 0.16 52.61 43. 6 2 A (Fe3+)0.63 −0.28 49.67 34. 7 6 B (Fe3+, Fe2+)

3

P er shp .0, 0.0

rmdnMriq

4

apwtcpronitFmftFtac

RRTTFRFpast

[

[

[

[[[[

0.54 −0.31 41.1

arameters used in table: Hyperfine magnetic field Hhf , quadrupole splitting �EQ , isomattern in the transmission Mössbauer spectra of Cu substituted (Fe3−xMnxO4, x = 0

oom-temperature Mössbauer spectra of Cu2+ and Cr3+ substitutedagnetites exhibit the additional presence of a quadrupole-split

oublet, with the hyperfine parameters typical of superparamag-etic magnetite particles [21]. The present Mössbauer spectra ofn2+ doped magnetites (Fe3−xMnxO4; x = 0.0, 0.01, and 0.10) also

eveal the similar features. However, for higher substituted Mn2+

on in magnetite Fe2.50Mn0.50O4 samples, we notice one additionaluadruple splitting that indicates less magnetic nature.

. Conclusions

The impact of Mn2+ doping in magnetites is investigated bynalyzing the measured structural and transport properties. Theolycrystalline samples Fe3−xMnxO4 (x = 0, 0.01, 0.10 and 0.50)ere prepared by a solid-state route reaction method. X-ray diffrac-

ion analysis shows the samples are in single phase and haveubic inverse spinel structure of all synthesized Fe3−xMnxO4 sam-les. The resistivity measurements carried out in the temperatureange 10 K < T < 300 K. The electrical resistivity confirms the firstrder–disorder phase transition for Fe3O4 and Fe2.99Mn0.01O4 mag-etites at about 123 K and 117 K, respectively. The abrupt change

n resistivity at low temperature is attributed to charge localiza-ion on the octahedral iron sites. However for Fe2.9Mn0.1O4 ande2.5Mn0.5O4 magnetites, the resistivity behavior does not docu-ent any substantial changes at low temperatures. Furthermore,

or Fe2.9Mn0.1O4 and Fe2.5Mn0.5O4 magnetites, the high resis-ivity at all measured temperatures as compared to Fe3O4 ande2.99Mn0.01O4 magnetites identifies increased disorder at B sublat-ices of Fe3O4. The activation energy for x = 0.10 and x = 0.50 samplesre εa = 72.41 meV and εa = 77.39 meV, respectively, could not beompared due to lack of data.

The spectroscopic aspects of Fe3−xMnxO4 are investigated byaman spectra to observe five phonons modes for all samples. Theaman phonon bands for parent are observed at A1g ∼= 669.7 cm−1,2g(2) ∼= 543.7 cm−1, Eg ∼= 433.5 cm−1, T2g(3) ∼= 307.4 cm−1 and2g(1) ∼= 188.2 cm−1, respectively. Mn doping concentration ine3O4 leads gradual changes in phonon modes. However, theaman active mode for higher Mn2+ doping concentration

−1

e3−xMnxO4 (x = 0.50) at ∼=641.5 cm is shifted as compared toarent Fe3O4 at ∼=669.7 cm−1.The laser power is fixed to 5 mW (forll the samples) causes the bands to broaden and to undergo amall shift to lower wave numbers (cm−1) as well as an increase inhe full width half maxima FWHM ( ) for A1g phonon modes with

[[[[[

21.7 6 B (Fe3+, Fe2+, Mn2+)

ift ı (relative to ˛ – Fe at 300 K), and relative areas corresponding to the component1, 0.10, and 0.50) Fe3O4 at T = 300 K.

the enhancement of Mn2+ ions doping concentration. Mössbauerspectra shows strong magnetic nature and this gives the Fe isin Ferric (+3) oxidation states. The spectra of Mn2+-substitutedmagnetites exhibit the additional presence of a quadrupled-splitdoublet with higher magnitude as compared to that of parentmagnetite. For Mn2+ doped Fe3−xMnxO4 (x = 0.01, 0.1, and 0.5), wenotice a sharper sextet for Fe3O4 that corresponds to Fe3+ state.The Mn2+ substitution at Fe3+ site results a broader pattern impliesa less well defined oxidation state of Fe3+ consistent with theprevious results of Fe3−xCr/CoxO4.

Acknowledgements

Financial assistance from CSR, Indore, India is gratefullyacknowledged. Authors are thankful to UGC-DAE CSR, Indore forproviding characterization facilities. Useful discussions with Prof.A. Gupta, Dr. D.M. Phase, Dr. R.J. Choudhary, Dr. G.S. Okram, Dr. V.Sathe and Dr. V.R. Reddy are gratefully acknowledged.

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