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Materials Chemistry and Physics 123 (2010) 434–438

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 and Cu2+-doped magnetite:e3−xCuxO4

inesh Varshneya,b,∗, A. Yogia

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

r t i c l e i n f o

rticle history:eceived 13 January 2010ccepted 26 April 2010

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

a b s t r a c t

The Cu substituted magnetites Fe3−xCuxO4 (x = 0, 0.01, 0.2, and 1.0) were prepared by solid-state reactionroute. The X-ray diffraction measurements confirm the cubic spinel structure of all doped samples ofFe3−xCuxO4. Resistivity measurements have been made in the range 77 K < T < 300 K. The polycrystallinesamples for the lower compositions (x = 0.0 and 0.01) exhibit the first order phase transition at the Verweytransition Tv = 123 (119) K. No transitions were seen for samples with higher doping concentration (x = 0.2and 1.0). The structural aspects of Cu substituted magnetites are further investigated by Raman spectra.The spectrum reveals five Raman active modes at room temperature consistent with the previous Ramanspectra. The changes in Raman spectrum as functions of Cu doping concentration are gradual. However,the Raman active mode for parent Fe3O4 at ∼=670 cm−1 is shifted at ∼=660 cm−1 for over doped Fe2CuO4,

eywords:agnetic materials

hemical synthesisössbauer spectroscopy-ray diffractionlectrical properties

inferring that Cu2+ ions are located mostly on the octahedral (B) sites. The transmission Mössbauer spec-troscopy was used to determine the site preference of the substitutions and their effect on the hyperfinemagnetic fields which confirms that for copper ferrite: Fe2CuO4 the Cu2+ ions are located mostly on theoctahedral (B) sites of the spinel structure.

© 2010 Elsevier B.V. All rights reserved.

agnetic properties

. Introduction

Magnetites (Fe3O4) and substituted magnetites have been aubject of immense interest due to their significant half-metallicharacteristic with 100% spin polarization and high Curie temper-ture (Tc) of 858 K [1]. The high spin polarization at the Fermi leveln ferromagnetic spinel magnetites causes a metallic minority spinhannel and a semiconductor majority spin channel [2]. The mag-etites are of technological importance as they are used as catalyst,

n chemical sensors, in microwave absorbers, in tunneling magne-oresistance (TMR) and in giant magnetoresistive devices (GMR)3].

Cations as Cr, Co, Ni, Cu, Zn, and Ag are successfully substitutedt Fe site with a motivation that the cation distribution betweenetrahedral (A) and octahedral (B) sites in Fe3O4 may cause changesn the electronic and magnetic properties. Cation as Cu doped mag-

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

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

254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2010.04.036

netites (Fe2CuO4) possesses a cubic close-packed arrangement ofoxygen ions, with Cu2+ and Fe3+ ions at two different crystallo-graphic sites. The doped structure is represented as (CuıFe1−ı)A

[Cu1−ıFe1+ı]BO4, with ı as the inversion parameter. It is noticed thatmost of the spinel magnetites are cubic; Fe2CuO4 is ferrimagneticwith a Neel temperature of about 780 K [4].

In cation-substituted magnetites, the Cu enters at octahedralsite [5], while to that the Zn exclusively enters at tetrahedralinterstices [6]. Furthermore, in ZnFe2O4 magnetites, the electri-cal conductivity takes place by electron transfer among Fe ions atoctahedral sites. The electrical resistivity of Zn doped magnetitesreveals that the underdoped samples x ≤ 0.035 show a first orderVerwey transition, for x = 0.035, the second order transformation isbarely detectable and for higher doping (0.13 ≤ x ≤ 0.29) the elec-trical resistivity show only a gentle variation. As far as the cationsas Cr, Co, Ni, Cu, and Ag doped magnetites; no systematic effortshave been made to address the electrical resistivity as functions ofdoping.

Raman spectroscopy is a powerful probe to reveal the vibra-tional and structural properties of materials. Raman measurementsof parent magnetites have identified five Raman active modes atroom temperature [7]. On the other hand, the unpolarized Ramanmeasurements on polycrystalline Fe3O4 have identified six Raman

mistry and Physics 123 (2010) 434–438 435

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in the ionic radius of Fe3+(0.64 Å) and Cu2+(0.72 Å) ions.The resistivity measurements over the range 77–300 K are

shown in Fig. 2 for cation-substituted magnetite Fe3−xCuxO4 (x = 0and 0.01) samples. In parent magnetite Fe3O4 sample, the resistivity

D. Varshney, A. Yogi / Materials Che

ands, an additional A1g band [8]. The Raman and reflectivity mea-urements on natural single crystals of Fe3O4 above and belowerwey transition (Tv) documents only four Raman modes [9]. Fur-

hermore, the Raman spectrum of the substituted magnetites ase2ZnO4 reveals four Raman active modes at room temperature [7].s compared to the parent Fe3O4, the doped Fe3−xZnxO4 (x = 0.015),

nfers the disappearance of weakest T2g mode at 193 cm−1. Wedmit that no systematic effort has been made on other Cr, Co, Ni,u, and Ag doped magnetites.

Mössbauer spectroscopy is a useful probe not only to investi-ate the magnetic phases but also to identify the structural aspectsnd magnetites as well doped ones seems to be an ideal illustra-ion to identify the Fe distribution at tetrahedral (A) and octahedralB) sites [10]. It is noticed that hyperfine parameters are unaffectedor small Cr substitution in magnetite, however larger quantitiesndicate the features of the FeCr2O4 spinel [11]. The Mössbauerpectrum for other cation substitutions infers that Co, Ni, and Znere distributed uniformly over the system while Mn, Cd and Cuave the tendency to accumulate on the particles surface [12,13].

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-ation’s are obtained from Raman spectroscopy. The above is

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

ents [13] 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 Cunters at tetrahedral or octahedral site in the resultant spinel struc-ure.

We thus aimed at studying the structural and electric transportroperties of the Cu substituted magnetites Fe3−xCuxO4 (x = 0, 0.01,.20, and 1.0) following X-ray diffraction (XRD), resistivity, Raman,nd Mössbauer spectroscopy.

. Experimental procedure

The polycrystalline samples with the composition Fe3−xCuxO4 (x = 0, 0.01, 0.2,nd 1.0) samples were prepared by the solid-state route reaction method [14]. Sto-chiometric amounts for Fe2O3, FeO and CuO, were mixed and heated (calcinations)t different temperatures [850, 950, and 1050 ◦C] in air for 24 h and the samples werexygen annealed at temperatures (950 and 1050 ◦C) and at atmospheric pressure for4 h with intermediate grindings. The pellets were finally sintered and annealed at050 ◦C in oxygen atmosphere. The samples were characterized by means of X-rayiffraction (XRD), resistivity and room temperature Raman and Mössbauer spec-roscopy. The XRD measurements were carried out with Cu K˛ (1.54 Å) radiationsing a Rigaku powder diffractometer equipped with a rotating anode scanning0.01 step in 2�) over the angular range 10–80◦ at room temperature.

The dc electrical resistivity measurements of the sample have been done from0 to 300 K using four-probe method. Indium contacts are made on the polishedurface of the sample. The sample is in the form of a rectangular rod. The sampleizes of Fe3−xCuxO4 (x = 0, 0.01, 0.2, and 1.0) are 0.756, 0.719, 0.714, and 0.736 mm,espectively. The SI temperature controller, Schlumberger multimeter and Advan-est current source are used for the measurement and the details of set up areeported elsewhere [15]. The Raman measurement on the pellets of 10 mm in diam-ter have been made by Jobin-Yovn Horiba Labram HR800 single monochromatoroupled with a Peltier cooled charge coupled device with He–Ne laser (632.81 nm)t room temperature. The Mössbauer measurements were carried on powder sam-les using a standard PC-based spectrometer equipped with a Weissel velocity driveperating in the constant acceleration mode.

. Results and discussion

The X-ray diffraction patterns have been collected on the sur-aces of the disks with a Rigaku diffractometer using Cu K˛adiation. Fig. 1 shows the representative �–2� scans of XRD for thearent and Cu2+ as cation substituted Fe3O4 and could be indexed ashat of cubic system with face centered lattice. For parent magnetite

Fig. 1. The X-ray diffraction patterns for Fe3−xCuxO4 (x = 0, 0.01, 0.20, and 1.00).

and Cu substituted Fe3O4 samples, the strongest Bragg peak occurat 2� ≈ 35.9◦ corresponding to (3 1 1) reflections. The above indexedpeak is consistent with the Joint committee for powder diffrac-tion (JCPD) data [16]. This implies that the cation Cu2+ replacesFe2+ (B site). The 2� values of diffraction patterns for higher doped(x = 0.2 and 1.0) samples are consistent with the parent and lightlydoped magnetites confirming the cubic spinel structure. However,the Bragg peaks looses intensity.

The calculated lattice parameter of parent magnetite Fe3O4 isestimated as 8.379 Å, which is close to the bulk lattice parame-ter (8.393 Å) [1,16,17]. The Cu substitution enhances the latticeparameter as 8.381, 8.386, and 8.391 Å for x = 0.01, 0.2, and 1.0respectively. The increase of the lattice constants with the increasedCu as cation doping concentration x is attributed to the difference

Fig. 2. Variation of resistivity of copper ferrites with temperature, plotted as log10 �versus 1000/T for Fe3−xCuxO4 (x = 0, 0.01).

436 D. Varshney, A. Yogi / Materials Chemistry and Physics 123 (2010) 434–438

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The room temperature 57Fe Mössbauer spectra of parent Fe3O4magnetite sample is shown in Fig. 6, whereas the spectra corre-sponding to Fe2.99Cu0.01O4, Fe2.8Cu0.2O4 and Fe2CuO4 are givenin Fig. 7. The Mössbauer spectrum is analysed by consideringtwo (three) symmetric Lorentzian-shaped sextets model, corre-

ig. 3. Variation of resistivity of Copper ferrites with temperature, plotted as log10 �ersus 1000/T for Fe3−xCuxO4 (x = 0.20, 1.00).

xhibits a discontinuity at about 123 K consistent with the knownerwey transition (the slope of the plot log10 � versus 1000/T isaximum) [18]. The sample Fe2.99Cu0.01O4 shows a discontinuity

n resistivity at about 119 K. Similar features are reported pre-iously for Fe2ZnO4 magnetites [6]. The transport properties ofhe cation-substituted magnetites are sensitive to the doping con-entration and Fig. 3 illustrates the resistivity behavior for x = 0.2nd 1.0 samples, respectively. It is further seen that for samplese2.8Cu0.2O4 and Fe2CuO4 the electrical characteristics are com-letely altered.

In Fig. 3, we have plotted log10 � as functions of 1000/T, whereoth the curves are much flatter as compared to Fig. 2 as well nonomalous variation is documented, only a gentle variation in theurves are noticed consistent with the previous reports for Fe2ZnO4agnetites [6]. For x = 0.2 and 1.0 samples, log10 � curves intersect

t a temperature of about 125 K in the vicinity of the Verwey tran-ition. The Cu doped samples (x = 0.2 and 1.0) have considerablyigher resistivity in the range 77 K < T < 300 K as compare to thearent and under doped (x = 0.01) magnetite samples. The partialr complete replacement of Fe3+ ions by Cu2+ ions is evidenced fromhe changes in the electrical resistivity due to charge localizationn the octahedral Fe sites. The viable conduction mechanism is thelectron hopping between two adjacent octahedral sites (B sites) ashe periodical ordering of Fe3+ and Fe2+ ions in the crystallographicattice results Fe2+ + Cu2+ ⇔ Fe3+ + Cu+ [19].

The resistivity measurements on magnetite single crystalse3−xMxO4 doped with M = Ni, Co, Mg, Al, Ga, and Ti shows thathe Verwey temperature shift as function of the substituent con-entration. Doped magnetites with Ni, Co, or Mg yield a transitionemperature shift towards lower temperature from 125 K [20]. Thepecific heat anomaly and the entropy changes for Al- and Ti-ubstituted magnetite at the Verwey transitions reveals a relativeeak dependence of the Tv shift as a function of the substituent

oncentration [21]. The thermoelectric power and conductivityeasurements on Fe3−xTixO4 doped single crystals of magnetite

eveal a sharp decrease at Tv and a marked rise on lowering theemperature. From the analysis of thermoelectric and conductiv-ty measurements it is argued that above and below Tv a thermalctivated mobility of the electrons is noticed, indicating hoppingonduction in both phases [22]. In the present investigations the

esults with Cu doping are consistent with the previous measure-ents on Fe3−xMxO4 (M = Ni, Co, Mg, Al, Ga, and Ti) [20–22]. Weust mention that these measurements are performed on single

rystals leading to a very sharp transition with well-defined Tv,

Fig. 4. Raman Shift as a function of wave number for Fe3−xCuxO4 (x = 0, 0.01).

however our results are on polycrystalline samples although yieldTv in the similar range but due to the presence of some impuritiessuch sharpness is not noticed.

The Raman measurements in the range 100–800 cm−1 at roomtemperature have been made for parent and Cu doped magnetites(Fe3−xCuxO4; x = 0.01, 0.2, and 1.0). Fig. 4 illustrates the Ramanspectrum for parent Fe3O4 and five Raman active modes are iden-tified as: A1g = 669.7, Eg = 543.7, T2g(1) = 433.5, T2g(2) = 307.4 andT2g(3) = 188.2 cm−1 according to group theory assignment and con-sistent with the previous reports [7–9]. For x = 0.01 sample, weakphonon features with a minor shift in the same frequency rangeare observed and is shown in Fig. 4. Furthermore, Fig. 5 illustratesthat for x = 0.2 sample, the phonon modes are further weakened,while to that for Fe2CuO4, all the five Raman active modes are moresharp and pronounced with a left side Raman shift. The changesobserved in Raman shift for Fe2CuO4 shows a crossover from a firstorder transition to a higher order transition. Thus all samples docu-ment a Raman-active phonon at 670 cm−1 (Table 1) confirming thecubic inverse spinel structure of magnetites. We note that similarfeatures are observed for the room temperature Raman spectra ofFe3−x ZnxO4 [7].

Fig. 5. Raman shift as a function of wave number for Fe3−xCuxO4 (x = 0, 0.01).

D. Varshney, A. Yogi / Materials Chemistry and Physics 123 (2010) 434–438 437

Table 1Raman active phonon mode.

Raman activemode

Fe3O4 [Ramanshift (cm−1)]

Fe3−xCuxO4 [Raman shift (cm−1)]

x = 0.01 x = 0.2 x = 1.0

A1g 669.7 669.3 668.4 660.8T2g(2) 543.7 543 543.1 500.5Eg 433.5 433.1 432.9 414.5T2g(3) 307.4 306.9 306.4 288.3T2g(1) 188.2 187.7 187 220.7

Fig. 6. Room temperature Mössbauer spectra of the parent Fe3−xCuxO4 (x = 0.0, and0.01) samples. The full lines are the A- and B- site Lorentzian-shaped sextets andtheir sum fitted to the experimental data.

Fig. 7. Room temperature Mössbauer spectra of Fe Cu O (x = 0.20, and 1.00). The

Table 2Mössbauer parameters derived from the spectra using the two (three) symmetric Lorentz

Fe3−xCuxO4 ı (mm s−1) �EQ (mm s−1) Hhf (T)

x = 0 0.26 −0.35 49.540.59 −0.16 47.13

x = 0.01 0.25 −0.33 50.290.66 −0.11 48.130.58 −0.21 47.05

x = 0.20 0.28 −0.04 50.870.64 −0.19 48.390.53 −0.33 44.23

x = 1.00 0.28 0.23 51.610.63 −0.11 48.780.60 −0.19 45.92

Hyperfine magnetic field, Hhf; quadrupole splitting, �(Q; isomer shift, ı (relative to �-transmission Mössbauer spectra of Cu substituted (Fe3−xCuxO4, x = 0.0, 0.01, 0.20, and 1.0

3−x x 4

full lines are the A- and B- site Lorentzian-shaped sextets and their sum fitted to theexperimental data.

sponding to tetrahedral (A) and octahedral (B) magnetic sublatticesfor parent (doped) magnetites. The room temperature Mössbauerspectra are fitted with NORMOS-SITE program and all samplesshow magnetic ordering. The obtained value of Chi-2 is mini-mum as possible and NORMOS-SITE program fitted good to theexperimental data and are illustrated in the same figures. The 57Fefitted Mössbauer parameters are listed in Table 2. We note thatthe deduced hyperfine parameters are with respect to natural ironmatching with literature [4,12]. The room temperature Mössbauer

2+

spectra of Cu -substituted magnetites exhibit the additional pres-ence of a quadrupole-split doublet, with the hyperfine parameterstypical of super-paramagnetic magnetite particles consistent withthe earlier experimental data [10].

ian-shaped sextets model for parent (doped) magnetites.

Relative area (%) Spectrum Assignment of site

36.3 6 A (Fe3+)63.7 6 B (Fe3+, Fe2+)

40.4 2 A (Fe3+)39.7 6 B (Fe3+, Fe2+)19.9 6 B (Fe3+, Fe2+, Cu2+)

42.2 2 A (Fe3+)36.8 6 B (Fe3+, Fe2+)21.0 6 B (Fe3+, Fe2+, Cu2+)

48.7 2 A (Fe3+)34.6 6 B (Fe3+, Fe2+)16.7 6 B (Fe3+, Fe2+, Cu2+)

Fe at 300 K); and relative areas corresponding to the component pattern in the0) Fe3O4 at T = 300 K.

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38 D. Varshney, A. Yogi / Materials Che

For Cu as cation-substituted magnetites, the peak with highyperfine field values is related to the Fe involved in the elec-ron delocalization process, without the presence of Cu as nearesteighbors. It is seen in Fig. 7, that the second peak correspondso those iron atoms surrounded by some Cu neighbors. The B siteoped sample shows that charge distributions become broader andccupy more area as compared to the A site doped sample. Thusith the increased Cu amount, the relative area decreases. The

bove indicates that Cu ions in Fe2CuO4 have entered preferen-ially into the B sites. The observed isomer shift values from roomemperature Mössbauer data clearly show the presence of FerricFe3+) state only. We may refer to the work of Ok et al. on Fe2CrO4,ho notices that the increasing broadening of the Mössbauer lines

f the B pattern with increasing doping of Cr in magnetite is mainlyue to the distributions of Cr ions among the six nearest-neighbor Bites around a B-site Fe ion [23]. The present Mössbauer spectra ofu doped magnetites (Fe3−xCuxO4; x = 0.01 and 0.2) also reveal theimilar features. However, for completely substituted magnetite,e notice one additional quadrupole splitting that indicates lessagnetic nature of Fe2CuO4.For Cr doped magnetites, Ok et al. find that the quadrupole split-

ings at the A and B sites are constant (x < 0.3) and independent ofr concentrations [23]. The above is attributed to the fact that the

onic radii of 0.64 Å for Fe3+ are nearly the same as the 0.63 Å ionicadii of Cr3+ and the local symmetries at both A and B sites areot changed appreciably with Cr3+ substitution. On the other hand

or Cu as cation-substituted magnetites shows that the quadrupoleplitting at the A and B sites are not constant and dependent ofu doping and is attributed to higher ionic radii of Cu (0.72 Å) asompared to concentrations the ionic radii of 0.64 Å for Fe3+. Thesomer shift for Cr doped magnetites shows that the isomer shift isonstant for x < 0.3 exhibiting ferric like charge state and for x ≈ 0.5ore ferric charge state is documented [23]. Similar are the features

or Cu doped magnetites.The Co-substituted magnetites Fe3−xCoxO4 have been studied

y Mössbauer spectroscopy at low and at high temperatures withmphasis on low doping concentrations (x ≤ 0.04). The hyperfinearameters obtained from the transmission spectra reveals thathe largest hyperfine field arises from the tetrahedral ferric ions24]. For Cu as cation-substituted magnetites the fitted A and Bites Lorentzian-shaped sextets and quadrupole splitting at the And B sites are consistent with the earlier results on Fe3−xCoxO4x < 0.3) [24]. Mössbauer parameters are characteristic of Cu substi-uted magnetite and indicate the presence of a single phase only.

e end up by stating that the isomorphic substitution as Cu ine2CuO4 occurs preferentially on octahedral coordination sites ofhe spinel structure.

. Conclusions

The X-ray diffraction pattern of Fe3−xCuxO4 (x = 0, 0.01, 0.2, and.0) shows that all prepared samples crystallize in the spinel cubictructure and are in single phase. The electrical resistivity confirms

he first order-disorder phase transition for Fe3O4 (Fe2.99Cu0.01O4)

agnetites at about 123 (119) K. The abrupt change in resistivity atow temperature is attributed to charge localization on the octahe-ral iron sites. However for Fe2.8Cu0.2O4 and Fe2CuO4 magnetites,he resistivity behavior does not document any substantial changes

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y and Physics 123 (2010) 434–438

at low temperatures. Furthermore, for Fe2.8Cu0.2O4 and Fe2CuO4samples, the high resistivity at all measured temperatures as com-pared to Fe3O4 and Fe2.99Cu0.01O4 magnetites identifies increaseddisorder at B sublattices of Fe3O4.

The Raman measurements of Fe3−xCuxO4 (x = 0, 0.01, 0.2,and 1.0) document five Raman active modes. The weak phononfeatures are observed for x = 0.01 and 0.2, while to that forFe2CuO4 all the Raman active modes are sharp and pronounced.The changes observed in Raman shift for Fe2CuO4 as well theresistivity behaviour shows a higher order structural transition.The room temperature 57Fe Mössbauer spectra of parent Fe3O4,Fe2.99Cu0.01O4, Fe2.8Cu0.2O4 and Fe2CuO4 show magnetic ordering.The spectra of Cu2+-substituted magnetites exhibit the additionalpresence of a quadrupole-split doublet with higher magnitude ascompared to that of parent magnetite. For Cu doped Fe3−xCuxO4(x = 0, 0.01, 0.2, and 1.0), we notice a sharper sextet for Fe3O4 thatcorresponds to Fe3+ state. The Cu2+ substitution at Fe3+ site resultsa broader pattern implies a less well-defined oxidation state of Fe3+

consistent with the previous 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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