A Mössbauer evaluation of cation distribution in titanomagnetites

*Corresponding author. Tel.: 1#316-978-3994; fax: 1#316-978-3350; e-mail: [email protected].

Journal of Magnetism and Magnetic Materials 191 (1999) 72—78

A Mossbauer evaluation of cation distributionin titanomagnetites

H.H. Hamdeh!,*, K. Barghout!, J.C. Ho!, P.M. Shand", L.L. Miller#

!Department of Physics, Wichita State University, Wichita, KS 67260, USA"Department of Physics, University of Northern Iowa, Cedar Falls, IA, 50614, USA

#Department of Physics and Ames Laboratory, Iowa State University, Ames, IA, 50011, USA

Received 24 June 1998; received in revised form 13 August 1998

Abstract

Peak area analyses of Mossbauer spectra were used to directly appraise models of cation distribution in synthetictitanomagnetites, Ti

xFe

3~xO

4(0(x(1). For x(0.30, room-temperature and zero-field data were sufficient for this

purpose. At higher Ti concentrations, however, they were inconclusive or ineffective because the Mossbauer signals fromFe ions on both tetrahedral and octahedral sites were inextricable. Instead, Mossbauer spectra were collected at 4.2 K ina 7 T magnetic field. In these spectra, the signal from Fe3` ions on the tetrahedral sites of the cubic spinels was effectivelydelineated, and on the basis of its area fraction a new cation distribution is proposed. ( 1999 Elsevier Science B.V. Allrights reserved.

PACS: 75.50.Gg; 76.80.#y

Keywords: Titanomagnetites; Cation distribution; Spinel; Mossbauer spectroscopy; Magnetization

Titanomagnetites have been of interest for manyyears because they are naturally occurring constitu-ents of igneous rocks such as basalts, and studies ofthe characteristics of the magnetization of suchrocks have provided a wealth of information on themagnetic history of the earth [1,2]. Apart fromtheir importance in the area of geophysics,titanomagnetites by themselves have been exten-sively investigated because of the interest inthe technological applications of magnetite and

other ferrites. The synthetic titanomagnetite seriesTi

xFe

3~xO

4(0(x(1) provides an excellent

system for studying the evolution of magnetic,electronic, and structural properties with titaniumconcentration.

Magnetite (Fe3O

4) is the ideal mixed-valence 3D

transition metal oxide which crystallizes in an in-verted cubic structure above the Verwey transitiontemperature. In the framework of an FCC lattice,Fe3` ions occupy tetrahedral (A) interstitial sitesand equal number of Fe3` and Fe2` ions occupyoctahedral (B) interstitial sites. Titanomagnetitesare solid solutions of magnetite and ulvospinel(Fe

2TiO

4) in which a fraction of Fe3` ions on the

0304-8853/99/$ — see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 3 4 0 - 0

B-sites of magnetite are substituted by Ti4` ions[3,4]. When a Ti4` ion is substituted for an Fe3ìon, another Fe3` ion on either sites must be con-verted to an Fe2` ion in order to maintain chargebalance. The consequent distribution of cations be-tween the two possible sites was shown to be crucialto the understanding of the complex magnetic be-havior of titanomagnetites. For example, the dissi-pation of magnetic energy at low temperatures hasbeen ascribed to the redistribution of Fe valencestates on B-sites due to electron transfer [5,6]. Also,the rapid increase of magnetocrystalline anisotropy[7,8] as well as the remanent and coercive fields[9,10] at low temperatures with increasing titaniumhave been related to the l—s coupling of the Fe2ìons on B-sites [11]. Further, the tetragonal elonga-tion [12] in titanium-rich titanomagnetites hasbeen attributed to the Jahn—Teller distortion result-ing from the A-site Fe2` ions [11]. Some workershave also reported on evidence of spin canting andspin glass behavior at high titanium concentrations(x'0.8) [13,14].

The need for an accurate knowledge of cationdistribution in titanomagnetites has motivated nu-merous studies, which resulted in four models.These models are represented by the following for-mulas where the round and square brackets denoteA- and B-sites, respectively. The first model, due toAkimoto [15], assumed an equal concentration ofFe3` ions on both sites which varies linearly with x:

(Fe3`1~x

Fe2`x

)[Fe3`1~x

Fe2`1

Ti4`x

]O2~4

(0)x)1).

The second model, proposed by Neel [16] andChevallier et al. [17] assumed a strong preferenceof Fe3` ions for the A sites for x)0.5, as postu-lated by Verwey and Heilmann [18]:

(Fe3`1

)[Fe3`1~2x

Fe2`1`x

Ti4`x

]O2~4

(0)x)0.5),

(Fe3`2~2x

Fe2`2x~1

)[Fe2`2~x

Ti4`x

]O2~4

(0.5(x)1).

The third model, due to O’Reilly and Banerjee [19],is given by

(Fe3`1

)[Fe3`1~2x

Fe2`1~x

Ti4`x

]O2~4

(0)x)0)0.2),

(Fe3`1.2~x

Fe2`x~0.2

)[Fe3`0.8~x

Fe2`1.2

Ti4`x

]O2~4

(0.2(x(0.8),

(Fe3`2~2x

Fe2`2x~1

)[Fe2`2~x

Ti4`x

]O2~4

(0.8)x)1)

and the fourth model, proposed by Kakol et al.[20] is written as

(Fe3`1

)[Fe3`1~2x

Fe2`1`x

Ti4`x

]O4

(0)x)0.2),

(Fe3`1.25~1.25x

Fe2`1.25x~0.25

)

[Fe3`0.75~0.75x

Fe2`1.25~0.25x

Ti4`x

]O4.

(0.2(x)1).

The latter two models were proposed on the basisof saturation magnetization measurements done at77 K.

Previous Mossbauer studies [21,22] in the con-centration range x(0.30 supported the Akimotomodel, though the agreement is not exact. At highertitanium concentrations, the complexity of room-temperature spectra collected in zero-field renderedthe technique inconclusive or completely ineffec-tive. In order to deconvolve the peaks of the Fe3`sextet on the A-sites, we have collected Mossbauerspectra of polycrystalline Ti

xFe

3~xO

4for x"0.30,

0.4, 0.5, 0.68, 0.8 and 0.96 at 4.2 K in an applied fieldof 7 T. Here the areal fraction ( f ) of this sextet wasmeasured accurately, and by preserving the chargebalance, the cation distribution can be expressed as

(Fe3`f(3~x)

Fe2`1~f(3~x)

)

[Fe3`2(1~x)~f(3~x)

Fe2`x`f(3~x)

Ti4`x

]O2~4

.

Consequently, this paper reports the first directevaluation of cation distribution in titanomagne-tites over the whole composition range.

Powders of titanomagnetites were prepared byheating stoichiometric amounts of a-Fe

2O

3and

TiO2

to 1000°C and quenching to room temper-ature. X-ray powder diffraction pattern showed ex-cellent phase purity for the samples with x'0.5,with the single phase being in the cubic space groupFd3m and having peaks between those of Fe

3O

4and Fe

2TiO

4. Some samples with smaller Ti con-

centrations (x"0.2, 0.3, 0.4, 0.5) contained smallamounts of residual a-Fe

2O

3.

Fig. 1a shows the X-ray powder diffraction pat-terns for the sample with x"0.2, which gives anindication of the extent of the a-Fe

2O

3impurity

phase in the low-x samples. Fig. 1b shows thesingle-phase pattern for x"0.68. Note that a sili-con standard was also mixed with the samples forthe purpose of lattice parameter refinement. The

H.H. Hamdeh et al. / Journal of Magnetism and Magnetic Materials 191 (1999) 72—78 73

Fig. 1. X-ray diffraction patterns for the powder samples ofTi

xFe

3~xO

4for (a) x"0.2 and (b) x"0.68. The peaks corre-

sponding to residual a-Fe2O

3are indicated for the sample with

x"0.2. Peaks corresponding to the silicon standard are alsoindicated. The other indexed peaks correspond to pure magnet-ite (Fe

3O

4).

(h k l) indices which are not preceded by the Si anda-Fe

2O

3identifiers are those corresponding to

Fe3O

4. It should also be noted that a Rietveld

refinement of the X-ray data for each sample yiel-ded lattice constants which exhibited a linear vari-ation with x, which is consistent with previousresults [21].

Mossbauer effect measurements were taken inthe transmission mode, with 50 mCi 57Co sourcemounted in a conventional constant accelerationspectrometer. The absorbers were prepared byevenly dispersing titanomagnetites powders be-tween two pieces of plastic tape. For low temper-ature and field measurements, the samples wereclamped to the finger of a liquid 4He flow cryostatand placed in the bore of a superconducting

solenoid magnet such that the field direction co-incided with the gamma-ray beam.

Mossbauer spectra of stoichiometric magnetite(x"0) taken at room temperature contain twodistinct hyperfine components with areal fractionsof 1 : 2 ratio. The sextet with 490 kG hyperfinemagnetic field (HMF) and 0.30 mm/s (relative topure Fe) isomer shift (IS) corresponds to the A-siteFe3` ions. The large sextet with HMF"460 kGand IS"0.65 mm/s is related to the B-site Fe2.5ìons, which arise from the rapid electron exchangebetween Fe3` and Fe2` ions [23]. Like Ti4`, cat-ion vacancies in non-stoichiometric magnetite(Fe

3~dO

4) selectively form at the B-sites [24] but

they replace the B-site Fe2` ions, which are con-verted to Fe3` ions. In such samples the A sextet is,in fact, two superimposed sextets, with the addi-tional component originating from the unpairedB-site Fe3` ions that are not involved in the elec-tron exchange process [25]. Indeed, the apparentarea ratio of the A and B sextets increases bya sizable amount for a small number of cationvacancies. However, for titanomagnetites, in whichFe3` ions are replaced by Ti4` and Fe2` ions,a new component appears under the shoulder ofthe B sextet external peaks. This additional sextetis labeled C in Fig. 2. The measured values,HMF"423 kG and IS"0.71 mm/s at x"0.10,show the Fe ions of component C to have a moreferrous character than those of B. This is becausepure Fe2` ions have an IS larger than 1 mm/s andHMF as small as 330 kG [26] due to contributionsfrom the orbital magnetic moment and the dipolarinteractions. Also, the broader line widths of com-ponent C suggest a slower electron exchange pro-cess than that of B, which is consistent with thelarger mobility activation energy with titaniumsubstitution [27]. For small titanium concen-trations (x(0.2), the areal fractions of the threesextets can be assessed correctly. To evaluate thefractions of Fe cations on A- and B-sites, arealfractions measured at room temperature are occa-sionally corrected by using their recoilless fractionratio of 0.94 [28]. This ratio, however, is not gener-ally accepted because it was obtained from naturalmagnetite rather than synthetic material. In thiswork, areal fractions are considered to be equal toFe fractions, and on the basis of their values one

74 H.H. Hamdeh et al. / Journal of Magnetism and Magnetic Materials 191 (1999) 72—78

Fig. 2. Mossbauer spectra at room temperature for the in-dicated samples. Continous lines are the calculated sub-spectraand their sum.

Fig. 3. Fraction of A-site Fe3` ions as a function of titaniumconcentration x. Error bars are based on the statistics of thefittings.

can readily conclude that C originates primarilyfrom B-site Fe ions. The changes in the areal frac-tion of A shown in Fig. 3 are too small for Moss-bauer effect measurements to detect the few Fe2ìons on the A-sites. There is, however, the possibili-ty of electron exchange between Fe3` and Fe2ìons on equivalent A-sites. In this case, these ionsmight contribute to either the B or the C sextets.Consequently, f would be larger than the one infer-red from the data. Again this effect may be negli-gible for x(0.2, but its severity would increase asA-site Fe2` ions become more abundant at highertitanium concentrations. Furthermore, other com-plications are evident in Fig. 2 due to the associatedchanges in the magnetic state. The line widths of thethree sextets increase at a higher rate, and theHMFs start to decrease appreciably for 0.3)x)0.5 and collapse to zero before x"0.8. Clearly,the uncertainties are too large to be ignored if f isobtained from the room temperature measure-ments. In an attempt to minimize or eliminate these

effects, Mossbauer spectra were collected in a 7 Tmagnetic field, allowing a more definite determina-tion of f. These values are presented in Fig. 3 for0.3)x)0.96.

Magnetic ordering in magnetite-based spinelsarises from the large negative exchange interactionbetween Fe ions occupying the A- and the B-sites.This interaction dominates the negative intra-sub-lattice exchange interactions, causing each of thetwo magnetic sublattices to be aligned. A non-zeromagnetization is measured because the total mag-netic moment of the B sublattice is typically greaterthan that of the A sublattice. The application ofa strong bias field will cause magnetic moment ofthe sublattice with the greater moment (‘majoritysublattice’) to align parallel with the field and mag-netic moment of the sublattice with the smallermoment (‘minority sublattice’) to align opposite tothe field. Since the HMF at the 57Fe nucleus isopposite to the magnetic moment, the absorptionlines in the Mossbauer sextet from the majoritysublattice will be pushed to lower velocities andthose from the minority sublattice will be pushed tohigher velocities. Results from such measurementsare shown in Fig. 4. As expected, all of the spectranow show distinct A sextets except for x"0.96where the signal from A-site Fe3` is very weak.Here, f is best obtained by fitting the first and sixthpeaks of the A sextet and integrating the area underthe whole spectrum. By assuming a spin structure


Fig. 4. Mossbauer spectra at 4.2 K in a 7 T applied field for theindicated samples.

Fig. 5. Variation of A- and B-sites Fe ions with titanium con-centration x. Symbols and continuous lines are based on dataand linearized approximation, respectively. Error bars are gen-erated by the statistics of the fittings.

that is collinear with the applied field, the areasunder the remaining peaks of the A sextet can beobtained from the intensity ratio of 1 : 0 : 1

3for the

three pairs of peaks I1,6

: I2,5

: I3,4

. This assump-tion is reasonable given the observed isolation ofthe external peaks of the A sextet, which is depen-dent on the spins alignment with the applied field.The computed value of f at x"0.3 is slightly largerthan, but in a fair agreement with, that obtainedfrom the room temperature spectrum. Whenevercomparison is possible, however, the difference isfound to be wider at higher titanium concentra-tions. In addition to providing a method for ex-tracting the Fe3` ions occupancy of the A-sites, thefield is strong enough to suppress the effects ofthermal excitations, especially at 4.2 K. These fluc-tuations arise from the interruption of the long-range order and the formation of small size andthermally unstable magnetic clusters. It is also con-ceivable that the applied field is strong enough tohinder the possible electron exchange process at the

A-sites. The cause of this ‘self-trapping’, which isfrequent in ionic crystals, is the lattice distortionassociated with the Jahn—Teller effect of degeneratestates. Such modifications by the applied fieldwould definitely account for the observed largerf values.

The dependence of the measured f values onx can be approximated by the following linearrelations:

f"0.333(1!0.25x) (0)x)0.2),

f"0.35(1!0.57x) (0.3(x)0.5),

f"0.50(1!x) (0.5(x)1).

When combined with the cation distribution for-mula, these relations lead to the variations of Fe3ànd Fe2` ions among the A- and B-sites presentedin Fig. 5. For comparison with previous models, thesaturation moments of Fig. 6 are obtained by as-suming 4.06 l

bfor Fe2` and 5 l

bfor Fe3`. Our

results agree best with the saturation moments


Fig. 6. Composition dependence of saturation moment for pres-ent and previous models. Symbols and continuous line are basedon present data and model. From top to bottom, the broken anddotted lines represent the models of Akimoto, Ka1 kol et al.,O’Reilly—Banerjee, and Neel—Chevallier, respectively.

measured at 77 K by Ka1 kol et al. [20] forx*0.5. At lower titanium concentrations, the ob-served reduction in their magnetization may becaused by complications to the magnetic state in-duced by the substitution of Fe2` and diamagneticTi4` for Fe3`. The difference could also be in-fluenced by the traces of a-Fe

2O

3detected by X-ray

diffraction. We were unable to make meaningfulcorrections because the Mossbauer signal from theFe3` ions of the a-Fe

2O

3fine grains is indistin-

guishable from that of A-site Fe3` ions. Usually,these signals are discernible at room temperaturewhen a-Fe

2O

3is in bulk form. In addition, the

residual a-Fe2O

3could generate cation vacancies

in the titanomagnetite primary phase, which in turnimpact the cation distribution. It should be notedthat the cation distributions in this kind of oxidesare sensitive to impurities and cations or anionsdeficiencies.

Summarizing, we have proposed a new model forthe distribution of Fe2` and Fe3` ions in polycrys-talline titanomagnetites based upon Mossbauerspectra for samples with titanium concentrations inthe range 0)x)0.96. Using this model of thecation distribution, the variation of the saturationmagnetization of titanomagnetites as a functionof x was calculated. There was good agreementwith magnetization measurements conducted byKa1 kol et al. for x)0.5, but our results deviatedfrom the experimental data for lower values of x.

Acknowledgements

The authors wish to thank Dr. C. Radhakrish-namurty of Tata Institute of Fundamental Re-search for providing the samples.

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A Mössbauer evaluation of cation distribution in titanomagnetites