Charge disproportionation transition under external magnetic field in La 1 3 Sr 2 3 FeO 2.96Sunghyun Yoon and Chul Sung Kim

Citation: Journal of Applied Physics 97, 10A318 (2005); doi: 10.1063/1.1855274 View online: http://dx.doi.org/10.1063/1.1855274 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/97/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in F 57 e Mössbauer study of first-order phase transition in LaVO 3 J. Appl. Phys. 105, 07D509 (2009); 10.1063/1.3063075 Evidence of magnetodielectric coupling in multiferroic Pb ( Fe 0.5 Nb 0.5 ) O 3 ceramics from ferroelectricmeasurements and electron paramagnetic resonance Appl. Phys. Lett. 93, 172902 (2008); 10.1063/1.3006433 Magnetic properties of Bi 0.5 Sr 0.5 Fe x Mn 1 x O 3 ( 0 x 0.7 ) J. Appl. Phys. 103, 053910 (2008); 10.1063/1.2888529 High pressure bulk synthesis and characterization of the predicted multiferroic Bi ( Fe 1 2 Cr 1 2 ) O 3 Appl. Phys. Lett. 90, 112909 (2007); 10.1063/1.2713757 Coercive Sm 2 Fe 17 N 3 : A model pinning system created by heavy ion irradiation J. Appl. Phys. 83, 6902 (1998); 10.1063/1.367563

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Charge disproportionation transition under external magnetic fieldin La 1/3Sr2/3FeO2.96

Sunghyun Yoona!

Department of Physics, Gunsan National University, Gunsan 573-701, Korea

Chul Sung KimDepartment of Physics, Kookmin University, Seoul 136-702, Korea

sPresented on 9 November 2004; published online 28 April 2005d

The effects of magnetic field on the charge disproportionationsCDd transition for La1/3Sr2/3FeO2.96

above its transition temperature are investigated using the external field Mössbauer spectroscopy.Without an external magnetic field, a completely paramagnetic singlet was obtained in thetemperature range of the averaged valence state above the transition temperature, which wasinterpreted as coming from the average valence Fe3.6+. Under the external magnetic field at 225 K,which is above the CD transition temperature, a doublet of considerable magnitude of splitting issuperimposed to the central singlet. The origin of the doublet can be explained by a magneticZeeman interaction. However, it is found that there is still a considerable fraction of the Fe nucleithat does not experience the applied field. This is understood as an evidence of the fast electrontransfer among Fe ions. The observed magnetic Zeeman splitting probably results from particular Feions in the neighbor of either an oxygen vacancy or a lattice imperfection such as the domainwall. © 2005 American Institute of Physics. fDOI: 10.1063/1.1855274g

I. INTRODUCTION

Unlike in the case of manganites, the carrier-doped ironperovskite La1/3Sr2/3FeO3−y is free from any structural insta-bility such as the Jahn–Teller distortion despite of itsFe4+sd4d nature. It is now believed that this is the result ofthe existence of charge-disproportionationsCDd transition.Takanoet al.1 found that the Fe ions could take indiscretecharge states between Fe3+ and Fe5+ depending on the com-position. Subsequently he suggested the existence of nonin-tegral oxidation state of iron in this material following adisproportionation of the type.

2Fe4+ ↔ Fe4+l + Fe4−l.

This CD transition was abrupt and was accompanied by botha simultaneous metal-to-insulator transition and antiferro-magnetic ordering. Battleet al.studied the detailed feature ofCD transition in Sr2LaFe3O3+y by a Mössbauer measure-ment, and found a first-order-like transition from a high-temperature paramagnetic average-valence state in which allFe ions are electronically equivalent to a low-temperatureantiferromagnetic mixed-valence state.2 A neutron-diffraction study showed that two kinds of Fe ions with va-lence states Fe3+ and Fe5+ were found in a ratio of 2:1 below200 K. In this CD state, a sequence ofFe3+s↑dFe3+s↑dFe5+s↑dFe3+s↓d Fe3+s↓dFe5+s↓d exists alongthe f111g direction of the pseudocubic perovskite unit cell.3

More lately, such a CD state was also verified by transmis-sion electron microscopy.4 Parket al. suggested that the dis-tortion of the FeO6 octahedra decreased the strength ofp-d

hybridization interaction, and the CD transition becameblurred.5 Mössbauer spectrum for stoichiometricLa1/3Sr2/3FeO3−y at 4.5 K under the external magnetic fieldof 7 T was introduced recently.6 It consisted of two broad-ened sextets with relative intensities of second and fifth ab-sorption peaks increased in the spectrum, which led to asuggestion that it was an antiferromagnet with a collinearspin structure.

Battle et al. already mentioned about the asymmetricline shape of the paramagnetic singlet at 290 K, which indi-cated that the obtained spectrum could consist of severalcomponents. Now, it will be of interest to observe the changein the Mössbauer spectra above the CD transition tempera-ture under an external field. In this paper, we present resultsof x-ray diffraction sXRDd and external field Mössbauerspectroscopy for La1/3Sr2/3FeO3−y in order to see how theexternal magnetic field influence the CD state above its tran-sition temperature.

II. EXPRIMENTS

The nonstoichiometric La1/3Sr2/3FeO3−y polycrystallinesample was prepared by the conventional solid-state reactionmethod starting with La2O3, Fe2O3, and SrO powders in ap-propriate proportions. In order to prevent any inaccuracy inthe composition caused by using carbonate powder contain-ing moisture, pure strontium-oxide powder was preciselyweighed in a glove box filled with N2 gas.

The XRD patterns of the samples were obtained withCuKa radiation. The patterns at room temperature weretaken at a slow scanning speeds0.5° advance in 2u /mind toenhance the resolution. To determine the nominal Fe4+ con-tent, chemical analyses were carried out by digestion in astandardized solution of Mohr salt in the presence of HCl

adAuthor to whom correspondence should be addressed; present address:Department of Physics, Gunsan National University, San 68 Miryong-dong, Gunson 573-701, Korea; Fax:182-62-469-4561; electronic mail:shyoon@kunsan.ac.kr

JOURNAL OF APPLIED PHYSICS97, 10A318s2005d

0021-8979/2005/97~10!/10A318/3/$22.50 © 2005 American Institute of Physics97, 10A318-1

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and by titration with K2Cr2O7 standard solution using theappropriate indicator. A Mössbauer spectrometer of a con-ventional transmission-type was used in the constant accel-eration mode. External magnetic field was applied parallel tothe direction ofg-ray propagation using a split magnet su-perconductor cryostat. A57Co source in a Rh matrix wasused at room temperature.

III. RESULTS AND DISCUSSION

Profile refinements were carried out by theGSAS pro-gram package7 with a space groupR/3c. In the process ofrefinements, it was assumed that there were no orderings ineither the La/Sr cations or the oxygen vacancies. Oxygenfraction was fixed to the value obtained from the chemicalanalysis. Finally, a profile function formed by a convolutionof a pseudo-Voigt shapescw profile 2 provided in the pack-aged was used. The final observed and calculated room-temperature XRD profiles are illustrated in Fig. 1. Rhombo-hedral lattice constantaR and edge angleaR are 5.4874 Åand 60.07°, respectively. Both the bond lengths Fe–O and thebond angles/O–Fe–O and/Fe–O–Fe arelisted in TableI. Using the values of the bond lengths listed, the calculatedtolerance factor is 0.999, which is very close to 1. From thechemical analysis, oxygen deficiency is found to be 0.04indicating that 1.3% of the oxygen site are vacant. Stoichio-metric formula for the sample above the CD transition tem-perature deduced from the chemical analysis isLa1/3Sr2/3Fe4+

0.6Fe3+0.4O2.96.

Figure 2 displays the Mössbauer spectra forLa1/3Sr2/3FeO2.96 at 225 K as a function of applied magnetic

field. An additional spectrum at 6 T was taken within a nar-rower velocity range to enhance the resolution of the spec-trum. Without an external magnetic field, a completely para-magnetic singlet was obtained in the temperature range ofthe averaged valence state above the transition temperature.This singlet came from the average valence Fe3.6+.8 Althoughthe content of Fe4+ was definitely determined from thechemical analysis, it was not possible to single out the ab-sorption line from Fe4+ in the spectra. This can be explainedby an itinerant nature of thes* band derived from theeg

electron in high-spin Fe4+ st2g3 eg

1d. An electron can hop fromone Fe to another, generating both the intermediate valencestate and the metallic conductivity. If Fe4+ ions are in high-spin states and their 3d electrons are localized, Jahn–Tellerdistortion should take place. However, this material avoidssuch a distortion through fast hopping, resulting in a vanish-ing electric quadrupole splitting.

As the strength of the magnetic field is increased, a dou-blet of larger splitting emerges and superimposes upon thecentral singlet. Two possible origins for the doublet can becounted. First, the doublet possibly stems from an electricquadrupole interaction of Fe4+ ions. The other possibility isthe magnetic Zeeman interaction of iron with the externalmagnetic field. However, as is clear in Fig. 3, the splitting isdirectly proportional to the applied field. At 6 T, symmetricaldoublet has a splitting of 2.1 mm/s. This splitting of theoutermost lines nearly coincides with the applied field value.So the first possibility is ruled out and the observed doubletis regarded as a part of the magnetic hyperfine spectrum. Asthe external magnetic field is parallel to theg-ray direction in

FIG. 1. The observeds¯d, calculateds2d, and difference curves in theXRD profile for La1/3Sr2/3FeO2.96.

TABLE I. Structural parameters, bonding lengths, and bonding angles forLa1/3Sr2/3FeO2.96 at room temperaturefSG:R/3c, Hexagonal setting,xsO4d:x position of oxygen atomg.

Fe3–O4 1.9439 Å36 /Fe3–O4–Fe3 173.8°La1/Sr2–O4 2.7457 Å36 /O4–Fe3–O4 180.0°

2.8511 Å33 90.14°2.6782 Å33 89.86°

aH=5.4927 Å c=13.4349 ÅxsO4d=0.519

FIG. 2. 57Fe Mössbauer spectra for La1/3Sr2/3FeO2.96 taken under variousmagnetic-field strengths.

10A318-2 S. Yoon and C. S. Kim J. Appl. Phys. 97, 10A318 ~2005!

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the instrument, the lines 2 and 5 are suppressedsat the sametime the lines 3 and 4 are overwhelmed by the strong centralsingletd.

If this is the case, however, it is remarkable that there isstill a considerable fraction of the Fe nuclei that does notexperience the applied field. The authors think of it as anevidence of the fast electron transfer among Fe ions. Whensuch a fast hopping of electrons exists, fluctuations of hyper-fine parameters on57Fe nucleus will take place, resulting in arelaxation effect as can be observed in the singlet spectrumwithout a field. Isomer-shift value for the singlet is,0.06 mm/s and effectively remains constant irrespective ofthe field strength. Furthermore, isomer shift of the magneticcomponent is almost identical to that of the central singletfor each spectrum. An investigation on the isomer-shift val-ues for these two components leads us to assign an interme-diate average valencesFe3.6+d due to hopping mechanism tothese components. According to the observation of Battleetal. that the asymmetry of the singlet under zero field in-creased with the oxygen deficiency, the masked subspectrathat gives rise to the asymmetry may be the origin of theabove-mentioned magnetic component. Then the observedmagnetic Zeeman splitting probably results from particularFe ions in the neighbor of either an oxygen vacancy or a

lattice imperfection such as the domain wall. Unfortunately,as the data for a stoichiometric sample for the present mate-rial are not available, it is difficult to precede a further de-scription. At any rate, it seems that the fast hopping of elec-tron makes the external magnetic field at the57Fe nucleusaverage out to zero. The fraction of the absorption area forthe magnetic phase increases linearly as the strength of theapplied magnetic field is increased. At 6 T, the magneticcomponent: singlet area ratio was about 53:47. The expectedfraction of the masked subspectra at 0 T was linearly ex-trapolated to be about 32%.

Incidentally, measurements were carried out in two dif-ferent routes:sid the magnetic field was applied after thesample was heated from 4.2 K to 225 K,sii d in reverse or-der. However, there were no substantial differences in theshape of the spectra between these two cases.

In conclusion, Mössbauer spectra for La1/3Sr2/3FeO2.96

above its CD transition temperature under external magneticfield were studied. Under the external magnetic field of 6 Tat 225 K, which was well above the CD transition tempera-ture, there was still a considerable amount of Fe nuclei thatdid not experience the applied magnetic field. This was ex-plained by the existence of a fast electron hopping, whichmade some fraction of Fe nuclei blind to the external mag-netic field.

ACKNOWLEDGMENT

This work was supported by Grant No. R05-2001-000-00128-0 from the Basic Research Program of the Korea Sci-ence and Engineering Foundation.

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FIG. 3. Variations of splitting of outermost linessbottomd and fraction ofmagnetic componentstopd for La1/3Sr2/3FeO2.96 at 225 K as a function ofapplied magnetic field. The expected fraction of the masked subspectra at0 T was linearly extrapolated to be about 32%.

10A318-3 S. Yoon and C. S. Kim J. Appl. Phys. 97, 10A318 ~2005!

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