6
Solid State Communications,Vol. 16, pp. 987—992, 1975. Pergamon Press. Printed in Great Britain CATION DISTRIBUTIONS OF Y-Fe-Al GARNETS P. Fischer and W. Halg Delegation fir Ausbildung und Hochschulforschung am Eidg. Institut für Reaktorforschung, CH-5303 Wurenlingen, Switzerland and P. Roggwiller Physik-Institut der Universität Zurich, Switzerland and E.R. Czerlinsky Air Force Cambridge Research Laboratories, Bedford, U.S.A. (Received 27 December 1974 by E.F. Bertaut) The cation distributions of polycrystalline, paramagnetic Y—Fe—Al gamets were determined by neutron diffraction and Mossbauer spectroscopy. The results of both methods agree well, but deviate systematically from corres- ponding values obtained by previous investigations. Néel ferrimagnetic order- ing of Y 3Fe3A12O12 at 4.5 K was found with ordered Fe 3~ spin moments considerably reduced below 5 PB~ 1. NEUTRON DIFFRACTION STUDY OF THE determination of the metal distribution due to the FERRIMAGNET SYSTEM Y 3Fe5~A1~Oi2 essentially different, constant scattering amplitudes. 4’5 Moreover by magnetic neutron diffraction at low tem- 1.1. Introduction peratures it is possible to verify the validity of Néel’s THE PRESENT investigation of the garnets model of ferrimagnetism.6 Y 3Fe5~Al~O12, 0 ~c~ 5 was performed in order to derive the mean distribution of iron and aluminium on In the following we shall present, after a discussion the tetrahedral and octahedral sites 24d and 16a re- of experimental aspects, the refind structure parameters spectively (space group Ia3d-O~°),’ which is important of Y—Fe—Al garnets and report on the magnetic struc- for the interpretation of magnetic properties of this ture of Y3Fe3A12O12. Finally the results are discussed “statistical” ferrimagnet system. A particular aim was in Part 3. to determine whether there exist similar systematic differences in the corresponding results obtained by 1.2. Experimental different methods as are known to occur in the case of Y—Fe—Ga garnets. 2 Previously Czerlinsky et aL Polycrystalline samples of Y 3Fe5_~ALOi2 with nominal compositions c = 1, 2, 3, and 4 were supplied investigated the iron—aluminium distribution of Y3Fe5~AL~O12 by Mossbauer spectroscopy. 3 by Trans-Tech, Inc., U.S.A. The results of chemical analysis and the lattice constants ax and aN deter- mined by X-ray and neutron diffraction are given in Nuclear neutron scattering by paramagnetic Table 1. The lattice constants correspond reasonably Y—Fe--A1 garnets allows a direct experimental to the nominal compositions.3’7 987

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Page 1: Cation distributions of YFeAl garnets

Solid StateCommunications,Vol.16, pp. 987—992,1975. PergamonPress. Printedin GreatBritain

CATION DISTRIBUTIONSOFY-Fe-Al GARNETS

P. FischerandW. Halg

Delegationfir Ausbildungund Hochschulforschungam Eidg. Institut für Reaktorforschung,CH-5303Wurenlingen,Switzerland

and

P. Roggwiller

Physik-Institutder UniversitätZurich, Switzerland

and

E.R. Czerlinsky

Air ForceCambridgeResearchLaboratories,Bedford, U.S.A.

(Received27 December1974by E.F. Bertaut)

The cationdistributionsof polycrystalline,paramagneticY—Fe—Al gametsweredeterminedby neutrondiffraction andMossbauerspectroscopy.Theresultsof bothmethodsagreewell, butdeviatesystematicallyfromcorres-pondingvaluesobtainedby previousinvestigations.Néel ferrimagneticorder-ing of Y3Fe3A12O12at 4.5 K wasfoundwith orderedFe

3~spinmomentsconsiderablyreducedbelow5 PB~

1. NEUTRON DIFFRACTION STUDY OF THE determinationof themetal distributiondueto theFERRIMAGNET SYSTEMY

3Fe5~A1~Oi2 essentiallydifferent,constantscatteringamplitudes.4’5

Moreoverby magneticneutrondiffraction at low tem-1.1. Introduction peraturesit is possibleto verify thevalidity of Néel’sTHE PRESENTinvestigationof the garnets modelof ferrimagnetism.6Y

3Fe5~Al~O12,0 ~ c ~ 5 wasperformedin ordertoderivethemeandistribution of iron andaluminiumon In the following we shallpresent,aftera discussionthetetrahedralandoctahedralsites24d and16a re- of experimentalaspects,the refindstructureparametersspectively(spacegroupIa3d-O~°),’whichis important of Y—Fe—Al garnetsandreporton themagneticstruc-for theinterpretationof magneticpropertiesof this ture of Y3Fe3A12O12.Finally the resultsare discussed“statistical” ferrimagnetsystem.A particularaim was in Part3.to determinewhetherthereexistsimilarsystematicdifferencesin thecorrespondingresultsobtainedby 1.2.Experimentaldifferentmethodsasare knownto occurin the caseof Y—Fe—Gagarnets.

2PreviouslyCzerlinskyet aL Polycrystallinesamplesof Y3Fe5_~ALOi2with

nominalcompositionsc = 1, 2, 3, and4 were suppliedinvestigatedthe iron—aluminiumdistribution ofY3Fe5~AL~O12by Mossbauerspectroscopy.

3 by Trans-Tech,Inc., U.S.A.The resultsof chemicalanalysisandthelattice constantsax andaNdeter-minedby X-ray andneutrondiffraction aregiven in

Nuclearneutronscatteringby paramagnetic Table1. ThelatticeconstantscorrespondreasonablyY—Fe--A1garnetsallowsa directexperimental

to thenominalcompositions.3’7987

Page 2: Cation distributions of YFeAl garnets

988 CATION DISTRIBUTIONSOF Y—Fe—Al GARNETS Vol. 16, No. 8

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Page 3: Cation distributions of YFeAl garnets

Vol. 16,No. 8 CATION DISTRIBUTIONS OFY—Fe—Al GARNETS 989

l0~~X,

Theneutronmeasurementsof the samplesenclosed ~ I!

in cylindrical vanadiumtubeswereperformedby meansof a doubleaxisspectrometerat reactorSaphir,usingneutronsof wavelength(2.364±0.002)A. In orderto obtain theparamagneticstatethe Y3Fe4A1O12sample [T~ = (415 —430)K] ~wasmeasuredat 476K,whereasin theothercasesit is sufficientfor thedeter- Iminationof thecationdistribution to‘measureat roomtemperature.

tion andincoherentscatteringaccordingto the~R (= ~ ~ ~O~’O

The observedintensitieswerecorrectedfor absorp-product of linearabsorptioncoefficientandsample Iradius)valueswhich were determinedby transmission

measurements.

1.3. Cation distributionofY3Fe5_0A10012

As an illustration of theresultsFig. I showsneutrondiffraction patternsof Y3Fe3A12O12at 293 K in theparamagneticstateandat 4.5K in the ferrimagneticphase.Concerningthereflection431 (dependingonlyon theoxygens)the 4.5K intensitieswerenormalized LI_i ~U~,~to the 293 K diagram,becausethetwo patternscouldnot be measuredunderequivalentconditions.

Themeasuredneutronintensitieswereanalysed ° I°

by meansof theprofile method.8The resultsare FIG. 1. Observed(points) andcalculated(lines)neutron

summarizedin Table1. We omittedthe instrumental diffraction patternsof Y3Fe3A12O12(correctedfor

parameters:scalefactor,zero-pointof thescattering absorption,backgroundsubtracted).I,~= neutronin-tensity,20 = scatteringangle.TheunderlinedMifier

anglesand threeresolutionparameters.Individual indicesmarkreflectionswhich areonly due to oxygen.isotropic temperaturesfactors(B1) were usedfor theequivalentpositions96h of oxygen,24cof yttrium aswell asfor the tetrahedralandoctahedralsites,respect- Concerningtherelativefractionft = X/c of alu-ively. The scatteringamplitudeh of Ywasrefined,too, minium ions at tetrahedralsitesthe resultsof bothbecauseof differentpublishedvalues:b~= 0.79,~ typesof refinementsdo not differ essentially.Theft0.75±0.02,~in unitsof l012 cm.The resulting mean curveasfunction of c is comparedin Fig. 2 to corres-scatteringlengthb~= 0.759±0.002, relativeto pondingresultsobtainedby othermethods.b0 = 0.580,~agreeswell with thepreviouslydeter-minedvalue.

5Correspondingto the formula The positionparametersx0 andYo of oxygen

varyalmostlinearly with compositionbetweenthey3(Fe3_xAlx)d(Fe2_C+XAL.~_X)COl2‘ valuesfor c = 0 and5, knownfrom X-ray diffraction.

1’1°

whered relatesto thetetrahedralsites24d anda to On theotherhandz0 indicatesa maximumfor inter-

the’octahedraloxygenvacancies16 a, it is possibleto mediatealuminiumconcentrations.obtainfrom theneutronintensitiesboth thecationdistribution (X) andthe concentrationc. Therefine- 1.4. Ferriinagneticorderingof Y3Fe3AI2012ment wasalsomadeby usingthec valuesknownfrom Already in 1956theNéel—ferrimagneticcharacterchemicalanalysis. of Y3Fe5O12hadbeenestablishedby neutrondiffrac-

tion.6 In the long-rangeorderedstatethe magnetic

Fe3~momentsalign ferromagneticallywithin each

Page 4: Cation distributions of YFeAl garnets

990 CATION DISTRIBUTIONS OFY—Fe—Al GARNETS Vol. 16,No. 8

ft1.0 thesaturationmomenti~all thoseiron ions, which are

exchange-linkedto at leastonefurther Fe3~,results0.9 theoretically(Gilleo—Gellermodel9)in ~e =

and~u~’e= 4.9412B. Furtherrestrictionof cooperativemagneticinteractionsto caseswithcoordinationto at0.8

leasttwo otheriron ions(Gilleo model12)yields~. 7= 4.85j.~and~e = 4.S3j~.Apparentlythese

0.6 assumptionsdo notexplainthe observedlow ordered

_______________________ momentsin zero externalmagneticfields. It is intended0.5

0 1 2 3 4 5 c to extendthemagneticneutrondiffraction measure-mentsto othercompositions,in orderto determine

FIG. 2. Meanrelativeamount(fe) on tetrahedralsitesvs the total amount(c) of Al ionsin Y—Fe—A1garnets. in moredetail thefunctionaldependenceof the—. —. correspondsto the statisticalFe,Al distribution. orderedmomenton thesubstitutionof iron by alu-• = resultsfrom the presentneutrondiffraction ex- minimum. A short abstractof thepresentwork wasperiments.• refersto a formerneutroninvestigation.9 publishedrecently.’3A, A = resultsfrom thepresentMössbauerspectroscopystudy (A concernsthe samplesmeasuredalso b neutrondiffraction). x = publishedMössbauerresults. The 2. INVESTIGATION OF CATION DISTRIBUTIONdotted line correspondsto valuesderivedfrom bulk IN YIA1G BY MOSSBAUER-EFFECTmagneticmeasurements.1

SPECTROSCOPY2.l.Introduction

sublatticé24d and l6a,andthereis antiparallelcoupl-ing betweenthe momentsof the two different sub- An investigationon aluminium-substitutedlattices.This ferrimagneticstructureresultsin magnetic yttrium iron garnetsby Mossbauer-effecttechniqueintensitycontributionsto nuclearneutronreflections showsa goodagreementin the cationdistributionexceptthosewhich are only dueto the oxygenions. comparedto theresultsobtainedby neutrondiffraction.In the presentcubic casethe spin orientationcannot The Mossbauerspectrawere takenwith powdersamplesbedetermined.Thecorrespondingintensityfactor producedby sinteringthemixturesof the oxidesaveragesto 2/3. Y

203, A1203,andFe203of extremelyhighpurity.Tensamplesof theunit formulaY3A1~Fe5~O12with

As canbeseenfrom Fig. 1, the samemagnetic c from 0—4.5 in stepsof 0.5were preparedandorderingat 4.5 K holdsalso for Y3Fe3A12O12,where measured.Moreoverthe samplesusedin theneutronthe netmagnetizationis almostzero.

7Thereareno diffraction experimentswere also investigated.purely magneticpeaks.Comparedto thenucleardia-gramat 293 K in theparamagneticstateone observes Above theNéel temperaturestheMossbauerstrongmagneticcontributionsto certainreflections, absorptionspectrashowtwo quadrupolsplit lines.particularlyto 220. Thesecanbeattachedto the 57Fe nuclei of atoms

locatedona- andd-sitesof the garnetsrespectively.

Usingthecationdistribution determinedin the The intensitiesof the absorptionlinesare proportionalparamagneticstateand themeasuredFe31 neutron to the numberof 57Featomson the correspondingform factorof MgFe

2O4~ theNée! modelhasbeen sites.Fromthe ratio of theseintensities,therelativerefined.The resultingorderedmagneticmomentper amountof A1

3’1 ions on thetetrahedralsitesis deter-Fe3’1 ion (on the 24dand16a sites)of approximate mined.magnitude3.7J.LB at 4.5K, i.e. practically thesaturationvalue,is considerablylower thanthe usualspin 2.2.Samplepreparationmomentof Fe3’1’ (3d5) of 5#B• Presumablythis re- Foreachsample,the threeoxides(Y

2O3, Al203,ductionis dueto decreasedexchangeinteractionsand Fe2O3)of highpurity were mixed in the appropriateassociatedconsiderablestatisticaldeviationsfrom the ratio. After drying the mixturesat 200°Ctheyweremeanspinorientationsbecauseof diamagneticalu- pressedto pills of about200 mgeach.Within a p!atinminium neighbours.Consideringfor contributionsto cruciblethesewere sinteredfour timesin a temperature

Page 5: Cation distributions of YFeAl garnets

Vol. 16,No.8 CATION DISTRIBUTIONSOFY--Fe—AlGARNETS 991

Table2. Durationand temperatureof thefour sinter- Eachspectrumconsistsof two quadrupolsplitingprocessesfor eachsample linescorrespondingto the octahedralandtetrahedral

sitesof the iron. With aleastsquaresfit programthec °C h °C h °C h °C h quadrupolsplittingand therelativeintensityof the

0,0.5,1.0 1400 1 1440 3 1460 12 1480 24 two doubletswere calculated.The obtainedvaluesfor1.5, 2.0, 2.5 1400 1 1440 3 1480 12 isoo 24 themeasuredabsorptionline intensity ratiosD/A are3.0,3.5 1400 1 1460 3 1500 12 1520 24 givenin Table3. AssumingequalDebye—Wallerfactors4.0,4.5 1400 1 1460 3 1520 12 1550 24 for 57Feon both latticesites,theareaunderthe

absorptionlinesareproportionalto theoccupationof thesitesby iron.

Table3. Measuredabsorptionline intensity,~~ztid~’D/Aof57Feon d-and a-sites. ThefractionofsubstitutionalAlon d-sitesis calculatedfromf~= [3 + (c — 2)D/A] / 3. DISCUSSION[(1 + D/A)cJ. * indicatestheMössbauerresultsob-tainedfromthesamplesusedin theneutrondiffraction Thecationdistributionsresultingfrom the present

experiments neutrondiffraction andMössbauerexperimentsagreewithin theexperimentalerrorswell. Both methods

c RatioD/A ft (Al3~1’) yield approximatelyequivalentresults.Theerrorsare

0.5 1.328±0.028 0.866±0.046 largestfor smallaluminiumconcentrations,where0.99* 1.192 0.828±0.022 presumablybulk magneticmeasurementsaremore1.0 1.192±0.030 0.825±0.025 sensitive.1.5 1.042±0.020 0.809±0.0101.96* 1.028 0.744±0.008 Recent(concerningthecationdistribution less2.0 0.980±0.016 0.758±0.006 accurate)X-ray diffraction measurementsof powder2.5 0.917 ±0.018 0.722±o.oos samplesof Y—Fe—Al garnets14also supportthepre-2.97* 0.905 0.685±o.oos sent results.3.0 0.790±0.015 0.708±0.0033.5 0.715±0.012 0.678±0.002 Formostof thecompositionrangec Al31’ ions3~97* 0.871 0.635±0.003 havea preferencefor tetrahedralsites(Fig. 2).This4.0 0.613±0.010 0.655±0.001 canbeexplainedqualitativelyby thefactthat the4.5 0.671±0.026 0.622±0.001 octahedralpositionsprovidea largervolume thanthe

tetrahedralsitesto the Fe3~ionswith greaterionicradiusthan theA13~1’ions.9

regulatedoven.After eachsinteringprocess,with tem-peratureanddurationgiven in Table2, the pills were Foraluminiumconcentrationsc> 1 the f~valuespowderedandintensivelymixed. In orderto avoid lat- determinedin thepresentinvestigationsare system-tice defects,after thelastsinteringthe temperature atically greaterthan thosederivedfrom bulk magneticwasdecreasedslowly (50°C/hr). measurements1andpublishedMössbauerexperiments.3

Presentlyit seemsdifficult to decidetowhich extent2.3. Experimental thesedeviationsare causedby sampledifferences,e.g.

In orderto haveno magnetichyperfinesplitting becauseof a temperaturedependenceof the cationin theMOssbauerspectratheseweretakenat sample distributions.Correspondinglythedeterminedft-valuestemperaturesabovethe Née!point. Theabsorbertem- andorderedmomentsreproduceonly approximatelyperaturesfor c = 0, 0.5, 1.0and1.5were 572, soo, themeasurednetmagnetization(e.g. — °•3PB results440and400Krespectivelyand300 K for the other insteadof 0~4PB for c = 1.86).~samples.As source10 mC of 51Co in a palladiummatrix wasused.It wasmountedon a loudspeaker In agreementwith conclusioniderivedbydrivenin the constantaccelerationmodewithavelo- Czerlinskyfrom Mössbauerexperiments3the cationcity rangeof±2.6mm/sec. distributionsof Y—Fe—Al garnetsderivedin the

presentstudycannotbe describedby Borghese’s

Page 6: Cation distributions of YFeAl garnets

992 CATION DISTRIBUTIONSOF Y—Fe—Al GARNETS Vol. 16,No.8

thermodynamicequation:15 gation AF andreactorSaphir,EIR for supportofthis work. Moreoverwe are indebtedto R. Keil and

(2 — c + X)X B. von Känel, EIR for chemicaland X-ray analysis.C = — X)(c — X) Finally it is a pleasureto acknowledgestimulating

discussionswith Profs.E. Brun, W. Kundig and F.with a singleconstantCoverthe whole composition Waldnerfrom the PhysicsDepartmentof the Universityrange(C 5 atlow and 2 athigh aluminiumconcen- of Zurich. The EIR, ETH and University computertrations). centersprovidedcomputertime.

Acknowledgements— Wethankour colleaguesof theneutronscatteringgroup,thetechnicalstaffsof dde-

REFERENCES

1. GELLER S.,Z. Krist. 125, 1(1967).

2. CZERLINSKY E.R.,Phys.StatusSolidi 34,483 (1969).

3. CZERLINSKY E.R. andMACMILLAN R.A.,Phys.StatusSolidi 41, 333 (1970).

4. BACON G.E.,ActaC,yst. A28, 357 (1972).

5. FISCHERP.,HALG W., STOLL E. and SEGMULLERA.,ActaCryst. 21, 765 (1966).

6. BERTAUT F., FORRATF., HERPIN A. andM~RIELP.,Compt.Rend.,Paris 243, 898 (1956).

7. GELLER S.,WILLIAMS H.J.,ESPINOSAG.P. andSHERWOODR.C.,BellSystemTech.Journal 43, 565 (1964).

8. RIETVELD H.M., ReportRCN 104 Petten,The Netherlands(1969);VON WARTBURG W., ReportAF-SSP46Wurenlingen(1970).

9. GILLEO M.A. andGELLER S.,Phys.Rev. 110, 73 (1958).

10. EULER F. andBRUCEJ.A.,ActaCryst. 19,971 (1965).

11. CORLISSL.M. andHASTINGS J.M.,Phys.Rev.90, 1013(1953).

12. GILLEO M.A., J. Phys.Chem.Solids 13,33 (1960).

13. FISCHERP.,HALG W., and CZERLINSKYE.R.,Helv.Phys.Acta 46,429(1973).

14. MONTANARINI M., Diss.No 5176,ETH Zurich (1973).

15. BORGHESE C., J. Phys.Chem.Solids28, 2225(1967).