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Mössbauer study of amorphous and nanocrystalline FeZrBCu alloys
NmoS~
Mataids. Vol. 6. pp. 957-960.1995
cqrigItQ1995GlstviScimceLtd Fviniodiothousk Allli@ra-ed
Pergamon
09654773195 $9.50 + .oo
0965.9773(95)00219-7
MiiSSBAUER STUDY OF AMORPHOUS AND NANOCRYSTALLINE FeZrBCu ALLOYS M. Kopcewicz, A. Grab&
Instituteof ElectronicMaterialsTechnology w6le 133,01-919Warsztwa,Poland P. Nowicki
Instituteof MaterialsScienceand Engineering WarsawUniversityof Technology,NarbuttaS&02-524 Warsmwa,Poland Abstrad-The microstructure and magnetic properties of the nanocrystalline bee phaseformed by annealing at 5004OoOCof the amorphousFe,,Zr$3,, and Fe7&r+3&u2 alloys are studied using Mossbauer spectroscopy. The unconventional $Uossbauer experiments in which the radio-frequency induced eflects (rf collapse and rf sidebank) are used allow the nanocrystalline a-Fe phase to be distinguishedfrom the magnetically harder microcrystalline a-Fe formed at higher annealing temperatures.The complete rf collapse of the magnetic hyperjne structure occurs only in the amorphousand nanocrystalline phases and is suppressedby the formation of microcrystalline a-Fe. The rf sidebands disappear when the nanocrystalline phase i s formed, revealing that magnetostriction vanishes. The rf-Mossbauer experimentsperformed as a function of the rf field intensity allowed the determination of the distribution of anisotropy fields related to the size distribution of the a-Fe grains. INTRODUCTION
The developmentof new materialswith excellentsoft magneticpropertiesand with high saturationmagnetizationis importantin view of their applicationsin technology.A new class of materials has been developedby utilizing the first stage of crystallization of amorphous alloys (1). Good examples of such materials are FeZrB and FeZrBCu nanocrystallinealloys formed by annealing the melt-spun amorphousalloys (2). These materials have superior properties (higher magnetizationand pemmability) than the FeCttNbSiB alloys developedinitially (1). The excellenceof soft magnetic properties is understoodin termsof the reductionof the apparentmagnetocrystalline anisotropydue to the formationof nanocrystallinegrains(3). ‘The unconventionaltechnique, which combines the M6ssbauere@ct with the phenomenainducedby an externalradio-frequencymagneticfield (rf collapseand sideband effects) (4), is employedto study structuraland magneticpropertiesof the F~+,Zr&$Sr,, alloys in the amorphousand nanocrystallinestates.The rf collapseof the magnetichyperfk structuredue to fast magnetizationreversalallows the short range order to be studiedvia 957
958
M KOPCEWICZ. A GFWASAND P Ncwcm
electric quadrupoleinteraction.The rf collapseis very sensitiveto small changesof local magnetic anisotropyfields, thus allowing the identificationof sofl nanocrystallineand magneticallyharder microcrystallinephasesformed due to annealing of the starting amorphousalloy. The rf sidebandseffect, directly related to magnetostriction,allows magnetostriction properties,e.g.the decrease of magnetostriction dueto the formationof the nanocrystalline phaseto be studied(4). The fiM (lssbauertechniquewas usedrecentlyin the studyof nanocrystalline FeCuNbSiBalloys(5). Althougha whole seriesof alloyswith x=6, 8, 12 and,y=O,2 were studiedonly the resultsfor the Fe,,Zr,B,, andFe,sZr7B,pC$alloyswill be discussed here.Thesealloyshave the highestCurietemperatures in this setof samples. EXPERJMENTAL
The FeslZr,B,, and Fe,sZr,Br2cU,amorphousalloys were preparedby the meltspinningtechnique.The ribbonswere4 m m wideandabout25 pm thick. In orderto form the nanocrystallinephasethe amorphousalloyswereannealedfor 1 h in the temperaturerange 430-78OOC. The DSC measurements were performedto establishthe first and second crystallizationpeaks. ConventionalM&batter measurements were performedfor the asquenchedand annealedsamplesprior to rf exposure.The M&sbauermeasurements were performedalsoduringthe exposureof the samplesto the rf field of 20 Oe at 60.8 MHz which wasappliedin the planeof the samplethat playedthe role of an absorber.Adequatecoolingof the samplepreventingexcessive heatingwasapplied. RESULTS m
DISCUSSION
AnnealingamorphousFeZrBandFeZrBCualloysinducedsubstantialchangesin their microstructureswhich are clearly seenin the conventionalM&sbauermeasurements. The spectrameasuredfor the as-quenched ribbonsare typical of amorphousalloys.The average hyperfinefields +I$ determinedfrom the fit of hyperflnefield distributionsare 16.4T and 16.9T for Fes,Zr,B,, and Fe&r,BlzcU,, respectively (Fig. 1A). AnnealingFeZrBCuat n 500°C causespartial crystallizationof amorphousalloysto the nanocrystalline beephaseas revealedby the appearance of the spectralcomponentwith sharp lines (Fig. lB-D). The parametersof this component(Hkf, isomershill IS) correspond exactlyto thoseof u-Fe. The increasein annealingtemperatureresultsin the increasein the abundance of the c+Fephase. The spectrameasuredfor the samplesannealedat 500°C~SaOOoC revealthe coexistence of the nanocrystallinea-Fe phasewith the remainingamorphousmatrix whosecompositionis, however,changedas comparedwith the startingalloy as shownby the increaseof +I,$ to 17.7 T. Annealing at 780°C causescompletecrystallizationof the FeZrBCu alloy to microcrystallinec+Fe. The presenceof Cu in the amorphousalloys decmasesthe crystallizationtemperatureand makesthe transformationto the nanocrystallinebee phase more efficient than in the caseof FeZrB. The relative spectmlcontributionof a-Fe after annealingat 6OO’Cis about 45% for FeZrBCuas comparedwith only 10% for FeZrB. However;the u-Fe phaseappearsin theFeZrBalloyafterannealingat 55O’C.
STUDYOF
M~SSBAUER
AMORPH~W
F~Z~BCUALLOYS
AND NANOCRYSTALLINE
959
TheMUssbatterspectrameasured for FeZrBCuduringrf exposure to the rf field of 20 Oe at 60.8 MHz are shownin Figs. IA-E’. A completecollapseof the magnetichyperfine structureto a quadrupoledoubletis observed for the as-quenched alloy (Fig. 1A’). The rf sidebands clearlyseenin this spec&umdecrease markedlyfor the sampleannealedat 500°C which showsthat the formationof the nanocrystalline phasecausesa substantial reductionof magnetostriction. Also the shapeof the centralcollapsed spectralcomponent changes. Instead of a quadrupole doubletonly, we observea superposition of a QS doublet(corresponding to the amorphous matrix) anda singleline (corresponding to sofl nanocrystalline a-Fe grains). The increasein the annealingtemperature andthe corresponding increasein the amountof c+Fegrainscausea dramaticchangein the rf collapsedspectrum.The component revealing the unresolvedpartially collapsedhyperflnestructureappears(Fig. 1C) besidethe fully collapsedcomponents. The spectralcontributionof the non-collapsed component,revealing for TA=600°C(Fig. ID’) a well resolvedhyperflnestructure,increases. This evidences the formationof the grainswith sufficientlylargemagneticanisotropyto preventthe rf collapse (noncollapsed component) in additionto the magnetically verysoft nanocrystalline grains of a-Fe for which a completerf collapseis observed.Annealingat 780°C causescomplete crystallizationof the amorphousalloy and the a-Fe phaseis so hardmagneticallythat 1.00
0.95
5 ii
0.96
ii 3 E
1.00 . , :* .. :; 1
c F-2 0.98 i
1.00
1.00 ?T!
;a< - 0.88
if
0.95
0.98
- 0.95
-b
l-O0
0.95
C’
1.00 -!r,~n;~.r\’ . .* f i . . . - - . : t ,
0.95 0.90 -0
0
6
-6
0
:’ . :
1*0° .* .. . E’ - 0.95 . 6
’[mm/s1 Fig. 1. Mossbauer spectrarecorded in the absence of the rf field (A-E) andduring rf exposure(A-E’) for Fe+r,B&~ alloyin the as-quenchedstate andafter annealing at SOO-78O’C.
_. -e
0
8
-6
0
6
VELOCITY [mm/s]
VELOCITY
Fig. 2. Mossbauerspectrarecordedasa function of the rf field intensity for the as-quenched (A-E) andannealed at 55O’C (A-E’) Fe7&r7BlzcU,alloy.
960
M itmxvdcz,
A GRABASAND
P F~MCKI
the rf collapsedoesnot occur(Fig. 1E’).Thus,annealingFeZrBCuat temperatures 550-600°C causesthe formation of grains with a broad distributionof the magneticanisotropyfields relatedto the broadsizedistributionof the a-Fe particlesembedded in the amorphousmatrix. This effect is clearly seenin Fig. 2 which showsthe dependence of the rf collapse on the rf field intensityfor the as-quenched FeZrBCualloy andannealedat 550°C.As canbe seenfrom Figs. 2A-C the decreaseof the rf field intensityfrom 20 Oe to 8 Oe doesnot markedlyaffect the shapeof the rf collapsedspecnumof the as-quenched sample.The collapsedspectrum starts to broadenat about 6 Ge (Fig. 2D). This showsthat in the as-quenched alloy the effectiveanisotropyfield is comparableto the rf field of 6 Ge whenit startsto suppressthe rf collapseeffect.In the caseof annealedsamplesthe decrease of the rf field intensityfrom 20 Oe to 16 Oe causesalready the appearanceof the hyperfine split partially collapsed component,leaving the central fully collapsedpart unaffected.With the decreaseof the rf field intensitysplitting of the non-collapsed componentbecomessimilar to that characteristic of the microcrystalline a-Fe (Hht=32.95T).The spectral contribution of the collapsed componentdecreasessimultaneously.Since the rf collapseis very sensitiveto the local anisotropyfield (t-f collapseoccursonly if the rf field is larger than the anisotropyfield) the increasein the hyperlinesplit componentwith decreasingintensityof the rf field is relatedto the increasein the anisotropyfields of the grains;the latter,in turn, increaseswith increasing grain size.Thus, the spectrain Figs. 2A’-E’provideevidencefor a broadsize distributionof the a-Fe grains. In the FeslZr,Blz alloy annealedat 500°Cfl<6000C the nanocrystallinebee a-Fe phaseyieldedmoreuniform grainsas revealedby the rf collapse.A more detaileddiscussion of the shapeof the collapsedspectraof Fe,,Zr,B,, as a functionof the annealingtemperature is given in ref. (6). In conclusion,the conventionalMossbauermeasurements allow identification of phasesformed due to annealingof the starting amorphousalloys, but do not provide informationconcerningthe magneticanisotropyof the grains.The fiM&sbauer experiments, in which the r-f collapseand sidebandeffectsare observed,permit us to distinguishthe soft nanocrystallinebee phase from the magnetically harder microcrystalline a-Fe. Some information concerninggrain size distribution can be obtained.The nanocrystallinea-Fe phaseis identitiedby the appearance of a singleline componentin the rf collapsedspectrum andby the disappearance of rf sidebands dueto vanishingmagnetos&non. Acknowledgement
Financialsupportfrom grantsITME Ol- l-0092-4and SRC3-P407-02 l-07 is acknowledged. References
1. Y. Yoshizawa,S. Oguma,K. Yamauchi,J. Appl. Phys.@, 6044(1988). 2. A. Makino, K. Suzuki,A. Inoue,T. Masumoto,Mat. Sci. Eng. A 179/A 180,127(1994). 3. G. Herzer,Mat. Sci. Eng. A 133,l(l991). 4. M. Kopcewicz,Struct.Chem.5 313(1991). 5. M. Kopcewicz,J. Jagielski,T. Graf, M. Fricke,J. Hesse,HyperfineInter. (1994),in print. 6. M. Kopcewicz,A Grab&, P. Nowicki, J. Magn.Magn.Mat., in print.
Mataids. Vol. 6. pp. 957-960.1995
cqrigItQ1995GlstviScimceLtd Fviniodiothousk Allli@ra-ed
Pergamon
09654773195 $9.50 + .oo
0965.9773(95)00219-7
MiiSSBAUER STUDY OF AMORPHOUS AND NANOCRYSTALLINE FeZrBCu ALLOYS M. Kopcewicz, A. Grab&
Instituteof ElectronicMaterialsTechnology w6le 133,01-919Warsztwa,Poland P. Nowicki
Instituteof MaterialsScienceand Engineering WarsawUniversityof Technology,NarbuttaS&02-524 Warsmwa,Poland Abstrad-The microstructure and magnetic properties of the nanocrystalline bee phaseformed by annealing at 5004OoOCof the amorphousFe,,Zr$3,, and Fe7&r+3&u2 alloys are studied using Mossbauer spectroscopy. The unconventional $Uossbauer experiments in which the radio-frequency induced eflects (rf collapse and rf sidebank) are used allow the nanocrystalline a-Fe phase to be distinguishedfrom the magnetically harder microcrystalline a-Fe formed at higher annealing temperatures.The complete rf collapse of the magnetic hyperjne structure occurs only in the amorphousand nanocrystalline phases and is suppressedby the formation of microcrystalline a-Fe. The rf sidebands disappear when the nanocrystalline phase i s formed, revealing that magnetostriction vanishes. The rf-Mossbauer experimentsperformed as a function of the rf field intensity allowed the determination of the distribution of anisotropy fields related to the size distribution of the a-Fe grains. INTRODUCTION
The developmentof new materialswith excellentsoft magneticpropertiesand with high saturationmagnetizationis importantin view of their applicationsin technology.A new class of materials has been developedby utilizing the first stage of crystallization of amorphous alloys (1). Good examples of such materials are FeZrB and FeZrBCu nanocrystallinealloys formed by annealing the melt-spun amorphousalloys (2). These materials have superior properties (higher magnetizationand pemmability) than the FeCttNbSiB alloys developedinitially (1). The excellenceof soft magnetic properties is understoodin termsof the reductionof the apparentmagnetocrystalline anisotropydue to the formationof nanocrystallinegrains(3). ‘The unconventionaltechnique, which combines the M6ssbauere@ct with the phenomenainducedby an externalradio-frequencymagneticfield (rf collapseand sideband effects) (4), is employedto study structuraland magneticpropertiesof the F~+,Zr&$Sr,, alloys in the amorphousand nanocrystallinestates.The rf collapseof the magnetichyperfk structuredue to fast magnetizationreversalallows the short range order to be studiedvia 957
958
M KOPCEWICZ. A GFWASAND P Ncwcm
electric quadrupoleinteraction.The rf collapseis very sensitiveto small changesof local magnetic anisotropyfields, thus allowing the identificationof sofl nanocrystallineand magneticallyharder microcrystallinephasesformed due to annealing of the starting amorphousalloy. The rf sidebandseffect, directly related to magnetostriction,allows magnetostriction properties,e.g.the decrease of magnetostriction dueto the formationof the nanocrystalline phaseto be studied(4). The fiM (lssbauertechniquewas usedrecentlyin the studyof nanocrystalline FeCuNbSiBalloys(5). Althougha whole seriesof alloyswith x=6, 8, 12 and,y=O,2 were studiedonly the resultsfor the Fe,,Zr,B,, andFe,sZr7B,pC$alloyswill be discussed here.Thesealloyshave the highestCurietemperatures in this setof samples. EXPERJMENTAL
The FeslZr,B,, and Fe,sZr,Br2cU,amorphousalloys were preparedby the meltspinningtechnique.The ribbonswere4 m m wideandabout25 pm thick. In orderto form the nanocrystallinephasethe amorphousalloyswereannealedfor 1 h in the temperaturerange 430-78OOC. The DSC measurements were performedto establishthe first and second crystallizationpeaks. ConventionalM&batter measurements were performedfor the asquenchedand annealedsamplesprior to rf exposure.The M&sbauermeasurements were performedalsoduringthe exposureof the samplesto the rf field of 20 Oe at 60.8 MHz which wasappliedin the planeof the samplethat playedthe role of an absorber.Adequatecoolingof the samplepreventingexcessive heatingwasapplied. RESULTS m
DISCUSSION
AnnealingamorphousFeZrBandFeZrBCualloysinducedsubstantialchangesin their microstructureswhich are clearly seenin the conventionalM&sbauermeasurements. The spectrameasuredfor the as-quenched ribbonsare typical of amorphousalloys.The average hyperfinefields +I$ determinedfrom the fit of hyperflnefield distributionsare 16.4T and 16.9T for Fes,Zr,B,, and Fe&r,BlzcU,, respectively (Fig. 1A). AnnealingFeZrBCuat n 500°C causespartial crystallizationof amorphousalloysto the nanocrystalline beephaseas revealedby the appearance of the spectralcomponentwith sharp lines (Fig. lB-D). The parametersof this component(Hkf, isomershill IS) correspond exactlyto thoseof u-Fe. The increasein annealingtemperatureresultsin the increasein the abundance of the c+Fephase. The spectrameasuredfor the samplesannealedat 500°C~SaOOoC revealthe coexistence of the nanocrystallinea-Fe phasewith the remainingamorphousmatrix whosecompositionis, however,changedas comparedwith the startingalloy as shownby the increaseof +I,$ to 17.7 T. Annealing at 780°C causescompletecrystallizationof the FeZrBCu alloy to microcrystallinec+Fe. The presenceof Cu in the amorphousalloys decmasesthe crystallizationtemperatureand makesthe transformationto the nanocrystallinebee phase more efficient than in the caseof FeZrB. The relative spectmlcontributionof a-Fe after annealingat 6OO’Cis about 45% for FeZrBCuas comparedwith only 10% for FeZrB. However;the u-Fe phaseappearsin theFeZrBalloyafterannealingat 55O’C.
STUDYOF
M~SSBAUER
AMORPH~W
F~Z~BCUALLOYS
AND NANOCRYSTALLINE
959
TheMUssbatterspectrameasured for FeZrBCuduringrf exposure to the rf field of 20 Oe at 60.8 MHz are shownin Figs. IA-E’. A completecollapseof the magnetichyperfine structureto a quadrupoledoubletis observed for the as-quenched alloy (Fig. 1A’). The rf sidebands clearlyseenin this spec&umdecrease markedlyfor the sampleannealedat 500°C which showsthat the formationof the nanocrystalline phasecausesa substantial reductionof magnetostriction. Also the shapeof the centralcollapsed spectralcomponent changes. Instead of a quadrupole doubletonly, we observea superposition of a QS doublet(corresponding to the amorphous matrix) anda singleline (corresponding to sofl nanocrystalline a-Fe grains). The increasein the annealingtemperature andthe corresponding increasein the amountof c+Fegrainscausea dramaticchangein the rf collapsedspectrum.The component revealing the unresolvedpartially collapsedhyperflnestructureappears(Fig. 1C) besidethe fully collapsedcomponents. The spectralcontributionof the non-collapsed component,revealing for TA=600°C(Fig. ID’) a well resolvedhyperflnestructure,increases. This evidences the formationof the grainswith sufficientlylargemagneticanisotropyto preventthe rf collapse (noncollapsed component) in additionto the magnetically verysoft nanocrystalline grains of a-Fe for which a completerf collapseis observed.Annealingat 780°C causescomplete crystallizationof the amorphousalloy and the a-Fe phaseis so hardmagneticallythat 1.00
0.95
5 ii
0.96
ii 3 E
1.00 . , :* .. :; 1
c F-2 0.98 i
1.00
1.00 ?T!
;a< - 0.88
if
0.95
0.98
- 0.95
-b
l-O0
0.95
C’
1.00 -!r,~n;~.r\’ . .* f i . . . - - . : t ,
0.95 0.90 -0
0
6
-6
0
:’ . :
1*0° .* .. . E’ - 0.95 . 6
’[mm/s1 Fig. 1. Mossbauer spectrarecorded in the absence of the rf field (A-E) andduring rf exposure(A-E’) for Fe+r,B&~ alloyin the as-quenchedstate andafter annealing at SOO-78O’C.
_. -e
0
8
-6
0
6
VELOCITY [mm/s]
VELOCITY
Fig. 2. Mossbauerspectrarecordedasa function of the rf field intensity for the as-quenched (A-E) andannealed at 55O’C (A-E’) Fe7&r7BlzcU,alloy.
960
M itmxvdcz,
A GRABASAND
P F~MCKI
the rf collapsedoesnot occur(Fig. 1E’).Thus,annealingFeZrBCuat temperatures 550-600°C causesthe formation of grains with a broad distributionof the magneticanisotropyfields relatedto the broadsizedistributionof the a-Fe particlesembedded in the amorphousmatrix. This effect is clearly seenin Fig. 2 which showsthe dependence of the rf collapse on the rf field intensityfor the as-quenched FeZrBCualloy andannealedat 550°C.As canbe seenfrom Figs. 2A-C the decreaseof the rf field intensityfrom 20 Oe to 8 Oe doesnot markedlyaffect the shapeof the rf collapsedspecnumof the as-quenched sample.The collapsedspectrum starts to broadenat about 6 Ge (Fig. 2D). This showsthat in the as-quenched alloy the effectiveanisotropyfield is comparableto the rf field of 6 Ge whenit startsto suppressthe rf collapseeffect.In the caseof annealedsamplesthe decrease of the rf field intensityfrom 20 Oe to 16 Oe causesalready the appearanceof the hyperfine split partially collapsed component,leaving the central fully collapsedpart unaffected.With the decreaseof the rf field intensitysplitting of the non-collapsed componentbecomessimilar to that characteristic of the microcrystalline a-Fe (Hht=32.95T).The spectral contribution of the collapsed componentdecreasessimultaneously.Since the rf collapseis very sensitiveto the local anisotropyfield (t-f collapseoccursonly if the rf field is larger than the anisotropyfield) the increasein the hyperlinesplit componentwith decreasingintensityof the rf field is relatedto the increasein the anisotropyfields of the grains;the latter,in turn, increaseswith increasing grain size.Thus, the spectrain Figs. 2A’-E’provideevidencefor a broadsize distributionof the a-Fe grains. In the FeslZr,Blz alloy annealedat 500°Cfl<6000C the nanocrystallinebee a-Fe phaseyieldedmoreuniform grainsas revealedby the rf collapse.A more detaileddiscussion of the shapeof the collapsedspectraof Fe,,Zr,B,, as a functionof the annealingtemperature is given in ref. (6). In conclusion,the conventionalMossbauermeasurements allow identification of phasesformed due to annealingof the starting amorphousalloys, but do not provide informationconcerningthe magneticanisotropyof the grains.The fiM&sbauer experiments, in which the r-f collapseand sidebandeffectsare observed,permit us to distinguishthe soft nanocrystallinebee phase from the magnetically harder microcrystalline a-Fe. Some information concerninggrain size distribution can be obtained.The nanocrystallinea-Fe phaseis identitiedby the appearance of a singleline componentin the rf collapsedspectrum andby the disappearance of rf sidebands dueto vanishingmagnetos&non. Acknowledgement
Financialsupportfrom grantsITME Ol- l-0092-4and SRC3-P407-02 l-07 is acknowledged. References
1. Y. Yoshizawa,S. Oguma,K. Yamauchi,J. Appl. Phys.@, 6044(1988). 2. A. Makino, K. Suzuki,A. Inoue,T. Masumoto,Mat. Sci. Eng. A 179/A 180,127(1994). 3. G. Herzer,Mat. Sci. Eng. A 133,l(l991). 4. M. Kopcewicz,Struct.Chem.5 313(1991). 5. M. Kopcewicz,J. Jagielski,T. Graf, M. Fricke,J. Hesse,HyperfineInter. (1994),in print. 6. M. Kopcewicz,A Grab&, P. Nowicki, J. Magn.Magn.Mat., in print.