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α-Fe magnets
NanoStmcturedMaterials,Vol. 10. No. 6. pp. 1013-1022.1998 E%evia science Ltd 0 1998 Acta MetallurgicaInc. Printedin the USA. All rights-cd 096s9173198 $19.00 + .oo
Pergamon
PII SO9659773(98)00139-l
THIN FILM NANOCOMPOSITE
NdzFel4B/a-Fe MAGNETS
I. Panagiotopoulos,and G.C. Hadjipanayis Department of Physics and Astronomy, University of Delaware,Newark DE 19716 (AcceptedJune II, 1998) Abstmcr~- NanocompositeNd2Fel&la-Fe magnets were prepared by either sputtering fromcastalloy targetswithincreasedFecontent,orbydepositingsuccessivelyfromaNdzoFe7dVbB9 and an iron target in a multilayerfashion. For the optimallyannealed samples magnetic data indicatethe existenceof a hardphase thatis exchange-coupledwiththe softphase (a-Fe) grains. These samples showed remanence enhancement.For longer annealing times a-Fe clustering leads to decoupling of the softphase. The TEM studiesshowed that in this case the grain size exceeds the theoreticallypredicted size (of about 10 nm)for optimumcoupling between the two phases. By depositingin an alternatinglayer structure,higher coercivitieswere obtaineddue to theadvantage(ofcontrollingthesoftphase contentas wellas thegrain size by the individuallayer thickness. 01!398 Acta MetallurgicaInc.
INTRODUCTION Nanocomposite materials,consistingof a fine mixture of a magneticallysoft phase with high magnetization exchange coupled to a high anisotropy hard phase have been widely studied recently due their potential application as high energy product low cost permanent magnets (1). These systems usually show smooth demagnetization curves and high values of reduced remanence at the expense of coercivity which is smaller than that of the hard phase. The early studies (2) have been made on melt-spun Nd4Fe77Bt9magnets where a consisted of a mixture of 15% Ndger4B with FesB and o-Fe (2).Melt-spun samplesconsistingof 30%Nd$et4B with u-Fe can giveacoercivityof2.7kGeandremanenceof 14SkGleading toanenergyproductof 14.45MGGe (3). Remanence enhancement was also observed in isotropic mechanically alloyed Sm7Fe93 nitride powder samples, consisting of a-Fe and Sm$e17Nx,which gave energy products as high as 25.6 MGGe (4). The realization of such a system in the thin film is of considerable theoretical and practical interest (5)‘sinceit offers the opportunity to optimize the properties of the sample by controlling the thickness of the individual layers. Al-Omari et al. (6) have studied the magnetic properties of CoSm/FeCo (hard/soft) multilayered structures prepared by sputtering and found that for soft layer thickness less than 30 nm the two phases are exchange coupled. Thin film Ndfle14B with a-Fe nanocomposite magnets have been prepared by sputtering deposition in trilayer form (7) with the soft layer sandwichedbetween the two hard magnetic phase layers.These 1013
1014
I PANAGIOTOPOULOS ANDGC HADJIPANAWS
samples also showed remanence enhancement. The coercivity as well as the reduced remanence are decreased with increasing soft layer thickness above 35 nm. Mulitlayer Ndflel4B/a_Fe samples prepared by sputtering (8) exhibit exchange springbehavior with high recoil permeability even for reversed fields that exceed the coercivity. Due to the heat-treatment necessary to crystallize the 2: 14:1phase interdiffusionand breakingof the layered structureis expected to some extent. In this work we extended our previous studies in nanocomposite ribbon materials (2) to samples in the form of thin films using two different approaches: either sputtering from a single target with increased Fe content or successively sputtering from two targets (Nd-Fe-B and u-Fe) in a multilayer fashion. EXPERIMENTAL Nd-Fe-Nb-Bfilms were prepared by magnetron sputtering on tantalum foil substrates from 1.3inchcasttargetswiththefollowingcompositions:N~e~~Ba,NdsFes3NbBs,Ndl~~9~Bs, NdI$e75NbBs and Nd2cFeTeNbBs.The targets were prepared by arc-melting stoichiometric amounts of the constituent elements in a high purity Ar atmosphere. The procedure was repeated several times to assure the homogeneity of the target material. The lower Nd compositions were chosen to give a microstructure consisting of a mixture of NdzFel4NbB and a-Fe grains. For brevity in what follows films sputteredfrom these targetswill be referredto as Nd6, Nd8, Nd12, Ndl5 and Nd20, respectively. The base pressure was 3~10~~Torr and the Ar pressure during
Figure 1. X-ray diffraction pattern of a film sputtered from the N&Fe7$IbB9 target and then annealed at 700°C for 5 minutes.
THIN FM Nmommsm
Nd,Fe,,B/u-Fe
MAGNETS
1015
sputtering was 5 mTorr. Another set of samples was prepared in the form of Nd-Fe-B/a-Fe multilayers. High sputtering rates were used (around 20 &ec). The as-sputtered films were amorphous and showed no hysteresis. These films were subsequently annealed (ex-situ) at temperatures between 650-750°C for 5-30 minutes. The hysteresis loops were measured in a SQUID magnetometer with a maximum field of 55 kGe. X-ray diflraction (XRD) spectra were measured with Cu Ka radiation in a Philips APD3520diffractometer.The microstructural studies were carried ‘out using a Jeol JEM-2000FX transmission electron microscope. The chemical composition was determined by PIXE Selected samples were sent for chemical analysis to Galbraith Laboratories Inc.
The rare earth content of the sputtered films is lower than that of the target as summarized in Table 1. Compositional differences between the target material and the sputtered films are expected when sputtering materialsthat consist of atoms with such different atomic weights as Nd, Fe and B (9).
TABLE 1 Summary of the Nd-Fe-NbB Film Samples Sample -16 Nd8 Nd12 Ndll5 Ndl20
Target composition Nd6Fes7NhB6 NdsFes&bBs Ndt2Fe7&&
Film composition Nd4.6Fess.3Nbo.7Bs.4 Nd6.3Fes4.9EJbo.sBx Nd9Fes&Jbl.lBx
Ndl&MJW NM’e7oN’bb
N42.2Fe77.3NbBx Ndl~b4.&h4Bx
TABLE 2 Summary of the Nd-Fe-Nb-B/FeSamples number of bilayers 40 808 150 180
Fe layer thickness (C) 145 72 72 72
Nd-Fe-Nb-B layer thickness (A) 562 281 112 56
Composition Ndt3.s Fe&bo.sBx Ndt3.3 Fes&lbo.sBx N41.2
F‘G6.6~2.2Bx
Nds.2 F%~Nbo.sBx
1016
I PANAGIOTOPOULOS ANDGC HADJIPANAMS
Figure 2. X-ray diffraction patterns for films sputtered from (a) Nd@eg$%BS, (b) NdsFessNbB8, (c) Ndl$e79NbBs targets and then annealed at 700°C for 5 minutes.
I
’
I
-20-15-10
1
I
’
-5
I
0
5
x
b
’
t
’
10 15 20
H (kOe) Figure 3. Hysteresis loops of film samples sputtered from various targets and then annealed at 700°C for 5 minutes. The measurement was done at 300 K.
THINFILMNANOCOMPOSITE Nd2Fe,,Wa-Fe MAGNETS
1017
Figure 11shows the X-ray diffraction pattern of a film sputtered from the NdtsFe7WBs target and annealed at 700°C for 5 minutes. The observed peaks can be indexed to the tetragonal NdSel4B phalse with unitcellparametersa= 8.81 Aandc=12.24 A, which giveac/aratioof 1.39. The Nd$elhEt phase shows a small degree of preferential orientation of the (105) direction perpendicular to the film plane. Figure 2 shows the X-ray diffraction patterns for films sputtered from targets with lower rare earth content (Nd6-Nd12) and annealed for 5 minutes at 700°C. A strong a-Fe peak is observed along with the peaks of the Nd$et4B phase. The relative intensity oftheNd2Fel~IBpeakstothatofthea-FepeakisreducedinsampleswithhigherFecontent.Finally only tbe a-Fe peak is seen in the X-ray diffraction pattern of the Nd6 film probably due to the fine size of the hard phase grains. However, reflections corresponding to the Nd2Fq4B phase were observed in the electron diffraction patterns of this sample. The existence of the magnetically hard NdzFetB phase is furthermore indicated by the hysteresis loops of these samples as shown in Figure 3 where coercivities in the range of 1.5 kOe were observed. The highest coercivity was obtained in tbe Nd20 sample and it is attributed to the presence of Nld-rich phase which is usually observed in Nd-rich magnets.
] -20
I
-10
1
I
0
10
20
H (kOe) Figure 4. Hysteresis loops for films sputtered from a NdsFessNbBs target and annealed at various temperatures between 620 and 740°C for 5 minutes. The measurement was done at 300 K.
I PANAGIOTOPOULOS ANDGC HADJIPANAYIS
Figure 5. TEM micrograph of a fiim sputtered from a Ndd;es7NbBe target and annealed at 700°C for 5 miIUtteK
Figure 4 shows the hysteresis loops for Nd8 films annealed at various temperatures between 620 and 740°C for 5 minutes. The samples annealed at 660 and 700°C have coercivities around 1.5 kOe and reduced remanence (Ma) of 0.63. This value of coercivity is low compared to the one (&=3.1 kOe) obtained in ribbons with approximately the same composition (5) which consist 50%wt by soft phase. The sample annealed at 740°C has slightly higher coercivity but a shoulder can be observed in the demagnetization part of the hysteresis loop. This can be attributed to an increase the grain size of the soft phase (a-Fe) at higher annealing temperatures and longer annealing times leading to the decoupling of some of the grains as it is reflected by the a shoulder in the demagnetization curve. Indeed some a-Fe clustering can be observed in Figure 5, showing a TEM micrograph of a Nd6 film annealed at 700°C for 5 minutes. This results in relatively large clusters of grains (about 50 nm) which cannot be optimally coupled to the hard phase. In an attempt to avoid the problem of a-Fe clustering, a different approach in the preparation of these films was ‘usedwhere Nd-Fe-Nb-B and a-Fe bilayers were sputtered successively from a Nd2cFe7eNbB9 and an Fe target, respectively. By changing the relative thickness of the layers one can control the composition of the sample. The samples prepared are summarized in Table 2. The number of bilayers was varied to keep the total thickness close to 2.8 pm for all the samples. These samples were subsequently crystallized by annealing ex situ.The hysteresis loops for these multilayersafter5 minutesannealingat675,700and725”C,respectivelyareshowninFigutes6-8. With the exception ofthe 7/5.5 the samples show reduced remanence 0.6-0.7 depending on the composition and annealing conditions. The sample 7/l 1 has much higher coercivity compared to sample Nd15 (Figure 3) which has a similar composition. This indicates that preparing the samples
1019
I
(14.956)
l
,p .1 (7/28) .*d.- . ..J
-60 -40 -20
0
(7/l 1)
20
40
.
60
H (kOe) Figure 6. Hysteresis loops of multilayers annealed at 680°C for 5 minutes. The numbers indicate the thickness of the Fe and Nd-Fe-Nb-Blayers respectively in nm. The measurements were done at 300 K.
in a multilayer fashion really enables to control the size and arrangementof the soft and hardphase grains. The shoulder observed in the demagnetizationcurve is minimum for the samples annealed at 700°C. However the shoulder is still significant especially for the sample with the higher a-Fe content. It is worth noting that the shoulder in the demagnetizationcurve is not observed when the loops are measured with the field applied perpendicularto the substrateof the film (Figure 9). This could be a result of the layered structure that would lead to a more effective coupling between the soft and hard phase along the direction perpendicular to the substrate plane. However a similar behavior has been reported for Nd$elfi films crystallized from amorphous deposits (10). Multilayered lsamplescompared to the samples sputtered by single targets with similar compositions present higher coercivities but also increased shoulder in the demagnetization curve. This is associated wifh partial oxidation of the films during sputtering with the substrate oscillating back and forth between the two targets. This interpretation has been verified by the X-ray diffraction patterns.
I PANAGI~T~PO~L~~ ANDGC Hmwwans
1020
CONCLUSIONS Nanocomposite film magnets were prepared by sputtering from cast alloy targets and then annealing en-situ in high vacuum. lko different approaches have been used: Sputtering from targets with increased Fe content or sputtering succesively from a Nd-Fe-B and a iron target in a multilayer fashion. The latter gives higher coercivities due to the advantage of controlling the soft phase content as well as the grain size by the individual layer thickness, but also allows some oxidation to occur during the time the target is rotated. Magnetic data indicate the existence of a hard phase that is exchange-coupled with the soft phase (u-Fe) grains for samples annealed for 5 minutes at 700°C. Annealing times long enough to fully crystallizethe high anisotropy tetragonalNd$eldB phase cannot be used in films with low ram earth percentage due to u-Fe clustering that leads to decoupling of the soft phase. The TEM studies show that in this case the grain size exceeds the theoretically predicted size (of about 10 nm) for optimum coupling between the two phases (1,5). The optimally annealed samples show remanence enhancement. A direct comparison with the theoretical predictions of micromagneticcalculations(5) is not feasible due to therandom easy magnetization axis and size distribution of the grains in our sample. Basic features as the decrease of coercive field with increasing soft phase are however observed.
(14356) l
C
00J .*a0’
(7/28)
/-.**oJ
(7/l 1)
$z (7/M)
I
,
I I
-60
-40
*
-20
,
,
I
0
20
40
’
60
H (kOe) Figure 7. Hysteresis loops of multilayers annealed at 700°C for 5 minutes. The numbers indicate the thickness of the Fe and Nd-Fe-Nb-Blayers respectively in nm. The measurements were done at 300 K.
THIN FILM NANOCWPOSITE NdpFe,,Bla-Fe MAGNETS
1021
Figure 8. Hysteresis loops of multilayers annealed at 725°C for 5 minutes. The numbers indicate the thickness of the Fe and Nd-Fe-Nb-Blayers respectively in nm. The measurements were done at 300 K.
-40 -30 -20 -10 0 10 20 30 40 H,, NW Figure 9. Hysteresis loop of a Fe(7nm)/Nd-Fe-Nb-B(5.5nm) multilayer measured with applied field perpendicular to the film substrate.
1022
I PANAGIOTOPOULOS ANDGC HADJPANAW
ACKNOWLEDGMENTS
We would Iike to thank Dr. C.P. Swarm for thePIXE measurements. Work supported by DOE # DE-FGO2-90ER454 13. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9. 10.
Kneller, E.F. and Hawig, R., IEEE Transactions Magnetism, 1991,27,2588. Coehom, R., de Mooij, D.B., Duchateau, J.P. and Bushow, K.H.J., Journal de Physique (Paris) Colloq., 1988,49, C8-889. Withanawaam, L., Hadjipanayis, G.C. andKrause, R.F., Journal ofApplied Physics, 1994,75( lo), 6646. Ding, J., McCormick, P.G. and Street, R., Journal of Magnetism and Magnetic Materials, 1993, 124, Ll. Skornski, R. and Coey, J.M.D., Physical Review B, 1993,48(21), 15812. Al-Gmari, L.A. and Selhnyer, D.J., Physical Review B, 1995,52,3441. Shindo,M., Ishizone,M.,Kato,H.,Miyazaki, T., Sakuma,A., JournalofMagnetismandMagnetic Materials, 1996, 161, Ll. Parhofer, S.M., Wecker, J., Kuhrt, C., Gieres, G., Schultz, L., IEEE TransactionMagnetism, 1996, 32,437. Cadieu, F.J., Chencinski, N., IEEE Transactions Magnetism, 1975, 11(2), 227. Gu, B.X., Homburg, H., Methessel, S. and Zhai, H.R., Physica Status Solidi (a), 1990, 120, 159.
Pergamon
PII SO9659773(98)00139-l
THIN FILM NANOCOMPOSITE
NdzFel4B/a-Fe MAGNETS
I. Panagiotopoulos,and G.C. Hadjipanayis Department of Physics and Astronomy, University of Delaware,Newark DE 19716 (AcceptedJune II, 1998) Abstmcr~- NanocompositeNd2Fel&la-Fe magnets were prepared by either sputtering fromcastalloy targetswithincreasedFecontent,orbydepositingsuccessivelyfromaNdzoFe7dVbB9 and an iron target in a multilayerfashion. For the optimallyannealed samples magnetic data indicatethe existenceof a hardphase thatis exchange-coupledwiththe softphase (a-Fe) grains. These samples showed remanence enhancement.For longer annealing times a-Fe clustering leads to decoupling of the softphase. The TEM studiesshowed that in this case the grain size exceeds the theoreticallypredicted size (of about 10 nm)for optimumcoupling between the two phases. By depositingin an alternatinglayer structure,higher coercivitieswere obtaineddue to theadvantage(ofcontrollingthesoftphase contentas wellas thegrain size by the individuallayer thickness. 01!398 Acta MetallurgicaInc.
INTRODUCTION Nanocomposite materials,consistingof a fine mixture of a magneticallysoft phase with high magnetization exchange coupled to a high anisotropy hard phase have been widely studied recently due their potential application as high energy product low cost permanent magnets (1). These systems usually show smooth demagnetization curves and high values of reduced remanence at the expense of coercivity which is smaller than that of the hard phase. The early studies (2) have been made on melt-spun Nd4Fe77Bt9magnets where a consisted of a mixture of 15% Ndger4B with FesB and o-Fe (2).Melt-spun samplesconsistingof 30%Nd$et4B with u-Fe can giveacoercivityof2.7kGeandremanenceof 14SkGleading toanenergyproductof 14.45MGGe (3). Remanence enhancement was also observed in isotropic mechanically alloyed Sm7Fe93 nitride powder samples, consisting of a-Fe and Sm$e17Nx,which gave energy products as high as 25.6 MGGe (4). The realization of such a system in the thin film is of considerable theoretical and practical interest (5)‘sinceit offers the opportunity to optimize the properties of the sample by controlling the thickness of the individual layers. Al-Omari et al. (6) have studied the magnetic properties of CoSm/FeCo (hard/soft) multilayered structures prepared by sputtering and found that for soft layer thickness less than 30 nm the two phases are exchange coupled. Thin film Ndfle14B with a-Fe nanocomposite magnets have been prepared by sputtering deposition in trilayer form (7) with the soft layer sandwichedbetween the two hard magnetic phase layers.These 1013
1014
I PANAGIOTOPOULOS ANDGC HADJIPANAWS
samples also showed remanence enhancement. The coercivity as well as the reduced remanence are decreased with increasing soft layer thickness above 35 nm. Mulitlayer Ndflel4B/a_Fe samples prepared by sputtering (8) exhibit exchange springbehavior with high recoil permeability even for reversed fields that exceed the coercivity. Due to the heat-treatment necessary to crystallize the 2: 14:1phase interdiffusionand breakingof the layered structureis expected to some extent. In this work we extended our previous studies in nanocomposite ribbon materials (2) to samples in the form of thin films using two different approaches: either sputtering from a single target with increased Fe content or successively sputtering from two targets (Nd-Fe-B and u-Fe) in a multilayer fashion. EXPERIMENTAL Nd-Fe-Nb-Bfilms were prepared by magnetron sputtering on tantalum foil substrates from 1.3inchcasttargetswiththefollowingcompositions:N~e~~Ba,NdsFes3NbBs,Ndl~~9~Bs, NdI$e75NbBs and Nd2cFeTeNbBs.The targets were prepared by arc-melting stoichiometric amounts of the constituent elements in a high purity Ar atmosphere. The procedure was repeated several times to assure the homogeneity of the target material. The lower Nd compositions were chosen to give a microstructure consisting of a mixture of NdzFel4NbB and a-Fe grains. For brevity in what follows films sputteredfrom these targetswill be referredto as Nd6, Nd8, Nd12, Ndl5 and Nd20, respectively. The base pressure was 3~10~~Torr and the Ar pressure during
Figure 1. X-ray diffraction pattern of a film sputtered from the N&Fe7$IbB9 target and then annealed at 700°C for 5 minutes.
THIN FM Nmommsm
Nd,Fe,,B/u-Fe
MAGNETS
1015
sputtering was 5 mTorr. Another set of samples was prepared in the form of Nd-Fe-B/a-Fe multilayers. High sputtering rates were used (around 20 &ec). The as-sputtered films were amorphous and showed no hysteresis. These films were subsequently annealed (ex-situ) at temperatures between 650-750°C for 5-30 minutes. The hysteresis loops were measured in a SQUID magnetometer with a maximum field of 55 kGe. X-ray diflraction (XRD) spectra were measured with Cu Ka radiation in a Philips APD3520diffractometer.The microstructural studies were carried ‘out using a Jeol JEM-2000FX transmission electron microscope. The chemical composition was determined by PIXE Selected samples were sent for chemical analysis to Galbraith Laboratories Inc.
The rare earth content of the sputtered films is lower than that of the target as summarized in Table 1. Compositional differences between the target material and the sputtered films are expected when sputtering materialsthat consist of atoms with such different atomic weights as Nd, Fe and B (9).
TABLE 1 Summary of the Nd-Fe-NbB Film Samples Sample -16 Nd8 Nd12 Ndll5 Ndl20
Target composition Nd6Fes7NhB6 NdsFes&bBs Ndt2Fe7&&
Film composition Nd4.6Fess.3Nbo.7Bs.4 Nd6.3Fes4.9EJbo.sBx Nd9Fes&Jbl.lBx
Ndl&MJW NM’e7oN’bb
N42.2Fe77.3NbBx Ndl~b4.&h4Bx
TABLE 2 Summary of the Nd-Fe-Nb-B/FeSamples number of bilayers 40 808 150 180
Fe layer thickness (C) 145 72 72 72
Nd-Fe-Nb-B layer thickness (A) 562 281 112 56
Composition Ndt3.s Fe&bo.sBx Ndt3.3 Fes&lbo.sBx N41.2
F‘G6.6~2.2Bx
Nds.2 F%~Nbo.sBx
1016
I PANAGIOTOPOULOS ANDGC HADJIPANAMS
Figure 2. X-ray diffraction patterns for films sputtered from (a) Nd@eg$%BS, (b) NdsFessNbB8, (c) Ndl$e79NbBs targets and then annealed at 700°C for 5 minutes.
I
’
I
-20-15-10
1
I
’
-5
I
0
5
x
b
’
t
’
10 15 20
H (kOe) Figure 3. Hysteresis loops of film samples sputtered from various targets and then annealed at 700°C for 5 minutes. The measurement was done at 300 K.
THINFILMNANOCOMPOSITE Nd2Fe,,Wa-Fe MAGNETS
1017
Figure 11shows the X-ray diffraction pattern of a film sputtered from the NdtsFe7WBs target and annealed at 700°C for 5 minutes. The observed peaks can be indexed to the tetragonal NdSel4B phalse with unitcellparametersa= 8.81 Aandc=12.24 A, which giveac/aratioof 1.39. The Nd$elhEt phase shows a small degree of preferential orientation of the (105) direction perpendicular to the film plane. Figure 2 shows the X-ray diffraction patterns for films sputtered from targets with lower rare earth content (Nd6-Nd12) and annealed for 5 minutes at 700°C. A strong a-Fe peak is observed along with the peaks of the Nd$et4B phase. The relative intensity oftheNd2Fel~IBpeakstothatofthea-FepeakisreducedinsampleswithhigherFecontent.Finally only tbe a-Fe peak is seen in the X-ray diffraction pattern of the Nd6 film probably due to the fine size of the hard phase grains. However, reflections corresponding to the Nd2Fq4B phase were observed in the electron diffraction patterns of this sample. The existence of the magnetically hard NdzFetB phase is furthermore indicated by the hysteresis loops of these samples as shown in Figure 3 where coercivities in the range of 1.5 kOe were observed. The highest coercivity was obtained in tbe Nd20 sample and it is attributed to the presence of Nld-rich phase which is usually observed in Nd-rich magnets.
] -20
I
-10
1
I
0
10
20
H (kOe) Figure 4. Hysteresis loops for films sputtered from a NdsFessNbBs target and annealed at various temperatures between 620 and 740°C for 5 minutes. The measurement was done at 300 K.
I PANAGIOTOPOULOS ANDGC HADJIPANAYIS
Figure 5. TEM micrograph of a fiim sputtered from a Ndd;es7NbBe target and annealed at 700°C for 5 miIUtteK
Figure 4 shows the hysteresis loops for Nd8 films annealed at various temperatures between 620 and 740°C for 5 minutes. The samples annealed at 660 and 700°C have coercivities around 1.5 kOe and reduced remanence (Ma) of 0.63. This value of coercivity is low compared to the one (&=3.1 kOe) obtained in ribbons with approximately the same composition (5) which consist 50%wt by soft phase. The sample annealed at 740°C has slightly higher coercivity but a shoulder can be observed in the demagnetization part of the hysteresis loop. This can be attributed to an increase the grain size of the soft phase (a-Fe) at higher annealing temperatures and longer annealing times leading to the decoupling of some of the grains as it is reflected by the a shoulder in the demagnetization curve. Indeed some a-Fe clustering can be observed in Figure 5, showing a TEM micrograph of a Nd6 film annealed at 700°C for 5 minutes. This results in relatively large clusters of grains (about 50 nm) which cannot be optimally coupled to the hard phase. In an attempt to avoid the problem of a-Fe clustering, a different approach in the preparation of these films was ‘usedwhere Nd-Fe-Nb-B and a-Fe bilayers were sputtered successively from a Nd2cFe7eNbB9 and an Fe target, respectively. By changing the relative thickness of the layers one can control the composition of the sample. The samples prepared are summarized in Table 2. The number of bilayers was varied to keep the total thickness close to 2.8 pm for all the samples. These samples were subsequently crystallized by annealing ex situ.The hysteresis loops for these multilayersafter5 minutesannealingat675,700and725”C,respectivelyareshowninFigutes6-8. With the exception ofthe 7/5.5 the samples show reduced remanence 0.6-0.7 depending on the composition and annealing conditions. The sample 7/l 1 has much higher coercivity compared to sample Nd15 (Figure 3) which has a similar composition. This indicates that preparing the samples
1019
I
(14.956)
l
,p .1 (7/28) .*d.- . ..J
-60 -40 -20
0
(7/l 1)
20
40
.
60
H (kOe) Figure 6. Hysteresis loops of multilayers annealed at 680°C for 5 minutes. The numbers indicate the thickness of the Fe and Nd-Fe-Nb-Blayers respectively in nm. The measurements were done at 300 K.
in a multilayer fashion really enables to control the size and arrangementof the soft and hardphase grains. The shoulder observed in the demagnetizationcurve is minimum for the samples annealed at 700°C. However the shoulder is still significant especially for the sample with the higher a-Fe content. It is worth noting that the shoulder in the demagnetizationcurve is not observed when the loops are measured with the field applied perpendicularto the substrateof the film (Figure 9). This could be a result of the layered structure that would lead to a more effective coupling between the soft and hard phase along the direction perpendicular to the substrate plane. However a similar behavior has been reported for Nd$elfi films crystallized from amorphous deposits (10). Multilayered lsamplescompared to the samples sputtered by single targets with similar compositions present higher coercivities but also increased shoulder in the demagnetization curve. This is associated wifh partial oxidation of the films during sputtering with the substrate oscillating back and forth between the two targets. This interpretation has been verified by the X-ray diffraction patterns.
I PANAGI~T~PO~L~~ ANDGC Hmwwans
1020
CONCLUSIONS Nanocomposite film magnets were prepared by sputtering from cast alloy targets and then annealing en-situ in high vacuum. lko different approaches have been used: Sputtering from targets with increased Fe content or sputtering succesively from a Nd-Fe-B and a iron target in a multilayer fashion. The latter gives higher coercivities due to the advantage of controlling the soft phase content as well as the grain size by the individual layer thickness, but also allows some oxidation to occur during the time the target is rotated. Magnetic data indicate the existence of a hard phase that is exchange-coupled with the soft phase (u-Fe) grains for samples annealed for 5 minutes at 700°C. Annealing times long enough to fully crystallizethe high anisotropy tetragonalNd$eldB phase cannot be used in films with low ram earth percentage due to u-Fe clustering that leads to decoupling of the soft phase. The TEM studies show that in this case the grain size exceeds the theoretically predicted size (of about 10 nm) for optimum coupling between the two phases (1,5). The optimally annealed samples show remanence enhancement. A direct comparison with the theoretical predictions of micromagneticcalculations(5) is not feasible due to therandom easy magnetization axis and size distribution of the grains in our sample. Basic features as the decrease of coercive field with increasing soft phase are however observed.
(14356) l
C
00J .*a0’
(7/28)
/-.**oJ
(7/l 1)
$z (7/M)
I
,
I I
-60
-40
*
-20
,
,
I
0
20
40
’
60
H (kOe) Figure 7. Hysteresis loops of multilayers annealed at 700°C for 5 minutes. The numbers indicate the thickness of the Fe and Nd-Fe-Nb-Blayers respectively in nm. The measurements were done at 300 K.
THIN FILM NANOCWPOSITE NdpFe,,Bla-Fe MAGNETS
1021
Figure 8. Hysteresis loops of multilayers annealed at 725°C for 5 minutes. The numbers indicate the thickness of the Fe and Nd-Fe-Nb-Blayers respectively in nm. The measurements were done at 300 K.
-40 -30 -20 -10 0 10 20 30 40 H,, NW Figure 9. Hysteresis loop of a Fe(7nm)/Nd-Fe-Nb-B(5.5nm) multilayer measured with applied field perpendicular to the film substrate.
1022
I PANAGIOTOPOULOS ANDGC HADJPANAW
ACKNOWLEDGMENTS
We would Iike to thank Dr. C.P. Swarm for thePIXE measurements. Work supported by DOE # DE-FGO2-90ER454 13. REFERENCES 1.
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