The magnetic properties of nanocomposite multilayer magnets of the (Nd,Dy)(Fe,Co,Nb,B) 5.5/M (M = Co,FeCo) on Ti-buffered Si substrates have been investigated. X-ray diffraction results reveal that the Nd2Fe14B-type phase in almost all the films is randomly oriented. Different thickness x (nm) for Co layers and y (nm) for FeCo layers were adopted in the multilayers. It is found that high remanence is achieved in the nanocomposite multilayer films consisting of the Nd2Fe14B-type phase and soft magnetic phase for Co with 6≥x≥4 and for FeCo with 10≥y≥6. The enhancement of the remanence in the nanocomposite multilayer films is attributed to the exchange coupling between the magnetically soft and hard phases. Increasing the soft magnetic components results in a continuously decreasing coercivity

Li, Xingzhong

Liu, J. Ping

Liu, W.

Sellmyer, David J.

Sun, X.K.

Zhang, Z.D.

English

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University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
David Sellmyer Publications Research Papers in Physics and Astronomy 
5-15-2002 
Structure and magnetic properties of sputtered 
(Nd,Dy)(Fe,Co,Nb,B) 5.5/M (M = FeCo,Co) multilayer magnets 
W. Liu 
University of Nebraska - Lincoln 
Z.D. Zhang 
Shenyang National Laboratory for Materials Science and International Centre for Materials Physics, 
Institute of Metal Research, Academia Sinica, Shenyang 110016, People's Republic of China 
J. Ping Liu 
University of Nebraska-Lincoln, pliu@uta.edu 
Xingzhong Li 
University of Nebraska-Lincoln, xli2@unl.edu 
X.K. Sun 
Shenyang National Laboratory for Materials Science and International Centre for Materials Physics, 
Institute of Metal Research, Academia Sinica, Shenyang 110016, People's Republic of China 
See next page for additional authors 
Follow this and additional works at: https://digitalcommons.unl.edu/physicssellmyer 
 Part of the Physics Commons 
Liu, W.; Zhang, Z.D.; Liu, J. Ping; Li, Xingzhong; Sun, X.K.; and Sellmyer, David J., "Structure and magnetic 
properties of sputtered (Nd,Dy)(Fe,Co,Nb,B) 5.5/M (M = FeCo,Co) multilayer magnets" (2002). David 
Sellmyer Publications. 46. 
https://digitalcommons.unl.edu/physicssellmyer/46 
This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in David Sellmyer Publications 
by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
Authors 
W. Liu, Z.D. Zhang, J. Ping Liu, Xingzhong Li, X.K. Sun, and David J. Sellmyer 
This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/
physicssellmyer/46 
Structure and magnetic properties of sputtered
Nd,DyFe,Co,Nb,B5.5 ÕM M˜FeCo,Co multilayer
magnets
W. Liua)
Shenyang National Laboratory for Materials Science and International Centre for Materials Physics,
Institute of Metal Research, Academia Sinica, Shenyang 110016, People’s Republic of China and Department
of Physics and Astronomy and Center for Materials Research and Analysis, University of Nebraska,
Lincoln, Nebraska 68588-0113
Z. D. Zhang
Shenyang National Laboratory for Materials Science and International Centre for Materials Physics,
Institute of Metal Research, Academia Sinica, Shenyang 110016, People’s Republic of China
J. P. Liu
Department of Physics and Astronomy and Center for Materials Research and Analysis, University of
Nebraska, Lincoln, Nebraska 68588-0113 and Institute for Micromanufacturing, Louisiana Tech University,
Ruston, Louisiana 71272
X. Z. Li
Department of Physics and Astronomy and Center for Materials Research and Analysis, University of
Nebraska, Lincoln, Nebraska 68588-0113
X. K. Sun
Shenyang National Laboratory for Materials Science and International Centre for Materials Physics,
Institute of Metal Research, Academia Sinica, Shenyang 110016, People’s Republic of China
D. J. Sellmyer
Department of Physics and Astronomy and Center for Materials Research and Analysis, University of
Nebraska, Lincoln, Nebraska 68588-0113
The magnetic properties of nanocomposite multilayer magnets of the (Nd,Dy)(Fe,Co,Nb,B)5.5 /M
(M5Co,FeCo) on Ti-buffered Si substrates have been investigated. X-ray diffraction results reveal
that the Nd2Fe14B-type phase in almost all the films is randomly oriented. Different thickness x ~nm!
for Co layers and y ~nm! for FeCo layers were adopted in the multilayers. It is found that high
remanence is achieved in the nanocomposite multilayer films consisting of the Nd2Fe14B-type phase
and soft magnetic phase for Co with 6>x>4 and for FeCo with 10>y>6. The enhancement of the
remanence in the nanocomposite multilayer films is attributed to the exchange coupling between the
magnetically soft and hard phases. Increasing the soft magnetic components results in a
continuously decreasing coercivity. © 2002 American Institute of Physics.
@DOI: 10.1063/1.1451581#
I. INTRODUCTION
Many experimental investigations of nanocomposite
magnets prepared by rapid quenching and mechanical alloy-
ing have been reported in the past ten years.1–5 Because of
the difficulties in controlling the morphology in nanocom-
posite magnets, the maximum energy products achieved have
been much smaller than those expected from the calcula-
tions. On the other hand, it is known that the morphology of
nanocomposite films can be relatively easily controlled, by
adjusting parameters like the thickness of hard and soft
phases, and by changing annealing temperature and time in
nanostructured multilayer magnets. Recently, some studies
on exchange coupling were carried out for nanostructured
CoSm/FeCo and PrCo/Co multilayer films prepared by sput-
tering and subsequent heat treatment.6,7 Magnetic properties
of exchange-coupled a-Fe/Nd–Fe–B multilayer magnets
were investigated by Shindo8 and the observations for the
Nd–Fe–B/Fe/Nd–Fe–B trilayers were reported by Parhofer
and Yang et al.9–11 It is found that further studies are neces-
sary in developing nanostructured multilayer magnets. In the
present article, we report the structural and magnetic proper-
ties of nanocomposite (Nd,Dy)(Fe,Co,Nb,B)5.5 /M
(M5Co,Fe65Co35) multilayer magnets synthesized by sput-
tering and subsequent annealing.
II. EXPERIMENT
(Nd,Dy)(Fe,Co,Nb,B)5.5 /M (M5Co,Fe65Co35)
multilayer films were prepared with a multiple-gun
dc- and rf-sputtering system by depositing the
(Nd0.9Dy0.1)(Fe0.77Co0.12Nb0.03B0.08)5.5 alloy and Co or FeCo
targets onto silicon substrates covered with a 20 nm Ti
buffer. The target of hard magnetic phase alloy was prepared
by sintering powders of the alloys and the others were com-
mercial products. Purities of all the materials of targets were
higher than 99.9%. The base pressure of the sputtering sys-
tem was 2 – 331027 Torr, and the Ar pressure during thea!Electronic mail: wliu@imr.ac.cn
JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 10 15 MAY 2002
78900021-8979/2002/91(10)/7890/3/$19.00 © 2002 American Institute of Physics
Downloaded 16 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
sputtering was 5 mTorr. The thickness of the films was mea-
sured by weighting the mass of the films. The as-deposited
films were heat-treated in a furnace with a vacuum of 2
31027 Torr. The crystalline structure of the phases in the
films was determined by x-ray diffractometry ~XRD! with
Cu Ka radiation. The nanostructure of a representative
sample was examined by transmission electron microscopy
~TEM!. Magnetic properties of the films were measured by
an alternating gradient force magnetometer and supercon-
ducting quantum interference device magnetometer. The hys-
teresis loops and the magnetic properties were recorded with-
out the demagnetizing correction.
III. RESULTS AND DISCUSSION
To study the structural and magnetic properties of mul-
tilayers, initially a hard phase single-layer film with compo-
sition of Ti~20 nm!/NdDyFeCoNbB~300 nm!/Ti~20 nm!/~Si
substrate! was investigated.
Figure 1 shows XRD patterns for the
Ti~20 nm!/NdDyFeCoNbB~300 nm!/Ti~20 nm!/~Si substrate!
thin film as-deposited and annealed at 625 °C for 5 min. In
the as-deposited film, the amorphous phase is formed. After
annealing at 625 °C for 5 min, the hard magnetic phase with
Nd2Fe14B-type structure is formed, accompanied with some
Nd-rich phase and a trace of NdO2 in the film.
Figure 2 gives hysteresis loops at room temperature of
the thin films of Ti(20 nm)/@NdDyFeCoNbB(15 nm)
Co(xnm)]320/Ti(20 nm)/(Si substrate) multilayers an-
nealed at 625 °C for 5 min. It can be seen from Fig. 2 that the
intrinsic coercivity of the single layer is as high as 17.2 kOe,
the remanence is about 333 emu/cc for x50, i.e., the single-
layer case. With increasing thickness of Co, the remanence
increases and coercivity decreases slightly. It is clearly seen
that for 4<x<6, the existence of Co layers among
Nd2Fe14B-type hard-phase layers results in an increase of
remanence by about 270 emu/cc and a decrease of the coer-
civity by about 4 kOe, compared to the case of single layer.
The enhanced remanence is due to exchange coupling be-
tween hard and soft phase and magnetic softening is caused
by the soft magnetic component.
Because of the higher saturation magnetization of
Fe65Co35 alloy than those of pure Co and Fe, the Fe65Co35
alloy is also chosen as the soft magnetic layer
component of the multilayer films. XRD patterns
for Ti(20 nm)/@NdDyFeCoNbB(15 nm)Fe65Co35(y nm)#
320/Ti(20 nm)/(Si substrate) multilayers annealed at
625 °C for 1 min are given in Fig. 3. In comparison with the
result of the single layer, it is found that there is a large
amount of a-Fe~Co! in the films and trace quantity of
Nd-rich phase and NdO2 besides the hard magnetic phase of
Nd2Fe14B type. The Nd-rich phase decreases quickly with
increasing thickness of FeCo layers. The XRD profiles of the
films reveal a random orientation of Nd2Fe14B-type phases.
Figure 4 shows hysteresis loops at room temperature for
the Ti(20 nm)/@NdDyFeCoNbB(15 nm)Fe65Co35(y nm)#
320/Ti(20 nm)/(Si substrate) multilayers annealed at
625 °C for 1 min. It can be seen that for a 2 nm thickness of
the FeCo layers, the magnetic behavior is similar to that of
single layer, because the hard phase is dominant in this
multilayer. However, the remanence is higher than that of
single layer. The saturation magnetization and remanence en-
hance with increasing thickness of FeCo alloy, and the rema-
nence is up to 692 emu/cc for y510. It is found that the
remanence of the film with y56 is slightly higher than that
of the above-mentioned Co-containing series due to the
higher magnetization of FeCo than Co. In contrast, the coer-
civity of the former is only 5.7 kOe, which is remarkably
lower than the latter. The ratio Mr /Ms for all the samples is
higher than 0.65. The enhancement of the magnetic proper-
ties in the nanocomposite multilayer films is attributed to the
FIG. 1. XRD patterns for the Ti~20 nm!/NdDyFeCoNbB~300 nm!/Ti~20
nm!/~Si substrate! thin film as-deposited and annealed at 625 °C for 5 min.
FIG. 2. Hysteresis loops at room temperature for the thin film of Ti-buffered
NdDyFeCoNbB and NdDyFeCoNbB/Co multilayers on the Si substrate an-
nealed at 625 °C for 5 min. Magnetic measurements are in the film plane.
7891J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Liu et al.
Downloaded 16 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
exchange coupling between the magnetically soft and hard
phases.
To study the nanostructure of the multilayer magnets,
a TEM observation with a plane view was conducted.
Figure 5 shows a TEM bright-field image of the Si/Ti~20
nm!/@A~15 nm!Fe65Co35(6 nm)]320/Ti(20 nm) multilayer
thin film annealed at 625 °C for 1 min. Two types of grain
morphology were observed, i.e., relatively small grains ~la-
beled as A! and large ones with irregular contrasts ~labeled as
B!. Selected-area electron diffraction analyses reveal that the
relatively small grains ~type A! represent the hard phase with
Nd2Fe14B-type structure and the large grains ~type B! repre-
sent the soft phase of the a-Fe~Co! with the cubic structure
in agreement with the results of XRD. Average grain size of
the hard phase is between 10 and 15 nm, but the average
grain size of the soft phase is larger than that. In order to
improve the magnetic properties of the multilayer nanocom-
posite magnets, it is necessary to effectively control the grain
size of the soft phase.
In conclusion, the structure and magnetic properties of
(Nd,Dy)(Fe,Co,Nb,B)5.5 /M (M5Co,FeCo) multilayer mag-
nets have been prepared by sputtering and subsequent heat
treatments. The hard magnetic phase of Nd2Fe14B-type co-
exists with a large amount of soft phase ~Co phase and FeCo
alloy! in the multilayer magnets. In comparison with the case
of single layer, the remanence of the multilayer magnets is
noticeably enhanced. The enhancement effect has been ob-
served in all of the multilayer magnets due to exchange cou-
pling between the soft and the hard phases.
ACKNOWLEDGMENTS
This work has been supported by the U.S. NSF under
Grant No. INT-9812082, DOE and DARPA/ARO, and the
National Natural Science Foundation of China under Project
Nos. 50071062, 59725103.
1 R. Coehoorn, D. B. de Mooji, and C. D. E. Waard, 80, 101 ~1989!.
2 J. Ding, P. G. McCormick, and R. Street, J. Magn. Magn. Mater. 124, 1
~1993!.
3 A. Manaf, R. A. Buckley, and H. A. Davies, J. Magn. Magn. Mater. 128,
302 ~1993!.
4 L. Withanawasam, A. S. Hurphy, G. C. Hadjipanayis, and R. F. Krause, J.
Appl. Phys. 75, 7065 ~1994!.
5 X. K. Sun, J. Zhang, Y. L. Chu, W. Liu, B. Z. Cui, and Z. D. Zhang, Appl.
Phys. Lett. 74, 1740 ~1999!.
6 I. A. Al-Omari and D. J. Sellmyer, Phys. Rev. B 52, 3441 ~1995!.
7 J. P. Liu, Y. Liu, R. Skomski, and D. J. Sellmyer, IEEE Trans. Magn. 35,
3241 ~1999!.
8 M. Shindo and M. Ishizone, J. Appl. Phys. 81, 4444 ~1997!.
9 S. Parhofer, J. Wecker, C. Kuhrt, and G. Gieres, IEEE Trans. Magn. 32,
4437 ~1996!.
10 S. Parhofer, G. Gieres, J. Wecker, and L. Schultz, J. Magn. Magn. Mater.
163, 32 ~1996!.
11 C. J. Yang, and S. W. Kim, J. Magn. Magn. Mater. 202, 311 ~1999!.
FIG. 3. XRD patterns for the thin film of Ti-buffered
NdDyFeCoNbB/Fe65Co35 multilayers on the Si substrate annealed at 625 °C
for 1 min.
FIG. 5. TEM bright-field image of the Si/Ti(20 nm)/
@A(15 nm)Fe65Co35(6 nm)#20/Ti/(20 nm) multilayer thin film annealed at
625 °C for 1 min.
FIG. 4. Hysteresis loops at room temperature for the thin films of Ti-
buffered NdDyFeCoNbB/Fe65Co35 multilayers on the Si substrate annealed
at 625 °C for 1 min. Magnetic measurements are in the film plane.
7892 J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Liu et al.
Downloaded 16 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp


Structure and magnetic properties of sputtered (Nd,Dy)(Fe,Co,Nb,B) \u3csub\u3e5.5\u3c/sub\u3e/M (M = FeCo,Co) multilayer magnets

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln David Sellmyer Publications Research Papers in Physics and Astronomy 5-15-2002 Structure and magnetic properties of sputtered (Nd,Dy)(Fe,Co,Nb,B) 5.5/M (M = FeCo,Co) multilayer magnets W. Liu University of Nebraska - Lincoln Z.D. Zhang Shenyang National Laboratory for Materials Science and International Centre for Materials Physics, Institute of Metal Research, Academia Sinica, Shenyang 110016, People's Republic of China J. Ping Liu University of Nebraska-Lincoln, pliu@uta.edu Xingzhong Li University of Nebraska-Lincoln, xli2@unl.edu X.K. Sun Shenyang National Laboratory for Materials Science and International Centre for Materials Physics, Institute of Metal Research, Academia Sinica, Shenyang 110016, People's Republic of China See next page for additional authors Follow this and additional works at: https://digitalcommons.unl.edu/physicssellmyer  Part of the Physics Commons Liu, W.; Zhang, Z.D.; Liu, J. Ping; Li, Xingzhong; Sun, X.K.; and Sellmyer, David J., "Structure and magnetic properties of sputtered (Nd,Dy)(Fe,Co,Nb,B) 5.5/M (M = FeCo,Co) multilayer magnets" (2002). David Sellmyer Publications. 46. https://digitalcommons.unl.edu/physicssellmyer/46 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in David Sellmyer Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Authors W. Liu, Z.D. Zhang, J. Ping Liu, Xingzhong Li, X.K. Sun, and David J. Sellmyer This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/physicssellmyer/46 Structure and magnetic properties of sputteredNd,DyFe,Co,Nb,B5.5 ÕM M˜FeCo,Co multilayermagnetsW. Liua)Shenyang National Laboratory for Materials Science and International Centre for Materials Physics,Institute of Metal Research, Academia Sinica, Shenyang 110016, People’s Republic of China and Departmentof Physics and Astronomy and Center for Materials Research and Analysis, University of Nebraska,Lincoln, Nebraska 68588-0113Z. D. ZhangShenyang National Laboratory for Materials Science and International Centre for Materials Physics,Institute of Metal Research, Academia Sinica, Shenyang 110016, People’s Republic of ChinaJ. P. LiuDepartment of Physics and Astronomy and Center for Materials Research and Analysis, University ofNebraska, Lincoln, Nebraska 68588-0113 and Institute for Micromanufacturing, Louisiana Tech University,Ruston, Louisiana 71272X. Z. LiDepartment of Physics and Astronomy and Center for Materials Research and Analysis, University ofNebraska, Lincoln, Nebraska 68588-0113X. K. SunShenyang National Laboratory for Materials Science and International Centre for Materials Physics,Institute of Metal Research, Academia Sinica, Shenyang 110016, People’s Republic of ChinaD. J. SellmyerDepartment of Physics and Astronomy and Center for Materials Research and Analysis, University ofNebraska, Lincoln, Nebraska 68588-0113The magnetic properties of nanocomposite multilayer magnets of the (Nd,Dy)(Fe,Co,Nb,B)5.5 /M(M5Co,FeCo) on Ti-buffered Si substrates have been investigated. X-ray diffraction results revealthat the Nd2Fe14B-type phase in almost all the films is randomly oriented. Different thickness x ~nm!for Co layers and y ~nm! for FeCo layers were adopted in the multilayers. It is found that highremanence is achieved in the nanocomposite multilayer films consisting of the Nd2Fe14B-type phaseand soft magnetic phase for Co with 6>x>4 and for FeCo with 10>y>6. The enhancement of theremanence in the nanocomposite multilayer films is attributed to the exchange coupling between themagnetically soft and hard phases. Increasing the soft magnetic components results in acontinuously decreasing coercivity. © 2002 American Institute of Physics.@DOI: 10.1063/1.1451581#I. INTRODUCTIONMany experimental investigations of nanocompositemagnets prepared by rapid quenching and mechanical alloy-ing have been reported in the past ten years.1–5 Because ofthe difficulties in controlling the morphology in nanocom-posite magnets, the maximum energy products achieved havebeen much smaller than those expected from the calcula-tions. On the other hand, it is known that the morphology ofnanocomposite films can be relatively easily controlled, byadjusting parameters like the thickness of hard and softphases, and by changing annealing temperature and time innanostructured multilayer magnets. Recently, some studieson exchange coupling were carried out for nanostructuredCoSm/FeCo and PrCo/Co multilayer films prepared by sput-tering and subsequent heat treatment.6,7 Magnetic propertiesof exchange-coupled a-Fe/Nd–Fe–B multilayer magnetswere investigated by Shindo8 and the observations for theNd–Fe–B/Fe/Nd–Fe–B trilayers were reported by Parhoferand Yang et al.9–11 It is found that further studies are neces-sary in developing nanostructured multilayer magnets. In thepresent article, we report the structural and magnetic proper-ties of nanocomposite (Nd,Dy)(Fe,Co,Nb,B)5.5 /M(M5Co,Fe65Co35) multilayer magnets synthesized by sput-tering and subsequent annealing.II. EXPERIMENT(Nd,Dy)(Fe,Co,Nb,B)5.5 /M (M5Co,Fe65Co35)multilayer films were prepared with a multiple-gundc- and rf-sputtering system by depositing the(Nd0.9Dy0.1)(Fe0.77Co0.12Nb0.03B0.08)5.5 alloy and Co or FeCotargets onto silicon substrates covered with a 20 nm Tibuffer. The target of hard magnetic phase alloy was preparedby sintering powders of the alloys and the others were com-mercial products. Purities of all the materials of targets werehigher than 99.9%. The base pressure of the sputtering sys-tem was 2 – 331027 Torr, and the Ar pressure during thea!Electronic mail: wliu@imr.ac.cnJOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 10 15 MAY 200278900021-8979/2002/91(10)/7890/3/$19.00 © 2002 American Institute of PhysicsDownloaded 16 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspsputtering was 5 mTorr. The thickness of the films was mea-sured by weighting the mass of the films. The as-depositedfilms were heat-treated in a furnace with a vacuum of 231027 Torr. The crystalline structure of the phases in thefilms was determined by x-ray diffractometry ~XRD! withCu Ka radiation. The nanostructure of a representativesample was examined by transmission electron microscopy~TEM!. Magnetic properties of the films were measured byan alternating gradient force magnetometer and supercon-ducting quantum interference device magnetometer. The hys-teresis loops and the magnetic properties were recorded with-out the demagnetizing correction.III. RESULTS AND DISCUSSIONTo study the structural and magnetic properties of mul-tilayers, initially a hard phase single-layer film with compo-sition of Ti~20 nm!/NdDyFeCoNbB~300 nm!/Ti~20 nm!/~Sisubstrate! was investigated.Figure 1 shows XRD patterns for theTi~20 nm!/NdDyFeCoNbB~300 nm!/Ti~20 nm!/~Si substrate!thin film as-deposited and annealed at 625 °C for 5 min. Inthe as-deposited film, the amorphous phase is formed. Afterannealing at 625 °C for 5 min, the hard magnetic phase withNd2Fe14B-type structure is formed, accompanied with someNd-rich phase and a trace of NdO2 in the film.Figure 2 gives hysteresis loops at room temperature ofthe thin films of Ti(20 nm)/@NdDyFeCoNbB(15 nm)Co(xnm)]320/Ti(20 nm)/(Si substrate) multilayers an-nealed at 625 °C for 5 min. It can be seen from Fig. 2 that theintrinsic coercivity of the single layer is as high as 17.2 kOe,the remanence is about 333 emu/cc for x50, i.e., the single-layer case. With increasing thickness of Co, the remanenceincreases and coercivity decreases slightly. It is clearly seenthat for 4<x<6, the existence of Co layers amongNd2Fe14B-type hard-phase layers results in an increase ofremanence by about 270 emu/cc and a decrease of the coer-civity by about 4 kOe, compared to the case of single layer.The enhanced remanence is due to exchange coupling be-tween hard and soft phase and magnetic softening is causedby the soft magnetic component.Because of the higher saturation magnetization ofFe65Co35 alloy than those of pure Co and Fe, the Fe65Co35alloy is also chosen as the soft magnetic layercomponent of the multilayer films. XRD patternsfor Ti(20 nm)/@NdDyFeCoNbB(15 nm)Fe65Co35(y nm)#320/Ti(20 nm)/(Si substrate) multilayers annealed at625 °C for 1 min are given in Fig. 3. In comparison with theresult of the single layer, it is found that there is a largeamount of a-Fe~Co! in the films and trace quantity ofNd-rich phase and NdO2 besides the hard magnetic phase ofNd2Fe14B type. The Nd-rich phase decreases quickly withincreasing thickness of FeCo layers. The XRD profiles of thefilms reveal a random orientation of Nd2Fe14B-type phases.Figure 4 shows hysteresis loops at room temperature forthe Ti(20 nm)/@NdDyFeCoNbB(15 nm)Fe65Co35(y nm)#320/Ti(20 nm)/(Si substrate) multilayers annealed at625 °C for 1 min. It can be seen that for a 2 nm thickness ofthe FeCo layers, the magnetic behavior is similar to that ofsingle layer, because the hard phase is dominant in thismultilayer. However, the remanence is higher than that ofsingle layer. The saturation magnetization and remanence en-hance with increasing thickness of FeCo alloy, and the rema-nence is up to 692 emu/cc for y510. It is found that theremanence of the film with y56 is slightly higher than thatof the above-mentioned Co-containing series due to thehigher magnetization of FeCo than Co. In contrast, the coer-civity of the former is only 5.7 kOe, which is remarkablylower than the latter. The ratio Mr /Ms for all the samples ishigher than 0.65. The enhancement of the magnetic proper-ties in the nanocomposite multilayer films is attributed to theFIG. 1. XRD patterns for the Ti~20 nm!/NdDyFeCoNbB~300 nm!/Ti~20nm!/~Si substrate! thin film as-deposited and annealed at 625 °C for 5 min.FIG. 2. Hysteresis loops at room temperature for the thin film of Ti-bufferedNdDyFeCoNbB and NdDyFeCoNbB/Co multilayers on the Si substrate an-nealed at 625 °C for 5 min. Magnetic measurements are in the film plane.7891J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Liu et al.Downloaded 16 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspexchange coupling between the magnetically soft and hardphases.To study the nanostructure of the multilayer magnets,a TEM observation with a plane view was conducted.Figure 5 shows a TEM bright-field image of the Si/Ti~20nm!/@A~15 nm!Fe65Co35(6 nm)]320/Ti(20 nm) multilayerthin film annealed at 625 °C for 1 min. Two types of grainmorphology were observed, i.e., relatively small grains ~la-beled as A! and large ones with irregular contrasts ~labeled asB!. Selected-area electron diffraction analyses reveal that therelatively small grains ~type A! represent the hard phase withNd2Fe14B-type structure and the large grains ~type B! repre-sent the soft phase of the a-Fe~Co! with the cubic structurein agreement with the results of XRD. Average grain size ofthe hard phase is between 10 and 15 nm, but the averagegrain size of the soft phase is larger than that. In order toimprove the magnetic properties of the multilayer nanocom-posite magnets, it is necessary to effectively control the grainsize of the soft phase.In conclusion, the structure and magnetic properties of(Nd,Dy)(Fe,Co,Nb,B)5.5 /M (M5Co,FeCo) multilayer mag-nets have been prepared by sputtering and subsequent heattreatments. The hard magnetic phase of Nd2Fe14B-type co-exists with a large amount of soft phase ~Co phase and FeCoalloy! in the multilayer magnets. In comparison with the caseof single layer, the remanence of the multilayer magnets isnoticeably enhanced. The enhancement effect has been ob-served in all of the multilayer magnets due to exchange cou-pling between the soft and the hard phases.ACKNOWLEDGMENTSThis work has been supported by the U.S. NSF underGrant No. INT-9812082, DOE and DARPA/ARO, and theNational Natural Science Foundation of China under ProjectNos. 50071062, 59725103.1 R. Coehoorn, D. B. de Mooji, and C. D. E. Waard, 80, 101 ~1989!.2 J. Ding, P. G. McCormick, and R. Street, J. Magn. Magn. Mater. 124, 1~1993!.3 A. Manaf, R. A. Buckley, and H. A. Davies, J. Magn. Magn. Mater. 128,302 ~1993!.4 L. Withanawasam, A. S. Hurphy, G. C. Hadjipanayis, and R. F. Krause, J.Appl. Phys. 75, 7065 ~1994!.5 X. K. Sun, J. Zhang, Y. L. Chu, W. Liu, B. Z. Cui, and Z. D. Zhang, Appl.Phys. Lett. 74, 1740 ~1999!.6 I. A. Al-Omari and D. J. Sellmyer, Phys. Rev. B 52, 3441 ~1995!.7 J. P. Liu, Y. Liu, R. Skomski, and D. J. Sellmyer, IEEE Trans. Magn. 35,3241 ~1999!.8 M. Shindo and M. Ishizone, J. Appl. Phys. 81, 4444 ~1997!.9 S. Parhofer, J. Wecker, C. Kuhrt, and G. Gieres, IEEE Trans. Magn. 32,4437 ~1996!.10 S. Parhofer, G. Gieres, J. Wecker, and L. Schultz, J. Magn. Magn. Mater.163, 32 ~1996!.11 C. J. Yang, and S. W. Kim, J. Magn. Magn. Mater. 202, 311 ~1999!.FIG. 3. XRD patterns for the thin film of Ti-bufferedNdDyFeCoNbB/Fe65Co35 multilayers on the Si substrate annealed at 625 °Cfor 1 min.FIG. 5. TEM bright-field image of the Si/Ti(20 nm)/@A(15 nm)Fe65Co35(6 nm)#20/Ti/(20 nm) multilayer thin film annealed at625 °C for 1 min.FIG. 4. Hysteresis loops at room temperature for the thin films of Ti-buffered NdDyFeCoNbB/Fe65Co35 multilayers on the Si substrate annealedat 625 °C for 1 min. Magnetic measurements are in the film plane.7892 J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Liu et al.Downloaded 16 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

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Structure and magnetic properties of sputtered (Nd,Dy)(Fe,Co,Nb,B) \u3csub\u3e5.5\u3c/sub\u3e/M (M = FeCo,Co) multilayer magnets

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