Co1−xFex alloy nanowires with 40 nm diam and x=0–1.0 were fabricated by electrodeposition in nanopores of alumina templates. The crystalline structure of the nanowires is concentration dependent and shows a transition from the cobalt hexagonal-closed-packed structure (hcp) to a face-centered-cubic structure (fcc) in the concentration range 

Fodor, Petru S.

Tsoi, Georgy M.

Wenger, Lowell E.

English

EngagedScholarship @ Cleveland State University

Cleveland State University
EngagedScholarship@CSU
Physics Faculty Publications Physics Department
5-15-2002
Fabrication and Characterization of Co1−xFex
Alloy Nanowires
Petru S. Fodor
Cleveland State University, p.fodor@csuohio.edu
Georgy M. Tsoi
Wayne State University
Lowell E. Wenger
Wayne State University, wenger@physics.wayne.edu
Follow this and additional works at: https://engagedscholarship.csuohio.edu/sciphysics_facpub
Part of the Physics Commons
How does access to this work benefit you? Let us know!
Publisher's Statement
© 2002 American Institute of Physics.
This Article is brought to you for free and open access by the Physics Department at EngagedScholarship@CSU. It has been accepted for inclusion in
Physics Faculty Publications by an authorized administrator of EngagedScholarship@CSU. For more information, please contact
library.es@csuohio.edu.
Repository Citation
Fodor, Petru S.; Tsoi, Georgy M.; and Wenger, Lowell E., "Fabrication and Characterization of Co1−xFex Alloy Nanowires" (2002).
Physics Faculty Publications. 201.
https://engagedscholarship.csuohio.edu/sciphysics_facpub/201
Fabrication and characterization of  alloy nanowires
P. S. Fodor, G. M. Tsoi, and L. E. Wenger
Citation:  91, 8186 (2002); doi: 10.1063/1.1449450
View online: http://dx.doi.org/10.1063/1.1449450
View Table of Contents: http://aip.scitation.org/toc/jap/91/10
Published by the American Institute of Physics
Fabrication and characterization of Co1ÀxFex alloy nanowires
P. S. Fodor, G. M. Tsoi, and L. E. Wenger
Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201
Co12xFex alloy nanowires with 40 nm diam and x50 – 1.0 were fabricated by electrodeposition in
nanopores of alumina templates. The crystalline structure of the nanowires is concentration
dependent and shows a transition from the cobalt hexagonal-closed-packed structure ~hcp! to a
face-centered-cubic structure ~fcc! in the concentration range 0,x,0.1. For an iron content greater
than 15 at % the structure becomes body-centered-cubic ~bcc!. The nanowires with an aspect ratio
of about 8 show a highly anisotropic magnetization with the easy axis along the length of the wire.
The squareness of the hysteresis loops is between 72% and 93% for magnetic fields parallel to the
wires while the squareness is less than 13% for fields perpendicular to the wires. The coercivity
along the easy axis reaches a maximum of 2150 Oe for x50.55 and is almost concentration
independent for 0.5,x,0.6. © 2002 American Institute of Physics. @DOI: 10.1063/1.1449450#
I. INTRODUCTION
Many of the magnetic devices used in today’s technolo-
gies are based on the magnetic properties of thin films or
bulk materials. With the advent of nanofabrication processes,
opportunities to use the unique properties associated with
nanostructured materials exist that were not previously
thought possible. However, conventional technologies used
to obtained nanostructures, such as e-beam lithography, x-ray
lithography, and scanning probe-based lithography are very
expensive and not always suitable for commercial applica-
tions.
A less expensive alternative for the production of nano-
structures is the electrodeposition of various materials in na-
noporous membranes. Examples of nanoporous membranes
that have been used for electrodeposition include polycar-
bonate membranes created by etching damage tracks of high-
energy particles and alumina membranes produced by the
anodic oxidation of aluminum.1 The advantage of the alu-
mina templates over other porous membranes is the possibil-
ity of obtaining them with ordered nanopores using
prepatterning2 or multiple anodization steps.3–6
The magnetic properties of transition metals electrode-
posited in alumina templates have been studied by several
groups.4,7–10 High coercivities and large values for the
squareness have been measured parallel to the wire axis in
these types of nanostructures with shape anisotropy being
found to be the dominant mechanism in determining the
magnetic properties. In the present study we have investi-
gated the crystallographic and magnetic properties for vari-
ous concentrations of Co12xFex alloys deposited in the pores
of an alumina matrix produced by anodization in oxalic acid.
II. EXPERIMENTS
High-purity 99.997% Al strips ~632030.25 mm3! were
used as the starting material. The strips were ultrasonically
cleaned for 15 min in acetone and then annealed in air at
500 °C for 2 h in order to increase the size of the crystalline
grains and remove any mechanical stress induced in the
samples during the cutting process of the strips. The samples
were then electropolished in a solution of perchloric acid,
ethanol, and 2-butoxyethanol at a current density
.500 mA/cm2 and a temperature of 5 °C. To obtain highly
ordered pores a two-step anodization process was employed.
In the first anodization step the Al strips were anodized at 40
V in 0.3 M oxalic acid at 5 °C for 2 h. The aluminum oxide
was then stripped off in a 0.6 M chromic acid and 0.4 M
phosphoric acid solution at 60 °C. Subsequently, the sample
was reanodized for 30 min using the same anodization con-
ditions as in the first step. A nonequilibrium anodization pro-
cess in which the voltage was reduced to about 8 V followed
the second anodization. This led to a reduction of the barrier
layer that formed between the porous alumina and the Al
substrate, thus facilitating the electrodeposition of the metals.
The Co12xFex alloy was then electrodeposited in the synthe-
sized template from a concentrated Watts-type electrolyte
containing 45 g/l boric acid and CoSO4 , CoCl2 , and FeSO4
salts in different ratios depending on the composition desired
for the final alloy. The electrodeposition was carried out at
37 °C with an ac voltage of 8 V rms and a frequency of 60 Hz
using a graphite counterelectrode.
Scanning electron microscopy ~SEM! was used to study
the morphology of the alumina templates. The crystalline
properties of the nanowires were investigated using x-ray
diffraction ~XRD! after the Al substrate of the samples was
dissolved using a saturated solution of mercury chloride. The
composition of the nanowires was determined by energy dis-
persive spectroscopy ~EDS! and the magnetic properties
were measured using a Quantum Design SQUID magneto-
meter.
III. RESULTS AND DISCUSSION
Figure 1 shows a SEM micrograph of the top surface of
an anodized alumina template after the two-step anodization
process. The average pore diameter is 40 nm while the inter-
pore distance is 95 nm. The pores are parallel with each other
through the entire thickness of the alumina layer and form a
hexagonally ordered array with typical ordered domain size
of 1 mm. The length of the electrodeposited nanowires is
determined from cross-sectional SEM to be 290 nm
JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 10 15 MAY 2002
81860021-8979/2002/91(10)/8186/3/$19.00 © 2002 American Institute of Physics
620 nm. The x-ray diffraction pattern for the Co nanowires
shows a hexagonal-closed-packed ~hcp! structure with a
(101¯0! texture as shown in Fig. 2. As the concentration of Fe
increases there are two crystallographic transitions. The first
one occurs in the 0,x,0.1 range where a mixed state be-
tween the hcp and fcc ~g! phases exists. For x50.1 the mix-
ing ratio of hcp:fcc is about 7:3. For an iron content greater
than 15 at % the only stable phase is the a phase ~body-
centered-cubic! with the lattice parameter being the same as
that for pure iron.
Typical hysteresis loops for the CoxFe12x nanowires are
shown in Fig. 3~a! with the easy magnetization axis being
along the length of the wires for all the samples. The value of
the coercivity field for the pure Co nanowires is 1400 Oe,
about 30% less than previous results for Co nanowires hav-
ing a larger aspect ratio.11 As the Fe content increases, the
coercivity rapidly increases reaching essentially a concentra-
tion independent plateau of approximately 2150 Oe in the
0.5,x,0.6 range before decreasing to a value of 1750 Oe for the pure Fe nanowires as shown in Fig. 3~b!. This maxi-
mum coercivity value is also about 30% smaller than coer-
civities found in previous studies on Co–Fe nanowires hav-
ing larger aspect ratios.12 The abrupt change in coercivity
with a small addition of iron might be associated with the
transition in the crystalline structure from the highly aniso-
tropic hcp lattice to cubic lattices with higher symmetry. This
conclusion is supported by the change in squareness along
the wire length, from 72% for the Co nanowire sample to
values larger than 80% for the other CoFe nanowire samples
as also seen in Fig. 3~b!. For fields normal to the wires, the
squareness is less than 13% for the Co12xFex nanowires
while the coercivities range between 450 and 750 Oe.
Measurements of the crystalline anisotropy in Co12xFex
bulk alloys indicate that for x50.55 the crystalline anisot-
ropy is zero.13 Thus the Co0.45Fe0.55 ensemble of nanowires
should be an ideal system to study how the magnetic prop-
erties of our nanowire arrays are governed by the shape an-
isotropy. Approximating the cylindrical nanowires as prolate
ellipsoids with an aspect ratio of 8, the theoretical coercivity
FIG. 1. SEM top view of a nanopore array in anodic alumina prepared by a
two-step anodization in 0.3 M oxalic acid. The average pore diameter and
interpore distance are 40 nm and 95 nm, respectively.
FIG. 2. X-ray diffraction pattern for Co12xFex nanowires arrays after re-
moving the Al substrate.
FIG. 3. ~a! Hysteresis curves for a Co0.45Fe0.55 nanowire array with fields
oriented perpendicular ~s! and parallel ~d! with respect with the nanowire
axis. ~b! Concentration dependence of the coercivity field Hc ~j! and
squareness ~h! along the nanowire axis.
8187J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Fodor, Tsoi, and Wenger
based solely on shape anisotropy would be 11 kOe (5(Nx
2Nz)M s) using the bulk value for the saturation magnetiza-
tion of 1923 G.14 This large overestimation of Hc as com-
pared to the measured value of 2150 Oe suggests other
mechanisms are also present. The reduction in the coercivity
might be due to nonuniform growth of the nanowires and to
concentration inhomogeneities that formed in the individual
nanowires during the electrodeposition process and give rise
to local composition variations as compared to the average
determined by EDS. Also the presence of any magnetostatic
interaction between the nanowires may have a significant
effect on the estimated coercivity force. Dipolar stray fields
from adjacent nanowires will oppose the magnetization of its
neighbors and thus reduce the applied field necessary to re-
verse their magnetization. To investigate whether any inter-
actions between the nanowires were present, modified Hen-
kel plots15 ~dM as a function of the applied field! derived
from the isothermal remanent magnetization M r(H) and dc
demagnetization remanence M d(H) curves can be used.
Negative values in dM are typically associated with magne-
tostatic interactions that tend to destabilize the magnetized
state, while positive values indicate the existence of ex-
change interactions between the individual particles. For the
Co0.45Fe0.55 nanowires, dM takes only negative values with a
peak at 2100 Oe ~see Fig. 4! suggesting magnetostatic inter-
actions dominate.
In conclusion, highly ordered Co12xFex alloy nanowires
arrays have been produced in anodic alumina templates. The
magnetic properties—coercivities and squareness of hyster-
esis loops—of the CoFe alloy nanowires are improved over
the properties of similar nanostructures composed of pure Co
and Fe. The concentration dependence of the coercivity force
along the nanowire axis appears to be related to the crystal-
line structure of the nanowires and the coercivity reaches a
maximum in the range in which the nanowires crystallize in
the body-centered-cubic structure.
ACKNOWLEDGMENT
This work is supported in part by the National Science
Foundation through Grant No. DGE-9870720.
1 C. R. Martin, Chem. Mater. 8, 1739 ~1996!.
2 H. Masuda, H. Yamada, M. Satoh, H. Asoh, M. Nakao, and H. Tamamura,
Appl. Phys. Lett. 71, 2770 ~1997!.
3 H. Masuda and K. Fukuda, Science 268, 1466 ~1995!.
4 H. Zeng, M. Zheng, R. Skomski, D. J. Sellmyer, Y. Liu, L. Menon, and S.
Bandyopadhyay, J. Appl. Phys. 87, 4718 ~2000!.
5 A. P. Li, F. Mu¨ller, A. Birner, K. Nielsch, and U. Go¨sele, Adv. Mater. 11,
483 ~1999!.
6 K. Nielsch, F. Mu¨ller, A. P. Li, and U. Go¨sele, Adv. Mater. 12, 582 ~2000!.
7 D. AlMawlawi, N. Coombs, and M. Moskovits, J. Appl. Phys. 70, 4421
~1991!.
8 F. Li and R. M. Metzger, J. Appl. Phys. 81, 3806 ~1997!.
9 M. Zheng, L. Menon, H. Zeng, Y. Liu, S. Bandyopadhyay, R. D. Kirby,
and D. J. Sellmyer, Phys. Rev. B 62, 12282 ~2000!.
10 M. Sun, G. Zangari, M. Shamuzzzoha, and R. M. Metzger, Appl. Phys.
Lett. 78, 2964 ~2001!.
11 R. M. Metzger, V. V. Konovalov, M. Sun, T. Xu, G. Zangari, B. Xu, M.
Benakli, and W. D. Doyle, IEEE Trans. Magn. 36, 30 ~2000!.
12 Coercivity is dependent on aspect ratio as K. I. Arai, H. W. Kang, and K.
Ishiyama, IEEE Trans. Magn. 27, 4906 ~1991! report Hc;2.8 kOe for
aspect ratio of 100.
13 R. M. Bosorth, J. Appl. Phys. 8, 575 ~1937!.
14 P. Weiss and R. Forrer, Ann. Phys. ~Paris! 12, 352 ~1929!.
15 P. E. Kelly, K. O’Grady, P. I. Mayo, and R. W. Chantrell, IEEE Trans.
Magn. 25, 3881 ~1989!.
FIG. 4. Henkel plot for alumina sample filled with electrodeposited
Co0.45Fe0.55 . dM is defined as dM5M d2(122M r), where M d is the re-
duced dc demagnetization remanence and M r is the normalized isothermal
remanence.
8188 J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Fodor, Tsoi, and Wenger


Fabrication and Characterization of Co1−xFex Alloy Nanowires

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Cleveland-Marshall College of Law

Cleveland State UniversityEngagedScholarship@CSUPhysics Faculty Publications Physics Department5-15-2002Fabrication and Characterization of Co1−xFexAlloy NanowiresPetru S. FodorCleveland State University, p.fodor@csuohio.eduGeorgy M. TsoiWayne State UniversityLowell E. WengerWayne State University, wenger@physics.wayne.eduFollow this and additional works at: https://engagedscholarship.csuohio.edu/sciphysics_facpubPart of the Physics CommonsHow does access to this work benefit you? Let us know!Publisher's Statement© 2002 American Institute of Physics.This Article is brought to you for free and open access by the Physics Department at EngagedScholarship@CSU. It has been accepted for inclusion inPhysics Faculty Publications by an authorized administrator of EngagedScholarship@CSU. For more information, please contactlibrary.es@csuohio.edu.Repository CitationFodor, Petru S.; Tsoi, Georgy M.; and Wenger, Lowell E., "Fabrication and Characterization of Co1−xFex Alloy Nanowires" (2002).Physics Faculty Publications. 201.https://engagedscholarship.csuohio.edu/sciphysics_facpub/201Fabrication and characterization of  alloy nanowiresP. S. Fodor, G. M. Tsoi, and L. E. WengerCitation:  91, 8186 (2002); doi: 10.1063/1.1449450View online: http://dx.doi.org/10.1063/1.1449450View Table of Contents: http://aip.scitation.org/toc/jap/91/10Published by the American Institute of PhysicsFabrication and characterization of Co1ÀxFex alloy nanowiresP. S. Fodor, G. M. Tsoi, and L. E. WengerDepartment of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201Co12xFex alloy nanowires with 40 nm diam and x50 – 1.0 were fabricated by electrodeposition innanopores of alumina templates. The crystalline structure of the nanowires is concentrationdependent and shows a transition from the cobalt hexagonal-closed-packed structure ~hcp! to aface-centered-cubic structure ~fcc! in the concentration range 0,x,0.1. For an iron content greaterthan 15 at % the structure becomes body-centered-cubic ~bcc!. The nanowires with an aspect ratioof about 8 show a highly anisotropic magnetization with the easy axis along the length of the wire.The squareness of the hysteresis loops is between 72% and 93% for magnetic fields parallel to thewires while the squareness is less than 13% for fields perpendicular to the wires. The coercivityalong the easy axis reaches a maximum of 2150 Oe for x50.55 and is almost concentrationindependent for 0.5,x,0.6. © 2002 American Institute of Physics. @DOI: 10.1063/1.1449450#I. INTRODUCTIONMany of the magnetic devices used in today’s technolo-gies are based on the magnetic properties of thin films orbulk materials. With the advent of nanofabrication processes,opportunities to use the unique properties associated withnanostructured materials exist that were not previouslythought possible. However, conventional technologies usedto obtained nanostructures, such as e-beam lithography, x-raylithography, and scanning probe-based lithography are veryexpensive and not always suitable for commercial applica-tions.A less expensive alternative for the production of nano-structures is the electrodeposition of various materials in na-noporous membranes. Examples of nanoporous membranesthat have been used for electrodeposition include polycar-bonate membranes created by etching damage tracks of high-energy particles and alumina membranes produced by theanodic oxidation of aluminum.1 The advantage of the alu-mina templates over other porous membranes is the possibil-ity of obtaining them with ordered nanopores usingprepatterning2 or multiple anodization steps.3–6The magnetic properties of transition metals electrode-posited in alumina templates have been studied by severalgroups.4,7–10 High coercivities and large values for thesquareness have been measured parallel to the wire axis inthese types of nanostructures with shape anisotropy beingfound to be the dominant mechanism in determining themagnetic properties. In the present study we have investi-gated the crystallographic and magnetic properties for vari-ous concentrations of Co12xFex alloys deposited in the poresof an alumina matrix produced by anodization in oxalic acid.II. EXPERIMENTSHigh-purity 99.997% Al strips ~632030.25 mm3! wereused as the starting material. The strips were ultrasonicallycleaned for 15 min in acetone and then annealed in air at500 °C for 2 h in order to increase the size of the crystallinegrains and remove any mechanical stress induced in thesamples during the cutting process of the strips. The sampleswere then electropolished in a solution of perchloric acid,ethanol, and 2-butoxyethanol at a current density.500 mA/cm2 and a temperature of 5 °C. To obtain highlyordered pores a two-step anodization process was employed.In the first anodization step the Al strips were anodized at 40V in 0.3 M oxalic acid at 5 °C for 2 h. The aluminum oxidewas then stripped off in a 0.6 M chromic acid and 0.4 Mphosphoric acid solution at 60 °C. Subsequently, the samplewas reanodized for 30 min using the same anodization con-ditions as in the first step. A nonequilibrium anodization pro-cess in which the voltage was reduced to about 8 V followedthe second anodization. This led to a reduction of the barrierlayer that formed between the porous alumina and the Alsubstrate, thus facilitating the electrodeposition of the metals.The Co12xFex alloy was then electrodeposited in the synthe-sized template from a concentrated Watts-type electrolytecontaining 45 g/l boric acid and CoSO4 , CoCl2 , and FeSO4salts in different ratios depending on the composition desiredfor the final alloy. The electrodeposition was carried out at37 °C with an ac voltage of 8 V rms and a frequency of 60 Hzusing a graphite counterelectrode.Scanning electron microscopy ~SEM! was used to studythe morphology of the alumina templates. The crystallineproperties of the nanowires were investigated using x-raydiffraction ~XRD! after the Al substrate of the samples wasdissolved using a saturated solution of mercury chloride. Thecomposition of the nanowires was determined by energy dis-persive spectroscopy ~EDS! and the magnetic propertieswere measured using a Quantum Design SQUID magneto-meter.III. RESULTS AND DISCUSSIONFigure 1 shows a SEM micrograph of the top surface ofan anodized alumina template after the two-step anodizationprocess. The average pore diameter is 40 nm while the inter-pore distance is 95 nm. The pores are parallel with each otherthrough the entire thickness of the alumina layer and form ahexagonally ordered array with typical ordered domain sizeof 1 mm. The length of the electrodeposited nanowires isdetermined from cross-sectional SEM to be 290 nmJOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 10 15 MAY 200281860021-8979/2002/91(10)/8186/3/$19.00 © 2002 American Institute of Physics620 nm. The x-ray diffraction pattern for the Co nanowiresshows a hexagonal-closed-packed ~hcp! structure with a(101¯0! texture as shown in Fig. 2. As the concentration of Feincreases there are two crystallographic transitions. The firstone occurs in the 0,x,0.1 range where a mixed state be-tween the hcp and fcc ~g! phases exists. For x50.1 the mix-ing ratio of hcp:fcc is about 7:3. For an iron content greaterthan 15 at % the only stable phase is the a phase ~body-centered-cubic! with the lattice parameter being the same asthat for pure iron.Typical hysteresis loops for the CoxFe12x nanowires areshown in Fig. 3~a! with the easy magnetization axis beingalong the length of the wires for all the samples. The value ofthe coercivity field for the pure Co nanowires is 1400 Oe,about 30% less than previous results for Co nanowires hav-ing a larger aspect ratio.11 As the Fe content increases, thecoercivity rapidly increases reaching essentially a concentra-tion independent plateau of approximately 2150 Oe in the0.5,x,0.6 range before decreasing to a value of 1750 Oe for the pure Fe nanowires as shown in Fig. 3~b!. This maxi-mum coercivity value is also about 30% smaller than coer-civities found in previous studies on Co–Fe nanowires hav-ing larger aspect ratios.12 The abrupt change in coercivitywith a small addition of iron might be associated with thetransition in the crystalline structure from the highly aniso-tropic hcp lattice to cubic lattices with higher symmetry. Thisconclusion is supported by the change in squareness alongthe wire length, from 72% for the Co nanowire sample tovalues larger than 80% for the other CoFe nanowire samplesas also seen in Fig. 3~b!. For fields normal to the wires, thesquareness is less than 13% for the Co12xFex nanowireswhile the coercivities range between 450 and 750 Oe.Measurements of the crystalline anisotropy in Co12xFexbulk alloys indicate that for x50.55 the crystalline anisot-ropy is zero.13 Thus the Co0.45Fe0.55 ensemble of nanowiresshould be an ideal system to study how the magnetic prop-erties of our nanowire arrays are governed by the shape an-isotropy. Approximating the cylindrical nanowires as prolateellipsoids with an aspect ratio of 8, the theoretical coercivityFIG. 1. SEM top view of a nanopore array in anodic alumina prepared by atwo-step anodization in 0.3 M oxalic acid. The average pore diameter andinterpore distance are 40 nm and 95 nm, respectively.FIG. 2. X-ray diffraction pattern for Co12xFex nanowires arrays after re-moving the Al substrate.FIG. 3. ~a! Hysteresis curves for a Co0.45Fe0.55 nanowire array with fieldsoriented perpendicular ~s! and parallel ~d! with respect with the nanowireaxis. ~b! Concentration dependence of the coercivity field Hc ~j! andsquareness ~h! along the nanowire axis.8187J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Fodor, Tsoi, and Wengerbased solely on shape anisotropy would be 11 kOe (5(Nx2Nz)M s) using the bulk value for the saturation magnetiza-tion of 1923 G.14 This large overestimation of Hc as com-pared to the measured value of 2150 Oe suggests othermechanisms are also present. The reduction in the coercivitymight be due to nonuniform growth of the nanowires and toconcentration inhomogeneities that formed in the individualnanowires during the electrodeposition process and give riseto local composition variations as compared to the averagedetermined by EDS. Also the presence of any magnetostaticinteraction between the nanowires may have a significanteffect on the estimated coercivity force. Dipolar stray fieldsfrom adjacent nanowires will oppose the magnetization of itsneighbors and thus reduce the applied field necessary to re-verse their magnetization. To investigate whether any inter-actions between the nanowires were present, modified Hen-kel plots15 ~dM as a function of the applied field! derivedfrom the isothermal remanent magnetization M r(H) and dcdemagnetization remanence M d(H) curves can be used.Negative values in dM are typically associated with magne-tostatic interactions that tend to destabilize the magnetizedstate, while positive values indicate the existence of ex-change interactions between the individual particles. For theCo0.45Fe0.55 nanowires, dM takes only negative values with apeak at 2100 Oe ~see Fig. 4! suggesting magnetostatic inter-actions dominate.In conclusion, highly ordered Co12xFex alloy nanowiresarrays have been produced in anodic alumina templates. Themagnetic properties—coercivities and squareness of hyster-esis loops—of the CoFe alloy nanowires are improved overthe properties of similar nanostructures composed of pure Coand Fe. The concentration dependence of the coercivity forcealong the nanowire axis appears to be related to the crystal-line structure of the nanowires and the coercivity reaches amaximum in the range in which the nanowires crystallize inthe body-centered-cubic structure.ACKNOWLEDGMENTThis work is supported in part by the National ScienceFoundation through Grant No. DGE-9870720.1 C. R. Martin, Chem. Mater. 8, 1739 ~1996!.2 H. Masuda, H. Yamada, M. Satoh, H. Asoh, M. Nakao, and H. Tamamura,Appl. Phys. Lett. 71, 2770 ~1997!.3 H. Masuda and K. Fukuda, Science 268, 1466 ~1995!.4 H. Zeng, M. Zheng, R. Skomski, D. J. Sellmyer, Y. Liu, L. Menon, and S.Bandyopadhyay, J. Appl. Phys. 87, 4718 ~2000!.5 A. P. Li, F. Mu¨ller, A. Birner, K. Nielsch, and U. Go¨sele, Adv. Mater. 11,483 ~1999!.6 K. Nielsch, F. Mu¨ller, A. P. Li, and U. Go¨sele, Adv. Mater. 12, 582 ~2000!.7 D. AlMawlawi, N. Coombs, and M. Moskovits, J. Appl. Phys. 70, 4421~1991!.8 F. Li and R. M. Metzger, J. Appl. Phys. 81, 3806 ~1997!.9 M. Zheng, L. Menon, H. Zeng, Y. Liu, S. Bandyopadhyay, R. D. Kirby,and D. J. Sellmyer, Phys. Rev. B 62, 12282 ~2000!.10 M. Sun, G. Zangari, M. Shamuzzzoha, and R. M. Metzger, Appl. Phys.Lett. 78, 2964 ~2001!.11 R. M. Metzger, V. V. Konovalov, M. Sun, T. Xu, G. Zangari, B. Xu, M.Benakli, and W. D. Doyle, IEEE Trans. Magn. 36, 30 ~2000!.12 Coercivity is dependent on aspect ratio as K. I. Arai, H. W. Kang, and K.Ishiyama, IEEE Trans. Magn. 27, 4906 ~1991! report Hc;2.8 kOe foraspect ratio of 100.13 R. M. Bosorth, J. Appl. Phys. 8, 575 ~1937!.14 P. Weiss and R. Forrer, Ann. Phys. ~Paris! 12, 352 ~1929!.15 P. E. Kelly, K. O’Grady, P. I. Mayo, and R. W. Chantrell, IEEE Trans.Magn. 25, 3881 ~1989!.FIG. 4. Henkel plot for alumina sample filled with electrodepositedCo0.45Fe0.55 . dM is defined as dM5M d2(122M r), where M d is the re-duced dc demagnetization remanence and M r is the normalized isothermalremanence.8188 J. Appl. Phys., Vol. 91, No. 10, 15 May 2002 Fodor, Tsoi, and Wenger

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Fabrication and Characterization of Co1−xFex Alloy Nanowires

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