Fabrication and magnetic properties of FePt/Ag multilayered nanowires

Article abstract of DOI:10.1016/j.jmmm.2010.07.005

FePt and FePt/Ag multilayered nanowires were fabricated by a pulseplating technique in nanoporous anodic alumina templates. The effect of Ag layers on the chemical ordering of FePt was investigated. It is found that the ordering rate of FePt is enhanced by introducing Ag layers in the FePt nanowire during post-deposition annealing. Measurements of the structure and magnetic properties of FePt 5 nm/Ag 1 nm multilayered nanowires reveal that the disorderorder transformation temperature of FePt is lowered to 350 °C. The possible reason for the enhancement in the ordering of FePt by introducing the Ag layers in the FePt nanowire is discussed.

Full text of DOI:10.1016/j.jmmm.2010.07.005

Journal of Magnetism and Magnetic Materials 322 (2010) 3555–3557
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Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Fabrication and magnetic properties of FePt/Ag multilayered nanowires
Huixin Wang a,n, Lide Zhang b, Lixin Li a, Erguang Jia a, Xiaoxia Zhao a
a Department of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454000, People’s Republic of China
b Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
FePt and FePt/Ag multilayered nanowires were fabricated by a pulse-plating technique in nanoporous
anodic alumina templates. The effect of Ag layers on the chemical ordering of FePt was investigated. It is
found that the ordering rate of FePt is enhanced by introducing Ag layers in the FePt nanowire during
post-deposition annealing. Measurements of the structure and magnetic properties of FePt 5 nm/Ag
1 nm multilayered nanowires reveal that the disorder–order transformation temperature of FePt is
lowered to 350 1C. The possible reason for the enhancement in the ordering of FePt by introducing the
Ag layers in the FePt nanowire is discussed.
Received 9 April 2010
Received in revised form
25 June 2010
Available online 11 July 2010
Keywords:
Anodic alumite film
Coercivity
& 2010 Elsevier B.V. All rights reserved.
Transformation into ordered phase
1. Introduction
nanowire arrays is as high as 700 1C. In this paper, we present a
simple and economical way of fabricating FePt/Ag multilayered
nanowires embedded inside an array of empty holes in anodized
aluminum disk by electrodeposition. We have shown that Ag
buffer layer significantly lowers the ordering temperature of FePt
nanowire arrays.
Recently with the upsurge of nano-technology, many kinds of
ordering nano-structured materials such as quantum dots,
nanotubes and nanowires are studied [1–3]. Nanopatterned
magnetic recording media offer a number of advantages over
conventional continuous magnetic recording media, including the
ability to provide a much higher recording density [4–6]. Various
forms of nanopattern formation have been tried in order to reach
the ꢀ10 nm resolution required for 41 Tb in.ꢁ2 recording
density. Self-ordered structures found in nature, such as anodized
aluminum oxide (AAO), self-assembled particles and porous Si,
can be used as templates to produce ordered arrays of small
nanometerscale magnetic elements. Compared with other tem-
plates, the hole size of AAO can be readily controlled by properly
adjusting the anodizing condition. The AAO template is also
thermally stable and shows hexagonally ordered porous struc-
tures with nanochannel density in the range 1011–1013/cm2 [7].
These merits make it an ideal template for preparing magnetic
nanowire arrays. Face-centered-tetragonal (fct)-structured FePt
clusters have large magnetocrystalline anisotropy, high magneti-
zation and chemical stability. They are promising as building
blocks for the fabrication of future high-density recording media
and nanocomposite permanent magnets [8–12]. In the recent
years, it has been found that CoPt and FePt alloy nanowire arrays
with perpendicular magnetic anisotropy and adequate coercivity
can be easily prepared by electrodeposition [13,14]. However, the
transition temperature from the chemically disordered face-
centered-cubic (fcc) phase to the ordered fct phase in these
2. Experimental procedures
The AAO templates were prepared using a modified two-step
anodization process [15]. Briefly summarized,
a high-purity
aluminum sheet (99.999%, 0.5-mm thick) was anodized in 0.3 M
oxalic acid solution at 8–10 1C at a bias of 40 V for 8 h then the
oxide film was dissolved in a mixed solution of 0.2 M H2CrO4 and
0.4 M H3PO4 at 60 1C. After removing the resultant aluminum
oxide film formed by the first anodization, one side of the Al plate
was coated with manicure. In other words, it was covered with a
layer of something like a nail polish. A second anodization was
performed for 6 h under the same conditions as the first one. The
as prepared template was dipped into 5% phosphoric acid for 1 h
to eliminate the obstacle film, and then an AuCu film was
sputtered onto one side of the template to act as a conductive
contact. The equiatomic AuCu alloy has lattice constants close to
those of equiatomic FePt alloy with either fcc or fct structure.
FePt/Ag multilayered nanowires were electrodeposited inside
an array of empty holes in the AAO template from two electrolyte
baths. The substrate was moved between different electrolyte
baths. The first bath contained 0.012 mol/l H2PtCl6, 0.1 mol/l
FeSO4 and 0.5 mol/l Na2 SO4 solution for plating the FePt layer.
The pH value was adjusted to 3 by adding H2SO4. The second bath
contained 0.1 mol/l AgNO3 for plating the Ag layer. The pH value
n Corresponding author. Fax: +86 391 3987811.
E-mail address: hxwang01@163.com (H. Wang).
0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2010.07.005

3556
H. Wang et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3555–3557
was adjusted to 4 by adding H3BO3. N2 gas flow was adjusted
underneath the cathode to intermix the electrolyte. To prevent
oxidation of the ferrous material to ferric material, when
depositing FePt, an inhibitor, such as citric acid or ascorbic acid,
was added to the solution. We applied a potential of ꢁ0.9 V to
deposit FePt and a potential of ꢁ0.3 V to deposit Ag. The
potentials were measured with respect to the reference electrode
of Ag/AgC1. The electrodeposition was controlled by a computer.
The layer thicknesses were adjusted by controlling the deposition
times.
of FePt 5 nm/Ag 1 nm multilayered nanowires is shown in Fig. 2.
From Fig. 2 we can find that the nanowires are abundant and very
uniform with a diameter of about 40 nm, which basically equals
the pore diameter of the porous alumina template used in the
experiment. Fig. 2 also shows several clusters of nanowires. The
nanowires are uncovered from the anodic alumina template, but
they are incompletely freestanding and stick together, which
results in clusters of nanowires.
Fig. 3 shows the temperature dependence of coercivity Hc in
FePt 5 nm/Ag t nm multilayered nanowires annealed for 30 min in
vacuum. With 1 nm Ag layers inserted in FePt nanowire, Hc is
more than 2 kOe after annealing at 350 1C. It is found that
3. Results and discussion
Fig. 1 shows FE-SEM image of the porous alumina template
used in this work. It can be seen that the template exhibits a two-
dimensional array with a hexagonal pattern. The pore diameter
and interpore distances are about 40 and 100 nm, respectively. To
observe the morphology of FePt/Ag multilayered nanowire arrays
by FE-SEM, after electrodeposition the sample was eroded by an
aqueous solution of 2 wt% NaOH in order to remove the upper
part or the whole anodic alumina membrane. The FE-SEM image
Fig. 3. Temperature dependence of Hc in FePt 5 nm/Ag t nm multilayered
nanowires.
Fig. 1. FE-SEM image of anodic alumina membranes.
Fig. 2. FE-SEM image FePt/Ag multilayered nanowire arrays.
Fig. 4. XRD patterns for FePt nanowires at various annealing temperatures.

H. Wang et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3555–3557
3557
associated with high strain energy is obtained at the interface
between the FePt magnetic layer and the Ag layer. During
annealing, higher strain energy is stored in the film, and this
extra driving force decreases the energy barrier of phase
transformation of FePt from fcc disordered phase to fct ordered
phase; (2) Ag is immiscible with either Fe or Pt. Instead, Ag tends
to segregate at the grain boundary of FePt, and may serve as
nucleation sites for fct FePt.
4. Conclusion
In summary, FePt nanowire and FePt/Ag multilayered nano-
wire arrays were successfully fabricated in the nanochannels of
AAO templates by electrodeposition. We have shown that
introducing Ag layers in FePt nanowire can have a strong effect
on chemical ordering, For FePt 5 nm/Ag 1 nm multilayered
nanowires, the disorder–order transformation temperature of
FePt is lowered to 350 1C. L10 FePt nanowires prepared by the
present process have great potential for 1D nanomagnetism
studies and may evolve as building blocks for ultra-high density
hard disk drives.
Fig. 5. XRD patterns for FePt 5 nm/Ag 1 nm multilayered nanowires at various
annealing temperatures.
introducing Ag layers in FePt nanowire significantly lowers the
annealing temperature Ta, which is necessary to get high
coercivity Hc. Especially, the coercivity Hc of FePt5 nm/Ag1 nm
multilayered nanowires exceeded 3 kOe after annealing at 400 1C.
These results can be understood from X-ray diffraction profiles in
Figs. 4 and 5.
Acknowledgment
This work was supported by the Natural Science Research
Project of Education Department of Henan Province: Basic
Research (Grant no. 2009A430012).
Fig.
4 shows the XRD spectra for FePt nanowires after
annealing at temperatures from 300 to 700 1C for 30 min in
vacuum. The (1 1 1) plane of fcc FePt is observed. Even after
annealing up to 450 1C, little change is observed. When annealed
at 600 1C, the fct (0 0 1) and the fct (1 1 0) superlattice reflections
appear. For the higher annealing temperature of Ta¼700 1C, the
intensities of superlattice reflections increase, and the diffraction
peak of the (2 0 0) plane splits into two peaks corresponding to
the fct (2 0 0) and the fct (0 0 2) planes. This indicates that cubic
fcc phase is transformed to a tetragonal fct order phase.
Fig. 5 shows XRD patterns of FePt 5 nm/Ag 1 nm multilayered
nanowires annealed at different temperatures for 30 min in
vacuum. Diffraction peaks from the L10 ordered phase were
clearly observed after annealing at 350 1C, indicating that the
ordering of the multilayered nanowires starts at or lower than
350 1C. When the FePt 5 nm/Ag 1 nm multilayered nanowires
were annealed at 500 1C, intensities of the superlattice reflections
further increased and the diffraction peak of the fct (2 0 0) plane
splits into two peaks corresponding to the fct (2 0 0) and (0 0 2)
planes. This means that cubic A1 phase is transformed to a
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