The effect of diameter on micro-magnetic properties of Fe 0.68Ni0.32 nanowire arrays

Article abstract of DOI:10.1016/j.jmmm.2003.12.1357

Nanowire arrays of the alloy, Fe_0.68Ni_0.32, with different nanowire diameters have been fabricated by the electrochemical deposition method. X-ray diffraction and M?ssbauer spectroscopy were employed to characterize the size effect on the structure and micro-magnetic properties of the nanowire array, respectively. The Fe_0.68Ni _0.32 arrays with different diameters are stabilized in a BCC structure with a [110] texture oriented along the long axis of the nanowires. The M?ssbauer spectra of the nanowire arrays with smaller diameters show an absence of the second and fifth peaks, which then emerge with increasing diameter. The size effects on the hyperfine parameters are discussed.

Full text of DOI:10.1016/j.jmmm.2003.12.1357

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 278 (2004) 323–327
The effect of diameter on micro-magnetic properties of
Fe0.68Ni0.32 nanowire arrays
Q.F. Liu*, J.B. Wang, Z.J. Yan, D.S. Xue1
Key Laboratory for Magnetism and Magnetic Materials of MOE, Department of Physics, Lanzhou University, Lanzhou 730000,
People’s Republic of China
Received 14 September 2003; received in revised form 9 December 2003
Abstract
Nanowire arrays of the alloy, Fe0.68Ni0.32, with different nanowire diameters have been fabricated by the
electrochemical deposition method.X-ray diffraction and M o.ssbauer spectroscopy were employed to characterize the
size effect on the structure and micro-magnetic properties of the nanowire array, respectively.The Fe 0.68Ni0.32 arrays
with different diameters are stabilized in a BCC structure with a [1 1 0] texture oriented along the long axis of the
nanowires.The M o.ssbauer spectra of the nanowire arrays with smaller diameters show an absence of the second and
fifth peaks, which then emerge with increasing diameter.The size effects on the hyperfine parameters are discussed.
r 2004 Elsevier B.V. All rights reserved.
PACS: 75.50.S; 76.80.+y
Keywords: Nanowire arrays; Mo.ssbauer spectrum
1. Introduction
macro-magnetic properties of these systems, the
micro-magnetic properties should be studied.
Mo.ssbauer effect measurements are a useful tool to
investigate the micro-magnetic properties of systems.
It can not only provide magnetic information, but
also gives local environment information at the iron
sites.Recently, it was found that the size dependence
of the macro-magnetic properties is very interesting in
Fe0.32Ni0.68 nanowire arrays [6].In the present article,
the size effect on the structure and micro-magnetic
properties of Fe0.32Ni0.68 nanowire arrays is studied.
The production of magnetic wires with diameter
on the nanometer scale has recently attracted
much attention because of their attractive mag-
netic properties when compared to bulk materials
and their potential technological applications in
magnetic recording media, sensors and other
devices [1–3].Much work has previously been
reported on the size effect on macro-magnetic
properties of nanowire arrays [4,5].In order to
understand the origin of these size effects on the
2. Experimental
*Corresponding author.Tel/.fax: +86-931-8912726.
E-mail address: liuqf@lzu.edu.cn (Q.F. Liu).
1 Also correspondence to.
Samples were fabricated by AC electrodeposit-
ing Fe2+ and Ni2+ ions into anodic alumina oxide
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2003.12.1357

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(AAO) templates with different pore size.The
AAO templates were fabricated by a two-step
anodic oxidation process.First, high purity Al is
degreased, polished and then oxidized for the first
time in a 0.5 M phosphoric acid solution with
constant voltage at room temperature (RT) for 1 h.
This anodized alumina is then stripped away using
a mixture of phosphoric (0.4 M) and chromic acid
(0.2 M) at 60ꢀC.The Al sheet is then oxidized a
second time under the same conditions as above.
In order to fabricate AAO templates with different
pore size, a different constant voltage is applied
during the oxidizing process.The Fe 0.68Ni0.32
nanowire arrays were fabricated by an electro-
deposition method using an electrolyte containing
FeSO4 ꢁ 7H2O, NiSO4 ꢁ 6H2O and H3BO4.The
ratio of Fe2+ and Ni2+ in the electrolyte was
about 1:4.The deposition process was conducted
at RT with an AC frequency of 200 Hz and voltage
of 13 V rms for about 3 min.For convenience, the
residual Al was then removed using a saturated
HgCl2 solution in order to do the following
measurements.
Fig.1. AFM image of AAO template.
70 nm.TEM results show that, for the samples
prepared, the diameter of the Fe0.68Ni0.32 nano-
wires varied from 16 to 100 nm and the length in
each case is about 5 mm.A representative TEM
result for the Fe0.68Ni0.32 nanowire sample with a
diameter of 16 nm has been shown elsewhere [7].
The dark image and selected-area diffraction
indicates that the nanowires have a polycrystalline
structure.
A SPM-9500 atomic force microscope (AFM)
and JEOL2000 transmission electron microscope
(TEM) were used to investigate the morphology of
the AAO templates and nanowires, respectively.
The structure of the Fe0.68Ni0.32 nanowire arrays
with the AAO templates was obtained with a
Rigaku D/max-2400 X-ray diffractometer (XRD)
(
using Cu Ka1 radiation (l ¼ 1:54056 A).The con-
tent of iron and nickel was assessed by atomic
absorption spectrometry.All M o.ssbauer spectra
were obtained at RT using a constant acceleration
spectrometer with a 57Co/Pd source.The direction
of the g-ray was perpendicular to the surface of the
AAO template, so the nanowire arrays were
parallel to the direction of the g-ray.All
Mo.ssbauer spectra were fitted with Lorentz line
shapes and the isomer shifts were determined
relative to a-Fe at RT.
XRD patterns of AAO templates filled with
Fe0.68Ni0.32 nanowire arrays with different dia-
meters are shown in Fig.2 .These patterns indicate
that the Fe0.68Ni0.32 nanowires have a BCC
structure with [1 1 0] as the texture direction, which
is parallel to the long axis of the nanowires.When
the diameter of the nanowires is between 60 and
100 nm, a weak diffraction peak corresponding to
the (2 1 1) plane can be seen, which means that the
diameter has little influence on the degree of
texture.In addition, the diffraction peaks become
wider than that for nanowires which are 16 nm in
diameter, which may indicate the existence of some
very small nanoparticles.The XRD result for the
Fe0.68Ni0.32 nanowire array is different from that
in the bulk material, for which the FCC structure
also can be found for this composition [8].
3. Results and discussion
Fig.1 shows a typical AFM image of the AAO
template, where the diameter of the pore is about
50 nm and the distance between the pores is about

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325
(110)
d =16 nm
1.00
d =16nm
(110)
d =60 nm
0.99
1.00
(211)
0.99
(110)
d =100 nm
d = 60nm
0.98
1.00
(200)
(211)
40
60
80
100
2θ ( °)
0.99
d = 100nm
Fig.2.XRD patterns for nanowire arrays of the alloy,
Fe0.68Ni0.32, with different nanowire diameters, d:
-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9
Relative velocity (mm/s)
Fig.3. M o.ssbauer spectra of Fe0.68Ni0.32 nanowire arrays with
different diameters.
Mo.ssbauer spectra obtained at RT for AAO
templates filled with Fe0.68Ni0.32 nanowire arrays
with different diameters, d; are shown in Fig.3 .It is
apparent that the Dm ¼ 0 lines, i.e., the second and
the fifth peaks, are absent in the Fe0.68Ni0.32
nanowire array with 16 nm diameter.When the
diameter increases to about 60 nm, the second and
fifth peaks begin to appear.The M o.ssbauer
spectrum for the sample with 100 nm diameter wires
is different from the former samples.There are now
two sub-spectra, one corresponding to a single peak
and the other to a magnetic six-peak pattern.From
examination of the fitting parameters obtained for
the Mo.ssbauer spectra, which are given in Table 1,
and other results for the nanoparticles [9], we
conclude that the single peak corresponds to a
paramagnetic Fe–Ni alloy.To further confirm this
interpretation Mo.ssbauer spectra at low temperature
should be obtained, which is now in progress.
It is well known that the relative intensities of
the first three resonance lines in the Mo.ssbauer
spectrum for a ferromagnetic material are [10]:
3
1
I1 : I2 : I3 ¼ ð1 þ cos2 yÞ : 2 sin2 y : ð1 þ cos2 yÞ;
2
2
ð1Þ
where y is the angle between the direction of the
hyperfine field at the nucleus and the g-ray.If y is
equal to 0ꢀ or 180ꢀ, the intensity ratios of the
peaks should be 3:0:1 (I2 : I1 ¼ 0).On the
other hand, the ratio will be 3:4:1 when the
direction of magnetic moments is perpendicular
to the g-ray.For a sample with random magnetic
moment distribution, the ratio will be 3:2:1
(I2 : I1 ¼ 0:66).From the above intensity ratios
and the experimental arrangement, it can be
concluded that for the array with 16 nm dia-
meter nanowires, the magnetic moments are either

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Table 1
Fitting parameters of Mo.ssbauer spectra for Fe0.68Ni0.32 alloy nanowire array with different diameters, in which FWHM, d; DEQ and
Hhf are full-width half-maxminum of the peak, isomer shift, quadruple splitting and hyperfine field, respectively
Linetype
Relative intensity (%)
FWHM (mm/s)
d (mm/s)
DEQ (mm/s)
Hhf (T)
I2=I1
16 nm
60 nm
100 nm
Six
Six
100
100
96
0.5970.01
0.5370.01
0.5270.01
0.3670.01
0.0170.01
0.0270.01
0.0270.01
ꢂ0.0170.01
0.0070.01
0.0070.01
0.0070.01
33.870.1
34.4470.1
34.3670.1
0
0.268
0.268
Six
Single
4
I2=I1 is the intensity ratio of the second and the first peak of Mo.ssbauer spectra.
parallel or antiparallel to the long axis of the
nanowires, which implies that the distribution of
the magnetic moments is controlled by the shape
anisotropy.With increasing diameter, a certain
fraction of the magnetic moments distribute
randomly.However, for a nanowire array with
60 nm diameter, the intensity ratio I2 : I1 is equal
to about 0.268 (shown in Table 1), which is
smaller than 0.66, but larger than 0. Assume that
the mean angle between the magnetic moment and
the long axis of nanowire is y1: From the above
XRD results and previous macro-magnetic mea-
surements [6], the easy axis for the nanowire
is seen to lie along the [1 1 0] direction, which is
parallel to the long axis of the nanowires.The easy
axis corresponding to the magnetocrystalline
anisotropy in bcc FeNi is along the [1 0 0]
direction, i.e. the angle between the long axis of
the nanowire (the [1 1 0] direction) and the easy
axis corresponding to the magnetocrystalline
anisotropy is 45ꢀ.The competition between the
shape anisotropy and magnetocrystalline aniso-
tropy will result in a mean angle y1 smaller than
45ꢀ, but larger than 0ꢀ.According to Eq.(1) and
the fitting results, y1 is calculated to be 11.4ꢀ,
which is in agreement with the above analysis.For
a nanowire array with 100 nm diameter wires, I2 :
I1 is almost equal to that of the nanowire array
with 60 nm diameter, which indicates that the
competition between the shape anisotropy and
magnetocrystalline anisotropy achieves a maxi-
mum.From the macro-magnetic measurement
results [6], we found that the coercivity and
the squareness display little change when the
diameter changes from 60 to 100 nm (the aspect
ratio changes from about 80 to 50).In addition,
micro-magnetic simulations for a single nanowire
indicate that when the aspect ratio is larger
than 10, the macro-magnetic properties show little
change [11].
The other parameters given in Table 1 show
interesting properties compared with FeNi fine
particles [12,13].The full-width at half-maximum
(FWHM) is larger than the normal value for the
iron Mo.ssbauer spectrum, i.e., 0.3 mm/s, which
indicates that the distribution of Ni atoms is
disordered in our Fe0.68Ni0.32 nanowire array
system.The FWHM decreases with increasing
nanowire diameter, which means that Ni atoms
are more disordered with decreasing diameter.
However, the FWHM of the sextets for diameters
of 60 and 100 nm shows little difference, which
implies that when the diameter is increased to
60 nm, the degree of disorder of Ni atoms is near a
maximum.The isomer shift ( d) and quadruple
splitting (DEQ) remain almost constant when the
diameter increases, indicating that the electronic
structure and crystal symmetry are not influenced
by the diameter.The hyperfine field ( Hhf ) changes
from 33.8 to 34.4 T, when the diameter changes
from 16 to 60 nm, and then remains constant.A
Mo.ssbauer investigation of iron nanowire arrays
with the same range of diameters showed that the
hyperfine fields had no change for all diameters
[14], which means that the change of the hyperfine
fields for the Fe0.68Ni0.32 nanowire array is mainly
caused by the introduction of Ni atoms.The
diameter has influence on the hyperfine field by
changing the degree of disorder in the distribution
of the Ni atoms.

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327
4. Conclusion
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Acknowledgements
This work is supported by NSFC (Grant No.
50171032) of China and doctor start-up fund of
Lanzhou University.