Magnetization of self-organized Ni-nanowires with peculiar magnetic anisotropy

Article abstract of DOI:10.1016/j.jmmm.2007.02.148

Porous silicon (PS) offering a quasi-regular pore arrangement in the meso-/macroporous regime acts as a matrix for incorporating ferromagnetic nanostructures. The electrochemically fabricated structures are self-assembled and exhibit pores with a diameter tunable between 30 and 100 nm. The subsequent electrodeposition of Ni into these pores leads to a Ni/Si-nanocomposite consisting of Ni-particles and Ni-wires, respectively. The ratio of particles and wires depends on the electrochemical deposition conditions like current density as well as current pulse duration. The different distributions cause a variation of the magnetization curves. Especially in the high field regime a negative magnetization branch occurs and decreases with increasing amount of particles. This peculiar high field magnetic behavior appears if the external magnetic field is applied perpendicular to the sample surface and is due to antiferromagnetic coupling between the Ni-nanostructures.

Full text of DOI:10.1016/j.jmmm.2007.02.148

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 316 (2007) 302–305
www.elsevier.com/locate/jmmm
Magnetization of self-organized Ni-nanowires with
peculiar magnetic anisotropy
Ã
P. Granitzera, , K. Rumpfa, P. Poltb, A. Reichmannb, M. Hofmayera, H. Krenna
¨
aInstitute of Physics, Karl Franzens University Graz, Universita¨tsplatz 5, 8010 Graz, Austria
bInstitute for Electron Microscopy, University of Technology Graz, Steyrergasse 17, 8010 Graz, Austria
Available online 1 March 2007
Abstract
Porous silicon (PS) offering a quasi-regular pore arrangement in the meso-/macroporous regime acts as a matrix for incorporating
ferromagnetic nanostructures. The electrochemically fabricated structures are self-assembled and exhibit pores with a diameter tunable
between 30 and 100 nm. The subsequent electrodeposition of Ni into these pores leads to a Ni/Si-nanocomposite consisting of Ni-
particles and Ni-wires, respectively. The ratio of particles and wires depends on the electrochemical deposition conditions like current
density as well as current pulse duration. The different distributions cause a variation of the magnetization curves. Especially in the high
field regime a negative magnetization branch occurs and decreases with increasing amount of particles. This peculiar high field magnetic
behavior appears if the external magnetic field is applied perpendicular to the sample surface and is due to antiferromagnetic coupling
between the Ni-nanostructures.
r 2007 Elsevier B.V. All rights reserved.
PACS: 75.60.Ej; 07.55.Jg
Keywords: Porous silicon; Electrodeposition; Nanocomposite; Ferromagnetism; Magnetometry
1. Introduction
In this work we use porous silicon (PS) as matrix in
which ferromagnetic nanowires are incorporated [6] and we
show that the magnetic behavior of this nanocomposite
system differs strongly from the known hystereses loops [7].
In the described Si/Ni-nanocomposite system two regions
of an applied external magnetic field can be distinguished.
The behavior in the low field regime is comparable with
that of other metal-incorporated systems [8]. But there
exists a peculiar magnetic behavior in the high field region
around a few Tesla which can be controlled by the
deposition parameters like current density and current
pulse duration. This system is interesting not only because
of silicon as basic material but also because of the low-cost
fabrication and the extraordinary magnetic properties.
Nanostructured magnetic materials are of great interest
nowadays because of their applicability for magnetic data
storage technology, spintronic as well as sensor devices [1].
One’s attention is mainly turned on highly ordered arrays
of nanowires and nanoparticles [2]. The fabrication of such
arrangements has been investigated since the last two
decades and still goes on. The magnetic behavior of
nanoscopic systems differs from those of bulk material and
therefore they are of great interest to be investigated
experimentally [3] as well as theoretically [4].
Nanomagnetic patterns can be obtained either by pre-
structuring of the substrate using lithography or by self-
assembling of the structures. A well known self-organized
template for nanowires is porous alumina [5] exhibiting a
honeycomb-like hexagonal arrangement of the pores.
2. Experimental details and structural investigations
The PS matrix is fabricated during an anodization
process using aqueous hydrofluoric acid solution as
electrolyte. The electrochemical parameters as well as the
doping density of the silicon wafer are correspondingly
Ã
Corresponding author. Tel.: +43 316 380 5199; fax: +43 316 380 9816.
E-mail address: petra.granitzer@uni-graz.at (P. Granitzer).
0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2007.02.148

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303
chosen [9,12] to achieve pores in the meso-/macroporous
regime. The fabricated PS system exhibits highly oriented
pores perpendicular to the surface and a sharp termination
between porous layer and crystalline silicon (Fig. 1). A
quite homogeneous pore distribution is obtained without
any pre-structuring. The self-assembled pore-arrangement
shows a quadratic-like ordering which is found by Fourier
transform image processing [12].
The EDX-maps in Fig. 2 are a rough survey and do not
resolve individual pores but the altered distribution of
nickel within the porous layer is sufficiently proved.
3. Magnetization and anisotropy of the PS/Ni-
nanocomposite
The pore diameter of a typical sample like that in Fig. 1
is about 50 nm and thus lies in the transition region
between mesopores and macropores [10]. A subsequent
electrochemical procedure is used for metal-deposition into
the template. In this work the channels are filled with Ni
employing pulsed deposition technique [11]. As electrolyte
a NiCl2 (185 g/l), H3BO3 (45 g/l) solution and a Watts
electrolyte (NiCl2 45 g/l, NiSO4 300 g/l, H3BO3 45 g/l) are
used, respectively. Depending on the electrochemical
deposition conditions (see Table 1) the filling of the pores
varies and leads to distinct magnetic behavior due to the
depth and width of the depression in the magnetization
curve at high fields (see Fig. 3). The Ni-content within the
channel-layer is demonstrated by EDX-mapping. Some
examples are presented in Fig. 2.
Magnetization measurements have been carried out by a
SQUID-magnetometer (Quantum Design MPMS XL7) in
the field range of ꢀ7 T and a temperature variation
between 4.2 and 300 K. Considering the hysteresis loops
in the low field regime (below 1 T) the magnetic behavior is
comparable with conventional nanocomposite systems like
metal filled porous alumina [8], which are mostly governed
by dipolar interwire interaction.
Structural investigations of the fabricated PS/Ni-nano-
composite using SEM and EDX as well as magnetization
measurements lead to the assumption that the metallic Ni
phases within the pores consist of Ni-particles and Ni-
wires. The Ni-particles are smaller than 150 nm in length
[12] and offer a spherical or ellipsoidal shape. Calculating
the critical radius rC of Ni-single-domain particles after
reference [13] yields to rC ꢁ 50 nm. Most of the particles
found are below 50 nm in diameter and thus they are single
domain. Determining the coercive field of a particle with a
diameter of 60 nm after Ref. [13], a value of 450 Oe is
achieved which is in good accordance with the measure-
ments. Table 2 gives a summary of different PS/Ni-samples
with coercivity HC, squareness MR=MS and an estimate of
the Ni-volume in cm3. The given low field hystereses
properties and the Ni-contents are related to the incorpo-
rated Ni-particles.
Fig. 1. Cleaved edge of a porous silicon layer of 25 mm. In the inset a tilted
view of the porous sample is displayed showing the oriented pores.
Table 2
HC, squareness and Ni-content of investigated Ni-filled PS samples for H-
field parallel (k) and normal (?) to the pores (wires)
Table 1
Electrochemical deposition parameters associated with the EDX-images in
Fig. 2
Ni ð10ꢂ6 cm3Þ
HC; k (Oe) HC; ? (Oe) MR; k=MS (%) MR; ? =MS (%)
350
280
260
220
190
160
170
150
50
39
52
45
22
19
26
31
3.813
2.859
1.9
Current density jðmA=cm2Þ
Sample
Pulse duration tpulse (s)
1.9
a
b
c
10
20
50
20
10
10
The Ni-content is derived from the first saturated magnetization plateau at
low fields.
Fig. 2. X-ray maps over the cleaved edge of the samples showing different Ni-distributions. (a) Accumulation of Ni near the surface region, (b) nearly
homogeneous distribution within the porous layer and (c) the pore-tips are preferentially filled with Ni. Higher brightness of the pixels means higher Ni-
content.

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Fig. 3. The solid curve and the dashed line show the magnetization for an external magnetic field normal to the surface but of two samples Ni-loaded with
different electrochemical parameters (solid: j ¼ 50 mA=cm2, pulse duration ¼ 10 s, dashed: j ¼ 50 mA=cm2, pulse duration ¼ 5 s). Both samples show a
depression of the magnetization of different depth at 4.8 and 5.2 T, respectively. The dotted curve corresponds to the sample represented by the solid line
but for a magnetic field parallel to the surface. In this magnetization direction the investigated samples do not show a depression.
In the high field region of a few Tesla a peculiar magnetic
behavior is found, showing a deep depression in the
magnetization signal (Fig. 3). This ‘‘negative peak’’ of the
magnetization curve at 5.2 T is due to an antiferromagnetic
interaction between the Ni-wires. Regarding dipolar
coupling between neighboring wires with an interpore
distance equal to the pore diameter leads to a sum of the
stray fields of nearest and next nearest wires of about 2.5 T
under the assumption of a quadratic array and a saturation
magnetization for each contributing wire of 0.6 T. This
value is too small to characterize the depression but it is
near the beginning of the negative slope of the magnetiza-
tion curve for some other samples.
In accordance with structural investigations and magne-
tization measurements the incorporated Ni is assumed to
consist of particles and wires like previously pointed out.
The low magnetic field behavior is due to the particles,
whereas the one at high magnetic fields corresponds to the
high aspect ratio Ni-wires. This conception of a bimodal
PS/Ni-composite is additionally endorsed by measure-
ments of samples prepared using different electrochemical
filling parameters leading to changed Ni-distributions. The
corresponding magnetization curves show different ‘‘nega-
tive peaks’’ in the high field range. If the Ni-filling of the
PS-template becomes stronger towards the pore-tips, the
depression of the magnetization curve increases in depth.
At last the magnetization increases up to full saturation.
Samples with Ni accumulated near the surface region are
missing this peculiar behavior. If the external magnetic field
is applied parallel to the sample surface the nanocomposite
system is not fully saturated due to the high demagnetizing
field perpendicular to the Ni-wires. In this case saturation
magnetization is given by the Ni-particles and no longer an
up-turn of the magnetization signal is observed up to an
available field of 7 T.
4. Conclusion
A quasi one-dimensional PS/Ni-nanocomposite system
has been fabricated during a two-step electrochemical
process for (i) forming the PS template and (ii) for filling
this silicon matrix with nickel. The ferromagnetic pre-
cipitations consist of Ni-particles as well as Ni-wires
whereas the latter ones cause strong anisotropic magnetic
responses. The particles offer a ‘‘soft’’ hysteresis in the low
field regime below 0.1 T whereas an antiferromagnetic
coupling of the wires leads to a ‘‘negative magnetization
slope’’ at magnetic fields of a few Tesla. This depression of
the magnetization curve can be influenced in depth, width
and slightly in the magnetic field signature (between 4.5
and 5.4 T) by various electrochemical filling parameters like
current density, current pulse duration and (not shown) by
temperature. The physical nature of negative magnetiza-
tion due to antiferromagnetic interwire exchange is not yet
clear but has been already reported in literature [14].
Acknowledgments
We thank the Austrian FWF-fund for his grant under
project P18593.

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