RESEARCH PAPER

Magnetic configurations of Ni–Cu alloy nanowires obtainedby the template method

Elena Matei • Ionut Enculescu •

Maria Eugenia Toimil-Molares • Aurel Leca •

Corneliu Ghica • Victor Kuncser

Received: 25 February 2013 / Accepted: 12 July 2013

� Springer Science+Business Media Dordrecht 2013

Abstract High aspect ratio nanowires of Ni–Cu

alloys have been synthesized by potentiostatic elec-

trochemical deposition in etched ion-track mem-

branes. The nickel-to-copper ratio in the nanowires

was controlled via the deposition potential and elec-

trochemical bath composition. We present a detailed

study of nanowire properties including morphology,

composition, and magnetic behavior. We report the

magnetic configurations measured as function of the

nanowire composition and discuss domain formation,

anisotropy aspects, and local easy axis distributions.

Keywords Ni–Cu magnetic nanowires �Flower-like spin structure � Switching field �Magnetic structure

Introduction

Nanowires with controlled dimension and composition

constitute unique model systems to investigate the

magnetic properties of one-dimensional systems.

Among the various methods available to synthesize

magnetic nanowires, (e.g., lithographic techniques,

chemical reduction, and hydrothermal method) the

template approach is frequently employed, and allows a

simultaneous and precise control on both morphology

and composition. With this method, the pores of a

certain template are filled with the material of interest.

Usually, anodic porous alumina (Sugawara et al. 1998),

molecular sieves (Ozin 1992), or ion-track membranes

(Martin 1994; Fert and Piraux 1999; Yi and Schwarz-

acher 1999; Toimil Molares et al. 2001; Ohgai et al.

2003; Matei et al. 2010) are used as templates. In

particular, the electrochemical deposition in etched ion-

track membranes provides an excellent and independent

control on nanowire dimensions (length and diameter)

and wire density. Arrays of magnetic nanowires of

materials, such as Co, Ni, and Fe have been synthesized,

mostly by electrochemical deposition, as well as more

complex multilayered nanowires exhibiting giant mag-

netoresistance effect in a current perpendicular to plane

geometry (Gravier et al. 2002; Enculescu et al. 2007).

In the present paper, we report on the synthesis of

Ni–Cu alloy nanowires and their magnetic character-

ization. The influence of the material composition on

their magnetic behavior has been investigated and is

explained in detail.

E. Matei � I. Enculescu (&) � A. Leca �C. Ghica � V. Kuncser (&)

National Institute of Materials Physics, Magurele, P.O.

Box MG 7, 77125 Bucharest, Romania

e-mail: encu@infim.ro

V. Kuncser

e-mail: kuncser@infim.ro

M. E. Toimil-Molares

Gesellschaft fuer Schwerionenforschung, Planckstrasse 1,

64291 Darmstadt, Germany

123

J Nanopart Res (2013) 15:1863

DOI 10.1007/s11051-013-1863-3

Experimental

Polycarbonate foils were irradiated with highly ener-

getic (*2 GeV) heavy ions applying a fluence of

109 ions/cm2. The tracks created by the ions in the

polymer were selectively etched to 70 nm diameter

pores, in an aqueous 5 M NaOH solution with

10 vol% methanol at 50 �C.

On one side of the template we sputtered a 50 nm

thick gold film and electrochemically reinforced it

with a 10 lm thick copper layer. This conductive layer

acts as a working electrode. The electrolyte was a

modified Watts bath containing NiSO4 (225 g/l),

NiCl2 (30 g/l), CuSO4 (2 g/l), H3BO3 (32.5 g/l), and

polyvinylpyrrolidone (PVP) (5 g/l). The copper con-

centration in the bath was *100 times lower than

nickel’s (very unbalanced concentrations), due to the

fact that it deposits at less electronegative potentials.

Polyvinylpyrrolidone improves pore wetting. The

electrodeposition was performed at 50 �C in a three-

electrode configuration and was controlled by a

PARSTAT 2276 potentiostat. A saturated calomel

electrode (SCE) and a 1 cm2 platinum plate were

employed as reference and auxiliary electrodes,

respectively.

Morphology, composition, and structural aspects

were analyzed by scanning electron microscopy

(SEM), energy dispersive X-ray spectroscopy (EDX),

and X-ray diffractometry (XRD) with Cu Ka radiation,

after dissolving the templates. On the same nanowires,

transmission electron microscopy (TEM) investiga-

tions have been performed using a JEM ARM 200F

electron microscope operated at 200 kV. The micro-

scope, provided with a Quantum SE Gatan image filter,

has been used for elemental mapping by energy-

filtered transmission electron microscopy (EFTEM).

Magnetic measurements have been performed by

SQUID magnetometry (MPMS XL from Quantum

Design) under the high-sensitivity reciprocal space

option, with nanowires grown in the template. The

magnetic field was applied both perpendicular and

parallel to the wires onto 0.08 cm2 rectangular sample

pieces.

Results and discussions

Figure 1a displays the electrochemical polarization

curves measured at a scan rate of 5 mV/s. At voltages

less negative than -750 mV, the current is relatively

low and corresponds to copper deposition. At more

electronegative potentials, the current steadily

increases and both nickel and copper are reduced.

Two peaks were observed during the first sweep,

attributed to catalytic effect of the substrate.

The chronoamperometric curves for a set of

samples electrodeposited at different overvoltages

are plotted in Fig. 1b. All curves display three

characteristic regions: (i) polarization, (ii) steady

current during deposition inside the pores, and (iii)

current increase when the pores are completely filled

and caps grow on membrane’s top surface (Kronmul-

ler et al. 1988). By integrating the current in region (ii),

we determined the charge employed during the

nanowire growth for all overvoltages. We obtain

similar values (\10 % differences), indicating that

almost equal amounts of mass were deposited, as

expected since templates of identical characteristics

were employed.

The composition of the nanowires was investigated

by EDX. Nanowires consisting of alloys with the

following average formula correspond to the analyzed

samples: Ni0.2Cu0.8 (S1 deposited at -800 mV),

Ni0.54Cu0.46 (S2 deposited at -900 mV), Ni0.75Cu0.25

(S3 deposited at -1,000 mV), and Ni0.92Cu0.08 (S4

deposited at -1,050 mV). These results demonstrate

that the nickel-to-copper ratio increases as the over-

potential becomes more electronegative (Fig. 1c).

Figure 1d shows a SEM image of Ni–Cu nanowires

dissolved from the template, displaying cylindrical

shape along their entire length and smooth walls. The

wire diameter is approximately 80 nm and the length,

corresponding to the membranes thickness, 30 lm.

Figure 1e presents the XRD patterns evidencing just a

Ni-like face-centered cubic (fcc) structure textured

along the [111] direction, for samples S2–S4 and two

fcc phases due to distinct Ni and Cu precipitates in

sample S1.

The low-magnification TEM image in Fig. 2a

reveals general morphological aspects concerning

the sample S1. The corresponding nanowires are

polycrystalline while the inserted selected area elec-

tron diffraction (SAED) pattern corresponding to the

TEM image does not indicate a preferential growth

along any well-defined crystallographic direction. We

have used EFTEM imaging in order to map out the

spatial distribution of the two elements in the sample

(Cu and Ni), and thus to reveal any possible phase

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segregation. The 3-windows method has been

employed to obtain the filtered images using the Ni

L3,2 and Cu L3,2 edges at 855 and 931 eV, respec-

tively. The specimen relative thickness, t/k, varies

between 0.9 and 1.1 for single nanowires (not shown

here), where t stands for specimen thickness and k for

the electron mean free path. The Ni L3,2 and Cu L3,2

maps (Fig. 2b and c, respectively) are rather comple-

mentary, clearly showing that most of the nanowires

contain single element domains, where either Cu or Ni

is present. The Ni or Cu complementary areas are

better illustrated in false colors by the composite

image in Fig. 2d where green and red areas correspond

to Ni and Cu domains, respectively. The same EFTEM

acquisition parameters (exposure time, pixel binning,

filter slit position and width) have been used for

mapping Cu and Ni in the case of sample S3 (Fig. 3).

One can notice the much weaker Cu signal compared

to sample S1. Areas showing a somewhat higher

copper content are most likely an artifact, as they

actually correspond to overlapping nanowires (relative

thickness t/k[ 2.4), modifying therefore the signal-

to-background ratio for the Ni and Cu edges. In our

opinion, the EFTEM data from Fig. 3 confirm that the

as-grown nanowires in sample S3 contain Ni and Cu

homogeneously distributed (no chemical segregation),

with a rather low Cu content, in agreement with SEM–

EDX and XRD results.

Figure 4 displays the magnetic hysteresis loops

measured at two different temperatures (10 and

300 K), in fields up to 10 kOe. The magnetic field

was applied parallel (filled symbols) and perpendic-

ular (empty symbols) to the wires, respectively. The

raw data of the first quadrant are plotted in the insets.

One can observe an enhanced diamagnetic contribu-

tion in case of measurements performed with the field

applied perpendicular to the foil, as compared to the

case of the field applied along the foil.

To analyze the wires’ ferromagnetic behavior, the

diamagnetic contribution of the template has to be

subtracted. The question is of either considering a

Fig. 1 a Electrochemical polarization curves for NiCu alloy

deposition. The sweep rate is for all curves 5 mV/s. b Chrono-

amperometric curves recorded during potentiostatic deposition

of NiCu nanowires at different potentials. c Copper and nickel

content versus deposition potential. d Representative SEM

image of 70 nm diameter Ni–Cu nanowires, dissolved from the

template. e XRD diffraction patterns (k = 0.154 nm) of the Ni–

Cu samples

c

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same diamagnetic contribution in both geometries

(estimated from the negative slope observed at higher

fields applied perpendicular to the foil plane) or a

geometry dependent contribution. An extrinsic reason

for such anisotropy was mentioned as the filling factor

used in the fitting routine of the SQUID magnetometer

by Ney in 2011 (the correction factor counting the

deviation from the point-dipole approximation is

different in the two geometries). An additional mea-

surement performed on a polycarbonate foil evidenced

a diamagnetic constant of about -2 9 10-8 emu/Oe,

for the field perpendicular to the foil and a diamagnetic

constant one order of magnitude lower, for the field

applied along the foil. Therefore, the diamagnetic

contribution of the template has been subtracted

differently in the two geometries, under the assump-

tion of neglecting relative contribution in case of field

perpendicular to the wires (parallel to the foil). The

loops (Fig. 4), plotted from -3,000 to 3,000 Oe, show

strong dependence on the measurement geometry. For

all samples excepting the one with the lowest Ni

content (S1), a clear saturation, approaching almost

the same value in both the measurement geometries,

can be observed. As expected, the saturation values

increase with Ni content. Taking into account the

geometry of the samples (e.g., membrane sur-

face = 0.08 cm2, pore diameter = 80 nm, pore

length = 30 mm, and pore density = 109 cm-2), we

estimated the volume of magnetic alloy at

115 9 10-6 cm3 on each cm2 area. Since Ni and Cu

present similar densities (about 8.9 g/cm3), the same

value can be considered for the alloy density,

independently on the content. Hence, the average

magnetic moment per formula unit can be evaluated. It

is plotted in Fig. 5a, together with a typical depen-

dence versus Ni content (dashed line), as experimen-

tally, empirically, and theoretically reported for

Ni–Cu alloys (as for example, by Richter and Eschrig

1988). The measured values deviate from the expected

dependence more consistent than relative errors of

\10 %, due to the experimental uncertainties. Accord-

ingly, for high Ni content alloy, i.e., high overvoltage

Fig. 2 a TEM image of the CuNi nanowires in sample S1 and

corresponding SAED pattern inserted; b EFTEM image

showing the Ni L3,2 elemental map; c EFTEM image showing

the Cu L3,2 elemental map; d composite image in false colors

showing the Cu and Ni distribution in the nanowires (red copper,

green nickel). (Color figure online)

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deposition, one can assume a lower density of filled

pores probably due to hydrogen bubbles evolution at

electrode, leading regularly to lower average magnetic

moments (nanometer sized pores might be assumed

while the TEM image clearly proves unbroken wires).

For the lowest Ni content, no magnetic moment should

be measured, contrary to our result for sample S1

which present a small but finite magnetic moment.

Nevertheless, this peculiar magnetic behavior of

sample S1 is a consequence of nanowire inhomoge-

neity in agreement with the precipitation of distinct Ni

and Cu phases, as evidenced by XRD and EFTEM.

A loop of rectangular shape would be expected in

the frame of the Stoner–Wohlfarth model, when the

magnetic field is parallel to the wires and a quasilinear

shape of negligible coercive field in the case of

perpendicular field orientation. This theory assumes

coherent rotation of the spins with respect to a uniaxial

direction of anisotropy (Kuncser et al. 2010). Our

experiments reveal intermediate situations depending

on the Cu content, which can be attributed to various

reasons, as also mentioned by Sellmeyer et al. (2001)

and Ross et al. (2002). The rounded loops, for

magnetic fields applied along the wires, involving

different coercive and saturation fields for a same

system, could be due to: (i) domain formation in the

wire, (ii) distributed switching fields related to either

distributed dipolar interactions or distributed anisot-

ropy constants, or (iii) angular distributed easy axis. In

case of nanowires, the shape anisotropy, Ks (propor-

tional to Ms2) plays an important role, being compa-

rable or higher than the crystalline anisotropy Ku. The

Fig. 3 a Low-magnification TEM image of the CuNi nano-

wires in sample S3; b EFTEM image showing the Ni elemental

map; c EFTEM image showing the Cu elemental map;

d composite image in false colors showing the Cu and Ni

distribution in the nanowires (red copper, green nickel). (Color

figure online)

J Nanopart Res (2013) 15:1863 Page 5 of 9

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domain structure in such a cylinder depends on both

the ratio Q = Ku/Ks as well as the ratio between its

diameter and the wall thickness (Sellmeyer et al. 2001;

Ross et al. 2002) defined via the exchange length

parameter kex ¼ pffiffiffiffiffiffiffiffiffi

A=Kp

, with A the exchange stiff-

ness coefficient and K the anisotropy constant. Satu-

ration magnetization values decreasing with Cu

content x, from about 300 emu/cm3 (for x = 0.08) to

45 emu/cm3 (for x = 0.8) have been estimated for the

involved systems (the values at RT are not signifi-

cantly different from the ones at 10 K). Correspond-

ingly, the evaluated shape anisotropy constants

decrease from about 30 9 104 erg/cm3 (for x =

0.08) to 2 9 104 erg/cm3 (for x = 0.46) and 0.6 9

104 erg/cm3 (for x = 0.8). Sample S1 (with x = 0.8),

where the presence of magnetic order can be explained

by an inhomogeneous distribution of magnetic atoms

(e.g., Ni clustering) will be out of further estimations.

Ku has been roughly evaluated for samples with

x \ 0.6, by correcting the Ni value (about

-5 9 104 erg/cm3, according to Mansuripur 1995)

via a factor (1 - x/0.61), similar as for the average

magnetic moment of the CuxNi1-x alloy (Ku = 0 was

assumed at x = 0.61, where the alloy loses its

magnetic moment). Just sub-unitary Q factors

Fig. 4 Magnetic hysteresis loops at 10 and 300 K, measured

with magnetic field applied parallel (filled symbols) and

perpendicular (empty symbols) to the wires, for S1 (a), S2 (b),

S3 (c), and S4 (d) samples. Inset raw data from first quadrant

Fig. 5 The average magnetic moment (a) and apertures of the

angular distribution of easy axis (b) versus the Cu content. Inset

values of the orientation ratios

Page 6 of 9 J Nanopart Res (2013) 15:1863

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increasing with the Cu content from 0.15 (for

x = 0.08) to 0.5 (from x = 0.46) have to be men-

tioned. Thus, the shape anisotropy plays the dominant

role in the switching process for all the examined

samples with x \ 0.6, in spite of the fact that at higher

Cu content, the crystalline anisotropy becomes more

competitive. The stiffness constant A is given by A ¼ðJzS2Þ=ð12dÞ (Mansuripur 1995), with J the exchange

integral, z, the number of magnetic neighbors, S the

spin of the magnetic atom, and d the average distance

between the magnetic atoms. While these parame-

ters are dependent on the composition of the Ni–Cu

alloys, an alternative expression provided by typical

connection between the exchange integral and the

Curie temperature, TC, is preferred: A ¼ ð3kBTCSÞ=ð12dðSþ 1ÞÞ. Within the assumptions that the average

spin values of the alloys are much lower than 1 (e.g.,

even for Ni the spin value is about 0.3) and the average

spin and Curie temperature can be expressed as values

for Ni multiplied by the correction factor of (1 - x/

0.61), it results: A ¼ ANið1� x=0:61Þ2. Values of

about 8 9 10-7 erg/cm have been reported for the

stiffness constant of Ni (Michels et al. 2000). Hence,

values of about 50 nm have been estimated for the

exchange length parameter kex, a ratio D/kex of about

1.2 (slightly decreasing with x) being obtained for the

80 nm diameter wires. According to micromagnetic

simulations (Ross et al. 2002), a flower-like spin

structure (lack of magnetic domains, but surface spins

slightly misaligned) corresponds to wires with D/

kex \ 4, high aspect ratio and low Q. It is worth

mentioning that an average structural coherence

length of about 350 nm was estimated from the line-

width of XRD patterns of the single phase S2–S4

samples, which can be interpreted as the length of the

crystallites, L0, along the cylinder axis. Hence, the

analyzed wires can be also seen as a chain of

crystallites, each one of high aspect ratio. According

to Sellmeyer et al. (2001), for kex/L0 \ 1 and D/kex

close to 1, a non-cooperative and one-dimensional

magnetic regime might be considered for each wire.

Thus, in a decreasing magnetic field applied along the

cylinder axis, after reaching the saturation, the mag-

netization is independently reversed in each crystallite

with a switching field which might be approached via

the phenomenological expression for coercivity,

Hc = aH0 ? bMs (Kronmuller et al. 1988), with H0

the Stoner–Wohlfarth switching field (2Ku/Ms), a

depending on the orientation of grain texture versus

the applied field and b equating in absolute value the

demagnetizing factor (0.5 and 2p can be roughly

considered for a and b in the present case). Accord-

ingly, for crystallites of low Q values (Q � 1), the

magnetocrystalline anisotropy term becomes negligi-

ble and the switching field have to be related just to the

shape anisotropy (Hsw = 2pMs). In spite of the non-

cooperative magnetic regime, the switching field in

such crystallites of high aspect ratio and low Q values

is almost the same, and during the magnetization

reversal (after the saturation), each wire can be

considered as a magnetic monodomain switching its

magnetization at a given field. Certainly, such a field

cannot be related to switching against dipolar interac-

tions. A general expression for the magnetostatic

energy of two entities with arbitrary shapes was given

by Beleggia et al. (2004). A simplified expression for

the interaction energy of cylindrical magnetic entities

with high aspect ratio and the geometrical parameter

q ¼ D=R (R = the average distance between the

entities, about 330 nm in the present case) lower than

0.25 (as is the case) is Eint ¼ ðUiUj=RÞ (Beleggia et al.

2004), with Ui ¼ aipMiD2=4 (ai ¼ �1 and Mi the

magnetization of entity i). An average switching field,

Hsw, was estimated, values lower than 1 Oe being

obtained and proving that the dipolar interaction is

negligible. Contrary, switching fields of hundreds Oe

are required for exceeding the shape anisotropy energy

(using the same procedure and the above mentioned

shape anisotropy constants). While the observed

coercive fields for applied fields parallel to the wires

are of the same order (about 160(20) Oe at RT and

400(20) Oe at 10 K), for all samples, the involved

switching fields have to be related mainly to the shape

anisotropy energy. A distribution of such switching

fields could have two origins: (i) a distribution of

anisotropy energies or (ii) an angular distribution of

the anisotropy axes. In the first case, the coercive field,

seen as an average switching field, should depend

significantly on the alloy composition, while

HswðxÞ / MSðxÞ. The magnetization at saturation

decreases from about 300 emu/cm3 in sample with

x = 0.08–80 emu/cm3 in sample with x = 0.46,

whereas the coercive field varies by \12 % at RT

and 5 % at 10 K, excluding origin (i) as a main

contribution. Hence, the specific geometry-dependent

hysteresis loops seem to be explained via an easy axis

J Nanopart Res (2013) 15:1863 Page 7 of 9

123

distribution (EAD), involving angular distributed

anisotropy axes (Kuncser et al. 2003). The angular

dispersion of the nanowires inside the polymer is

physically better than few degrees (it is worth noticing

that contrary to structural and morphologic measure-

ments, the magnetic measurements were performed

with the nanowires fixed in the polymer template; and

due to the prevalence of the shape anisotropy, the

magnetization should align along the nanowire length,

respecting therefore the very narrow angular distribution

of the ion-tracks). Thus, the observed angular distribu-

tion of the spin directions may be related to a flower-like

spin structure, depending on composition. The param-

eters of the angular distribution can be estimated by

using the orientation ratio, OR, defined as the ratio

between the remanent magnetization in parallel and

perpendicular geometry, respectively, (Kuncser et al.

2003). In the simplest case of an uniform angular spin

distribution (Kuncser and Keune 2011; the probability

distribution is a finite constant for spin orientations up to

an angular deviation a0 and zero outside this angular

interval), it can be proven that a0 = 2arctg(OR). The

obtained a0 values are shown in Fig. 5b, whereas the

starting experimental values of OR are shown in the

inset. It can be observed that except the sample with the

highest Cu content (x = 0.8), the apertures are between

10� and 25� (just the static regime at the lowest

temperature is considered) and increase with the Cu

content. That is not surprisingly, given the increase of

the Q factor with the Cu content, leading therefore to a

challenging contribution of the magnetocrystalline

energy. The sample of highest Cu content presents an

angular aperture of about 90�, standing for a randomly

distributed spin orientation, typical to disordered ferro-

magnets. However, its magnetism cannot be discussed

according to the above considerations due to the peculiar

heterogeneous phase structure.

Conclusions

Nanowires of Ni–Cu alloys of narrow size distribution

and high aspect ratio (80 nm diameter and 30 lm

length) were grown by electrodeposition in etched

ion-track membranes. The alloy composition was

varied from 8 to 80 % of Cu, by the deposition

potential. The compositional dependent magnetic

configuration was analyzed starting from the geom-

etry-dependent magnetization reversal and suitable

estimations with respect to the average magnetic

moment, anisotropy constants and energies, exchange

length parameters, interactions between nanowires,

switching fields, and angular apertures of EADs. The

estimated average magnetic moment of the alloys

does not follow the typical linear decrease versus the

Cu content, due to lower average densities of the wires

(gas evolution at higher deposition potential) and the

heterogeneous composition (at the highest Cu con-

tent). Estimations of reference material parameters

give support for nanowires consisting of a chain of

polycrystals of high aspect ratio and non-cooperative

magnetic response. Due to the predominance of the

shape anisotropy over the magnetocrystalline anisot-

ropy in any crystallite, the magnetic reversal in each

wire takes place at an almost unique switching field,

similar as for a magnetic monodomain wire. A flower-

like spin structures and the presence of an EAD which

depends on the alloy content were observed. The

angular aperture of the distribution increases at higher

Cu content, due to a more competitive contribution of

the magnetocrystalline anisotropy.

Acknowledgments The financial support through the

exploratory research project PCE IDEI 75/2011 is highly

acknowledged.

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