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DOI: 10.1002/adfm.200701010

PAPER

Shape Anisotropy and Magnetization Modulation in HexagonalCobalt Nanowires**

By Zuwei Liu, Pai-Chun Chang, Chia-Chi Chang, Evgeniy Galaktionov, Gerd Bergmann, and Jia G. Lu*

Ferromagnetic cobalt nanowires with high-crystalline quality are synthesized using a low-voltage electrodeposition method.

High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) results show that the nanowires are

uniform in size, and consist of predominantly hexagonal close-packed (hcp) structure with the magnetocrystalline easy axis

(c-axis) perpendicular to the wire axis. Superconducting quantum interference device (SQUID) measurements illustrate the

dominance of shape anisotropy, manifested by the weak temperature dependence of the enhanced coercive field along the wire

axis. Furthermore, the magnetic structures of individual, segmented, or intersected nanowires are studied using magnetic

force microscopy. This reveals a strong dipole at the two ends of the wire, together with a spatial magnetization modulation along

the wire. Based on theoretical modeling, such intrinsic modulation is attributed to magnetization frustration due to the

competition between the magnetocrystalline polarization along the easy axis and the shape anisotropy along the wire axis.

1. Introduction

Development of novel magnetic materials has played an

important role in modern day science and technology.

Nanomagnetism research sparks interests in fundamental

physics and renders promising technological applications.

Compared with bulk and thin film, nanowires with high aspect

ratio not only have the advantage of enhanced surface-

to-volume ratio but also possess favorable dipolar magnetic

properties. In addition, magnetic nanowires with good

conductivity are ideal for studying one-dimensional electron

spin dynamics[1–3] and magnetic domain-wall dynamics.[4–9]

Technologically, vertical magnetic nanowire arrays embedded

in an insulating matrix may serve as a high density patterned

perpendicular magnetic medium. This type of patterned

medium not only supersedes the storage capacity of conven-

tional longitudinal media, but also achieves superior signal-

to-noise ratio with reduced transition position jitter [10] as

compared to thin film perpendicular recording media. These

advantages make magnetic nanowires potential elements for

[*] Z. Liu, C.-C. Chang, G. Bergmann, J. G. LuDepartment of Physics & Astronomy, University Southern CaliforniaLos Angeles, CA90089-0484 (USA)E-mail: jialu@usc.edu

P.-C. ChangDepartment of Electrical Engineering, University Southern CaliforniaLos Angeles, CA90089-0484 (USA)

[**] Zuwei Liu and Pai-Chun Chang contributed equally to the manuscript.The authors are indebted to Dr. Tore Niermann and Dr. XiaoshengFang for HRTEM imaging, Dr. James O’Brien for SQUID measure-ment, Profs. Richard Thompson, Hans Bozler, Carsten Ronning, andMarkus Muenzenberg for helpful assistances. The work is supportedby NSF grants ECS 0729612 and DMR 0742225.

Adv. Funct. Mater. 2008, 18, 1573–1578 � 2008 WILEY-VCH Verlag

high-performance memory storage. Furthermore, employing

magnetic nanostructures in integrated magneto-electronic

devices[11], biosensors,[12] and for in vivo cellular study [13]

holds great promise. Even though there have been many

investigations into magnetic nanowires, there is still much to be

explored and understood. In our research, one system we have

studied is cobalt, which has large magnetic moments and

effective magnetic interactions. This paper presents the

synthesis and material characterizations of Co nanowires

exhibiting a strong dipole with a magnetization modulation

along the longitudinal wire axis.

2. Synthetic Approach

The template-assisted electrodeposition method[14,15] has

proven to be a successful technique to synthesize vertically

aligned metallic nanowires. By controlling the deposition

conditions and the geometry of porous anodic aluminum oxide

(AAO) template, this method enables better stoichiometric

composition and geometric dimension of the as-grown

nanowires. Previous reports based on this method suffer from

poor filling rate due to the high deposition voltage[16,17]

required to overcome the oxide barrier layer. Since the

hydrogen evolution that originates from the reduction–

oxidation reaction easily blocks the AAO channels and thus

hinders the nanowire growth. In our experiment, we employ a

flip-chip process to remove the barrier layer before electro-

deposition, thus the deposition potential is lowered and

high-quality crystalline nanowires are obtained with filling

rate as high as 90% in the AAO template. (Detailed process is

described in the Experimental Section.)

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Figure 1. SEM images of Co nanowire arrays. a) Top view of Co nanowire embedded in AAOtemplate showing high filling rate. b) Without AAO template, bundles of Co nanowire stand alone onthe working electrode without support. Region enclosed the white circle indicates the exposedworking electrode posts.

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A top view of AAO template with Co nanowires embedded

in the pores is shown in Figure 1a. The average response of

such an ensemble array of Co nanowires has been stu-

died.[18–23] In order to characterize individual wires for better

understanding of their properties, the as-prepared template

with embedded Co nanowires is placed in NaOH solution to

remove the AAO matrix. Figure 1b shows bundles of Co

nanowires lie on the substrate after the removal of the matrix,

the bottom electrode can be clearly

observed in the form of dots because of

the electrode metal deposition into the

AAO pores.

Figure 2. a) XRD pattern of Co nanowire sample compared with hcp Co powder (PDF#05-0727) withthe strongest peak from (002). There are peaks contributed from Au and weaker ones from Ti thatexist in sample suspension. b) TEM image shows a Co nanowire with high aspect ratio and diameteraround 90 nm. c) HRTEM image exhibits the hcp crystal structure and the stacking direction along(01–10), where the 10-layer spacing is around 2.2 nm. d) Electron diffraction pattern shows thesingle-crystalline structure and corresponding growth direction (01–10).

3. Material Characterization

3.1. Structural Characterization

X-ray diffraction (XRD) results for Co

nanowires are shown in Figure 2a, which

matches with the hexagonal phase of

standard Co powder pattern (PDF#05-

0727). The strongest peak (002) indicates

that the most dominant plane is (001).

Energy dispersive X-ray spectroscopy

(data not shown) shows that the nano-

wires are pure Co, without Au or Ti

impurities. Figure 2b is a transmission

electron microscopy (TEM) image dis-

playing a single Co nanowire having

diameter around 90 nm and length over

10 mm. By adjusting the deposition rate

during the growth, long-range single-

crystalline Co nanowires have been

achieved. Although some segmental

grains can be observed, the elongated

grain size is usually on the order of 10 mm.

To confirm further the crystalline struc-

ture, a high-resolution transmission elec-

www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

tron microscopy (HRTEM) image and

select-area electron diffraction (SAED)

pattern, shown in Figure 2c and d, reveal

the dominant hexagonal close-packed

(hcp) crystalline structure. They indicate

that the stacking direction is along [01–10]

with interplane distance around 0.22 nm,

and the c-axis [0001] (easy axis) is per-

pendicular to the wire axis. In addition, on

the surface of the wire, a thin oxide layer

of �3 nm was observed.

3.2. Electrical Transport

Measurements

Hcp Co is a typical ferromagnetic

metal. To characterize the electrical property, Au/Ti electro-

des are patterned using photolithography onto single Co

nanowires. Figure 3a shows a nanowire (length 12mm and

diameter 90 nm) with four metal contacts. Standard four-probe

resistivity measurements are performed. The linear voltage

versus current characteristics indicate that good Ohmic

contacts are achieved. As the current is applied on electrodes

Iþ and I� as labeled in the figure, the voltages on electrodes

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Z. Liu et al. /Magnetization Modulation in Cobalt Nanowires

Figure 3. a) Microscope image of a nanowire and four lithography-patterned electrodes withequal spacing (�2 mm). b) Standard four-probe transport measurement shows Ohmic behaviorwith an estimated resistivity �50 mV � cm.

Vþ and V� are measured, yielding a resistance of 130V. Using

simple Drude model, the resistivity is estimated to be about

50mV � cm from the imaged nanowire geometry, and the

electron mean free path is around 1 nm based on the Fermi

energy EF¼ 0.81 Rydberg.[24] The resistivity is an order of

magnitude larger than the bulk value, and of the same order as

that of a 15 nm thickCo thin filmdeposited on SiO2 substrate.[25]

4. Magnetic Properties

4.1. Magnetic Hysteresis Response

Owing to the reduced dimension and elongated structure,

magnetic nanowires possess unique magnetic properties.

SQUID and magnetic force microscopy (MFM) have been

performed to investigate the magnetic structure of the

as-synthesized Co nanowires. As shown in Figure 4a, the

coercivity (Hc) measured in a magnetic field perpendicular

(transverse) to the wire axis is determined to be 15Oe,

comparable to the bulk value of �20Oe. The coercivity is

Figure 4. a) Hysteresis behavior of Co nanowire (NW) with magnetic field H parallel and perpenanowire at different temperatures illustrate the weak temperature dependence of coercivity.

Adv. Funct. Mater. 2008, 18, 1573–1578 � 2008 WILEY-VCH Verlag GmbH & Co. KGaA,

found to increase with decreasing diameter

(data not shown). In contrast, when magnetic

field is applied parallel (longitudinal) to the

wire axis, Hc increases by two orders of

magnitude to 1.1 kOe. This enhanced long-

itudinalHc manifests that the wire axis is the

preferred overall magnetization direction.[26]

The shape anisotropy dominance has been

further validated from the temperature

dependence of the hysteresis response.

Unlike the bulk material, temperature has

shown to have little effect on the coercivity.

As plotted in Figure 4b, the coercivity

remains almost constant as the temperature

increases from 1.8K to room temperature.

This implies that the shape anisotropy

prevails over the magnetization fluctuation.

4.2. Spatial Magnetization Modulation

The magnetic structures of Co nanowires were studied using

the MFM technique (Veeco Dimension V). To conduct a

systematic study, all nanowires presented here are grown under

the same synthesis conditions and possess similar magnetic

properties. To determine the contribution from the cobalt

oxide layer, nanowires that go through oxidation process are

measured, and they give nomagnetic signal. Figure 5a shows an

atomic force microscopy (AFM) image of a straight wire that

has a length around 16mm and a diameter of 90 nm. The MFM

tip (field strength of �10Oe) has a magnetization vertical to

the nanowires lying horizontal on SiO2 substrate. Since the

MFM response is proportional to the second derivative of the

stray field of the sample at the tip position, the vertical

component of the field is mainly measured. To confirm that the

magnetic signals are intrinsic, measurements were performed

with a MFM tip magnetized subsequently in two opposite

polarization directions. The corresponding MFM images and

phase-change profiles are displayed in Figure 5b. In addition to

the strong dipoles at the two ends of the wire, this nanomagnet

ndicular to wire axis. b) Hysteresis responses of Co

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Z. Liu et al. /Magnetization Modulation in Cobalt Nanowires

Figure 5. a) AFM image of a single nanowire with diameter of 90 nm andlength of 16mm. b) Detailed magnetic structure of single nanowire probedby a MFM tip with two opposite polarizations (top and bottom panels). Thecross-section phase-change profiles along the nanowire show quasiper-iodic modulation with identical period (T) around 700 nm. c) Schematicshowing the magnetization M (indicated by red arrows) modulates sinu-soidally along the wire axis.

Figure 6. a) Two sets of AFM and MFM images of one Co nanowirebefore and after sectioning by applying a voltage, showing a dipole at thesectioning points B and C. b) Phase profiles along the nanowire before andafter sectioning, indicating a similar modulation period T.

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exhibits finer dark–bright quasiperiodic magnetization mod-

ulations along the wire. The modulation period T is estimated

to be �700 nm. In contrast, by probing the nanowire with tip

polarized in the opposite orientation, the magnetic signal and

the phase profile are reversed, as demonstrated in the bottom

panel of Figure 5b, but maintain the same periodicity.

Furthermore, such spatial modulation persists after applying

an external field (�0.5 T) perpendicular to the wire axis.

From theoretical studies[18,26], this magnetization modula-

tion originates from the competition between the magneto-

crystalline polarization along the easy axis and the shape-

anisotropy energy along the wire axis. In a simple model, if

Co wire was an isotropic ferromagnet then the magnetization

M would align parallel to the wire axis. In the case when M

forms an angle u with the wire axis, the wire experiences a

shape demagnetization energy Ud (u)¼(U0/2)sin2u, where

U0¼m0M02V/2, M0 is the saturation magnetization, and V is

the volume. The alignment of the magnetization parallel to the

wire axis is favored as Ud¼ 0 for u¼ 0. However, since the

hcp-structured Co possesses a large crystalline anisotropy

energy in which the easy axis is perpendicular to the wire axis, a

magnetization along the wire axis, costs an energy of

Uca¼ k1þ k2, where k1¼ 0.34U0 and k2¼ 0.13U0 represent

www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH

Co anisotropy.[27] In [26], this problem is investigated

analytically. It is found, in the thin wire limit, that the angle

u between the magnetization and the wire axis is modulated

sinusoidally along the wire axis. Figure 5c depicts a schematic

of the magnetization modulation.

To investigate further the spatial modulation of magnetiza-

tion, a Co nanowire is imaged before and after sectioning into

separate segments. By applying 5V across a 5mm long Co

nanowire, the wire can be easily burned into two shorter

segments. Left panels in Figure 6a show themorphology before

and after sectioning.One can clearly see that the nanowire is cut in

the middle. By recording the phase change along the wire, the

corresponding MFM images (right panels in Fig. 6a) reveal a

pronounced pair of poles with opposite orientations (Fig. 6b) at

the cut points (labeled B and C).

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Figure 7. a) AFM image of a shorter nanowire in contact with a longer Conanowire in the dashed circle. b) MFM image demonstrates the magne-tization alteration due to the interaction between the long and the shortnanowires. Inset shows induced transverse magnetic structure.

Such uniquemagnetizationmodulation is found to change in

the case of nanowires with kinks (data not shown), and when

two Co nanowires come in contact with each other, their

interaction induces magnetic structure variations. As shown in

the AFM topography image of Figure 7a, two Co nanowires

touch within the dashed circle. The correspondingMFM image

shown in Figure 7b presents the magnetic structure resulting

from the interaction between the long and the short wires. The

long wire here displays magnetic poles of same polarity at the

wire ends, in contrast to the dipoles of opposite polarity

observed on individual wires. The short wire acts as if it has

magnetically ‘‘cut’’ the long wire, so around their contact point a

head-to-head configuration prevails, as depicted by the dashed

white arrows. More interestingly, the interaction of the two

nanowires induces strong transverse magnetic structures in the

long wire below the contact point. It is illustrated by the

head-to-tail white arrows enlarged in the inset of Figure 7b. The

origin of this phenomenon can be attributed to that the short wire

pins the magnetic poles (across the wire width) at the contact

point, and the interaction is strong enough to form transverse

magnetic structures. This rich phenomenon needs future detailed

theoretical modeling to elucidate the underlying mechanisms.

5. Conclusions

Hexagonal Co nanowires with diameter around 90 nm have

been synthesized using a low voltage electrodeposition method.

Structural characterizations indicate that the as-synthesized Co

nanowire mainly has an hcp single-crystalline structure with

easy axis perpendicular to the wire axis. The resistivity of the

nanowire is estimated to be 50 mV � cm. Investigating the

magnetic properties, SQUID data show strong shape aniso-

tropy manifested in the weak temperature dependence of the

Adv. Funct. Mater. 2008, 18, 1573–1578 � 2008 WILEY-VCH Verl

hysteresis responses for field along the wire axis. In addition,

fromMFM imaging the existence of distinct dipoles of opposite

polarity at the wire ends is observed as well as a spatial

magnetization modulation along the wire with a period around

700 nm. Such modulation is shown to originate from the

competition between the magnetocrystalline polarization

along the easy axis and the dominant shape anisotropy along

the wire axis.

6. Experimental

Highly ordered porous AAO templates are prepared by a two-stepanodization method. High purity aluminum chips (99.999%) are firstelectropolished in a mixture of perchloric acid (HClO4) and ethylalcohol (C2H5OH). The first anodization process is conducted in oxalicacid (H2C2O4) to form the first layer of aluminum oxide. In this step,the pores are not uniformly distributed. This first layer is then removedin a solution of phosphoric and chromic acid, leaving close-packedorifices on the Al chip surface. The second anodization is performedunder the same condition as the first anodization. Consequently,uniformly distributed pores with diameter around 90 nm and aninterpore distance of ca. 150 nm are formed in the AAO template.

For the flip-chip process, a thin Ti/Au bilayer is first coated on thetop side of AAOwith revealed pores, serving as the working electrode.The template is then flipped over and epoxy-bonded to a poly-vinylchloride plastic substrate and submerged into HgCl2 to removethe unwanted Al layer which is now on the top side. Afterwards, thealuminum oxide barrier layer is removed using an ion-millingpore-opening process.

The electrodeposition of Co nanowires is performed in thegalvanostat with CoSO4 electrolyte at room temperature. Theamplitude of the applied AC current is increased gradually to maintaina deposition potential at 1V for 3 h. This method enables a relativelyslow growth rate that improves the pore filling rate and wirecrystallinity.

Received: September 04, 2007Revised: February 20, 2008

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