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Electrochimica Acta
jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta
lectrodeposition of hexagonal Co nanowires with largeagnetocrystalline anisotropy
. Cattaneoa,1, S. Franza,∗, F. Albertinib, P. Ranzierib, A. Vicenzoa,2, M. Bestetti a, P.L. Cavallotti a,2
Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Via Mancinelli 7, 20131 Milano, ItalyCNR-IMEM, Parco Area delle Scienze 37/A, 43100 Parma, Italy
r t i c l e i n f o
rticle history:eceived 11 April 2012eceived in revised form 11 August 2012ccepted 16 August 2012vailable online xxx
eywords:obaltanowires
a b s t r a c t
Cobalt nanowires were fabricated by electrodeposition from a sulfamate-based electrolyte into anodicaluminum oxide templates having pore size of 27 nm, 50 nm and 250 nm. The electrolyte and operatingconditions were chosen with the aim of achieving hcp-Co with basal [0 0 1] preferred orientation. The typeof structure and the orientation of the Co nanowires were found to depend on the pore size of the oxidetemplate. Based on XRD and TEM analysis, nanowires formed in the 27 nm template showed the growthof oriented crystals with either prismatic or basal orientation; nanowires grown in the 50 nm templatewere oriented along the [0 0 1] direction, resulting in the alignment of the easy axis with the wire axis;the easy axis was perpendicular to the nanowire axis for nanowires grown in the 250 nm template with
a prismatic [1 0 0] + [1 1 0] texture. According to AGFM and MFM measurements individual Co pillarswith average diameter of 27 or 50 nm behaved as single magnetic domains with the magnetizationaxis parallel to the wire axis. Coercivity as high as 2700 Oe were measured for Co NWs having averagediameter of 27 nm. The magnetostatic configuration of planar array of Co nanowires, strictly depending
the
lline
on the texture quality andshape and magnetocrysta
. Introduction
Magnetic nanostructured materials are becoming more andore attractive for a range of applications related to recent devel-
Magnetic nanowires (NWs) represent a valid solution in termsf shape, size and magnetic properties for a variety of these novelechnologies, either in order to follow the continuous miniaturiza-ion of device components in a low-cost regime or to comply withize and shape requirements imposed by the application.
Among the different techniques used to grow nanowires, thelectrochemical deposition in anodic aluminum oxide (AAO) tem-late has emerged as a simple, cost-effective and efficient techniqueo synthesize high quality NW-arrays with both controlled size and
ength depending on the AAO template features [7–9].
In this research area, a greater interest has been shownn the fabrication of ferromagnetic cobalt NWs, due to the
1 Current address: Spectroscopy of Solids and Interfaces, Institute of Moleculesnd Materials, Radboud University, Nijmegen, Heyendaalseweg 135, 6525 AJijmegen, The Netherlands.2 ISE Member.
possibility of tailoring their magnetic properties by controllingthe crystal structure of the metal [10–24]. In fact, Ni and Fe NWshave room temperature stable face-centered cubic (fcc) and body-centered cubic (bcc) crystal structure, respectively, with shapeanisotropy typically overcoming the magnetocrystalline contri-bution. As a result, the easy magnetization direction is orientedparallel to the wire axis. Co NWs, on the other hand, may be grownwith either fcc or hexagonal close packed (hcp) crystal structure[10,15,20–24]. In the case of fcc-Co NWs, shape anisotropy domi-nates over crystal anisotropy, as for Ni and Fe NWs, while for hcp-CoNWs, the crystal anisotropy energy density (4.5 × 106 erg/cm3) [25]is of the same order of magnitude of the shape anisotropy energydensity (6 × 106 erg/cm3) [26]. Therefore, the crystallographic ori-entation of the hcp structure is expected to have a determininginfluence on the total effective anisotropy [15–18,20–22,24,27,28].Accordingly, in recent work the enhancement of the magnetic prop-erties of Co NWs has been primarily pursued by the refinementand optimization of process parameters for the control of crystalstructure and orientation [20–24]. Concurrently with the latter, adifferent approach has been also attempted by studying the effectof alloying modification on the magnetic properties of NW arrays,investigating in particular binary systems such as Co–Cr [29], Co–Fe
[30] or Co–Ni [31].
In this study, we report on the fabrication of hcp-Co NWs by elec-trodeposition (ECD) from a sulfamate electrolyte into porous AAOmembranes of different pore size. The NW shape depends obviously
58 L. Cattaneo et al. / Electrochimica Acta 85 (2012) 57– 65
F s (c,
odm[dpItabtasbgt
oa
ig. 1. SEM cross section (a, d, g) and top-view (b, e, h) of AAO membranes and Co NW
n the template geometrical features. Accordingly, the average NWiameter is roughly equal to the average pore diameter of the alu-ina template and the NW length depends on deposition time
18,22]. Structural properties of NWs deposited into membranesepend strongly on ECD parameters, namely the electrolyte com-osition, pH and the type of current–voltage control [16–18,24,32].
n this respect, it is stressed that the sulfamate electrolyte used inhe present work was shown previously to give hcp-Co films with
strong [0 0 1] texture [33–38]. The choice of the electrodepositionath was therefore motivated by the objective of growing tex-ured hcp-Co NWs, thereby obtaining a strong magnetocrystallinenisotropy and the achievement of high magnetic performance andtability. However, the crystal structure of ECD NWs is expected toe affected by the peculiar conditions of the template constrainedrowth, being possibly either weakened or disrupted, compared to
he conditions of thin film formation.
The issues briefly outlined above, namely the combined effectf shape and magnetocrystalline anisotropy on properties of NWrrays, are explored in the present work, performing a detailed
f, i) embedded into template of different pore sizes: 27, 50 and 250 nm, respectively.
analysis of the relationship between structure, shape and magneticproperties of Co NW arrays using a range of diverse techniques.
2. Experimental
AAO templates with a thickness of 30 �m, average pore diam-eter of 27, 50 and 250 nm, and inter-pore distance of about 50,120 and 400 nm, respectively, were used for the fabrication ofNW arrays. Prior to electrodeposition, a 15-nm thick Au layer wasdeposited by sputtering on the back side of AAO membranes toprovide electrical contact. The AAO templates were purchased fromNanomaterials S.r.l. (Milan, Italy).
Co NWs were electrodeposited from 1 M Co(NH2SO3)2 (cobaltsulfamate) electrolyte at pH 6.2. All solutions were prepared by ana-
lytical grade reagents, namely cobalt carbonate and sulfamic acid,and distilled water (milli-Q system) and pre-treated with activecharcoal at 90 ◦C for 1 h, to remove organic contaminants and inor-ganic colloidal particles.
ochimica Acta 85 (2012) 57– 65 59
gbrm(a
bmfiaweCPgAicc
F
wrbp
iGMitwncm
3
3
dsvttr(12AtTehmfpaw
A
the AAO membrane grown from sulfuric acid. Furthermore, HR-TEM imaging of the structure along the wire axis suggests that thegrowth of individual NW may evolve toward the formation of sin-gle crystallites (Figs. 3d and 4a), possibly with a high density of
Table 1Texture indexes Fhkl of Co NW arrays as a function of NW diameter.
L. Cattaneo et al. / Electr
The electrodeposition of Co NWs was performed in a 250 mllass cell, at 30 ◦C and under stirring with a magnetically drivenar of 0.5 cm in length, at 300 rpm. A three-electrode cell configu-ation was used. The counter-electrode was a cobalt plated titaniumesh and the reference electrode was a saturated calomel electrode
SCE). The electrodeposition was carried out in potentiostatic modet −1.1 V versus SCE, using an Amel Mod. 7050 Potentiostat.
The morphology of AAO membranes and Co NWs was examinedy scanning electron microscopy (SEM, Zeiss X1540) and trans-ission electron microscopy (TEM, Tecnai TEM equipped with a
eld emission gun and a spherical aberration corrector, operatingt 200 keV). For TEM examination, specimens planar views (PV)ere prepared using an Ar+ ion beam thinning procedure to achieve
lectron transparency. Crystallographic structure and texture of theo NWs were assessed by X-ray diffraction (XRD) using a PhilipsW1830 instrument, with Cu K�1 radiation and Bragg-Brentanoeometry. XRD was performed on Co NWs embedded into the hostAO template in the 2� angular range of 30–90◦. The method of
nverse pole figures [39,40] was used for an assessment of therystallographic orientation of NWs array, calculating the textureoefficient Fhkl for (hkl) plane according to the following expression:
hkl = Ihkl/IRhkl
(1/n)∑
hklIhkl/IRhkl
(1)
here Ihkl is the measured intensity of reflection (hkl), IRhkl
is theeflection intensity of a random powder sample and n is the num-er of measured reflections, i.e. the (1 0 0), (0 0 2), (1 0 1) and (1 1 0)eaks.
Magnetic measurements were performed on Co NWs embeddednto the AAO membrane, at room temperature, using an Alternatingradient Force Magnetometer (AGFM, MicroMag 2900, Princetoneasurements Corporation). Hysteresis loops were measured both
n parallel and in perpendicular applied field mode, with respecto the main axis of the Co NWs. Co NWs arrays and single Co NWsere further examined by atomic force microscopy (AFM) and mag-etic force microscopy (MFM, Dimension 3100 with Nanoscope IVaontroller, Digital Instruments) in order to asses and compare theorphology and the magnetic domain configuration.
. Results and discussion
.1. Morphological and structural characterization
Geometric parameters of AAO templates and Co NWs wereetermined by SEM imaging. Representative SEM micrographshowing the fractured cross section and – in the insets – the topiew of the templates are reported in Fig. 1, side by side withhe micrographs of the corresponding NWs still embedded in theemplate. The average diameter of Co NW (in the following alsoeferred to as dNW) was estimated as 27 ± 2 nm (Fig. 1c), 50 ± 4 nmFig. 1f) and 250 ± 54 nm (Fig. 1i); the length was, respectively,3.6, 14.4 and 9.3 �m, for NWs formed in template of 27, 50 and50 nm average pore size, respectively. Moreover, features of theAO membranes obtained from different electrolytes and relevant
o NWs growth were noted by SEM examination (Fig. 1a, d, and g).he template from phosphoric acid anodizing (250 nm pore diam-ter) showed an irregular arrangement of pores, a comparativelyigh pore size dispersion, and growth faults, mostly due to poreserging (not shown here). On the other hand, though templates
rom the oxalic or sulfuric acid process had an overall uniform mor-hology – in terms of pores size and arrangement – features such
s not perfectly parallel channels, high roughness of the inner poreall and branching of pore channels were observed.
Fig. 2 shows the XRD patterns of Co NW arrays embedded intoAO template of different pore diameter. Apparently, Co NW arrays
Fig. 2. XRD patterns of Co NWs array embedded in AAO template with different poresize, as indicated in the graph. In the inset above, the standard pattern of hcp-Copowder is plotted for comparison (JCPDS #05-0727).
had a polycrystalline hcp structure with varying preferred orienta-tion (PO) depending on NW diameter. Texture indexes for Co NWswith diameter of 27, 50 and 250 nm, and length of 13.6, 14.4 and9.3 �m, respectively, are reported in Table 1.
NW array formed into the 50 nm pore size template showed arelatively strong orientation with the [0 0 1] direction aligned alongthe wire axis. The 27 nm NW array and 250 nm NW arrays bothshowed a comparatively more complex texture. NWs formed inthe 27 nm template showed apparently a two-component texture[1 0 0] + [0 0 1], resulting in the [0 0 1] direction either aligned withor tilted relative to the nanowire axis. On the contrary, for Co NWsformed in the 250 nm template, given the [1 0 0] + [1 1 0] preferredorientation, the [0 0 1] direction was predominantly normal to thewire axis.
The morphology and crystal structure of Co NWs were furtherinvestigated by bright field HR-TEM analysis, both in the planeparallel and perpendicular to the wire axis. HR-TEM micrographsand the related fast-Fourier transform (FFT) analysis are shown inFigs. 3–7.
Figs. 3 and 4 show HR-TEM micrographs of 27 nm diameter CoNWs embedded in the AAO template. Based on the HR-TEM micro-graphs and the corresponding FFT analysis, Co NWs were found tohave the c-axis of the hcp structure either oriented along the wireaxis (Fig. 3b and b′) or tilted about 30◦ with respect to the wire axis(Fig. 3c, c′ and d). These observations strongly suggest that individ-ual nanowires may show different textured growth, giving eithera basal or a prismatic orientation, in agreement with the obser-vation of a two-component texture by XRD. The development ofdifferent texture components may be tentatively related to differ-ent factors, having an impact on nucleation process, namely thehigh roughness of the inner pore wall or the small pore size of
dNW (nm) F10.0 F00.2 F10.1 F11.0
27 2.1 1.6 0.2 0.150 0.7 2.6 0.4 0.3
250 2.3 0.5 0.1 1.0
60 L. Cattaneo et al. / Electrochimica Acta 85 (2012) 57– 65
F n TEMm ctivelyN s mag
sol
papoNp
ig. 3. TEM images of Co NWs with average diameter of 27 nm. (a) Low magnificatioicrograph of Co NW section oriented along the [0 0 1] and [1 1 0] zone axis, respeW showing the [0 0 2] direction tilted with respect to the NW axis; square detail i
tacking faults along the c direction (Fig. 4). The defective structuref the crystal may in turn account for the broadening of diffractionines as obviously inferred from XRD patterns in Fig. 2.
TEM examination of Co NWs with average diameter of 50 nmrovided direct evidence in support of the results of XRD analysisnd further detail about the structure of individual NW. The basal
referred orientation of Co NWs is demonstrated by the high res-lution TEM image and the corresponding FFT image in Fig. 5. TheW textured growth appears as a result of a competitive growthrocess. In fact, as shown in Fig. 6 and already noted above, the
Fig. 4. (a) HR-TEM micrograph of a Co NW of 27 nm average diameter (longitu
micrograph the Co NW array embedded into the AAO template; (b) and (c) HR-TEM, together with their FFT analysis (b′) and (c′); (d) is the HR-TEM side view of one
nified in (e) with relative FFT in (e′).
structure of an individual NW may evolve from fine polycrystallineto seemingly single crystalline, moving away from the bottom ofthe template toward pore openings. Grain boundaries were in factdetected only close to the NW root, while no grain boundaries couldbe detected along most of the NW length. Moreover, as shownin Fig. 6b and more specifically in Fig. 6c, within the NW region
with polycrystalline structure surviving close to the root, the [0 0 1]orientation remains aligned with the wire axis through differentgrains. This transitional evolution is arguably related to the com-bined effects of nucleation and growth processes. Stemming from
dinal cross section). (b) Digital zoom of the area within the white square.
L. Cattaneo et al. / Electrochim
Fig. 5. TEM images of Co NWs with average diameter of 50 nm. (a) Low magni-fiH(
tl[sdpt
FN
cation TEM micrograph the Co NW array embedded into the AAO template; (b)R-TEM micrograph of a single Co NW; (c) detail from image (b); (d) FFT analysis of
c).
he polycrystalline Co film initially formed on the sputtered Auayer – on the backside of the AAO template – growth centers with0 0 1] preferred orientation prevail, merge together and finally
pread out through the pore channel. The latter process obviouslyetermines the growth texture of NWs. Interestingly, for the 50 nmore size template, the observed [0 0 1] texture corresponds tohe characteristic texture of the cobalt sulfamate electrolyte [35];
ig. 6. (a) HR-TEM micrograph of a Co NW having an average diameter of 50 nm (longituW in a different position. (b) HR-TEM image of the square region encircled in (a), corres
ica Acta 85 (2012) 57– 65 61
moreover, the pore size of the template corresponds closely to thecharacteristic crystallite size of cobalt deposits from the same elec-trolyte [35].
At variance with both the above cases, Co NWs with averagediameter of 250 nm were found to have a polycrystalline structureof hcp-Co without any apparent relationship between crystal ori-entation and growth direction (Fig. 7). Consequently, the texturetype assignment was decided based exclusively on the results ofthe XRD analysis.
3.2. Magnetic characterization
As already mentioned in the introduction, the magnetocrys-talline anisotropy energy density of the hcp-Co phase is comparableto the shape anisotropy energy density [15–18,20–22,24,27,28].Thus, the effective magnetic anisotropy of a Co NW array is deter-mined by both the magnetocrystalline anisotropy and the shapeanisotropy of individual NW, together with the magnetostaticinteractions among NWs.
Accordingly, for Co NWs with average diameter of 27 and50 nm (and aspect ratio of about 500 and 280, respectively),showing either a [0 0 1] texture component or a [0 0 1] texture,respectively, the alignment of the easy axis with the wire axis isexpected, meaning that the shape anisotropy collaborates with themagnetocrystalline anisotropy in maintaining the magnetizationaligned along the wires axis. On the other hand, larger diame-ter NWs (250 nm diameter and aspect ratio of about 40), with[1 0 0] + [1 1 0] texture, present a tilted easy axis, meaning that thecrystal anisotropy works in the opposite direction (perpendicularto the wire axis) with respect to the shape anisotropy.
Following the above analysis, the magnetic state of individualnanowires was studied by MFM, in conjunction with the charac-terization of the magnetic properties of NW arrays by AGFM.
Fig. 8 shows two-dimensional surface AFM and MFM imagesof arrays of Co NWs with average diameter of 50 (Fig. 8a and b)and 250 nm (Fig. 8c and d). The AFM topographic image in Fig. 8aconfirmed the hexagonal ordering of the NW array, following thecharacteristic ordering of the AAO porous template. Significantly,grain boundaries were not detected at the NW surface, as in the
topographic image no contrast was observed within the same NW.The MFM image obtained by scanning the same area of Fig. 8a isshown in Fig. 8b. The round dark and bright regions in the MFMscan are due to the magnetic response of the Co NWs, whose
dinal cross section). The inset is a low magnification TEM image showing the sameponding to the root of the NW, and in (c) the corresponding FFT image.
62 L. Cattaneo et al. / Electrochimica Acta 85 (2012) 57– 65
F gnifics
mopsaei
pmtosbmw
FM
ig. 7. HR-TEM images of Co NWs with average diameter of 250 nm. Left: a low maection of a single NW.
agnetization is parallel to the NW main axis, pointing either upr down with respect to the array surface. Similarly to topogra-hy imaging, no contrast could be observed within the same NW,uggesting a single magnetic domain structure. These observationsgree with the results of TEM analysis in indicating that the growthvolution of nanowires with average diameter of 50 nm terminatedn the formation of [0 0 1] textured crystallites.
On the other hand, AFM imaging of Co NWs grown in AAO tem-lates with average pore diameter of 250 nm definitely showed theark of a polycrystalline structure, as can be inferred by the con-
rast within the same NW in AFM images (Fig. 8d). The latter findingbviously agrees with the evidence provided by TEM analysis, as
hown in Fig. 7. Expectedly, a multi-domain structure was imagedy MFM scanning of the same area (Fig. 8e and f). Arguably, theagnetization lay in the plane normal to the NW main axis andas randomly oriented.
ig. 8. AFM and MFM micrographs of Co NWs embedded into the AAO templates: (a) AFMFM scan presented in (b); (d) AFM image of 250 nm Co NWs; (e) MFM scan of the same
ation image showing the distribution of NWs into the template; right: image of the
The magnetic properties of NW arrays revealed by AFGM mea-surements are presented and discussed in the following, withreference to the competing effects of shape and magnetocrystallineanisotropy and the NWs magnetostatic interaction.
To account for the combined influence of shape and magne-tocrystalline contribution in Co NW arrays having different texture,the effective anisotropy constant was estimated from the paralleland perpendicular magnetization curves according to the followingexpression:
Keff = Ms∫ mtot
H|| dm − Ms∫ mtot
H⊥ dm (2)
mtot 0
mtot 0
where Ms is the saturation magnetization, mtot is the total magneticmoment at saturation, and H|| and H⊥ are the applied field in the
image of 50 nm Co NWs; (b) MFM image of the same area; (c) digital zoom of the area; and (f) digital zoom from the MFM image.
L. Cattaneo et al. / Electrochimica Acta 85 (2012) 57– 65 63
Table 2Anisotropy effective constant and fields as a function of NW aspect ratio. Data estimated according to Eq. (2) and De La Torre Medina’s [42] model (see Eqs. (3)–(5)).
dNW (nm) NW aspect ratio Keff (erg/cm3) Ha measured (Oe) Hms = 2�M(1 − 3f) (Oe) HMC (Oe)
6 801551411
dr
NictaKrpt
twtca
H
wsw
emmbbcNa〈
〈
Fp
27 509 2.38 × 1050 280 2.49 × 106
250 38 –0.11 × 106
irection parallel and perpendicular to the rotation axis of the NW,espectively.
In Table 2, the calculated Keff are reported as a function of theWs aspect ratio (AR), for the three samples under study. The Keff
ncreases from negative to positive values as the AR increases. Thishange in sign from negative to positive can be attributed to the facthat there is an initial competition between the magnetocrystallinenisotropy term and the shape anisotropy term, leading to lowereff at low AR. As the AR increases, the magnetization easy axiselated to the magnetocrystalline anisotropy effect switches fromerpendicular to parallel, in cooperation with the shape anisotropyerm, resulting in higher Keff [20,22,41].
For the assessment of the magnetic properties of NW arrays,he magnetization curves recorded in perpendicular configurationere corrected for the magnetostatic energy contribution following
he De La Torre Medina’s model [42]. Accordingly, the hysteresisurves measured along the hard axis (Fig. 9b) were corrected using
demagnetizing field calculated with the equation:
ms = 2�M(1 − 3f ) (3)
here f is the surface filling factor and M the magnetization. Theurface filling factors used are 0.26, 0.24 and 0.20 for the samplesith NWs diameter of 27 nm, 50 nm and 250 nm, respectively.
This method, called anisotropy field distribution (AFD), allowsxtrapolating the total anisotropy field Ha from the experimentalagnetization curves, which is the field needed to saturate theaterial along the magnetization hard direction. Ha is described
y two contributions: the magnetostatic field Hms which includesoth the self-demagnetizing and the mean field dipolar interactionontribution, and the magnetocrystalline field HMC. In the case ofWs having the c-axis perpendicular to the wire axis (Co NWs with
verage diameter of 250 nm) the measured average anisotropy fieldHa〉 is [42]:
Ha〉 = Hms − HMC
2(4)
ig. 9. Hysteresis loops of Co NW array at different NW diameter, as indicated in the legearallel (a) and perpendicular (b) to the wire axis.
while in the case of nanowires with the c-axes parallel to their axis(Co NWs with 27 and 50 diameter size) the average effective fieldis [42]:
〈Ha〉 = Hms + HMC (5)
Magnetization curves recorded in the direction parallel to thenanowire axis need a minor correction due to the negligible demag-netizing factor of a NW along the rotation axis and to the low fillingfactor of the membrane. The only contribution to the demagnetiz-ing field in the direction parallel to the nanowire can be expectedfrom the inter-wire dipolar interaction. The demagnetized field inthis direction was estimated by calculating an effective demagne-tizing factor [29]. Hysteresis curve corrected for the magnetostaticcontribution are reported in Fig. 10. Data on coercivity and rema-nence are reported in Table 3, as a function of the wires diameter.
In agreement with previous studies [10–12,14,15,27], a strongreduction of the coercivity was observed as the diameter increases(Table 3). More interestingly, the dependence of the magnetizationeasy axis (EA) on magnetocrystalline anisotropy, as previously pre-dicted on the basis of XRD and HR-TEM analysis, was reconfirmed.The EA of Co NWs having average diameter of 27 nm and 50 nm liesalong the wire main axis, showing both high coercivity and rema-nence ratio (see Table 3). These results remove all doubt about thecrystallographic orientation, especially for the case of 27 nm diam-eter NWs, because only a highly [0 0 1] oriented sample can showremanence ratio over 80% and a coercivity fields as high as 2620 Oe,which were reported in a few instances for polycrystalline Co NWs[20–24] and never reported for single crystal nanowires.
The EA in Co NWs having diameter of around 250 nm appearsto be affected both by the magnetocrystalline anisotropy, which isaligned perpendicularly to the wire main axis [1 0 0], and the shape
anisotropy, lying parallel to the wire main-axis. These results are ingood agreement with the study of Henry et al. [27], where despitean high aspect ratio and consequently a strong shape anisotropyterm, a square hysteresis loop was obtained only for wires with
nd. Hysteresis loops were measured by applying the magnetic field in the direction
64 L. Cattaneo et al. / Electrochimica Acta 85 (2012) 57– 65
Fig. 10. Hysteresis loops of Co NW array at different wire diameter, as indicated in the legend. The magnetization is plotted versus the internal field after subtracting theappropriate magnetostatic term (see text for details).
Table 3Coercivity Hc and remanence Mr/Ms as a function of the NWs diameter, in both perpendicular and parallel configuration (see text for details).
dNW/nm Hc/Oe perpendicular tothe NW
Hc/Oe parallelto the NW
Mr/Ms perpendicular tothe NW
Mr/Ms parallelto the NW
dafma
HHotca
nosciwlpagtw
ta
4
NB
27 1570 2620
50 525 1230
250 380 400
iameter lower than 50 nm, with the c-axis preferentially orientedlong the wire axis; whereas for diameter larger than 50 nm theyound a perpendicular alignment. Moreover, from MFM measure-
ents, NWs with diameter smaller than 50 nm resulted to behaves single-domain.
Table 2 (columns 4–6) shows the extrapolated anisotropy fielda and the estimated magnetocrystalline HMC and shape-dipolarms fields as a function of the Co NWs aspect ratio. It can bebserved that as the AR decreases (i.e. as the diameter increases)he total anisotropy field Ha is reduced. If we calculate then the twoomponents, magnetostatic and magnetocrystalline, it is notice-ble that each sample presents a slightly different scenario.
Both the 27 and the 50 nm diameter NWs show a dominant mag-etocrystalline term, meaning that the crystallographic anisotropyvercomes the shape anisotropy even though they are of the sameign so they collaborate aligning the EA along the wire axis. In thease of 250 nm diameter NWs, it is found that Hms is the dom-nant anisotropy contribution. Hms and HMC have opposite sign,
here HMC was calculated from Eq. (4). This means that, for theargest diameter NWs, the magnetocrystalline axis rotates fromarallel to perpendicular to the wire axis, competing with the shapenisotropy to determine the final EA. This simple analysis results inood agreement with the previous evaluation of the Keff validatinghe magnetic configuration for each sample, in strong correlationith the structural analysis.
It can be concluded that in the case of the Co NWs here described,he magnetization EA direction is determined by both the shapenisotropy term and the crystallographic texture.
. Conclusions
In the present work the synthesis by electrodeposition of CoWs with large magnetocrystalline anisotropy was described.ased on the results of XRD and HR-TEM analysis, Co NW arrays
0.29 0.870.29 0.50.26 0.1
showed either the coexistence of crystals oriented along the [1 0 0]and [0 0 1] direction or with a strong [0 0 1] texture, for average NWdiameter of 27 and 50 nm, respectively. The structure of Co NWswith average diameter of 250 nm was fully polycrystalline with thec-axis normal to the wire axis. AFM/MFM measurements providedcomplementary evidence to the above conclusions showing thatCo NWs of 27 nm and 50 nm diameter size were single magneticdomains. The easy axis of magnetization was perpendicular to thetemplate surface and coercivity fields higher than 2.6 kOe, amongthe highest values reported for arrays of NWs of this size, weremeasured.
By extrapolating the shape and crystallographic contributionsto the total anisotropy field, it appears that both terms play a majorrole in determining the equilibrium magnetic configuration, strictlyconnected to the texture quality and the relative aspect ratio. Thesetwo parameters can be definitely chosen to tailor each possiblemagnetic configuration in such type of nano and micro-structuredmaterials.
Acknowledgments
The authors acknowledge financial support from the EuropeanUnion under the Framework 6 program under a contract for anIntegrated Infrastructure Initiative, Reference 026019 ESTEEM.
The authors also are grateful to C. Soldano for SEM analysis,Nanomaterials.it S.r.l. (Italy) for technical assistance and M. Savoinifor fruitful discussions.
References
[1] D.H. Reich, M. Tanase, A. Hultgren, L.A. Bauer, C.S. Chen, G.J. Meyer, Journal ofApplied Physics 93 (2003) 7275.
[2] K.-B. Lee, S. Park, C.A. Mirkin, Angewandte Chemie International Edition 43(2004) 3048.
ochim
[[
[
[
[
[
[
[
[
[
[
[
[
[[
[
[
[
[
[
[
[
[[[[[
[
[
[
L. Cattaneo et al. / Electr
[3] M. Safi, M. Yan, M.A. Guedeau-Boudeville, H. Conjeaud, V. Garnier-Thibaud, N.Boggetto, A. Baeza-Squiban, F. Niedergang, D. Averbeck, J.F. Berret, ACS Nano 5(2011) 5354.
[4] M. Liu, J. Lagdani, H. Imrane, C. Pettiford, J. Lou, S. Yoon, V.G. Harris, C. Vittoria,Nian X. Sun, Applied Physics Letters 90 (2007) 103105.
[5] E. Piccin, R. Laocharoensuk, J. Burdick, E. Carrilho, J. Wang, Analytical Chemistry79 (2007) 4720.
[6] Y. Rheem, C.M. Hangarter, E.H. Yang, D.Y. Park, N.V. Myung, B. Yoo, IEEE Trans-actions on Nanotechnology 7 (2008) 251.
[7] H. Masuda, K. Fukuda, Science 268 (1995) 1466.[8] H. Masuda, H. Yamada, M. Satoh, et al., Applied Physics Letters 71 (1997) 2270.[9] J. Sarkar, G.G. Khan, A. Basumallik, Bulletin of Materials Science 30 (2007) 271.10] A. Fert, L. Piraux, Journal of Magnetism and Magnetic Materials 200 (1999) 338.11] R.M. Metzger, V.V. Konovalov, M. Sun, T. Xu, G. Zangari, B. Xu, M. Benakli, W.D.
Doyle, IEEE Transactions on Magnetics 36 (2000) 30.12] H. Zeng, M. Zheng, R. Skomski, D.J. Sellmyer, Y. Liu, L. Menon, S. Bandyopadhyay,
Journal of Applied Physics 87 (2000) 4718.13] H. Zeng, R. Skomski, L. Menon, Y. Liu, S. Bandyopadhyay, D.J. Sellmyer, Physical
Review B 65 (2002) 134426.14] J.U. Cho, J. Wu, J.H. Min, S.P. Ko, J.Y. Soh, Q.X. Liu, Y.K. Kim, Journal of Magnetism
and Magnetic Materials 303 (2006) 281.15] R. Ferre, K. Ounadjela, J.M. George, L. Piraux, S. Dubois, Physical Review B 56
(1997) 14066.16] Z. Liu, P.C. Chang, C.C. Chang, E. Galaktionov, G. Bergmann, J.G. Lu, Advanced
Functional Materials 18 (2008) 1573.17] Y. Ren, Q.F. Liu, S.L. Li, J.B. Wang, X.H. Han, Journal of Magnetism and Magnetic
Materials 321 (2009) 226.18] A. Ramazani, M. Almasi Kashi, M. Alikhani, S. Erfanifam, Materials Chemistry
and Physics 112 (2008) 285.19] S. Sharma, A. Barman, M. Sharma, L.R. Shelford, V.V. Kruglyak, R.J. Hicken, Solid
State Communications 149 (2009) 1650.20] K.R. Pirota, F. Beron, D. Zanchet, T.C.R. Rocha, D. Navas, J. Torrejon, M. Vazquez,
M. Knobel, Journal of Applied Physics 109 (2011) 083919.21] Y. Ren, J. Wang, Q. Liu, Y. Dai, B. Zhang, L. Yan, Journal of Materials Science 46
(2011) 7545.
[[
[
ica Acta 85 (2012) 57– 65 65
22] L.G. Vivas, R. Yanes, O. Chubykalo-Fesenko, M. Vazquez, Applied Physics Letters98 (2011) 232507.
23] X.W. Wang, Z.H. Yuan, H.F. Luo, Solid State Sciences 13 (2011) 1211.24] A. Ramazani, M.A. Kashi, G. Seyedi, Journal of Magnetism and Magnetic Mate-
rials 324 (2012) 1826.25] D.M. Paige, B. Szpunar, B.K. Tanner, Journal of Magnetism and Magnetic Mate-
rials 44 (1984) 239.26] R.M. Bozorth, Ferromagnetism, D. Van Nostrand Company Inc., Princeton, NJ,
1951.27] Y. Henry, K. Ounadjela, L. Piraux, S. Dubois, J.-M. George, J.-L. Duvail, European
Physical Journal B 20 (2001) 35.28] L. Belliard, J. Miltat, A. Thiaville, S. Dubois, J.L. Duvail, L. Piraux, Journal of Mag-
netism and Magnetic Materials 190 (1998) 1.29] N.B. Chaure, J.M.D. Coey, Journal of Magnetism and Magnetic Materials 303
(2006) 232.30] M.A. Kashi, A. Ramazani, F. Es’haghi, S. Ghanbari, A.S. Esmaeily, Physica B 405
(2010) 2620.31] M.A. Kashi, A. Ramazani, N. Akhshi, A.S. Esmaeily, Japanese Journal of Applied
Physics 51 (2012) 025003.32] K. Nielsch, F. Müller, A.-P. Li, U. Gösele, Advanced Materials 12 (2000) 582.33] T. Chen, P.L. Cavallotti, Applied Physics Letters 41 (1982) 205.34] T. Chen, P.L. Cavallotti, IEEE Transactions on Magnetics 18 (1982) 1125.35] A. Vicenzo, P.L. Cavallotti, Electrochimica Acta 49 (2004) 4079.36] P.L. Cavallotti, M. Bestetti, S. Franz, A. Vicenzo, Transactions of the Institution
of Metal Finishing 88 (2010) 28.37] L. Cattaneo, A. Vicenzo, S. Franz, M. Bestetti, P.L. Cavallotti, ECS Transactions 25
(2010) 135.38] P.L. Cavallotti, L. Nobili, S. Franz, A. Vicenzo, Pure and Applied Chemistry 83
