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Electrochemical synthesis and magnetic characterization of periodically modulated Co nanowires

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Nanotechnology Nanotechnology 25 (2014) 145301 (7pp)


Electrochemical synthesis and magnetic characterization of periodically modulated Co nanowires I Minguez-Bacho1,2 , S Rodriguez-López1,3 , M Vázquez1 , M Hernández-Vélez3 and K Nielsch4 1 2 3 4

Instituto de Ciencia de Materiales de Madrid, CSIC, E-28049 Madrid, Spain Nanyang Technological University, School of Physical and Mathematical Sciences, 637371, Singapore Universidad Autónoma de Madrid, Departmento de Fisica Aplicada, E-28049 Madrid, Spain Institute of Applied Physics, University of Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany

E-mail: [email protected] Received 27 October 2013 Accepted for publication 5 February 2014 Published 12 March 2014


The synthesis of templates with modulated pore channels by combined mild and hard anodization processes is described. The hard anodization pulses, implemented during anodization, are controlled not only in time length and amplitude, but also in shape: square and exponential signals have been applied. Electrodeposition of Co is subsequently performed to obtain uniform and modulated diameter nanowire arrays. Square and exponential modulated diameter nanowires are imaged by scanning electron microscopy and hcp hexagonal polycrystalline structure is confirmed in all Co nanowires. Magnetic behavior strongly depends on nanowire shape and is interpreted considering the modification of magnetostatic interactions between wires induced by local stray fields from magnetic charges at the ends of the wider segments in modulated wires. As a consequence, magnetization processes under parallel and perpendicular field configurations denote the contribution of both thin and wide segments. Keywords: modulated diameter nanowires, Co nanowire arrays, anodic alumina, hard anodization, structural and magnetic characterization (Some figures may appear in colour only in the online journal)

The use of ordered arrays of magnetic nanoparticles and nanowires has become a fashionable alternative for a number of technological applications in advanced magnetic information storage, microwave devices or biofunctionalization [1–4]. Such nanowires should exhibit particular magnetic requirements to fit the expected applications. In that regard, magnetic response should be tailored by choosing a suitable magnetic element or alloy composition, and geometrical characteristics as for example diameter and length and interwire distance in the case of ordered nanowires. While lithography techniques have been used widely to fabricate arrays of nanomagnets, much attention is being also paid nowadays to the electrochemical route to synthesize tem0957-4484/14/145301+07$33.00

plates which afterwards must be filled with suitable magnetic material. The shape and arrangement of nanopores in the template is important in determining the properties of the final nanowires. Consequently, an optimized control of the template’s synthesis is important. Two modes, galvanostatic and potentiostatic, are usually employed for the synthesis of nanoporous templates based on Al2 O3 . Typically, one distinguishes between low-voltage long-time mild anodization (MA) and high-voltage short-time hard anodization (HA). In most cases, aluminum foils are employed as precursor material where anodization processes are used to make porous templates. The filling process is performed in a continuous way 1

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(generally, under potentiostatic conditions) [5] or in pulsed electrodeposition (commonly, galvanostatic pulses) [6, 7]. The HA process has attracted much attention in academic research. Initially, studies were focused on the fabrication of alumina nanochannels and on their formation mechanism [8, 9]. It was found that voids within the pore walls formed during the oxide growth process. A few years later, research studies by Chu et al focused on the formation of hexagonally ordered nanostructures by the HA process [10]. The HA process can be carried out by different methods and their combination. Chu et al used sulfuric acid based electrolyte and a two-step anodization process—first, galvanostatic anodization where the voltage increases until it reaches certain values, followed by potentiostatic anodization at that voltage. Other processes are carried out only in galvanostatic mode with an aged (by addition of Al2 (SO4 )3 ) sulfuric acid based solution [11], or only in potentiostatic mode, although that involves an increase of the applied voltage up to HA conditions [12]. Usually, HA conditions come with a high heat generation at the pore bottom due to the high current densities traversing the barrier layer. To reduce the anodization temperature below 0 ◦ C it is necessary to achieve a stable high field anodization process [13]. That implies the use of a solvent such as ethanol, which avoids freezing of the electrolytic bath. The origin of the nanochannel diameter oscillations that form during anodization processes have been ascribed to different factors [14–16]. One of them is the evolution of gas bubbles provoking plastic deformation of the nanochannels at the pore bottom [15]. Alternatively, the modulations have been proposed to arise from the different speed of ions at the metal/oxide and oxide/electrolyte interfaces [16]. Finally, other authors attribute such oscillations to volume contractions of Al(OH)3 formed near the ridges of the aluminum and accumulation of voids at the junctions of the alumina cells [14]. These modulations can also be controlled by external parameters that allow control of the geometry of pores: the shape, length and diameters of the modulated nanochannels. Lee et al introduced controlled modulation of diameters by a complete exchange of anodization conditions, i.e. acid electrolyte, applied voltage and anodization temperature, for each modulated segment [12]. This method is quite complex due to the large number of variables involved, and bearing in mind in addition that the interpore distance must remain the same. Novel methodologies have been proposed to obtain modulated pore diameter nanostructures based on a combination of MA and HA. Such methodologies have given rise to new anodization procedures such as pulsed anodization (PA) by Lee et al [17–19] or cyclic anodization by Losic et al [20, 21]. These processes can also be performed in potentiostatic or galvanostatic mode as well as with sulfuric or oxalic acid based electrolytes. The modulated nanostructures not only provide the possibility to use them as templates for novel 1D modulated nanostructures [22–24], but also as 3D nanostructures [18, 21, 25]. First studies on the magnetic properties of Py nanowires with modulated diameter have been recently reported [24, 26]. The objective of the present study was the synthesis of modulated nanopores by means of combined mild and hard

Table 1. Controlled experimental parameters applied in the

fabrication of the nanoporous anodic alumina film used as templates.

anodization processes. Electrochemical parameters have been very carefully tuned so that we have been able to achieve final nanopores with square or triangular/exponential modulation. The nanopores are then filled by controlled electrodeposition of Co resulting in arrays of Co cylindrical nanowires with modulated section. In contrast to very soft Py nanowires, Co nanowires are known to exhibit high magnetic moment and very significant crystalline anisotropy that determines the magnetization reversal process [27, 28]. The diameter modulation of such Co nanowires is confirmed to strongly tune the magnetic properties of the array. 1. Experimental details 1.1. Synthesis of templates with controlled geometry: periodic alternating voltage pulses with different lengths, periods and shapes

The experiments of this section are based on the anodization of aluminum foils by periodically alternating pulses of 25 V, called MA pulses, and of between 35 and 39 V for the HA pulses. The starting substrate is an aluminum foil nanopatterned after 16 h of conventional first anodization at 25 V after previous removal of the anodic alumina layer. We have used a refrigerated electrochemical cell not only at its base in contact with the aluminum foil, but also in its walls in order to maintain stable temperature of the system at 0 ◦ C under mechanical stirring. A specific software was developed with Labview programming to allow for the tailoring of the shape of the HA pulse. In particular, we have considered square and exponential shaped pulses. In addition, it is also possible to tune the amplitude of the HA pulse, its length or period as well as the number of pulses. In table 1, the main parameters used for our experiments during the anodization process are collected. We have used three different templates. The first one corresponds to the typical anodization conditions in sulfuric acid electrolyte [29, 30] with straight nanochannels (sample label A25). The second and third templates correspond to pulsed anodization processes, with square (SQR) and exponential (EXP) profile pulses, respectively. The nanoporous anodic alumina film (NAAF) templates with modulated nanochannels have been grown with a ‘sandwich’ structure in order to improve their mechanical stability, as is schematically shown in figure 1(a). The MA and PA 2

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Figure 1. (a) Schematic diagram of the proposed system, ‘sandwich’ like architecture, composed of two MA sections holding the PA section. (b) Current density evolution caused by the corresponding applied voltage sequence to achieve the proposed structure.

Figure 2. Current density evolution and applied voltage signal for individual (a) square and (b) exponential pulse of voltage.

sections of the square pulses are shown in figure 1(b), similar to the sequence of exponential pulses (not shown here). Figure 2 shows the detail of current density and voltage evolution during a single pulse for squared and exponential signal of anodization voltage. After the anodization process, the aluminum substrate is etched and removed with an aqueous solution composed of 0.1 M of copper chloride (CuCl2 ) and 3 M of hydrochloric acid (HCl). Later, only the barrier layer side is exposed to the etching aqueous solution of H3 PO4 at 5 wt% in order to dissolve the alumina barrier layer. As the barrier layer is dissolved, the etching solution travels through the nanochannels, reaching a pH indicator paper placed on the open porous side. This process is repeated twice to ensure the complete removal of the barrier layer, so that the NAAFs become nanoporous anodic alumina membranes (NAAMs). This method significantly reduces the exposure time to the etching solution since it avoids immersion of the sample. Since the alumina generated during the HA pulses is thus etched faster than that during the MA pulses [18], the pores are slightly wider in the HA segments, while the widening in the MA segments is negligible. The initial NAAFs are thick enough to avoid mechanical breaking of the alumina during the etching. After the removal of the barrier layer, a 150 nm thick gold layer is deposited by sputtering before electrochemical deposition under potentiostatic conditions to fill the pores.

1.2. Potentiostatic electrodeposition of Co

The electrolyte containing the Co source consists of an aqueous solution composed by 0.9 M CoSO4 and 0.5 M H3 BO3 . The electrochemical deposition was carried out under potentiostatic conditions by applying −1.0 V versus Ag/AgCl reference electrode. The temperature of the electrolyte is maintained at 30 ◦ C with a slow stirring of 150 rpm in order to maintain homogeneous pH at 4.5 and temperature of the electrolyte during the electrochemical deposition process. Subsequently, the nanochannels are filled by electroplating, so that hexagonal arrays of Co nanowires are prepared characterized by periodic changes in diameter. 2. Results and discussion 2.1. Geometrical characteristics of modulated Co nanowires

The samples were characterized by SEM imaging (FEI Quanta 650 FEG) to confirm the geometry characteristics of the electrodeposited Co nanowires and particularly their expected modulation in diameter along the nanochannels. For comparison, figure 3 shows the SEM image of straight Co nanowires electrodeposited into the A25 type template. Figure 4 shows micrographs of the uniform electrodeposition of Co into the modulated section of the template with squared PA. The strips corresponding to the modulated 3

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Figure 4. SEM images of cross-sections of electrodeposited Co

nanowires inside NAAM with squared PA: (a) whole thickness of the sample, (b) strips formed by Co electrodeposited HA segments, and (c) detailed micrograph with modulation of Co nanowires. Figure 3. SEM image showing a cross-section of electrodeposited Co nanowires in the NAAF inside straight nanochannels (label A25).

segments during the HA pulses are brighter because of the larger amount of Co (i.e., larger diameter) in comparison to the MA segments. In figure 4(b) we also can observe that the segments do not have equal lengths. We attribute this fact to mechanical instabilities and plastic deformations on the aluminum substrate. Figure 4(c) shows the nanowire modulation in more detail: nanowires are narrower in the MA segments than in the HA segments. Small modulations in the HA section can be also seen in figure 4(c), which can be attributed to the strong increase of the current density. This effect was also mentioned in a previous report [12]. Micrographs of Co electroplated nanowires on nanochannels anodized with exponential HA pulses are presented in figure 5. The homogeneity of the Co electroplating is shown in figure 5(a) where the bright stripes formed by the Co filling the HA sections are also observed. The PA section is shown in figure 5(b). Note that the Co nanowires in the HA segments reproduce a similar shape to that shown in figures 4(b) and (c). After collecting the visual information from the SEM images, we conclude that the geometric features of the three nanowire arrays are as schematically shown in figure 6. The overall length of the electroplated nanowires ranges between 50 and 60 µm and the center to center interwire distance is kept constant at 66 ± 2 nm. The diameter of the nanowires within the MA sections is approximately 26 nm. In the exponential HA segments the modulation of the diameter of the nanowires varies from 30 nm in the narrower part up to 55 nm in the wider part, approximately. In the case of the squared HA segments, the nanowire diameters are 40–45 nm. The average length of the HA segments for the exponential and squared pulses are 0.65 ± 0.2 µm and 0.95 ± 0.3 µm, respectively. On the other hand, the average lengths of the MA segments within the PA

Figure 5. SEM cross-sections of electrodeposited Co nanowires

inside NAAM with exponential PA: (a) total thickness of the sample, and (b) PA section with magnification of two zones where the modulations come from the exponential HA pulses.

section are 0.38 µm for both samples, which fits quite well with the expected value for the MA process on conventional sulfuric acid electrolyte anodizations. 2.2. Structure determination of modulated Co nanowires

X-ray diffractograms of the electroplated Co nanowires are shown in figure 7. We observe similar XRD spectra for all three Co nanowire arrays with three peaks denoting an hcp polycrystalline structure of nanowires. The peak observed at 41.61◦ can be ascribed to the (110) phase, which should appear 4

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Figure 8. Orientation parameter for straight and modulated Co

nanowires. 2.3. Magnetic characterization of modulated Co nanowires

The magnetic properties of the Co nanowire arrays were measured in a vibrating sample magnetometer (EV7 VSM from LOT-Oriel Company) for a range of different configurations of the applied field between parallel and perpendicular orientation with regard to the nanowires axes. Their hysteresis loops are shown in figure 9. Parallel and perpendicular configurations of applied field are defined with respect to the nanowires axis. Uniform 25 nm diameter Co nanowires grown onto the A25 template are seen to exhibit an easy magnetization direction parallel to the nanowire axis as deduced from the high coercivity and relative high remanence of parallel loop in comparison to the very reduced values of the perpendicular configuration in figure 9(a). In the case of modulated nanowires, figures 9(b) and (c), the direction of the magnetic easy axis is not as clearly established as for homogeneous diameter nanowires. While higher remanence for the perpendicular configuration would denote an easy magnetization direction perpendicular to the nanowires, the coercivity remains higher for the parallel field configuration. That holds for both modulated configurations, particularly for the nanowires with square modulation (see figure 9(c)). Such evolution is seemingly ascribed to the oscillating anisotropy introduced by the modulated diameter of nanowires. A summary of coercivity and normalized remanence values for the different samples is presented in figure 10. The magnetic behavior suggests the presence of two competing magnetic phases having axial and perpendicular magnetic anisotropy eventually related to the MA and HA segments, respectively. A first interpretation would suggest a reduction of axial anisotropy in the HA segments due to change in shape and/or magneto-crystalline anisotropy. However, it is known that uniform nanowires having longer diameter, with aspect ratio comparable to the HA segments of the modulated nanowires, retain the magnetization-easy axis along the wire. Similarly, the magneto-crystalline anisotropy remains constant along the nanowire since the length of each segment is well above the minimum length necessary to observe any change in the crystalline phase of electroplated Co nanowires. The change of effective anisotropy observed in modulated nanowires should be more properly a consequence of the

Figure 6. Schematic representation of geometrical features of the

electroplated nanowires using different templates: (a) uniform diameter template; modulated diameter within (b) exponential and (c) squared HA pulses template. S1 = 40 nm; S2 = 35–30 nm; S3 = 10–5 nm; S4 = 25–20 nm.

Figure 7. X-ray diffraction spectra of electroplated Co nanowires into straight and modulated nanochannels.

at 41.55◦ . The peak which appears at 47.46◦ can be ascribed to the (101), which should appear at 47.41◦ . The second-order peak of (100), i.e. (200), is also present at 90.40◦ . From the analysis of the diffractograms we have obtained the orientation parameter for each sample, and these are collected in figure 8. The forming crystals of Co nanowires are clearly oriented in the (100) direction. Although the crystalline structure is the same and there is practically no difference in the XRD spectra for the three samples, the orientation parameter indicates a slight increase of the crystal orientation for the modulated nanowires. The straight nanowires have an orientation parameter of 0.80 for the (100) direction, while that parameter is higher than 0.90 for the samples prepared with exponential HA pulses, and it reaches almost 0.98 for the sample with squared HA pulses, i.e. almost monocrystalline nanowires. 5

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charges should accumulate at the borders with the shorter diameter MA segments. Such charges, acting as local effective magnetostatic field, promote magnetization rotation towards the perpendicular direction in the HA segments to reduce the stray field energy. That leads to increased perpendicular remanence and coercivity values. This scenario is further confirmed for the Co nanowire arrays electroplated inside the squared HA pulse template. Here the increased perpendicular remanence and coercivity is more noticeable as a consequence of a nearly doubled density of the mentioned border regions between MA and HA segments. In this sample, the fractional volume of wider segments (see figure 9(c)) (with some effective perpendicular anisotropy) is of about 45% of the total nanowire length, which is approximately 30% higher than for the exponential HA pulse sample template. From a magnetic point of view, this system can be thought of as an array of vertical pillars intercalated by virtually continuous horizontal planes. In short, the stronger local stray fields at the ends of higher diameter segments significantly increase local magnetostatic interactions between the segments of neighboring wires. In order to obtain complementary information on the magnetization process, the angular dependence of the magnetic hysteresis loops has been also measured. Figure 11 shows the angular dependence of parallel and perpendicular coercivity and remanence values for the three Co nanowire arrays. Homogeneous diameter Co nanowires show a clear uniaxial parallel anisotropy, as mentioned above, with continuously decreasing coercivity and normalized remanence as the angle increases. Here, magnetostatic interactions are relatively moderated, and the longitudinal magnetization process of such polycrystalline Co nanowires has been proposed to take place by nucleation of a vortex at the nanowire end and subsequent propagation of the domain wall along the wires [27, 28]. The angular evolution of coercivity and remanence are clearly different for the exponential and squared modulated nanowires. Particularly for squared modulated nanowires, the changes along the angular dependence are much more reduced, denoting the relevance of the magnetostatic interactions pointed out above. When approaching the perpendicular field configuration, remanent magnetization significantly increases and coercivity changes trend. Two different magnetization contribution processes are suggested for each field configuration arising from MA and HA segments. The complex magnetization behavior deduced from the angular magnetic measurements suggests the need for a deeper analysis.

Figure 9. Hysteresis loops under parallel (black) and perpendicular (red) magnetic field configurations for Co nanowire arrays grown with uniform pore diameter (a); and with exponential (b) and squared (c) HA pulse modulated nanowires.

modified magnetostatic coupling between the HA segments of neighboring parallel nanowires. Assuming a homogeneous axial magnetization inside each HA segment, local magnetic

Figure 10. Coercive field (a) and normalized remanent magnetization (b) for parallel and perpendicular applied magnetic field

configurations. 6

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Figure 11. Coercive field (a) and normalized remanent magnetization (b) for Co nanowire arrays with uniform diameter (A25) and with

exponential (EXP) and squared (SQR) modulated diameter nanowires.

3. Conclusions

[7] Sousa C T, Leitao D C, Proenca M P, Apolinario A, Correia J G, Ventura J and Araujo J P 2011 Nanotechnology 22 315602 [8] Pu L, Bao X M, Zou J P and Feng D 2001 Angew. Chem. Int. Edn 40 1490 [9] Mei Y F, Wu X L, Shao X F, Huang G S and Siu G G 2003 Phys. Lett. A 309 109 [10] Chu S Z, Wada K, Inoue S, Isogai M and Yasumori A 2005 Adv. Mater. 17 2115 [11] Zhao S Y, Chan K, Yelon A and Veres T 2007 Adv. Mater. 19 3004 [12] Lee W, Ji R, Gosele U and Nielsch K 2006 Nature Mater. 5 741 [13] Li Y B, Zheng M J and Ma L 2007 Appl. Phys. Lett. 91 073109 [14] Li Y, Ling Z Y, Chen S S, Hu X and He X H 2010 Chem. Commun. 46 309 [15] Schwirn K, Lee W, Hillebrand R, Steinhart M, Nielsch K and Gosele U 2008 ACS Nano 2 302 [16] Lee W, Kim J C and Gosele U 2010 Adv. Funct. Mater. 20 21 [17] Lee W, Scholz R and Gosele U 2008 Nano Lett. 8 2155 [18] Lee W, Schwirn K, Steinhart M, Pippel E, Scholz R and Gosele U 2008 Nature Nanotechnol. 3 234 [19] Lee W and Kim J C 2010 Nanotechnology 21 485304 [20] Losic D and Lillo M 2009 Small 5 1392 [21] Losic D 2009 Langmuir 25 5426 [22] Pitzschel K, Moreno J M, Escrig J, Albrecht O, Nielsch K and Bachmann J 2009 ACS Nano 3 3463 [23] Pitzschel K, Bachmann J, Martens S, Montero Moreno J M, Kimling J, Meier G, Escrig J, Nielsch K and Goerlitz D 2011 J. Appl. Phys. 109 073109 [24] Salem M S, Sergelius P, Corona R M, Escrig J, Gorlitz D and Nielsch K 2013 Nanoscale 5 3941 [25] Sulka G D and Hnida K 2012 Nanotechnology 23 075303 [26] Rotaru A, Lim J H, Lenormand D, Diaconu A, Wiley J B, Postolache P, Stancu A and Spinu L 2011 Phys. Rev. B 84 134431 [27] Vivas L G, Escrig J, Trabada D G, Badini-Confalonieri G A and Vazquez M 2012 Appl. Phys. Lett. 100 252405 [28] Vivas L G, Ivanov Y P, Trabada D G, Proenca M P, Chubykalo-Fesenko O and Vazquez M 2013 Nanotechnology 24 105703 [29] Masuda H, Hasegwa F and Ono S 1997 J. Electrochem. Soc. 144 L127 [30] Li A P, Muller F, Birner A, Nielsch K and Gosele U 1998 J. Appl. Phys. 84 6023

Magnetic properties are reported for uniform and modulated diameter Co nanowires electroplated under careful choice of mild and hard anodization pulses inside three different templates: nanowires with uniform diameter, and two, squared and exponential, of modulated diameter. It has been demonstrated that the hard anodization pulses during anodization play a decisive role in the morphology of the template, and hence in the morphology and magnetic properties of synthesized nanowires. SEM images show the different shapes of the nanowires and XRD their polycrystalline structure with hcp hexagonal symmetry. These features have a strong influence on the magnetic properties. Strong local stray fields distribute at the ends of wider segments of modulated nanowires that determine enhancement of magnetostatic interactions between neighboring nanowires. As a consequence, magnetization processes under parallel and perpendicular field configurations denote the contribution of both thin and wide segments. In contrast, straight nanowires show a single magnetization process with enhanced uniaxial longitudinal anisotropy. Acknowledgments

The support of the Ministerio de Economia y Competitividad of Spain under projects MAT2010-02798-C05-01 and C0503 is acknowledged. Ignacio M´ınguez Bacho thanks the Spanish National Research Council (CSIC) for a JAE-preDoc fellowship cofinanced by the European Social Fund. References [1] Magnin D, Callegari V, Matefi-Tempfli S, Matefi-Tempfli M, Glinel K, Jonas A M and Demoustier-Champagne S 2008 Biomacromolecules 9 2517 [2] Kou X M, Fan X, Dumas R K, Lu Q, Zhang Y P, Zhu H, Zhang X K, Liu K and Xiao J Q 2011 Adv. Mater. 23 1393 [3] Vila L, Darques M, Encinas A, Ebels U, George J M, Faini G, Thiaville A and Piraux L 2009 Phys. Rev. B 79 2055 [4] Bran J 2012 Mater. Today 15 351 [5] Proenca M P, Sousa C T, Ventura J, Vazquez M and Araujo J P 2012 Electrochim. Acta 72 215 [6] Nielsch K, Muller F, Li A P and Gosele U 2000 Adv. Mater. 12 582 7

Electrochemical synthesis and magnetic characterization of periodically modulated Co nanowires.

The synthesis of templates with modulated pore channels by combined mild and hard anodization processes is described. The hard anodization pulses, imp...
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