Novel magnetic materials prepared by electrodeposition techniques: arrays of nanowires and multi-layered microwires

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1 Journal of Alloys and Compounds 369 (2004) Novel magnetic materials prepared by electrodeposition techniques: arrays of nanowires and multi-layered microwires K.R. Pirota a,d.navas a, M. Hernández-Vélez a,b, K. Nielsch c, M. Vázquez a, a Instituto de Ciencia de Materiales, CSIC, Madrid, Spain b Applied Physics Department, Autónoma University of Madrid, Madrid, Spain c Materials Science & Engineering Department, Massachusetts Institute of Technology, Cambridge, MA , USA Abstract The fabrication process by electrodeposition routes and the study of general magnetic properties is reported for two types of nanostructured magnetic materials: (a) nickel-filled highly-ordered nanoporous alumina templates, and (b) electrodeposited Ni layers onto glass coated amorphous microwires. Arrays of Ni nanowires, about 30 nm in diameter and separated by about 100 nm, are obtained by electrodeposition into the pores of alumina membranes prepared by two-steps anodization process from highly pure aluminum substrates. Morphological studies have been performed by high resolution scanning electron microscopy (HRSEM). The study includes the optimization of preparation parameters and the magnetic characterization of the hexagonally arranged nanowire arrays, i.e. the influence of the pore diameter and the interwire distance on the coercivity of the whole nanowire array. On the other hand, multi-layered magnetic microwires have been prepared in the following sequence: a nanometric Au coat is first sputtered onto Pyrex coated FeSiB amorphous microwires followed by electrodeposition of a 500 nm thick Ni external cover. While in as-cast microwires the hysteresis loop is squared shaped (magnetic bistability), in the case of the multilayer microwire, a transverse magnetic anisotropy is induced when reducing the measuring temperature as a consequence of the stresses induced by the different thermal expansion coefficients of the various layers Elsevier B.V. All rights reserved. Keywords: Micro and nanowires; Electrodeposition techniques; Magnetic properties 1. Introduction The fabrication of nanoscale structures is attracting a great interest, due to its large potential for applications in single-electron devices [1], optical media [2], or high-density magnetic memory devices [3]. Disordered anodic alumina has been used as a template material for decades for various applications [4]. In 1995 Masuda et al. discovered that under certain fabrication condition 2D-polycrystalline alumina pore arrays with a monodisperse pore diameter can be obtained via self-assembly [5,6]. Highly-ordered nanoporous alumina is suitable and inexpensive template material for the large scale fabrication of magnetic nanowires arrays. In the past arrays of nanocolumns with a low aspect ration have been man- Corresponding author. Tel.: ; fax: address: mvazquez@icmm.csic.es (M. Vázquez). ufactured by e-beam and imprint lithography techniques [7,8]. These nanostructures are very promising for the application in high-density magnetic storage devices, with areal densities beyond the predicted superparamagnetic limit for conventional unpatterned longitudinal recording (< Gbits/m) [9]. This predicted limit is reduced for a nanopatterned perpendicular media, where each nanomagnet (e.g., nanowire) can store one bit of information [10]. Such an ideal magnetic medium would consist of ferromagnetics islands of nanometric dimensions arranged in an ordered 2D nanosized lattice. For example, an areal density of about Gbit/m 2 can be achieved by a hexagonally arranged array of nanomagnets with a lattice constant of about 50 nm. Thus the areal density of this patterned media can be, at least, one order of magnitude higher than that obtained in conventional longitudinal media. From the processing point of view, anodization of aluminium plates constitutes a much cheaper procedure for the synthesis of nanometric porous structures, with /$ see front matter 2003 Elsevier B.V. All rights reserved. doi: /j.jallcom

2 K.R. Pirota et al. / Journal of Alloys and Compounds 369 (2004) closed-packet cells and centered pores in a local hexagonal arrangement. Thus hexagonally ordered patterns of about micrometric size domains can be obtained [7] where optimum domain size can be tailored by suitable choosing of the anodization time. Subsequent filling of nanoporous is performed after surface preparation of the oxide barrier layer at pore bottom by electrodeposition technique of a variety of materials. In this way, arrays of magnetic Ni, Co, Fe nanowires can be fabricated. Our interest here has been to show in detail the fabrication procedure for the preparation of arrays of Ni nanowires with given geometrical characteristics and their corresponding general magnetic characteristics. On the other hand, an electrodeposition technique has been also employed to fabricate multi-layered magnetic microwires starting from amorphous glass covered microwires. These amorphous microwires consist of a structurally amorphous ferromagnetic metallic nucleus (with typical alloy composition FeSiB) a few micron in diameter coated by a Pyrex-like insulating coating a few micron thick. They are prepared by quenching and drawing technique, and have recently attracted a great interest owing to their applications in various magnetic sensor devices [11,12]. This is a consequence of their outstanding magnetic characteristics as for example magnetic bistable behavior (exhibited by magnetiostrictive Fe base microwires), or giant magnetoimpedance effect (non-magnetostrictive Co base microwires). Other advantages of these systems are their small dimensions and reliability on electrical, mechanical and corrosive external effects due to the Pyrex coating layer. In the last few years, systematic studies have been reported on these materials [13,14]. In this work, our aim has been to show the important influence on the magnetic properties when external coatings are added to the insulating cover of the microwires. Thus, we report on the fabrication and magnetic characterization of novel magnetic multilayer microwires [15]. In this case, we focus our study on additional Ni cover electroplated onto FeSiB glass-coated microwire after suitable surface preparation. 2. Experimental 2.1. Preparation of Ni nanowires arrays In order to fabricate the magnetic nanowire arrays, nanoporous alumina membranes with hexagonal ordering have been prepared by a two-step anodisation process [5]. Aluminum foils with high purity (99.999%, from Goodfellows) have been used as starting material. The Al foils were degreased and electropolished in a mixture of perchloric acid (HClO 4 ) and ethanol C 2 H 5 OH by applying a constant voltage of 20 V and magnetic stirring in order to reduce the surface roughness, before it was placed in an anodisation cell. The anodization processes were done inside the an- Fig. 1. (a) Aluminium substrate; (b) first anodization; (c) aluminium template; (d) second anodization; (e) widening and thinning the barrier layer; (f) magnetic metal filling. odization cell by applying a constant voltage of 40 V and using an oxalic 0.3 M acid solution. The anodization was carried out at 2 C to prevent attacks of the electrolyte on the pore structure. A typical scheme of the samples preparation is shown in Fig. 1, while the typical current time behavior during both anodization processes is shown in Fig. 2. Before proceeding with the second anodization step, the porous alumina was removed by a wet chemical etching (phosporic and chromic acids solution). Subsequently, the pore channels were widened by an isotropic chemical etching procedure. The thickness of the alumina barrier layer (see scheme in Fig. 1) has been reduced from about 50 nm thickness after the second anodisation process and 30 nm thickness after the widening process down to less than 8 nm. Magnetic metals electrodeposition can be done by different modes as: constant current pulses, constant voltage pulses, current-voltage mixture pulses [17], and by alternating pulses [18]. In the present work Ni nanowires were grown inside alumina nanoporous by using constant current pulses in the following sequence: (i) 25 ma pulse for 10 ms, (ii) +5 ma pulse for 10 ms, and (iii) 0 ma for 1 s. Fig. 3 shows the voltage evolution whereas the negative current pulse was applied. In this figure three zones can be observed in which the fully filling of the nanopores is reached at the third zone. The electrolyte of Watts bath [16] used for the electrodeposition was: 300 g/l

3 20 K.R. Pirota et al. / Journal of Alloys and Compounds 369 (2004) x10-2 Current (A) 1.0x10-2 (a) Time (s) 1.0x x10-3 Current (A) 6.0x x x10-3 (b) Time (s) Fig. 2. Current time curve for the first (a) and second (b) anodization processes, at constant potential of 40 V. NiSO 4 6H 2 O, 45 g/l NiCl 2 6H2O, 45 g/l H 3 BO 3. The temperature and ph solution values were kept at 35 C and 4.5, respectively Preparation of multilayer microwires Depending on the metallic alloy composition the microwires exhibit large (Fe base) or vanishing (CoFe base) magnetostriction constant which determines the easy magnetization direction from magnetoelastic origin. Magnetostriction couples with the strong internal stresses arising mainly from the different thermal expansion coefficients of nucleus and coating so, giving rise to very noticeably magnetoelastic anisotropies that determines the easy magnetization direction. This easy direction is parallel to the longitudinal axis or circumferential for positive and negative magnetostriction respectively. Consequently, tailoring of the alloy composition determines firstly the magnetic behavior of the microwires. In the present work, only Fe 72.5 Si 12.5 B 15 microwires have been considered. An original technique has been developed to tailor the magnetic behavior by coating the microwires with additional layers [15]. A traditional sputtering system was used for depositing gold coatings onto the Pyrex cover of microwires

4 K.R. Pirota et al. / Journal of Alloys and Compounds 369 (2004) Voltage (V) 10 8 Zone 1ª Zone 2ª Zone 3ª Counts Fig. 3. Voltage absolute value measured during the negative current pulse of the electrodeposition process. (other metals are also possible to be sputtered). The thickness of these thin metallic coatings can be controlled throughout by the argon pressure within the sputtering chamber as well as through the deposition time (see Fig. 4a). Then, the sputtered Au layer is used as a substrate for a subsequent electrodeposition process. In this work, the sputtered coating thickness was limited to 30 nm working only as a conductive film to make possible the wire electroplating. Afterwards, an electrodeposition process was carried out by using a small electrochemical cell designed in our laboratory, consisting of a cylindrical tube 2 cm in diameter with one closed end and circular platinum electrode. Fig. 4b represents a schematic view of the multilayer microwire after the electrodeposition. In the present case, electroplated material is Ni, although other kinds of metals magnetic or not can be electroplated as well Structural and magnetic characterization A first structural characterization of the sample was first performed by scanning electron microscopy (SEM), and high resolution scanning electron microscopy (HRSEM), specially for the arrays of Ni nanowires. A superconducting quantum interference device magnetometer, SQUID, with low-field extra sensitivity has been used for the magnetic characterization for both wire species. In the case of Ni nanowires grown in alumina membranes, more attention was paid to room temperature hysteresis loops, while for multilayer microwires an experimental study is shown as a function of measuring temperature (in the range of K) with an emphasis in the changes of the loop shape, and magnetic parameters as coercivity and anisotropy field with the electroplated material. Fig. 4. (a) Schematic view of the multi-layered microwire. (b) Growing rate as a function of the plasma current for different sputered metals.

5 22 K.R. Pirota et al. / Journal of Alloys and Compounds 369 (2004) Results and discussion 3.1. Ni nanowires arrays The morphological characterization of the alumina membranes prior the electrodeposition processes is given in Fig. 5a and b, showing HRSEM and SEM images of the membrane surfaces after 3 and 24 h first anodization process, respectively. Some defects points are observed probably due to contamination and stressing effects. Also, Fig. 5. (a) Top view HRSEM image of the alumina membrane after 3 h first anodization process. (b) Alumina membrane top view SEM image after 24 h of first anodization and 2 h second anodization processes. (c) Back side SEM image of the alumina membrane with 24 h first anodization time after being removed from the aluminium substrate. a different ordering degree in those samples is obtained: longer anodization times improves the 2D-arrrangment of the nanopores. Moreover, a noticeable pore deformation is detected for short duration of the first anodization process. Particularly, the formation of ordered poly-domain regions can be identified in the case of Fig. 5b. Fig. 5c shows the back side SEM image of the membrane after 24 h of anodization after its detachment from the aluminum substrate. In this figure, the poly-domain formation is easier to identify, as well as, the regular formation of crests and valleys which constitute a negative pattern from that formed on the aluminum foils just prior the second anodization process. Consequently, the surface topographic predetermines the nanoporous alumina order degree obtained during the second anodization process. In fact, after a 2 h second anodization time, straight and ordered nanopore arrays have been obtained with membrane thickness of around 2 m. The process of the nanopores growth starts with the formation of the barrier layer, which finally make up the bottom side of the nanopores correspondingly to the minimum value of the current time curve in Fig. 2. Later on, the oxide dissolution and formation rates reach an equilibrium condition at the barrier layer, the current and thickness of the barrier layer remaining at stable values. It is known that under those conditions, the current is approximately proportional to the anodization voltage and the interpore distances (2.6 nm/v), and the layer thickness (1.3 nm/v) are proportional to the applied voltage [17]. In this regard, from the images presented in this work, it is confirmed that long first anodization time considerably improves the regularity and homogeneity of the subsequent nanostructured ordering. Fig. 6a and b show the surfaces of the samples after Ni electrodeposition. Note that a relatively wide size distribution of the pore diameters is detected, along with some zones where Ni clusters in thick and isolated islands have been grown on the external surfaces of the alumina template. These facts arise from an uncontrolled nanopore filling during the Ni electrodeposition process giving rise to the nanopores wrapping. That drawback can be overcome by bombarding the samples using a vacuum chamber with an Argon ion beam at Pa and devastating rate of 135 nm/min. During the electrodeposition method, the magnetic metal (Ni 2+ ) is deposited in the nanopores during the negative current pulse. The voltage increased further due to the potential of the barrier layer, and the metal ions concentration decreases near the deposition interface (see Fig. 3). The voltage is first fixed at 5 V and then quickly increased to 9 V. In Fig. 5, first zone corresponds to the deposition of Ni at the bottom of the pores. In the second zone located from 9 to 12.5 V corresponds to the deposition along the length of the nanopores. Note the smaller slope in this zone. The noisily aspect of the curve in third zone is ascribed to the filling of nanopores with Ni reach-electrolyte starting at that point. This is ascribed to the mentioned wrapping effect of the pores.

6 K.R. Pirota et al. / Journal of Alloys and Compounds 369 (2004) Fig. 6. (a) Top view HRSEM image of the Ni filled nanoporous alumina. First anodization time was 24 h. (b) Top view HRSEM image of the Ni filled nanoporous alumina. First anodization time was 72 h. The influence of the fist anodization time on the magnetic properties of arrays of Ni nanowires is shown in Fig. 7 where results correspond to 24 and 72 h time of first anodization. Magnetic field was applied parallel to the longitudinal axis of the nanowires, 30 nm in diameter, with hexagonal arrangement being 100 nm the distance among them. From the different values of saturation magnetic moments, the nanowires lengths were determined to be 0.7 and 1.6 m for the samples with 24 and 72 h of first anodization, respectively. A clear difference is observed in the two hysteresis loops, which is firstly ascribed to the ordering degree of the nanowires array. Also, coercivity takes a larger value, 960 Oe, for the sample with larger first anodization time in comparison with 720 Oe for the other sample. This increase in coercivity is correlated with the magnetic interactions among nanowires [19] Multi-layered microwires The hysteresis loops of an as-cast Pyrex-coated amorphous Fe 72.5 Si 12.5 B 15 microwire is shown in Fig. 8a, while Fig. 8b shows the hysteresis loop after sputtering a 10 nm thick Au cover plus an additional 500 nm thick Ni layer. Diameter of metallic nucleus was 21.4 m and the total

7 24 K.R. Pirota et al. / Journal of Alloys and Compounds 369 (2004) x10 0 Nomalized Magnetization (emu) 5.0x x x10 0 (a) -2x10 3-1x x10 3 2x10 3 Magnetic Field (Oe) 1.0x10 0 Normalized Magnetization (emu) 5.0x x x10 0 (b) -3x10 3-2x10 3-1x x10 3 2x10 3 3x10 3 Magnetic Field (Oe) Fig. 7. (a) Hysteresis loop of the array of Ni nanowires for a membrane anodised for 24 h. (b) Hysteresis loop of the array of Ni nanowires for a membrane anodised for 72 h. diameter (before Ni electroplating) was 32.0 m. Results are obtained in the temperature range from 5 to 300 K. The difference is obvious. Firstly, as-cast microwire shows square-shaped hysteresis loops typical of bistable magnetic behavior [11] in the whole temperature range. As previously reported, it is a consequence of the remagnetization process by a single very large Barkhausen jump caused by the depinning of a domain wall from one end of the microwire [20]. The temperature dependence of coercivity is plotted as a function of temperature in the inset of Fig. 8a. Asobserved, a nearly linear behavior is obtained which has been analysed elsewhere [20] to be a consequence of the temperature dependence of different magnetic energy terms (mainly domain wall energy which is a function of stray fields and magnetoelastic energy contributions). Fig. 8b shows the temperature dependence of hysteresis loops for the same microwire after electrodeposition of a 500 nm thick Ni layer, as explained above. Here, it is astonishing to notice the quite different shape of the loops. It is clear that the Ni layer induces a transverse anisotropy resulting in nearly non-hysteretic loops with defined transverse anisotropy field which increases with the temperature

8 K.R. Pirota et al. / Journal of Alloys and Compounds 369 (2004) M (emu) K 50K 100K 150K 200K 250K 300K H C (Oe) Temperature (K) (a) Magnetic Field (Oe) M (emu) T= 5K 50 K 100 K 150 K 200 K 250 K 300 K (b) H (Oe) Fig. 8. (a) Hysteresis loop of an as-cast glass-coated amorphous FeSiB microwire at different temperatures. Inset shows the evolution of coercivity with measuring temperature. (b) Temperature dependence of hysteresis loops for a multi-layered microwire (after 30 nm Au sputtered coat plus 500 nm Ni electroplated layer). also nearly linearly. This linear dependence seems to be correlated with the compressive tensile stresses induced by the Ni layer as a consequence of the different thermal expansion coefficients of the layers as has been analyzed recently [21]. Anisotropy fields reach values of the order of 500 Oe which corresponds to a maximum equivalent stress of about 800 MPa. Note finally that the saturation magnetic moment of the multilayer microwire is larger than for the as-cast microwire which can be ascribed precisely to the electrodeposited Ni layer. 4. Conclusions The fabrication of Ni nanowires electrodeposited onto nanoporous alumina membranes previously prepared by using a two-step anodization method is reported. The self-organised nanoporous play the role of templates where Ni nanowires arrays are afterwards electrodeposited. In this way, hexagonally ordered arrangements of nanowires are obatined with nearly 30 nm diameter and 100 nm separation among them.

9 26 K.R. Pirota et al. / Journal of Alloys and Compounds 369 (2004) Particularly, the effect of the ordering degree achieved on the templates depending on the first anodization time has been proved to be crucial to determine magnetic properties of the array as for example the coercivity. Changes observed in coercivity depending on the first anodization time seem to be correlated with the degree of achieved final ordering of the array, and with the corresponding dipolar magnetic interactions. On the other hand, a different electrodeposition technique has been employed to grow Ni layer (around 500 nm thick) onto a tiny Au layer previously sputtered onto the as-cast Pyrex-coated FeSiB amorphous microwires. The noticeable influence of this Ni layer on the temperature dependence of hysteresis loops has been clearly confirmed. In fact, this has been ascribed to the internal stresses induced in the amorphous nucleus when reducing the temperature as a consequence of the different thermal expansion coefficients of the different layers. In summary, we believe that both electrodeposited magnetic materials open a clear oportunity to be used as sensing elements in various sensor families devices. Acknowledgements Authors wish to thank Prof. J.L. Martínez from Instituto de Ciencia de Materiales de Madrid, CSIC, for magnetic measurements, and to Dr. B. García from Applied Physics Department at Autónoma University of Madrid, for his valuable help for recording the HRSEM and SEM images of the samples. References [1] A.A. Tager, J.M. Xu, M. Moskovits, Phys. Rev. B 33 (1997) [2] E. Wäckelgard, J. Phys. Cond. Matter 8 (1996) [3] F. Li, R.M. Metzger, W.D. Doyle, IEEE Trans. Magn. 33 (1997) [4] P. Czokan, in: M.G. Fontana, R.W. Staehle (Eds.), Advances in Corrosion Science and Technology, vol. 7, Plenum Press, New York, 1980, p [5] H. Masuda, K. Fukuda, Science 268 (1995) [6] A.P. Li, F. Müller, A. Birner, K. Nielsch, U. Gösele, Adv. Mater. 11 (1999) 6. [7] J.F. Smyth, S. Cali, T.R. Kohler, J. Appl. Phys. 69 (1991) [8] S.Y. Chou, P.R. Krauss, L. Kong, J. Appl. Phys. 79 (1996) [9] S. Charap, P.L. Lu, Y. He, IEEE Tran. Magn. 33 (1997) 978. [10] K. Nielsch, R.B. Wehrspohn, S.F. Fischer, H. Kronmüller, J. Barthel, J. Kirschner, U. Gösele, Mat. Res. Soc. 636 (2001). [11] M. Vázquez, A. Zhukov, J. Magn. Magn. Mater. 160 (1996) 223. [12] H. Chiriac, G. Pop, A.T. Ovari, Phys. Rev. B 52 (1995) [13] H. Chiriac, R.A. Ovari, Prog. Mater. Sci. 40 (1997) 333. [14] A. Zhukov, J. González, J.M. Blanco, M. Vázquez, V.S. Larín, J. Mater. Res. 15 (2000) [15] K. Pirota, D. Navas, M. Hernández-Vélez, M. Vázquez, Adv. Funct. Mater., in press. [16] O.P. Watts, Trans. Am. Electrochem. Soc. 29 (1916) 395. [17] K. Nielsch, F. Müller, A.P. Li, U. Gösele, Adv. Mater. 12 (2000) 582. [18] R.M. Metzger, V.V. Konovalov, M. Sum, T. Xu, G. Zangari, B. Xu, M. Benakli, W.D. Doyle, IEEE Trans. Magn. 36 (2000) 1. [19] M. Vázquez, K. Nielsch, P. Vargas, J. Velázquez, D. Navas, K. Pirota, E. Vogel, J. Cartes, R. Whersphon, U. Gösele, Physica B, in press. [20] M. Vázquez, A.P. Zhukov, K.L. García, K.R. Pirota, R. Varga, M. Hernández-Vélez, J.L. Martínez, J. Non-crystalline Solids, in press. [21] M. Vázquez, A.P. Zhukov, K.R. Pirota, R. Varga, K.L. García, J.L. Martínez, J. Mater. Sci. Eng., in press.