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Comparative study in fabrication and magnetic properties of FeNi alloy nanowires and nanotubes
Journal of Magnetism and Magnetic Materials 331 (2013) 162–167
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Comparative study in fabrication and magnetic properties of FeNi alloy nanowires and nanotubes Xiuli Zhang a,c, Huimin Zhang a,b,c, Tianshan Wu a,c, Ziyue Li a,c, Zhijun Zhang a,c, Huiyuan Sun a,c,n a
College of Physics Science and Information Engineering, Hebei Normal University, Shijiazhuang 050024, China College of Materials Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050016, China c Key Laboratory of Advanced Films of Hebei Province, No. 20 Road East of 2nd Ring South, Yuhua District, Shijiazhuang, Hebei 050024, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 August 2012 Received in revised form 25 September 2012 Available online 23 November 2012
FeNi alloy nanowires and nanotubes have been fabricated in the pores of the anodized aluminum oxide (AAO) templates by direct current electrodeposition with different potentials. The crystal structure and micrograph of FeNi alloy nanowires and nanotubes were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, and the content was confirmed by energy dispersive spectrometry. The results indicate that there is a critical potential in the formation of nanowires and nanotubes. FeNi alloy nanowires and nanotubes both present the (111) preferred orientation, and the content of Fe is reduced with the decrease of electrodeposition potentials. The magnetic behaviors of FeNi alloy nanowires and nanotubes with different ratios are investigated. The coercivity of FeNi alloy nanowires and nanotubes exhibits disparate behavior, which may attribute to their grain size and distinctive structure. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: FeNi alloy Nanowire Nanotube Electrodeposition Anodized aluminum oxide Magnetization reversal mode
1. Introduction In last few decades, nanostructures have been attracting considerable attention for their fantastic properties of surface effect, small size effect, quantum effect and macro-quantum tunnel effect [1,2]. Nanowires and nanotubes, especially ferromagnetic nanowires and nanotubes raise special interest due to their fundamental importance and potential applications in sensors, spintronics devices, magnetic recording media, magnetic logic, high-density magnetic random access memory, biological transportation, etc. [3–11]. In order to synthesize nanowires and nanotubes, a variety of techniques have been developed, such as vapor-based growth techniques, arc process, the supercritical fluid method, physical sputtering, self-assembly route, the sol– gel method, template-based electrodeposition for nanowires and thermal decomposition of precursors, hydrothermal synthesis, plasma enhanced chemical vapor deposition, atomic layer deposition, chemical reduction, and template-based electrodeposition for nanotubes [12–18]. Among these techniques, anodic aluminum oxide (AAO) template-based electrodeposition is widely adopted not only because it has preferable insulativity, chemical
n Corresponding author at: College of Physics Science and Information Engineering, Hebei Normal University, Shijiazhuang 050024, China. Tel.: þ86 133 638 15166; fax: þ 86 311 807 87300. E-mail address: [email protected] (H. Sun).
and heat stability but also because it is convenient, versatile and inexpensive. FeNi alloy which is usually called permalloy shows a series of physical characteristics with the variation of ratios [19–21], and it is one of the universal soft magnetic materials which has an extensive application. Till date, both FeNi alloy nanowires and nanotubes have been studied widely [22,23]; however, the comparative study in fabrication and magnetic reversal process which is important for the magnetic properties have not been intensively discussed. In this paper, we successfully fabricated FeNi alloy nanowires and nanotubes in AAO template by dc electrodeposition. The morphology, composition, and magnetic properties of FeNi alloy nanowires and nanotubes are also investigated.
2. Experimental section Highly ordered anodized aluminum oxide (AAO) templates were prepared by a two step anodization from 99.999% aluminum foil. Firstly, Al foils were annealed at 400 1C for 2 h in an argon atmosphere, and then were electropolished in a mixture of HClO4:C2H5OH¼174 (V/V) for 5 min. Secondly, the prepared Al foils were anodized in 0.3 M oxalic acid at a constant potential of 45 V and 5 1C for 6 h. The resulting alumina film was chemically removed in a mixed solution of phosphoric acid (6 wt%) and chromic acid (4 wt%) at 30 1C for 12 h. Subsequently, the samples were re-anodized for 12 h under the same conditions as those in the first step. Finally, the remaining Al substrate was removed by
0304-8853/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.11.033
X. Zhang et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 162–167
saturated CuCl2 solution, after that the template was dipped into phosphoric acid (6 wt%) for 3 h to remove barrier layer, at last the template was washed with deionized water in ultrasonic cleaner. The thickness and the pore diameter of obtained templates are 45 mm and 70 nm respectively. Electrodeposition was performed in a three-electrode cell under constant potential at room temperature. A very thin Cu membrane was deposited by sputtering on one side of the template serving as the working electrode, a saturated calomel electrode (SCE) as the reference, and graphite pole as the counter electrode. FeNi alloy nanowires and nanotubes with different ratios were fabricated under different potentials as follows: 0.8 V, 1.0 V, 1.2 V in electrolyte composed of 15 g/L FeSO4 7 H2O, 225 g/L NiSO4 6 H2O, 45 g/L H3BO3, 3 mg/L C6H8O6 for nanowires; 1.25 V, 1.75 V, 2.50 V in electrolyte composed of 6 g/L FeSO4 7 H2O, 26 g/L NiSO4 6 H2O, 25 g/L H3BO3, and 3 mg/L C6H8O6 for nanotubes. The pH value of electrolytes was adjusted to 3.0 by dilute sulfuric acid. All the samples were immersed in 4 M NaOH aqueous solution before SEM and TEM observations to dissolve the alumina layer. The morphologies of the nanowires and nanotubes were characterized by scanning electron microscopy (SEM, Hitachi S-4800). The structural analysis of the ferromagnetic nanowires and nanotubes was obtained from transmission electron microscopy (TEM, Hitachi
163
H-7650) and X-ray diffraction (XRD, PANalytical X’pert PRO MPD) with Cu Ka radiation. The compositions were analyzed by energy dispersive spectrometry (EDS). The magnetic properties of the samples were tested by a vibrating sample magnetometer (VSM) of the Physical Property Measurement System (PPMS, Quantum design Model 6000).
3. Results and discussion Fig. 1(a) and (b) shows SEM images of the surface and cross section of a typical AAO template which was prepared by twostep anodization in 0.3 M oxalic acid, it can be seen that the pores are uniform and the diameter is about 70 nm, and the inner wall of the pores are smooth, none branch is observed. The SEM images of FeNi alloy nanowires and nanotubes are exhibited in Fig. 1(c) and (e). It is obvious that nanowires and nanotubes have uniform diameter which is in accordance with the AAO pore size. Fig. 1(d) and (f) is corresponding TEM image of nanowires and nanotubes, TEM images indicate that the outer diameter (d) of nanowires and nanotubes both are around 70 nm which is consistent with SEM images of Fig. 1(a) and (b), and the wall thickness of the nanotubes is about 15 nm. In addition, it can be
Fig. 1. SEM images for the (a) surface and (b) cross section of AAO template, (c) FeNi alloy nanotubes and (e) nanowires; TEM images for FeNi alloy (d) nanotubes and (f) nanowires.
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X. Zhang et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 162–167
concluded from Fig. 1 that there exists a critical potential in the formation of nanowires and nanotubes. Nanotubes can be obtained when the potential is equal to or greater than the critical value, on the contrary nanowires can be formed. The detailed influence factors need to be researched further. The typical XRD patterns of FeNi alloy nanowires and nanotubes electrodeposited under different potentials are presented in Fig. 2(a) and (b). Though deposited under different potentials, the structure of nanowires is the same. Compared with standard patterns, the nanowires have face-center-cubic (fcc) structure, and present the (111) preferred orientation characteristics. The crystal structure is the same in case of nanotubes, but when the potential decreases to 2.50 V, the sample exhibits poor crystallization as shown in Fig. 2(b). According to the Scherrer Equation, the size of particles can be calculated: D¼
lk Bcos y
ð1Þ
where l is the wavelength of X-ray, k is the Scherrer constant ( ¼0.89), B is the full width half maximum which can be found from XRD pattern, y is the angle of diffraction. The grain sizes of FeNi alloy nanowires which were electrodeposited under different potentials of 0.8 V, 1.0 V, and 1.2 V are 10.6 nm, 20.8 nm and 101.6 nm, respectively. Whereas the grain sizes of nanotubes electrodeposited under 1.25 V, and 1.75 V are
Fig. 2. XRD patterns of as-prepared FeNi alloy (a) nanowires and (b) nanotubes embedded in AAO template.
17.3 nm, and 15.8 nm (the nanotubes prepared under 2.50 V cannot be calculated because of its bad crystallization), respectively. It is obvious that the grain sizes of nanowires increase with the decrease of potentials, however the case in nanotubes is opposite, that is, as there is decrease of potentials the grain sizes also decrease. As is well-known that nucleation is a very important process in metal deposition, the competition between growth and nucleation determines the granularity of the deposit [24]. The higher the nucleation rate during deposition, the finer are the crystal grains of the deposit. Consequently, we suppose that in the process of electrodepositing nanowires the concentration of electrolyte and the potential are higher so that the rate of nucleation is lower than the rate of growth, and the crystallization is better; however, in the case of nanotubes, the concentration of electrolyte and the potential are higher, which result in the low nucleation rate than the growth rate, leading to poorer crystallization in nanotubes. The EDS spectra of as-prepared nanowires and nanotubes are given in Fig. 3(a) and (b), respectively. Both spectra indicate that the nanowires and nanotubes indeed consist of Fe and Ni. The Al and O peaks arise from the small amount of residual alumina after the sample was rinsed. In conclusion, FeNi alloy nanowires and nanotubes can be successfully fabricated by dc electrodeposition. The EDS spectrum could not only confirm the components but also can give the relative content for different elements. As shown in Table 1, the Ni and Fe content are different with the change of potentials. From Table 1, it is obvious that compared with the ratios of the metal ion (Ni2 þ : Fe2 þ ) in corresponding electrolytes, the proportions of elements in nanowires and nanotubes are much bigger, this may due to the anomalous codeposition of iron and nickel during the electrodeposition [25]. Although the standard electrode potential of iron (0.44 V) is more negative than nickel ( 0.25 V), the iron deposition occurs preferentially so that the content of iron in nanowires and nanotubes is much higher than that in electrolyte. The cause of anomalous codeposition may attribute to the formation of hydrogen in the process of electrodeposition, which can lead to a rise of the electrolyte pH value,
Fig. 3. EDS spectra of FeNi alloy (a) nanowires and (b) nanotubes.
X. Zhang et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 162–167
hydroxide is generated by Fe2 þ and OH , as a result Fe2 þ electrodeposit prior to Ni2 þ , meanwhile the deposition of Ni2 þ is suppressed, which results in the lower deposition speed of Ni2 þ . As there is decrease of deposition voltage, the content of Fe in nanowires and nanotubes decreases gradually, which is consistent with the literature [21]. The main reason is when deposition voltage decreases, the cathodic current density gets bigger and the reduction reaction of Fe2 þ which is controlled by diffusion rate of ions will be restrained thus the content of iron in nanowires and nanotubes also decreases.
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Fig. 4(a) and (b) shows the magnetic hysteresis loops at room temperature for FeNi alloy nanowires and nanotubes. The nanowires and nanotubes which is located inside the AAO were measured in order to take advantage of the orientation of them and it should be noted that the AAO templates have no influence on their magnetic performances. The symbols O and?represent the applied magnetic field is parallel and perpendicular to the nanowires and nanotubes axis, respectively. Table 1 also shows the magnetic properties of obtained nanowires and nanotubes, it proves that squarenesses (Mr/Ms) are
Table 1 The ratios and magnetic parameters, coercivity (Hc), and remanent squareness (SQ) of FeNi alloy nanowires and nanotubes electrodeposited under different potentials in AAO with the field applied parallel (T) and perpendicular (?) to the axis. Products
Potential (V)
Fe (at.) Ni (at.)
HcT (Oe)
Hc? (Oe)
SQO
SQ?
Nanowires Nanowires Nanowires Nanotubes Nanotubes Nanotubes
0.8 1.0 1.2 1.25 1.75 2.50
32:68 15:85 10:90 28:72 14:86 10:90
280 731 456 486 458 438
55 88 130 25 31 35
0.075 0.614 0.197 0.384 0.319 0.328
0.050 0.033 0.043 0.0065 0.0108 0.0165
Fig. 4. Magnetic hysteresis loops of FeNi alloy (a) nanowires and (b) nanotubes with different ratios; the red dots denote field applied parallel to axis and the black dots denote field applied perpendicular to axis.
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changed with the ratios of FeNi alloy for both of the nanowires and nanotubes, and the remanent squarenesses (SQ) in the parallel direction are higher than that in the perpendicular direction, which suggests that the FeNi alloy nanowires and nanotubes have superior parallel magnetic characteristics. Because the shape anisotropy of our nanowires and nanotubes (aspect of 100) is much higher than magnetocrystalline anisotropy [26], we suppose that the shape anisotropy is predominant here and the easy axis of shape anisotropy is oriented along the axis of nanowires and nanotubes. Because the pore density of the AAO template is about 1010 cm 2, it is interesting to analyze magnetization reversal processes for the aspects of high-density magnetic recording. Generally, there are two kinds of magnetization reversal modes which can be modeled by coherent rotation and curling. For magnetic nanowires and nanotubes, the magnetization reversal mechanism depends upon the outer diameter of the nanocylinders. For a specific material, there exists a critical diameter dc for the transition from coherent rotation to curling. The critical diameter dc is given by [16] ! A1=2 dc ¼ 2:08 ð2Þ Ms where A is the exchange stiffness (about 10 6 erg/cm for Fe, Ni and FeNi alloy) and Ms is the saturation magnetization. For nanowires and nanotubes with diameters larger than the critical diameter, the magnetization reversal process can be described by the curling mode. The curling mode of magnetization reversal in an infinite cylinder predicts [27] að1 þ aÞ Hc ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Hk ð3Þ a2 þ ð1 þ 2aÞcos2 y 2 where a ¼ 1:08 dc =d and y is the angle between the nanotubes and the field. Thus the Hc in the parallel direction can be calculated. Comparing the Hc in experiment and in theory from Table 2, it is found that the results of curling mode consist with nanotubes perfectly, but there is slight difference with nanowires, and only the sample deposited under 1.2 V coincides with theoretical value. It is easy to discover that the grain size of nanowires deposited under 1.2 V (which is 101.6 nm) is larger than the diameter of AAO template, therefore, because of the restrain of nanopores, the magnetic moment in the granules may present a vortex distribution, and can be described by the curling mode, but for the nanowires deposited under 0.8 V and 1.0 V, the grain sizes (which are 10.6 nm and 20.8 nm) are much smaller in comparison with the AAO template diameter, they may have several particles of the same height of a nanowire, so their magnetization reversal modes are dissimilar with curling. For nanotubes electrodeposited under different potentials, the grain sizes are approximate to each other and similar to the wall thickness, it is conjectured that the particles distribute as a spherical ring of the same height of single nanotube, as a result, the magnetization reversal mode of nanotubes agrees with the curling mode.
Fig. 5. The angle dependence of coercivity of nanotubes prepared under 1.25 V.
In order to confirm the conclusion further, we measured the dependence of coercivity with different angles. Because all the nanotubes and the nanowires fabricated under 1.2 V coincided with curling mode and the tendencies are alike, we just present the figure of nanotubes prepared under 1.25 V, as shown in Fig. 5. It is clear that there is a very good agreement when the field orientation y r 7751, for field applied along the hard axis, a minimum is obtained which is opposite to the theoretical prediction of maximum coercive field, which is consistent with the results reported by Han et al. [16]. In ferromagnet, multidomain structure will be formed if the particle is bigger. With the decrease of the particles, the magnetic domain will be reduced in order to diminish the domain wall energy. Once the critical size is achieved, the particle will become single domain, but if the particle size is less than the critical size, the samples will exhibit superparamagnetism. The single domain system possesses much higher coercive force. For spherical particles, the critical sizes for Fe, Co, Ni are 17 nm, 11.4 nm, 39 nm, respectively. Compared with the critical size above, it can be learned that the sample of nanowires prepared with 1.0 V (Fe15Ni85, 20.8 nm) is the closest to the critical size of Ni, and its coercive force is much higher than two other samples indeed, the sample electrodeposited under 0.8 V (Fe32Ni68, 10.6 nm) appear superparamagnetic because of its smaller grain size and has lower coercive force, while due to the multidomain structure, the coercive force of nanowires electrodeposited under 1.2 V (Fe10Ni90, 101.6 nm) is smaller than Fe15Ni85 but larger than Fe32Ni68. Whereas the grain size of nanotubes are close to each other (Fe28Ni72 and Fe14Ni86 are 17.3 nm and 15.8 nm, respectively) and similar to their wall thicknesses (15 nm) but much smaller than the critical size of Ni. Therefore the coercive force variation of nanotubes may attribute to their special tubular structure and the decrease of Fe.
Table 2 The coercive field HcJ calculated by Eq. (3) and in experiment.
4. Conclusions Products
Potential (V)
Hk (Oe)
Ms (emu/ cm3)
HcT (in theory) HcT (in (Oe) experiment) (Oe)
Nanowires Nanowires Nanowires Nanotubes Nanotubes Nanotubes
0.8 1.0 1.2 1.25 1.75 2.50
3715 3826 3026 4303 3248 2826
876 668 607 827 656 607
401 526 453 487 454 422
280 731 456 486 458 438
In summary, FeNi alloy nanowires and nanotubes have been successfully fabricated in the pores of the anodized aluminum oxide (AAO) templates by direct current electrodeposition with different potentials. The diameter and content can be independently modulated by varying the anodization parameters and electrodeposition parameters. It is observed that the content of Fe in FeNi alloy nanowires and nanotubes reduced with the decrease
X. Zhang et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 162–167
of electrodeposition potentials. Magnetic measurements show that the magnetic anisotropic properties are strongly dependent on the length of FeNi alloy nanowires and nanotubes. So the variation of magnetic properties with the length of nanowires and nanotubes should be further investigated. Simultaneously, the realization of modulating ratios of Fe and Ni by varying the potentials will be valuable in future applications.
Acknowledgments This work is supported by the Natural Science Foundation of Hebei Province (Grant no. A2012205038). References [1] R.W. Siegel, E. Hu, M.C. Roco, Nanostructure science and technology: a worldwide study, National Science and Technology Council (NSTC) Committee on Technology, 1999. [2] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan, One-dimensional nanostructures: synthesis, characterization, and applications, Advanced Materials 5 (2003) 353–389. [3] D. Davis, M. Zamanpour, M. Moldovan, D. Young, E.J. Podlaha, Electrodeposited, GMR CoNiFeCu nanowires and nanotubes from electrolytes maintained at different temperatures, Journal of the Electrochemical Society 157 (6) (2010) 317–322. [4] J.Y. Chen, N. Ahmad, D.W. Shi, W.P. Zhou, X.F. Han, Synthesis and magnetic characterization of Co–NiO–Ni core–shell nanotube arrays, Journal of Applied Physics 110 (2011) 073912. [5] X.Y. Zhang, G.H. Wen, Y.F. Chan, R.K. Zheng, X.X. Zhang, N. Wang, Fabrication and magnetic properties of ultrathin Fe nanowire arrays, Applied Physics Letters 83 (2003) 3341–3343. [6] S.G. Yang, H. Zhu, D.L. Yu, Z.Q. Jin, S.L. Tang, Y.W. Du, Preparation and magnetic property of Fe nanowire array, Journal of Magnetism and Magnetic Materials 222 (2000) 97–100. [7] X.W. Wang, Z.H. Yuan, B.C. Fang, Template-based synthesis and magnetic properties of Ni nanotube arrays with different diameters, Materials Chemistry and Physics 125 (2011) 1–4. [8] C. Pham-Huu, N. Keller, C. Estourne s, G. Ehret, J.M. Grene che, M.J. Ledoux, Microstructural investigation and magnetic properties of CoFe2O4 nanowires synthesized inside carbon nanotubes, Physical Chemistry Chemical Physics 5 (2003) 3716–3723. [9] Y.L. Li, S.L. Tang, R. Xie, Y. Wang, M. Yang, et al., Fabrication and magnetic properties of free-standing Ni nanotube arrays with controllable wall thickness, Applied Physics Letters 100 (2012) 052402.
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[10] Kristen E. La Flamme, Ketul C. Popat, Lara Leoni, Erica Markiewicz, Thomas J. La Tempa, Brian B. Roman, Craig A. Grimes, Tejal A. Desai, Biocompatibility of nanoporous alumina membranes for immunoisolation, Biomaterials 28 (2007) 2638–2645. [11] Aliasger K. Salem, Peter C. Searson, Kam W. Leong, Multifunctional nanorods for gene delivery, Nature Materials 2 (2003) 668–671. [12] M. Herna ndez-Ve´lez, Nanowires and 1D arrays fabrication: an overview, Thin Solid Films 495 (2006) 51–63. [13] D.J. Sellmyer, M. Zheng, R. Skomski, Magnetism of Fe, Co and Ni nanowires in self-assembled arrays, Journal of Physics: Condensed Matter 13 (2001) 433–460. [14] D.M. Davis, Electrodeposition of Magnetic Nanowires and Nanotubes, B.S., Texas Tech University, Lubbock, Texas, USA, 2005. [15] A. Huczko, Template-based synthesis of nanomaterials, Applied Physics A 70 (2000) 365–376. [16] X.F. Han, S. Shamaila, R. Sharif, J.Y. Chen, H.R. Liu, D.P. Liu, Structural and magnetic properties of various ferromagnetic nanotubes, Advanced Materials 21 (2009) 4619–4624. [17] J. Escrig, J. Bachmann, J. Jing, M. Daub, D. Altbir, K. Nielsch, Crossover between two different magnetization reversal modes in arrays of iron oxide nanotubes, Physical Review B 77 (2008) 214421. [18] F.S. Li, D. Zhou, T. Wang, Y. Wang, L.J. Song, C.T. Xu, Fabrication and magnetic properties of FeCo alloy nanotube array, Journal of Applied Physics 101 (2007) 014309. [19] Steven D. Leith, Shirley Ramli, Daniel T. Schwartz, Characterization of NixFe1 x (0.10o xo 0.95) electrodeposition from a family of sulfamatechloride electrolytes, Journal of the Electrochemical Society 146 (4) (1999) 1431–1435. [20] X.L. Fu, Y. Wang, P.G. Li, L.M. Chen, H.Y. Zhang, Q.Y. Tu, L.H. Li, W.H. Tang, Large-scale fabrication and magnetic properties of Ni80Fe20 nanowire arrays, Chinese Physical Society 54 (2005) 1693–1696. [21] H.Q. Li, F. Ebrahimi, Tensile behavior of a nanocrystalline Ni–Fe alloy, Acta Materialia 54 (2006) 2877–2886. [22] H.R. Khan, K. Petrikowski, Anisotropic structural and magnetic properties of arrays of Fe26Ni74 nanowires electrodeposited in the pores of anodic alumina, Journal of Magnetism and Magnetic Materials 215–216 (2000) 526–528. [23] D. Zhou, L.H. Cai, F.S. Wen, F.S. Li, Template synthesis and magnetic behavior of FeNi alloy nanotube arrays, Chinese Journal of Chemical Physics 20 (2007) 821–825. [24] E. Budevski, G. Staikov, W.J. Lorenz, Electrocrystallization nucleation and growth phenomena, Electrochimica Acta 45 (2000) 2559–2574. [25] S.J. Geng, Y.D. Li, D. Xiang, S.G. Zhou, Electrodeposition of Fe–Ni alloy coating on ferritic stainless steel, Transactions of Nonferrous Metals Society of China 20 (2010) 226–230. [26] Y. Peng, H.L. Zhang, S.L. Pan, H.L. Li, Magnetic properties and magnetization reversal of a-Fe nanowires deposited in alumina film, Journal of Applied Physics 87 (2000) 7405–7408. [27] G.C. Han, B.Y. Zong, P. Luo, Y.H. Wu, Angular dependence of the coercivity and remanence of ferromagnetic nanowire arrays, Journal of Applied Physics 93 (2003) 9202–9207.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Comparative study in fabrication and magnetic properties of FeNi alloy nanowires and nanotubes Xiuli Zhang a,c, Huimin Zhang a,b,c, Tianshan Wu a,c, Ziyue Li a,c, Zhijun Zhang a,c, Huiyuan Sun a,c,n a
College of Physics Science and Information Engineering, Hebei Normal University, Shijiazhuang 050024, China College of Materials Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050016, China c Key Laboratory of Advanced Films of Hebei Province, No. 20 Road East of 2nd Ring South, Yuhua District, Shijiazhuang, Hebei 050024, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 August 2012 Received in revised form 25 September 2012 Available online 23 November 2012
FeNi alloy nanowires and nanotubes have been fabricated in the pores of the anodized aluminum oxide (AAO) templates by direct current electrodeposition with different potentials. The crystal structure and micrograph of FeNi alloy nanowires and nanotubes were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, and the content was confirmed by energy dispersive spectrometry. The results indicate that there is a critical potential in the formation of nanowires and nanotubes. FeNi alloy nanowires and nanotubes both present the (111) preferred orientation, and the content of Fe is reduced with the decrease of electrodeposition potentials. The magnetic behaviors of FeNi alloy nanowires and nanotubes with different ratios are investigated. The coercivity of FeNi alloy nanowires and nanotubes exhibits disparate behavior, which may attribute to their grain size and distinctive structure. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: FeNi alloy Nanowire Nanotube Electrodeposition Anodized aluminum oxide Magnetization reversal mode
1. Introduction In last few decades, nanostructures have been attracting considerable attention for their fantastic properties of surface effect, small size effect, quantum effect and macro-quantum tunnel effect [1,2]. Nanowires and nanotubes, especially ferromagnetic nanowires and nanotubes raise special interest due to their fundamental importance and potential applications in sensors, spintronics devices, magnetic recording media, magnetic logic, high-density magnetic random access memory, biological transportation, etc. [3–11]. In order to synthesize nanowires and nanotubes, a variety of techniques have been developed, such as vapor-based growth techniques, arc process, the supercritical fluid method, physical sputtering, self-assembly route, the sol– gel method, template-based electrodeposition for nanowires and thermal decomposition of precursors, hydrothermal synthesis, plasma enhanced chemical vapor deposition, atomic layer deposition, chemical reduction, and template-based electrodeposition for nanotubes [12–18]. Among these techniques, anodic aluminum oxide (AAO) template-based electrodeposition is widely adopted not only because it has preferable insulativity, chemical
n Corresponding author at: College of Physics Science and Information Engineering, Hebei Normal University, Shijiazhuang 050024, China. Tel.: þ86 133 638 15166; fax: þ 86 311 807 87300. E-mail address: [email protected] (H. Sun).
and heat stability but also because it is convenient, versatile and inexpensive. FeNi alloy which is usually called permalloy shows a series of physical characteristics with the variation of ratios [19–21], and it is one of the universal soft magnetic materials which has an extensive application. Till date, both FeNi alloy nanowires and nanotubes have been studied widely [22,23]; however, the comparative study in fabrication and magnetic reversal process which is important for the magnetic properties have not been intensively discussed. In this paper, we successfully fabricated FeNi alloy nanowires and nanotubes in AAO template by dc electrodeposition. The morphology, composition, and magnetic properties of FeNi alloy nanowires and nanotubes are also investigated.
2. Experimental section Highly ordered anodized aluminum oxide (AAO) templates were prepared by a two step anodization from 99.999% aluminum foil. Firstly, Al foils were annealed at 400 1C for 2 h in an argon atmosphere, and then were electropolished in a mixture of HClO4:C2H5OH¼174 (V/V) for 5 min. Secondly, the prepared Al foils were anodized in 0.3 M oxalic acid at a constant potential of 45 V and 5 1C for 6 h. The resulting alumina film was chemically removed in a mixed solution of phosphoric acid (6 wt%) and chromic acid (4 wt%) at 30 1C for 12 h. Subsequently, the samples were re-anodized for 12 h under the same conditions as those in the first step. Finally, the remaining Al substrate was removed by
0304-8853/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.11.033
X. Zhang et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 162–167
saturated CuCl2 solution, after that the template was dipped into phosphoric acid (6 wt%) for 3 h to remove barrier layer, at last the template was washed with deionized water in ultrasonic cleaner. The thickness and the pore diameter of obtained templates are 45 mm and 70 nm respectively. Electrodeposition was performed in a three-electrode cell under constant potential at room temperature. A very thin Cu membrane was deposited by sputtering on one side of the template serving as the working electrode, a saturated calomel electrode (SCE) as the reference, and graphite pole as the counter electrode. FeNi alloy nanowires and nanotubes with different ratios were fabricated under different potentials as follows: 0.8 V, 1.0 V, 1.2 V in electrolyte composed of 15 g/L FeSO4 7 H2O, 225 g/L NiSO4 6 H2O, 45 g/L H3BO3, 3 mg/L C6H8O6 for nanowires; 1.25 V, 1.75 V, 2.50 V in electrolyte composed of 6 g/L FeSO4 7 H2O, 26 g/L NiSO4 6 H2O, 25 g/L H3BO3, and 3 mg/L C6H8O6 for nanotubes. The pH value of electrolytes was adjusted to 3.0 by dilute sulfuric acid. All the samples were immersed in 4 M NaOH aqueous solution before SEM and TEM observations to dissolve the alumina layer. The morphologies of the nanowires and nanotubes were characterized by scanning electron microscopy (SEM, Hitachi S-4800). The structural analysis of the ferromagnetic nanowires and nanotubes was obtained from transmission electron microscopy (TEM, Hitachi
163
H-7650) and X-ray diffraction (XRD, PANalytical X’pert PRO MPD) with Cu Ka radiation. The compositions were analyzed by energy dispersive spectrometry (EDS). The magnetic properties of the samples were tested by a vibrating sample magnetometer (VSM) of the Physical Property Measurement System (PPMS, Quantum design Model 6000).
3. Results and discussion Fig. 1(a) and (b) shows SEM images of the surface and cross section of a typical AAO template which was prepared by twostep anodization in 0.3 M oxalic acid, it can be seen that the pores are uniform and the diameter is about 70 nm, and the inner wall of the pores are smooth, none branch is observed. The SEM images of FeNi alloy nanowires and nanotubes are exhibited in Fig. 1(c) and (e). It is obvious that nanowires and nanotubes have uniform diameter which is in accordance with the AAO pore size. Fig. 1(d) and (f) is corresponding TEM image of nanowires and nanotubes, TEM images indicate that the outer diameter (d) of nanowires and nanotubes both are around 70 nm which is consistent with SEM images of Fig. 1(a) and (b), and the wall thickness of the nanotubes is about 15 nm. In addition, it can be
Fig. 1. SEM images for the (a) surface and (b) cross section of AAO template, (c) FeNi alloy nanotubes and (e) nanowires; TEM images for FeNi alloy (d) nanotubes and (f) nanowires.
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concluded from Fig. 1 that there exists a critical potential in the formation of nanowires and nanotubes. Nanotubes can be obtained when the potential is equal to or greater than the critical value, on the contrary nanowires can be formed. The detailed influence factors need to be researched further. The typical XRD patterns of FeNi alloy nanowires and nanotubes electrodeposited under different potentials are presented in Fig. 2(a) and (b). Though deposited under different potentials, the structure of nanowires is the same. Compared with standard patterns, the nanowires have face-center-cubic (fcc) structure, and present the (111) preferred orientation characteristics. The crystal structure is the same in case of nanotubes, but when the potential decreases to 2.50 V, the sample exhibits poor crystallization as shown in Fig. 2(b). According to the Scherrer Equation, the size of particles can be calculated: D¼
lk Bcos y
ð1Þ
where l is the wavelength of X-ray, k is the Scherrer constant ( ¼0.89), B is the full width half maximum which can be found from XRD pattern, y is the angle of diffraction. The grain sizes of FeNi alloy nanowires which were electrodeposited under different potentials of 0.8 V, 1.0 V, and 1.2 V are 10.6 nm, 20.8 nm and 101.6 nm, respectively. Whereas the grain sizes of nanotubes electrodeposited under 1.25 V, and 1.75 V are
Fig. 2. XRD patterns of as-prepared FeNi alloy (a) nanowires and (b) nanotubes embedded in AAO template.
17.3 nm, and 15.8 nm (the nanotubes prepared under 2.50 V cannot be calculated because of its bad crystallization), respectively. It is obvious that the grain sizes of nanowires increase with the decrease of potentials, however the case in nanotubes is opposite, that is, as there is decrease of potentials the grain sizes also decrease. As is well-known that nucleation is a very important process in metal deposition, the competition between growth and nucleation determines the granularity of the deposit [24]. The higher the nucleation rate during deposition, the finer are the crystal grains of the deposit. Consequently, we suppose that in the process of electrodepositing nanowires the concentration of electrolyte and the potential are higher so that the rate of nucleation is lower than the rate of growth, and the crystallization is better; however, in the case of nanotubes, the concentration of electrolyte and the potential are higher, which result in the low nucleation rate than the growth rate, leading to poorer crystallization in nanotubes. The EDS spectra of as-prepared nanowires and nanotubes are given in Fig. 3(a) and (b), respectively. Both spectra indicate that the nanowires and nanotubes indeed consist of Fe and Ni. The Al and O peaks arise from the small amount of residual alumina after the sample was rinsed. In conclusion, FeNi alloy nanowires and nanotubes can be successfully fabricated by dc electrodeposition. The EDS spectrum could not only confirm the components but also can give the relative content for different elements. As shown in Table 1, the Ni and Fe content are different with the change of potentials. From Table 1, it is obvious that compared with the ratios of the metal ion (Ni2 þ : Fe2 þ ) in corresponding electrolytes, the proportions of elements in nanowires and nanotubes are much bigger, this may due to the anomalous codeposition of iron and nickel during the electrodeposition [25]. Although the standard electrode potential of iron (0.44 V) is more negative than nickel ( 0.25 V), the iron deposition occurs preferentially so that the content of iron in nanowires and nanotubes is much higher than that in electrolyte. The cause of anomalous codeposition may attribute to the formation of hydrogen in the process of electrodeposition, which can lead to a rise of the electrolyte pH value,
Fig. 3. EDS spectra of FeNi alloy (a) nanowires and (b) nanotubes.
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hydroxide is generated by Fe2 þ and OH , as a result Fe2 þ electrodeposit prior to Ni2 þ , meanwhile the deposition of Ni2 þ is suppressed, which results in the lower deposition speed of Ni2 þ . As there is decrease of deposition voltage, the content of Fe in nanowires and nanotubes decreases gradually, which is consistent with the literature [21]. The main reason is when deposition voltage decreases, the cathodic current density gets bigger and the reduction reaction of Fe2 þ which is controlled by diffusion rate of ions will be restrained thus the content of iron in nanowires and nanotubes also decreases.
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Fig. 4(a) and (b) shows the magnetic hysteresis loops at room temperature for FeNi alloy nanowires and nanotubes. The nanowires and nanotubes which is located inside the AAO were measured in order to take advantage of the orientation of them and it should be noted that the AAO templates have no influence on their magnetic performances. The symbols O and?represent the applied magnetic field is parallel and perpendicular to the nanowires and nanotubes axis, respectively. Table 1 also shows the magnetic properties of obtained nanowires and nanotubes, it proves that squarenesses (Mr/Ms) are
Table 1 The ratios and magnetic parameters, coercivity (Hc), and remanent squareness (SQ) of FeNi alloy nanowires and nanotubes electrodeposited under different potentials in AAO with the field applied parallel (T) and perpendicular (?) to the axis. Products
Potential (V)
Fe (at.) Ni (at.)
HcT (Oe)
Hc? (Oe)
SQO
SQ?
Nanowires Nanowires Nanowires Nanotubes Nanotubes Nanotubes
0.8 1.0 1.2 1.25 1.75 2.50
32:68 15:85 10:90 28:72 14:86 10:90
280 731 456 486 458 438
55 88 130 25 31 35
0.075 0.614 0.197 0.384 0.319 0.328
0.050 0.033 0.043 0.0065 0.0108 0.0165
Fig. 4. Magnetic hysteresis loops of FeNi alloy (a) nanowires and (b) nanotubes with different ratios; the red dots denote field applied parallel to axis and the black dots denote field applied perpendicular to axis.
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changed with the ratios of FeNi alloy for both of the nanowires and nanotubes, and the remanent squarenesses (SQ) in the parallel direction are higher than that in the perpendicular direction, which suggests that the FeNi alloy nanowires and nanotubes have superior parallel magnetic characteristics. Because the shape anisotropy of our nanowires and nanotubes (aspect of 100) is much higher than magnetocrystalline anisotropy [26], we suppose that the shape anisotropy is predominant here and the easy axis of shape anisotropy is oriented along the axis of nanowires and nanotubes. Because the pore density of the AAO template is about 1010 cm 2, it is interesting to analyze magnetization reversal processes for the aspects of high-density magnetic recording. Generally, there are two kinds of magnetization reversal modes which can be modeled by coherent rotation and curling. For magnetic nanowires and nanotubes, the magnetization reversal mechanism depends upon the outer diameter of the nanocylinders. For a specific material, there exists a critical diameter dc for the transition from coherent rotation to curling. The critical diameter dc is given by [16] ! A1=2 dc ¼ 2:08 ð2Þ Ms where A is the exchange stiffness (about 10 6 erg/cm for Fe, Ni and FeNi alloy) and Ms is the saturation magnetization. For nanowires and nanotubes with diameters larger than the critical diameter, the magnetization reversal process can be described by the curling mode. The curling mode of magnetization reversal in an infinite cylinder predicts [27] að1 þ aÞ Hc ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Hk ð3Þ a2 þ ð1 þ 2aÞcos2 y 2 where a ¼ 1:08 dc =d and y is the angle between the nanotubes and the field. Thus the Hc in the parallel direction can be calculated. Comparing the Hc in experiment and in theory from Table 2, it is found that the results of curling mode consist with nanotubes perfectly, but there is slight difference with nanowires, and only the sample deposited under 1.2 V coincides with theoretical value. It is easy to discover that the grain size of nanowires deposited under 1.2 V (which is 101.6 nm) is larger than the diameter of AAO template, therefore, because of the restrain of nanopores, the magnetic moment in the granules may present a vortex distribution, and can be described by the curling mode, but for the nanowires deposited under 0.8 V and 1.0 V, the grain sizes (which are 10.6 nm and 20.8 nm) are much smaller in comparison with the AAO template diameter, they may have several particles of the same height of a nanowire, so their magnetization reversal modes are dissimilar with curling. For nanotubes electrodeposited under different potentials, the grain sizes are approximate to each other and similar to the wall thickness, it is conjectured that the particles distribute as a spherical ring of the same height of single nanotube, as a result, the magnetization reversal mode of nanotubes agrees with the curling mode.
Fig. 5. The angle dependence of coercivity of nanotubes prepared under 1.25 V.
In order to confirm the conclusion further, we measured the dependence of coercivity with different angles. Because all the nanotubes and the nanowires fabricated under 1.2 V coincided with curling mode and the tendencies are alike, we just present the figure of nanotubes prepared under 1.25 V, as shown in Fig. 5. It is clear that there is a very good agreement when the field orientation y r 7751, for field applied along the hard axis, a minimum is obtained which is opposite to the theoretical prediction of maximum coercive field, which is consistent with the results reported by Han et al. [16]. In ferromagnet, multidomain structure will be formed if the particle is bigger. With the decrease of the particles, the magnetic domain will be reduced in order to diminish the domain wall energy. Once the critical size is achieved, the particle will become single domain, but if the particle size is less than the critical size, the samples will exhibit superparamagnetism. The single domain system possesses much higher coercive force. For spherical particles, the critical sizes for Fe, Co, Ni are 17 nm, 11.4 nm, 39 nm, respectively. Compared with the critical size above, it can be learned that the sample of nanowires prepared with 1.0 V (Fe15Ni85, 20.8 nm) is the closest to the critical size of Ni, and its coercive force is much higher than two other samples indeed, the sample electrodeposited under 0.8 V (Fe32Ni68, 10.6 nm) appear superparamagnetic because of its smaller grain size and has lower coercive force, while due to the multidomain structure, the coercive force of nanowires electrodeposited under 1.2 V (Fe10Ni90, 101.6 nm) is smaller than Fe15Ni85 but larger than Fe32Ni68. Whereas the grain size of nanotubes are close to each other (Fe28Ni72 and Fe14Ni86 are 17.3 nm and 15.8 nm, respectively) and similar to their wall thicknesses (15 nm) but much smaller than the critical size of Ni. Therefore the coercive force variation of nanotubes may attribute to their special tubular structure and the decrease of Fe.
Table 2 The coercive field HcJ calculated by Eq. (3) and in experiment.
4. Conclusions Products
Potential (V)
Hk (Oe)
Ms (emu/ cm3)
HcT (in theory) HcT (in (Oe) experiment) (Oe)
Nanowires Nanowires Nanowires Nanotubes Nanotubes Nanotubes
0.8 1.0 1.2 1.25 1.75 2.50
3715 3826 3026 4303 3248 2826
876 668 607 827 656 607
401 526 453 487 454 422
280 731 456 486 458 438
In summary, FeNi alloy nanowires and nanotubes have been successfully fabricated in the pores of the anodized aluminum oxide (AAO) templates by direct current electrodeposition with different potentials. The diameter and content can be independently modulated by varying the anodization parameters and electrodeposition parameters. It is observed that the content of Fe in FeNi alloy nanowires and nanotubes reduced with the decrease
X. Zhang et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 162–167
of electrodeposition potentials. Magnetic measurements show that the magnetic anisotropic properties are strongly dependent on the length of FeNi alloy nanowires and nanotubes. So the variation of magnetic properties with the length of nanowires and nanotubes should be further investigated. Simultaneously, the realization of modulating ratios of Fe and Ni by varying the potentials will be valuable in future applications.
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