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Magnetic and magnetization properties of electrodeposited fcc CoPt nanowire arrays
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Journal of Magnetism and Magnetic Materials 320 (2008) 1803–1809 www.elsevier.com/locate/jmmm
Magnetic and magnetization properties of electrodeposited fcc CoPt nanowire arrays S. Shamailaa, R. Sharifa, S. Riaza, M. Maa, M. Khaleeq-ur-Rahmanb, X.F. Hana, a
State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China b Physics Department, University of Engineering and Technology, Lahore 54890, Pakistan Received 13 April 2007; received in revised form 3 January 2008 Available online 6 March 2008
Abstract Magnetic and magnetization properties of fcc Co1xPtx (xp0.3) alloy nanowires fabricated by electrodeposition into self-synthesized anodic alumina templates are investigated. Magnetization curves, measured for varying wire geometries, show a crossover of easy axis of magnetization from parallel to perpendicular to the nanowire axis as a function of the diameter and length. The measured values of coercivity (Hc) and remanent squareness (SQ) of CoPt nanowire arrays, as a function of angle (y) between the field and wire axis, support the crossover of easy axis of magnetization. The curling mode of the magnetization reversal process is observed for CoPt nanowire arrays. At low temperatures, the easy axis for magnetization of the nanowires is observed to deviate from the room-temperature orientation. r 2008 Elsevier B.V. All rights reserved. PACS: 75.50.y; 75.60.d; 75.75.+a Keywords: Magnetic nanowire; Electrodeposition; Magnetic hysteresis; Angular dependant coercivity; Cobalt platinum
1. Introduction Ferromagnetic nanowires exhibit unique and tunable magnetic and magnetization properties due to their inherent shape anisotropy. The practical application of these magnetic nanowires in the field of nanotechnology is an attractive topic among researchers. This field represents an exciting and rapidly expanding area of research that crosses the borders between physical and engineering sciences. These ideas have driven scientists to develop methods for making nanostructures [1]. The magnetic properties of nanowires have been investigated with particular emphasis on three areas: (1) factors that determine the effective easy axis of the wires, (2) magnetization reversal processes within the arrays and (3) magnetic interaction between the wires [2]. Among ferromagnetic alloy nanowires, CoPt and FePt have been extensively investigated not only from a fundamental but Corresponding author. Tel.: +86 10 826 480 63.
E-mail address: [email protected] (X.F. Han). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.02.183
also from a technological point of view due to their applications in magnetic recording media [3–5] and also their integration capability in functional micro or nanodevices (MEMS/NEMS) [6]. CoPt alloy has potential applications in the perpendicular recording media due to its large anisotropy. It has attracted great interest recently because of its various crystalline phases and its extraordinary properties. These phases can be obtained under specific experimental conditions and have different applications [7]. For example, the CoPt alloy with L10 (fct) structure has very high magnetic anisotropy and is a promising candidate for fine permanent magnets and for high-density magnetic recording media [3,8]. Moreover, various phases of the CoPt alloy provide a good opportunity for us to study the transformation and interplay of different phases [5,7]. The CoPt alloy is also a fine candidate as a magneto-optical material [9,10]. Electrodeposition of platinum and platinum-based alloys is complicated by the catalyzed co-deposition of hydrogen [11,12], and it is only very recently that CoPt nanowires are being successfully electrodeposited in nanoporous alumina
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membrane. There have been limited reports of electrodeposition of CoPt alloy nanowires [2–11]. The low-cost alternating-current (AC) electrodeposition method of fabrication of the nanowires into self-assembled alumite templates has facilitated the researchers to investigate their fundamental magnetic and magnetization properties [13]. These alumite templates consist of highly ordered cylindrical pores with small diameters and an Al layer at the bottom of the pores. Its structure is described as a closedpacked array of columnar cells, each containing a central pore of which the size and interval can be controlled by changing the forming conditions. In recent years, there has been interest in this material as a medium for creating uniform nanostructures since its pores can function as ‘nanotemplates’ in which small metal and semiconductor particles can be electrochemically deposited [14]. The ferromagnetic nanowires fabricated in such templates have high coercivity (Hc) and remanence with an easy axis parallel to the nanowires axis due to the dominant role of shape anisotropy in effective magnetic anisotropy, which can be minimized by increasing the diameter of the templates. For large-diameter nanowire arrays, the shape anisotropy becomes less dominant and can be compared with the magnetocrystalline anisotropy and magnetostatic interactions. These nanowires show relatively lower Hc and squareness, and the easy axis can be either perpendicular or parallel to the wire axis [15]. The coherent and curling modes of rotation are observed for the nanowire diameters lesser and greater than the thickness of the domain wall (lw), respectively [16]. Magnetic properties with desired values of Hc and remanent squareness (SQ) are the important requirements of perpendicular recording media [17]. Therefore studies about Hc and SQ and the factors affecting these values are required. In view of having the potential to be used in perpendicular magnetic recording media, CoPt nanowires have been fabricated into the selfassembled templates using the AC electrodeposition method. The effect of diameter, length, field orientation and temperature on magnetic and magnetization properties of the nanowires are systematically investigated for CoPt nanowire arrays, for the present report. The curling mode of the magnetization reversal process has been observed for CoPt nanowire arrays. Magnetic and magnetization properties are determined from a competition among the shape and magnetocrystalline anisotropies, and the dipolar magnetostatic interactions. A crossover of the easy axis of magnetization as a function of diameter and length has been observed for the fcc CoPt nanowire arrays.
AC-deposition was performed using a standard doubleelectrode bath; the AAO template was used as one electrode, and the platinum wire was used as the other, with a sinusoidal voltage having a root-mean-square (rms) value of 23 V for different periods of time. X-ray diffraction (XRD) was used to monitor the crystal structure of the nanowire arrays. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to investigate the morphology and size of the templates and the nanowires. Magnetic properties of the samples with an applied external magnetic field at various angles were investigated using vibrating sample magnetometer (VSM) and with a superconducting quantum interference device (SQUID). The contents of the CoPt nanowires were determined using induction-coupled plasma spectrometry (ICP). The analysis showed that the composition of individual sample is almost homogeneous. Samples with the composition Co1xPtx (xp0.3) were prepared. For the present report, samples with x ¼ 0.1 where S1, S2, S3 with an average diameter (d) of 40 nm and average interpore distance (D) of 95 nm and L1, L2, L3, L4 with an average diameter (d) of 200 nm and average interpore distance (D) of 250 nm are selected. SEM, TEM, XRD and VSM measurements were carried out at room temperature, and temperature-dependent measurements were done with a SQUID between liquid helium temperature and room temperature. 3. Results and discussion Fig. 1 shows some characteristics of the as-deposited CoPt nanowires with average diameters (d) of 40 and 200 nm. Figs. 1(a) and (b) are the TEM images of isolated CoPt nanowires with diameters d ¼ 200 and 40 nm, respectively. TEM images of the nanowire samples were taken after releasing them from the alumina membrane,
2. Experimental details The highly ordered anodic aluminum oxide (AAO) templates were prepared as described previously [9]. The arrays of CoPt nanowires were electrodeposited into the AAO templates by the AC-electrochemical deposition method in an electrolyte consisting of PtCl4, CoSO4 7H2O, and H3BO3 at a pH value of about 3.0. The
Fig. 1. TEM images of isolated CoPt nanowires of diameter (a) 200 nm and (b) 40 nm. (c) SEM image of AAO template with diameter of about 40 nm. (d) SEM image of CoPt nanowires in the supported alumina template.
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Fig. 2. XRD patterns of aligned CoPt nanowires in the AAO template of diameter (a) 40 nm and (b) 200 nm.
by dissolving it for about 12 h in 1 M NaOH solution. Figs. 1(c) and (d) are the SEM images of CoPt nanowires in the supported AAO template. To examine the microstructure of these nanowires, the sample was fractured to produce cross-sectional regions that expose the CoPt nanowires on the fractured surface of the AAO template. Fig. 2 shows the XRD pattern of aligned CoPt nanowires in the AAO templates of diameters (a) 40 nm and (b) 200 nm. For XRD measurements, the Al substrates were etched away by an amalgamation process using a saturated HgCl2 aqueous solution and washed thoroughly with distilled water. In the as-synthesized conditions the XRD patterns show that the samples of CoPt nanowires comprise of an fcc phase with the identifiable diffraction peaks, namely (1 1 1) and (2 0 0) of which the (1 1 1) reflection is the most intense. For 200 nm CoPt nanowires the (2 2 0) peak has also been detected, together with the diffraction peaks (1 1 1) and (2 0 0). There is a peak shift of the dominant plane (1 1 1) for the nanowires (200 nm), as can be seen in Fig. 2(b). This shift and the additional peak (2 2 0) may be attributed to the more random distribution of grains [11] with a disordered fcc structure for 200 nm diameter. The intensity of the (1 1 1) peak is much larger than those of other peaks for both diameters. Moreover, the Co (1 1 1) diffraction peak is also detected at 2y ¼ 44.51 for 40 nm diameter. These XRD results suggest that the CoPt nanowires are of fcc polycrystalline structure with randomly oriented grains. Different factors affecting the magnetization behavior of the CoPt nanowires are investigated. These factors include nanowire geometry i.e., wire diameter and length. The typical room-temperature magnetic hysteresis (M–H) curves for 40 and 200 nm CoPt nanowires, with the external field applied parallel and perpendicular to the nanowire axis, are shown in Figs. 3(a–c) and (d–f),
Fig. 3. M–H curves for CoPt nanowire arrays of diameter 40 (a–c) and 200 nm (d–f) with different wire lengths.
respectively. The difference between the perpendicular and parallel M–H curves defines the uniaxial anisotropy for CoPt nanowire arrays. For the 40 nm CoPt nanowires (Fig. 3(a)) Hc and SQ in the parallel (J) geometry are larger than that in the perpendicular (?) geometry attributing to the parallel alignment of the magnetic easy axis along the wire axis [12]. The value of Hc in the two geometries approaches each other for the 200 nm CoPt nanowires, as shown in Fig. 3(d). However, SQ in the perpendicular geometry is larger than that in the parallel geometry. Therefore, the easy axis of magnetic anisotropy favors to
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Table 1 Magnetic properties for CoPt nanowire arrays with diameters 40 and 200 nm having different wire lengths Samples
S1 S2 S3 L1 L2 L3 L4
Diameter (nm)
40 40 40 200 200 200 200
Length (mm)
2 8 20 0.8 3 10 15
Hc (Oe)
SQ (Mr/Ms)
DHs (Hs||Hs?)
J
?
J
?
2032.9 1438.1 1051.1 652.1 106.8 189.5 44.4
575.1 721.6 555.2 302.7 109.1 141.2 83.24
0.93 0.88 0.84 0.38 0.1 0.07 0.01
0.15 0.20 0.21 0.14 0.12 0.18 0.03
be aligned perpendicular to the wire axis. This variation in the alignment of easy axis for the two diameters shows a crossover of easy axis of magnetization as a function of diameter. The values of magnetic parameters Hc, SQ and the difference of the saturation fields (DHs) are presented in Table 1. Comparison of the parameters, given in Table 1 shows the effect of the wire diameter and length on the magnetic properties of CoPt nanowire arrays. The alignment of easy axis can also be specified by the sign of DHs, here DHs ¼ Hs||Hs? [15]. Here HsJ is the saturation field when the magnetic field is applied parallel to the nanowire axis and Hs? is the saturation field when the magnetic field is applied perpendicular to the nanowires axis. The negative sign of DHs indicates the easy axis along the nanowire axis and positive sign shows the orientation of easy axis perpendicular to the nanowire axis. Figs. 3(a–f) show the effect of wire geometry on the magnetic properties for CoPt nanowire samples of the two diameters. For CoPt nanowire samples with d ¼ 40 nm, the easy axis remains parallel to the wire axis in the whole investigated range of lengths (Figs. 3(a–c)). For wire arrays with d ¼ 200 nm a different behavior can be observed (Figs. 3(d–f)), for which parallel magnetization is favored for very short wires; whereas, perpendicular magnetization is favored for long nanowires. Thus, in fcc CoPt nanowires, it is observed that when the wire length is larger than a critical value, the parallel wire axis crosses over from easy to hard as also was observed by others for Co nanowire arrays [18]. This crossover is clear from the shape of M–H curves as well as from the sign of DHs of these samples (Table 1). For sample L3 in Fig. 3(d) the easy axis of nanowires lies perpendicular to the nanowire axis, DHs is positive in this case. It is also observed for L3 that HcJ is a little bit larger than Hc? (Table 1), which may be due to the random distribution of grains as shown by XRD results. For sample L2, DHs decreases, which can also be observed from the shape of the M–H curve (see Fig. 3(e)). For L1 with small wire length DHs is negative indicating that easy axis is now parallel to the wire axis (Fig. 3(f)). This effect can be used to turn the parallel wire axis from easy to hard, by modifying the length. Additional evidence for this crossover of easy axis is provided by the angular dependence of
2164.45 2667.19 3375.59 717.45 1092.47 4018.07 397.95
the Hc and SQ of CoPt nanowire samples measured as a function of wire diameter and length (Fig. 4). Hc(y) and SQ(y) curves show bell-shaped or otherwise bell-shaped behavior corresponding to the easy axis of their magnetization. Bell-shaped curves for samples S1, S2 and S3 with d ¼ 40 nm and otherwise bell-shaped curves for samples L2, L3, L4 with d ¼ 200 nm confirm the crossover of easy axis from parallel to perpendicular as a function of diameter. Furthermore, the bell-shaped behavior of sample L1 of length ¼ 800 nm with the easy axis parallel to the wire axis (see Fig. 3(f)) shows the crossover of easy axis as a function of length. Figs. 4(a, b) also show that values for both Hc and SQ are large for samples with d ¼ 40 nm. These large values of Hc and SQ, and negative sign of nHs for samples S1, S2 and S3 are due to the dominant shape anisotropy. The values of Hc and SQ are comparatively small for samples L1, L2, L3 and L4 with d ¼ 200 nm having interwire distance of 250 nm. This decrease in Hc and SQ with increasing diameter is due to the decrease of shape anisotropy and an increase of the magnetostatic dipole interactions, because the interwire distance of these samples is not much larger than the wire diameter [19]. The overall anisotropic field (Hk) is mainly determined by following three contributions: (1) the shape anisotropy field (2pMs), which will induce a magnetic easy axis parallel to the nanowire axis, (2) magnetostatic dipole interaction field among the wires, which will induce a magnetic easy axis perpendicular to the nanowire axis and (3) the magnetocrystalline anisotropy field (Hma). The total effective anisotropic field (Hk) is given as follows [20]: H k ¼ 2pM s 6:3pM s r2 L=D3 þ H ma ,
(1)
where Ms is the saturation magnetization, r is the radius of the wire, L is the length and D is the interwire distance. The second term in Eq. (1) is the total dipole field acting on one wire due to all other wires. Eq. (1) predicts that as the wire length increases, Hk linearly decreases to zero when L ¼ Lc ¼ 2D3 =ð6:3r2 Þ,
(2)
while neglecting the contribution of Hma. For L4Lc, Hk is negative pointing out that there is a crossover of easy axis for magnetization from parallel to perpendicular to the nanowires axis. For CoPt nanowire arrays with
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Fig. 4. Angular dependence of (a) coercivity (Hc(y)) and (b) remanence (SQ(y)) of CoPt nanowire arrays where y is the angle between the field direction and the nanowire axis.
d ¼ 200 nm, r ¼ 100 nm and D ¼ 250 nm; we calculate Lc1 mm. The shape anisotropy field is weak in this case; therefore when L4Lc, Hk is negative and crossover of easy axis of magnetization from parallel to perpendicular to the nanowire axis is observed. For CoPt nanowire arrays with d ¼ 40 nm and D ¼ 95 nm the shape anisotropy is more dominant resulting in positive Hk, even for L4calculated Lc, because the dipolar interaction in this case may be much less than that of nanowires with d ¼ 200 nm and D ¼ 250 nm. The M–H curves at different temperatures are also measured to observe the temperature-dependent magnetic properties of CoPt nanowires. The M–H curves at temperatures (T) 5–300 K for 40 and 200 nm CoPt nanowires are shown in Fig. 5. It is observed that the shape of M–H curves changes for both diameters when the samples are cooled down to 5 K. For d ¼ 40 nm, shearing of the M–H loop for parallel geometry and expanding of M–H loop for perpendicular geometry suggest that at low temperature the easy axis is no longer fully parallel to the wire axis. For d ¼ 200 nm, SQ in the perpendicular geometry is much larger than that in the parallel geometry attributing to the perpendicular alignment of easy axis (see Fig. 3(d)). The change in the shape of M–H curve (see Figs. 5(c, d)) at T ¼ 5 K reveals that the easy axis of nanowire arrays with d ¼ 200 nm tends to turn over from perpendicular to the parallel direction. At low temperature, magnetocrystalline anisotropy becomes dominant towards the effective anisotropy caused by the spin–orbit interactions [21]. This magnetocrystalline anisotropy tends to rotate the orientation of easy axis of magnetization from high-temperature orientation. This anisotropy of the nanowires decreases with an increase in temperature. Therefore, at high temperature, for d ¼ 40 nm shape
Fig. 5. M–H curves for CoPt nanowire arrays of diameter 40 (a, b) and 200 nm (c, d) measured at T ¼ 5 and 300 K.
anisotropy becomes prevalent resulting in the orientation of easy axis along the parallel direction, and for d ¼ 200 nm magnetostatic interaction increases resulting in the alignment of easy axis perpendicular to the nanowire axis. Figs. 6(a, b) show the temperature dependence of Hc measured parallel and perpendicular to the 40 and 200 nm CoPt nanowires. Hc values show different variations with temperature for two diameters. This different behavior can be explained by the competition between the shape anisotropy and the temperature-dependent magnetocrystalline anisotropy for both diameters. For d ¼ 200 nm, difference in the values of Hc (DHc ¼ HcJHc?) is large at T ¼ 5 K and it decreases with increasing temperature. This decrease in HcJ and Hc? with increasing temperature is due
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Fig. 6. Temperature dependence of coercivity (Hc) for CoPt nanowire arrays of diameter (a) 40 and (b) 200 nm measured with field parallel and perpendicular to the nanowires.
to the thermal relaxation over the anisotropy energy barrier [16]. For d ¼ 40 nm, Hc in the two geometries tends to approach each other at low temperatures and DHc increases with increasing temperature. Moreover, HcJ does not decrease monotonically as temperature increases, but it shows a minimum, whereas Hc? decreases monotonically which is in accordance with the report by Paulus et al. [16]. The decrease in Hc (for d ¼ 40 nm) in both directions at low temperatures is due to thermal relaxation over the anisotropy energy barrier similar to that of d ¼ 200 nm, but at high temperatures, increase in HcJ and a further decrease of Hc? is due to the dominant shape anisotropy for these nanowires. The two most common magnetization reversal modes can be modeled by coherent rotation or curling. For magnetic nanowires the magnetization reversal mechanism depends upon the diameter of the nanowires. For a specific material, the critical diameter for the transition from coherent rotation to curling is given by dc ¼ 2.08(A1/2/Ms) where A is the exchange stiffness and Ms is the saturation magnetization [22]. For the nanowires with diameter larger than the critical diameter, the magnetization reversal process can be described by the curling mode, and Hc decreases with increasing diameter of the nanowires. Mallet et al. have calculated the critical diameter dc ¼ 14 nm for fcc CoPt nanowires [12]. Since the diameter of our samples (40–200 nm) is larger than the critical diameter (14 nm) of CoPt nanowires for coherent rotation, the reversal is expected to occur through the curling mode. The equation describing the dependence of Hc on the diameter of nanowires in the curling mode is given as follows [12]: H c ¼ 2pkA=½M s r2 þ 2KU=½M s
(3)
where r is the radius of the nanowire and k is a constant related to the shape of the material (1.08 for an infinite
cylinder). The right term 2KU/Ms is Hma with uniaxial anisotropy constant KU. For the present CoPt nanowire arrays the values of Hc decrease with an increase in diameter (Table 1). Eq. (1) can explain the variations in the magnetic behavior of the nanowire arrays as a function of diameter. Hc(y) curves (cf. Fig. 5(a)) are consistent with the easy axis orientation, but they show some complexities with the curling reversal mode. Such complexities can be attributed to the strong dependence of effective anisotropy field on the shape anisotropy, especially in case of nanowires having d ¼ 40 nm with D ¼ 95 nm. Similar angular dependence of Hc (manifesting curling mode) has also been observed in Refs. [11,23]. Therefore by considering the effective anisotropy energy, Hc(y) cannot account for the observed reversal mode. The present as-deposited CoPt nanowires have fcc structure with preferred orientation of (1 1 1) along the nanowires. A large variation in the values of Hc and SQ is observed depending upon diameter, length, field direction and temperature. These factors are involved in the magnetic characterization of the CoPt nanowires. The orientation of easy axis of magnetization can be deterP mined by the total energy E in the parallel and P perpendicular geometries; where E can be obtained by taking into account the magnetostatic interaction energy Edi, the demagnetization energy Ede and the magnetocrystalline anisotropy EK, where EdiJ4Edi?, EdeJoEde?, EKJoEK? [11]. For large diameters with small P interwire P distances the dipolar interaction is increased, EJ E?, and the easy axis favors to be alignedPperpendicular to the P wire axis [18]. For small diameters EJo E?, and the easy axis favors to be aligned along the wire axis. It should also be pointed that other reasons cannot be excluded such as the orientation of crystallographic axes and grain size of the nanowires. At low temperatures, the easy axis for the
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magnetization of the nanowires deviates from its orientation at room temperature due to the increase in the magnetocrystalline anisotropy. The results discussed here reveal that the fcc CoPt nanowire arrays with easy axis of magnetization either parallel or perpendicular to the nanowire axis, having the desired values of Hc and SQ for perpendicular recording media, can be obtained by modifying the diameter and length of the nanowires. Furthermore, we are currently being studying the effects of the composition and magnetic field annealing upon the microstructure and magnetic properties of CoPt nanowire arrays to be used in the magnetic recording media. Enhancement of the magnetic properties through compositional control and crystallographic orientation of the nanowires with magnetic field annealing has been observed and will be reported elsewhere. 4. Conclusion The as-prepared CoPt nanowires are of fcc polycrystalline structure with preferred orientation of (1 1 1) along the nanowires. Magnetic and magnetization properties of fcc CoPt alloy nanowires are determined by the competition among the shape and magnetocrystalline anisotropies and the dipolar magnetostatic interactions. A large variation in the values of Hc (from HcJ ¼ 2032 Oe for d ¼ 40 nm to HcJ ¼ 44 Oe for d ¼ 200 nm) and SQ (from SQJ ¼ 0.93 for d ¼ 40 nm to SQJ ¼ 0.07 for d ¼ 200 nm) is observed depending upon different factors involved in the magnetic properties of the CoPt nanowires. The curling mode of the magnetization reversal process for CoPt nanowires is perceived on the basis of diameter of the nanowires. A crossover of easy axis of magnetization for CoPt nanowires is observed as a function of wire diameter and length. At low temperatures, the easy axis for the magnetization of the nanowires is observed to deviate from room-temperature orientation due to the dominant role of magnetocrystalline anisotropy. Acknowledgments The project was supported by the State Key Project of Fundamental Research of Ministry of Science and Technology (MOST, No. 2006CB932200). X.F. Han thanks the partial support of the National Natural Science
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Foundation (NSFC No.10574156, 50325104, 50528101 and 50721001). The authors S. Shamaila and R. Sharif are on study leave from the University of Engineering and Technology, Lahore, Pakistan. References [1] Z.L. Wang, Nanowires and Nanobelts, Springer, New York, 2006. [2] L. Wuxia, G.A. Jones, Y. Peng, T.H. Shen, J. Appl. Phys. 97 (2005) 104306. [3] N. Yasui, A. Imada, T. Den, Appl. Phys. Lett. 83 (2003) 3347. [4] M.R. Visokay, R. Sinclair, Appl. Phys. Lett. 66 (1995) 1692. [5] Y. Sui, L. Yue, R. Skomski, X.Z. Li, J. Zhou, D.J. Sellmyer, J. Appl. Phys. 93 (2003) 7571. [6] Y. Dahmane, L. Cagnon, J. Voiron, S. Pairis, M. Bacia, L. Ortega, N. Benbrahim, A. Kadri, J. Phys. D 39 (2006) 4523. [7] W. Chen, Z. Li, G.B. Ji, S.L. Tang, M. Lu, Y.W. Du, Solid State Commun. 133 (2005) 235. [8] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, M. Schwickert, J.-U. Thiele, M.F. Doerner, IEEE Trans. Magn. 36 (2000) 10. [9] L. Wuxia, Y. Peng, J. Zhang, G.A. Jones, T.H. Shen, J. Phys: Conf. Ser. 17 (2005) 20. [10] Y.H. Huang, H. Okumura, G.C. Hadjipanayis, D. Weller, J. Appl. Phys. 91 (2002) 6869. [11] T.R. Gao, L.F. Yin, C.S. Tian, M. Lu, H. Sang, S.M. Zhou, J. Magn. Magn. Mater. 300 (2006) 471. [12] J. Mallet, K. Yu-Zhang, C.L. Chien, T.S. Eagleton, P.C. Searson, Appl. Phys. Lett. 84 (2004) 3900. [13] A.K.M. Bantu, J. Rivas, G. Zaragoza, M.A. Lo0 pez-Quintela, M.C. Blanco, J. Appl. Phys. 89 (2001) 3393. [14] Y. Peng, D.H. Qin, R.J. Zhou, H.L. Li, Mater. Sci. Eng. B 77 (2000) 246. [15] M. Ciureanu, F. Beron, L. Clime, P. Ciureanu, A. Yelon, T.A. Ovari, R.W. Cochrane, F. Normandin, T. Veres, Electrochim. Acta 50 (2005) 4487. [16] P.M. Paulus, F. Luis, M. Kroll, G. Schmid, L.J. de Jongh, J. Magn. Magn. Mater. 224 (2001) 180. [17] M.L. Plumer, J.V. Ek, D. Weller, The Physics of Ultra-High-Density Magnetic Recording, Springer, New York, 2006. [18] J. Rivas, A.K.M. Bantu, G. Zaragoza, M.C. Blanco, M.A. Lo0 pezQuintela, J. Magn. Magn. Mater. 249 (2002) 220. [19] H.L. Su, G.B. Ji, S.L. Tang, Z. Li, B.X. Gu, Y.W. Du, Nanotechnology 16 (2005) 429. [20] G.C. Han, B.Y. Zong, P. Luo, Y.H. Wu, J. Appl. Phys. 93 (2003) 9202. [21] A. Aharoni, Introduction to the Theory of Ferromagnetism, Oxford, New York, 2000. [22] H. Zeng, R. Skomski, L. Menon, Y. Liu, S. Bandyopadhyay, D.J. Sellmyer, Phys. Rev. B 65 (2002) 134426. [23] G.T.A. Huysmans, J.C. Lodder, J. Wakui, J. Appl. Phys. 64 (1988) 2016.
Journal of Magnetism and Magnetic Materials 320 (2008) 1803–1809 www.elsevier.com/locate/jmmm
Magnetic and magnetization properties of electrodeposited fcc CoPt nanowire arrays S. Shamailaa, R. Sharifa, S. Riaza, M. Maa, M. Khaleeq-ur-Rahmanb, X.F. Hana, a
State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China b Physics Department, University of Engineering and Technology, Lahore 54890, Pakistan Received 13 April 2007; received in revised form 3 January 2008 Available online 6 March 2008
Abstract Magnetic and magnetization properties of fcc Co1xPtx (xp0.3) alloy nanowires fabricated by electrodeposition into self-synthesized anodic alumina templates are investigated. Magnetization curves, measured for varying wire geometries, show a crossover of easy axis of magnetization from parallel to perpendicular to the nanowire axis as a function of the diameter and length. The measured values of coercivity (Hc) and remanent squareness (SQ) of CoPt nanowire arrays, as a function of angle (y) between the field and wire axis, support the crossover of easy axis of magnetization. The curling mode of the magnetization reversal process is observed for CoPt nanowire arrays. At low temperatures, the easy axis for magnetization of the nanowires is observed to deviate from the room-temperature orientation. r 2008 Elsevier B.V. All rights reserved. PACS: 75.50.y; 75.60.d; 75.75.+a Keywords: Magnetic nanowire; Electrodeposition; Magnetic hysteresis; Angular dependant coercivity; Cobalt platinum
1. Introduction Ferromagnetic nanowires exhibit unique and tunable magnetic and magnetization properties due to their inherent shape anisotropy. The practical application of these magnetic nanowires in the field of nanotechnology is an attractive topic among researchers. This field represents an exciting and rapidly expanding area of research that crosses the borders between physical and engineering sciences. These ideas have driven scientists to develop methods for making nanostructures [1]. The magnetic properties of nanowires have been investigated with particular emphasis on three areas: (1) factors that determine the effective easy axis of the wires, (2) magnetization reversal processes within the arrays and (3) magnetic interaction between the wires [2]. Among ferromagnetic alloy nanowires, CoPt and FePt have been extensively investigated not only from a fundamental but Corresponding author. Tel.: +86 10 826 480 63.
E-mail address: [email protected] (X.F. Han). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.02.183
also from a technological point of view due to their applications in magnetic recording media [3–5] and also their integration capability in functional micro or nanodevices (MEMS/NEMS) [6]. CoPt alloy has potential applications in the perpendicular recording media due to its large anisotropy. It has attracted great interest recently because of its various crystalline phases and its extraordinary properties. These phases can be obtained under specific experimental conditions and have different applications [7]. For example, the CoPt alloy with L10 (fct) structure has very high magnetic anisotropy and is a promising candidate for fine permanent magnets and for high-density magnetic recording media [3,8]. Moreover, various phases of the CoPt alloy provide a good opportunity for us to study the transformation and interplay of different phases [5,7]. The CoPt alloy is also a fine candidate as a magneto-optical material [9,10]. Electrodeposition of platinum and platinum-based alloys is complicated by the catalyzed co-deposition of hydrogen [11,12], and it is only very recently that CoPt nanowires are being successfully electrodeposited in nanoporous alumina
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membrane. There have been limited reports of electrodeposition of CoPt alloy nanowires [2–11]. The low-cost alternating-current (AC) electrodeposition method of fabrication of the nanowires into self-assembled alumite templates has facilitated the researchers to investigate their fundamental magnetic and magnetization properties [13]. These alumite templates consist of highly ordered cylindrical pores with small diameters and an Al layer at the bottom of the pores. Its structure is described as a closedpacked array of columnar cells, each containing a central pore of which the size and interval can be controlled by changing the forming conditions. In recent years, there has been interest in this material as a medium for creating uniform nanostructures since its pores can function as ‘nanotemplates’ in which small metal and semiconductor particles can be electrochemically deposited [14]. The ferromagnetic nanowires fabricated in such templates have high coercivity (Hc) and remanence with an easy axis parallel to the nanowires axis due to the dominant role of shape anisotropy in effective magnetic anisotropy, which can be minimized by increasing the diameter of the templates. For large-diameter nanowire arrays, the shape anisotropy becomes less dominant and can be compared with the magnetocrystalline anisotropy and magnetostatic interactions. These nanowires show relatively lower Hc and squareness, and the easy axis can be either perpendicular or parallel to the wire axis [15]. The coherent and curling modes of rotation are observed for the nanowire diameters lesser and greater than the thickness of the domain wall (lw), respectively [16]. Magnetic properties with desired values of Hc and remanent squareness (SQ) are the important requirements of perpendicular recording media [17]. Therefore studies about Hc and SQ and the factors affecting these values are required. In view of having the potential to be used in perpendicular magnetic recording media, CoPt nanowires have been fabricated into the selfassembled templates using the AC electrodeposition method. The effect of diameter, length, field orientation and temperature on magnetic and magnetization properties of the nanowires are systematically investigated for CoPt nanowire arrays, for the present report. The curling mode of the magnetization reversal process has been observed for CoPt nanowire arrays. Magnetic and magnetization properties are determined from a competition among the shape and magnetocrystalline anisotropies, and the dipolar magnetostatic interactions. A crossover of the easy axis of magnetization as a function of diameter and length has been observed for the fcc CoPt nanowire arrays.
AC-deposition was performed using a standard doubleelectrode bath; the AAO template was used as one electrode, and the platinum wire was used as the other, with a sinusoidal voltage having a root-mean-square (rms) value of 23 V for different periods of time. X-ray diffraction (XRD) was used to monitor the crystal structure of the nanowire arrays. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to investigate the morphology and size of the templates and the nanowires. Magnetic properties of the samples with an applied external magnetic field at various angles were investigated using vibrating sample magnetometer (VSM) and with a superconducting quantum interference device (SQUID). The contents of the CoPt nanowires were determined using induction-coupled plasma spectrometry (ICP). The analysis showed that the composition of individual sample is almost homogeneous. Samples with the composition Co1xPtx (xp0.3) were prepared. For the present report, samples with x ¼ 0.1 where S1, S2, S3 with an average diameter (d) of 40 nm and average interpore distance (D) of 95 nm and L1, L2, L3, L4 with an average diameter (d) of 200 nm and average interpore distance (D) of 250 nm are selected. SEM, TEM, XRD and VSM measurements were carried out at room temperature, and temperature-dependent measurements were done with a SQUID between liquid helium temperature and room temperature. 3. Results and discussion Fig. 1 shows some characteristics of the as-deposited CoPt nanowires with average diameters (d) of 40 and 200 nm. Figs. 1(a) and (b) are the TEM images of isolated CoPt nanowires with diameters d ¼ 200 and 40 nm, respectively. TEM images of the nanowire samples were taken after releasing them from the alumina membrane,
2. Experimental details The highly ordered anodic aluminum oxide (AAO) templates were prepared as described previously [9]. The arrays of CoPt nanowires were electrodeposited into the AAO templates by the AC-electrochemical deposition method in an electrolyte consisting of PtCl4, CoSO4 7H2O, and H3BO3 at a pH value of about 3.0. The
Fig. 1. TEM images of isolated CoPt nanowires of diameter (a) 200 nm and (b) 40 nm. (c) SEM image of AAO template with diameter of about 40 nm. (d) SEM image of CoPt nanowires in the supported alumina template.
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Fig. 2. XRD patterns of aligned CoPt nanowires in the AAO template of diameter (a) 40 nm and (b) 200 nm.
by dissolving it for about 12 h in 1 M NaOH solution. Figs. 1(c) and (d) are the SEM images of CoPt nanowires in the supported AAO template. To examine the microstructure of these nanowires, the sample was fractured to produce cross-sectional regions that expose the CoPt nanowires on the fractured surface of the AAO template. Fig. 2 shows the XRD pattern of aligned CoPt nanowires in the AAO templates of diameters (a) 40 nm and (b) 200 nm. For XRD measurements, the Al substrates were etched away by an amalgamation process using a saturated HgCl2 aqueous solution and washed thoroughly with distilled water. In the as-synthesized conditions the XRD patterns show that the samples of CoPt nanowires comprise of an fcc phase with the identifiable diffraction peaks, namely (1 1 1) and (2 0 0) of which the (1 1 1) reflection is the most intense. For 200 nm CoPt nanowires the (2 2 0) peak has also been detected, together with the diffraction peaks (1 1 1) and (2 0 0). There is a peak shift of the dominant plane (1 1 1) for the nanowires (200 nm), as can be seen in Fig. 2(b). This shift and the additional peak (2 2 0) may be attributed to the more random distribution of grains [11] with a disordered fcc structure for 200 nm diameter. The intensity of the (1 1 1) peak is much larger than those of other peaks for both diameters. Moreover, the Co (1 1 1) diffraction peak is also detected at 2y ¼ 44.51 for 40 nm diameter. These XRD results suggest that the CoPt nanowires are of fcc polycrystalline structure with randomly oriented grains. Different factors affecting the magnetization behavior of the CoPt nanowires are investigated. These factors include nanowire geometry i.e., wire diameter and length. The typical room-temperature magnetic hysteresis (M–H) curves for 40 and 200 nm CoPt nanowires, with the external field applied parallel and perpendicular to the nanowire axis, are shown in Figs. 3(a–c) and (d–f),
Fig. 3. M–H curves for CoPt nanowire arrays of diameter 40 (a–c) and 200 nm (d–f) with different wire lengths.
respectively. The difference between the perpendicular and parallel M–H curves defines the uniaxial anisotropy for CoPt nanowire arrays. For the 40 nm CoPt nanowires (Fig. 3(a)) Hc and SQ in the parallel (J) geometry are larger than that in the perpendicular (?) geometry attributing to the parallel alignment of the magnetic easy axis along the wire axis [12]. The value of Hc in the two geometries approaches each other for the 200 nm CoPt nanowires, as shown in Fig. 3(d). However, SQ in the perpendicular geometry is larger than that in the parallel geometry. Therefore, the easy axis of magnetic anisotropy favors to
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Table 1 Magnetic properties for CoPt nanowire arrays with diameters 40 and 200 nm having different wire lengths Samples
S1 S2 S3 L1 L2 L3 L4
Diameter (nm)
40 40 40 200 200 200 200
Length (mm)
2 8 20 0.8 3 10 15
Hc (Oe)
SQ (Mr/Ms)
DHs (Hs||Hs?)
J
?
J
?
2032.9 1438.1 1051.1 652.1 106.8 189.5 44.4
575.1 721.6 555.2 302.7 109.1 141.2 83.24
0.93 0.88 0.84 0.38 0.1 0.07 0.01
0.15 0.20 0.21 0.14 0.12 0.18 0.03
be aligned perpendicular to the wire axis. This variation in the alignment of easy axis for the two diameters shows a crossover of easy axis of magnetization as a function of diameter. The values of magnetic parameters Hc, SQ and the difference of the saturation fields (DHs) are presented in Table 1. Comparison of the parameters, given in Table 1 shows the effect of the wire diameter and length on the magnetic properties of CoPt nanowire arrays. The alignment of easy axis can also be specified by the sign of DHs, here DHs ¼ Hs||Hs? [15]. Here HsJ is the saturation field when the magnetic field is applied parallel to the nanowire axis and Hs? is the saturation field when the magnetic field is applied perpendicular to the nanowires axis. The negative sign of DHs indicates the easy axis along the nanowire axis and positive sign shows the orientation of easy axis perpendicular to the nanowire axis. Figs. 3(a–f) show the effect of wire geometry on the magnetic properties for CoPt nanowire samples of the two diameters. For CoPt nanowire samples with d ¼ 40 nm, the easy axis remains parallel to the wire axis in the whole investigated range of lengths (Figs. 3(a–c)). For wire arrays with d ¼ 200 nm a different behavior can be observed (Figs. 3(d–f)), for which parallel magnetization is favored for very short wires; whereas, perpendicular magnetization is favored for long nanowires. Thus, in fcc CoPt nanowires, it is observed that when the wire length is larger than a critical value, the parallel wire axis crosses over from easy to hard as also was observed by others for Co nanowire arrays [18]. This crossover is clear from the shape of M–H curves as well as from the sign of DHs of these samples (Table 1). For sample L3 in Fig. 3(d) the easy axis of nanowires lies perpendicular to the nanowire axis, DHs is positive in this case. It is also observed for L3 that HcJ is a little bit larger than Hc? (Table 1), which may be due to the random distribution of grains as shown by XRD results. For sample L2, DHs decreases, which can also be observed from the shape of the M–H curve (see Fig. 3(e)). For L1 with small wire length DHs is negative indicating that easy axis is now parallel to the wire axis (Fig. 3(f)). This effect can be used to turn the parallel wire axis from easy to hard, by modifying the length. Additional evidence for this crossover of easy axis is provided by the angular dependence of
2164.45 2667.19 3375.59 717.45 1092.47 4018.07 397.95
the Hc and SQ of CoPt nanowire samples measured as a function of wire diameter and length (Fig. 4). Hc(y) and SQ(y) curves show bell-shaped or otherwise bell-shaped behavior corresponding to the easy axis of their magnetization. Bell-shaped curves for samples S1, S2 and S3 with d ¼ 40 nm and otherwise bell-shaped curves for samples L2, L3, L4 with d ¼ 200 nm confirm the crossover of easy axis from parallel to perpendicular as a function of diameter. Furthermore, the bell-shaped behavior of sample L1 of length ¼ 800 nm with the easy axis parallel to the wire axis (see Fig. 3(f)) shows the crossover of easy axis as a function of length. Figs. 4(a, b) also show that values for both Hc and SQ are large for samples with d ¼ 40 nm. These large values of Hc and SQ, and negative sign of nHs for samples S1, S2 and S3 are due to the dominant shape anisotropy. The values of Hc and SQ are comparatively small for samples L1, L2, L3 and L4 with d ¼ 200 nm having interwire distance of 250 nm. This decrease in Hc and SQ with increasing diameter is due to the decrease of shape anisotropy and an increase of the magnetostatic dipole interactions, because the interwire distance of these samples is not much larger than the wire diameter [19]. The overall anisotropic field (Hk) is mainly determined by following three contributions: (1) the shape anisotropy field (2pMs), which will induce a magnetic easy axis parallel to the nanowire axis, (2) magnetostatic dipole interaction field among the wires, which will induce a magnetic easy axis perpendicular to the nanowire axis and (3) the magnetocrystalline anisotropy field (Hma). The total effective anisotropic field (Hk) is given as follows [20]: H k ¼ 2pM s 6:3pM s r2 L=D3 þ H ma ,
(1)
where Ms is the saturation magnetization, r is the radius of the wire, L is the length and D is the interwire distance. The second term in Eq. (1) is the total dipole field acting on one wire due to all other wires. Eq. (1) predicts that as the wire length increases, Hk linearly decreases to zero when L ¼ Lc ¼ 2D3 =ð6:3r2 Þ,
(2)
while neglecting the contribution of Hma. For L4Lc, Hk is negative pointing out that there is a crossover of easy axis for magnetization from parallel to perpendicular to the nanowires axis. For CoPt nanowire arrays with
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Fig. 4. Angular dependence of (a) coercivity (Hc(y)) and (b) remanence (SQ(y)) of CoPt nanowire arrays where y is the angle between the field direction and the nanowire axis.
d ¼ 200 nm, r ¼ 100 nm and D ¼ 250 nm; we calculate Lc1 mm. The shape anisotropy field is weak in this case; therefore when L4Lc, Hk is negative and crossover of easy axis of magnetization from parallel to perpendicular to the nanowire axis is observed. For CoPt nanowire arrays with d ¼ 40 nm and D ¼ 95 nm the shape anisotropy is more dominant resulting in positive Hk, even for L4calculated Lc, because the dipolar interaction in this case may be much less than that of nanowires with d ¼ 200 nm and D ¼ 250 nm. The M–H curves at different temperatures are also measured to observe the temperature-dependent magnetic properties of CoPt nanowires. The M–H curves at temperatures (T) 5–300 K for 40 and 200 nm CoPt nanowires are shown in Fig. 5. It is observed that the shape of M–H curves changes for both diameters when the samples are cooled down to 5 K. For d ¼ 40 nm, shearing of the M–H loop for parallel geometry and expanding of M–H loop for perpendicular geometry suggest that at low temperature the easy axis is no longer fully parallel to the wire axis. For d ¼ 200 nm, SQ in the perpendicular geometry is much larger than that in the parallel geometry attributing to the perpendicular alignment of easy axis (see Fig. 3(d)). The change in the shape of M–H curve (see Figs. 5(c, d)) at T ¼ 5 K reveals that the easy axis of nanowire arrays with d ¼ 200 nm tends to turn over from perpendicular to the parallel direction. At low temperature, magnetocrystalline anisotropy becomes dominant towards the effective anisotropy caused by the spin–orbit interactions [21]. This magnetocrystalline anisotropy tends to rotate the orientation of easy axis of magnetization from high-temperature orientation. This anisotropy of the nanowires decreases with an increase in temperature. Therefore, at high temperature, for d ¼ 40 nm shape
Fig. 5. M–H curves for CoPt nanowire arrays of diameter 40 (a, b) and 200 nm (c, d) measured at T ¼ 5 and 300 K.
anisotropy becomes prevalent resulting in the orientation of easy axis along the parallel direction, and for d ¼ 200 nm magnetostatic interaction increases resulting in the alignment of easy axis perpendicular to the nanowire axis. Figs. 6(a, b) show the temperature dependence of Hc measured parallel and perpendicular to the 40 and 200 nm CoPt nanowires. Hc values show different variations with temperature for two diameters. This different behavior can be explained by the competition between the shape anisotropy and the temperature-dependent magnetocrystalline anisotropy for both diameters. For d ¼ 200 nm, difference in the values of Hc (DHc ¼ HcJHc?) is large at T ¼ 5 K and it decreases with increasing temperature. This decrease in HcJ and Hc? with increasing temperature is due
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Fig. 6. Temperature dependence of coercivity (Hc) for CoPt nanowire arrays of diameter (a) 40 and (b) 200 nm measured with field parallel and perpendicular to the nanowires.
to the thermal relaxation over the anisotropy energy barrier [16]. For d ¼ 40 nm, Hc in the two geometries tends to approach each other at low temperatures and DHc increases with increasing temperature. Moreover, HcJ does not decrease monotonically as temperature increases, but it shows a minimum, whereas Hc? decreases monotonically which is in accordance with the report by Paulus et al. [16]. The decrease in Hc (for d ¼ 40 nm) in both directions at low temperatures is due to thermal relaxation over the anisotropy energy barrier similar to that of d ¼ 200 nm, but at high temperatures, increase in HcJ and a further decrease of Hc? is due to the dominant shape anisotropy for these nanowires. The two most common magnetization reversal modes can be modeled by coherent rotation or curling. For magnetic nanowires the magnetization reversal mechanism depends upon the diameter of the nanowires. For a specific material, the critical diameter for the transition from coherent rotation to curling is given by dc ¼ 2.08(A1/2/Ms) where A is the exchange stiffness and Ms is the saturation magnetization [22]. For the nanowires with diameter larger than the critical diameter, the magnetization reversal process can be described by the curling mode, and Hc decreases with increasing diameter of the nanowires. Mallet et al. have calculated the critical diameter dc ¼ 14 nm for fcc CoPt nanowires [12]. Since the diameter of our samples (40–200 nm) is larger than the critical diameter (14 nm) of CoPt nanowires for coherent rotation, the reversal is expected to occur through the curling mode. The equation describing the dependence of Hc on the diameter of nanowires in the curling mode is given as follows [12]: H c ¼ 2pkA=½M s r2 þ 2KU=½M s
(3)
where r is the radius of the nanowire and k is a constant related to the shape of the material (1.08 for an infinite
cylinder). The right term 2KU/Ms is Hma with uniaxial anisotropy constant KU. For the present CoPt nanowire arrays the values of Hc decrease with an increase in diameter (Table 1). Eq. (1) can explain the variations in the magnetic behavior of the nanowire arrays as a function of diameter. Hc(y) curves (cf. Fig. 5(a)) are consistent with the easy axis orientation, but they show some complexities with the curling reversal mode. Such complexities can be attributed to the strong dependence of effective anisotropy field on the shape anisotropy, especially in case of nanowires having d ¼ 40 nm with D ¼ 95 nm. Similar angular dependence of Hc (manifesting curling mode) has also been observed in Refs. [11,23]. Therefore by considering the effective anisotropy energy, Hc(y) cannot account for the observed reversal mode. The present as-deposited CoPt nanowires have fcc structure with preferred orientation of (1 1 1) along the nanowires. A large variation in the values of Hc and SQ is observed depending upon diameter, length, field direction and temperature. These factors are involved in the magnetic characterization of the CoPt nanowires. The orientation of easy axis of magnetization can be deterP mined by the total energy E in the parallel and P perpendicular geometries; where E can be obtained by taking into account the magnetostatic interaction energy Edi, the demagnetization energy Ede and the magnetocrystalline anisotropy EK, where EdiJ4Edi?, EdeJoEde?, EKJoEK? [11]. For large diameters with small P interwire P distances the dipolar interaction is increased, EJ E?, and the easy axis favors to be alignedPperpendicular to the P wire axis [18]. For small diameters EJo E?, and the easy axis favors to be aligned along the wire axis. It should also be pointed that other reasons cannot be excluded such as the orientation of crystallographic axes and grain size of the nanowires. At low temperatures, the easy axis for the
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magnetization of the nanowires deviates from its orientation at room temperature due to the increase in the magnetocrystalline anisotropy. The results discussed here reveal that the fcc CoPt nanowire arrays with easy axis of magnetization either parallel or perpendicular to the nanowire axis, having the desired values of Hc and SQ for perpendicular recording media, can be obtained by modifying the diameter and length of the nanowires. Furthermore, we are currently being studying the effects of the composition and magnetic field annealing upon the microstructure and magnetic properties of CoPt nanowire arrays to be used in the magnetic recording media. Enhancement of the magnetic properties through compositional control and crystallographic orientation of the nanowires with magnetic field annealing has been observed and will be reported elsewhere. 4. Conclusion The as-prepared CoPt nanowires are of fcc polycrystalline structure with preferred orientation of (1 1 1) along the nanowires. Magnetic and magnetization properties of fcc CoPt alloy nanowires are determined by the competition among the shape and magnetocrystalline anisotropies and the dipolar magnetostatic interactions. A large variation in the values of Hc (from HcJ ¼ 2032 Oe for d ¼ 40 nm to HcJ ¼ 44 Oe for d ¼ 200 nm) and SQ (from SQJ ¼ 0.93 for d ¼ 40 nm to SQJ ¼ 0.07 for d ¼ 200 nm) is observed depending upon different factors involved in the magnetic properties of the CoPt nanowires. The curling mode of the magnetization reversal process for CoPt nanowires is perceived on the basis of diameter of the nanowires. A crossover of easy axis of magnetization for CoPt nanowires is observed as a function of wire diameter and length. At low temperatures, the easy axis for the magnetization of the nanowires is observed to deviate from room-temperature orientation due to the dominant role of magnetocrystalline anisotropy. Acknowledgments The project was supported by the State Key Project of Fundamental Research of Ministry of Science and Technology (MOST, No. 2006CB932200). X.F. Han thanks the partial support of the National Natural Science
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Foundation (NSFC No.10574156, 50325104, 50528101 and 50721001). The authors S. Shamaila and R. Sharif are on study leave from the University of Engineering and Technology, Lahore, Pakistan. References [1] Z.L. Wang, Nanowires and Nanobelts, Springer, New York, 2006. [2] L. Wuxia, G.A. Jones, Y. Peng, T.H. Shen, J. Appl. Phys. 97 (2005) 104306. [3] N. Yasui, A. Imada, T. Den, Appl. Phys. Lett. 83 (2003) 3347. [4] M.R. Visokay, R. Sinclair, Appl. Phys. Lett. 66 (1995) 1692. [5] Y. Sui, L. Yue, R. Skomski, X.Z. Li, J. Zhou, D.J. Sellmyer, J. Appl. Phys. 93 (2003) 7571. [6] Y. Dahmane, L. Cagnon, J. Voiron, S. Pairis, M. Bacia, L. Ortega, N. Benbrahim, A. Kadri, J. Phys. D 39 (2006) 4523. [7] W. Chen, Z. Li, G.B. Ji, S.L. Tang, M. Lu, Y.W. Du, Solid State Commun. 133 (2005) 235. [8] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, M. Schwickert, J.-U. Thiele, M.F. Doerner, IEEE Trans. Magn. 36 (2000) 10. [9] L. Wuxia, Y. Peng, J. Zhang, G.A. Jones, T.H. Shen, J. Phys: Conf. Ser. 17 (2005) 20. [10] Y.H. Huang, H. Okumura, G.C. Hadjipanayis, D. Weller, J. Appl. Phys. 91 (2002) 6869. [11] T.R. Gao, L.F. Yin, C.S. Tian, M. Lu, H. Sang, S.M. Zhou, J. Magn. Magn. Mater. 300 (2006) 471. [12] J. Mallet, K. Yu-Zhang, C.L. Chien, T.S. Eagleton, P.C. Searson, Appl. Phys. Lett. 84 (2004) 3900. [13] A.K.M. Bantu, J. Rivas, G. Zaragoza, M.A. Lo0 pez-Quintela, M.C. Blanco, J. Appl. Phys. 89 (2001) 3393. [14] Y. Peng, D.H. Qin, R.J. Zhou, H.L. Li, Mater. Sci. Eng. B 77 (2000) 246. [15] M. Ciureanu, F. Beron, L. Clime, P. Ciureanu, A. Yelon, T.A. Ovari, R.W. Cochrane, F. Normandin, T. Veres, Electrochim. Acta 50 (2005) 4487. [16] P.M. Paulus, F. Luis, M. Kroll, G. Schmid, L.J. de Jongh, J. Magn. Magn. Mater. 224 (2001) 180. [17] M.L. Plumer, J.V. Ek, D. Weller, The Physics of Ultra-High-Density Magnetic Recording, Springer, New York, 2006. [18] J. Rivas, A.K.M. Bantu, G. Zaragoza, M.C. Blanco, M.A. Lo0 pezQuintela, J. Magn. Magn. Mater. 249 (2002) 220. [19] H.L. Su, G.B. Ji, S.L. Tang, Z. Li, B.X. Gu, Y.W. Du, Nanotechnology 16 (2005) 429. [20] G.C. Han, B.Y. Zong, P. Luo, Y.H. Wu, J. Appl. Phys. 93 (2003) 9202. [21] A. Aharoni, Introduction to the Theory of Ferromagnetism, Oxford, New York, 2000. [22] H. Zeng, R. Skomski, L. Menon, Y. Liu, S. Bandyopadhyay, D.J. Sellmyer, Phys. Rev. B 65 (2002) 134426. [23] G.T.A. Huysmans, J.C. Lodder, J. Wakui, J. Appl. Phys. 64 (1988) 2016.