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Magnetic properties of Fe0.95Pd0.05 nanowire arrays
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 320 (2008) 2305– 2309
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties of Fe0.95Pd0.05 nanowire arrays Haining Hu a,b,, Chunhu Yang a, Jinglan Chen c, Guangheng Wu c a b c
Department of Mathematics and Physics, Shanghai University of Electric Power, Shanghai 200090, PR China Applied Surface Physics Laboratory and Department of Physics, Fudan University, Shanghai 200433, PR China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, PR China
a r t i c l e in fo
abstract
Article history: Received 3 August 2007 Received in revised form 11 April 2008 Available online 2 May 2008
Fe0.95Pd0.05 nanowires were fabricated by the electrodeposition in porous anodic aluminum oxide templates and post-annealed at 300–700 1C. Transmission electron microscopy observations demonstrated the isolated nanowires to have polycrystalline structure. Magnetic measurements, however, showed improvement of both coercivity and squareness with the addition of 5 at% Pd in the Fe nanowires as well as proper annealing temperatures of about 500 1C. & 2008 Elsevier B.V. All rights reserved.
PACS: 75.75 81.07 81.07.B Keywords: Nanowire Electrodeposition Magnetic property Structure
1. Introduction
2. Experimental procedure
The FePt, CoPt, and FePd binary alloy systems have recently attracted considerable interest because of their applications in high-density perpendicular magnetic recording media. In fact, there are many reports on FePt and CoPt nanowires [1–5] as well as numerous investigations on FePd alloys utilized in bulk as well as thin film forms [6–8]. These binary alloys, in particular, can form a chemically ordered L10 phase that exhibits large uniaxial magnetocrystalline anisotropy. However, with regard to FePd nanowires, excluding this group’s previous study [9,10], these have been significantly underreported. In this paper, we investigated the array of highly ordered FePd nanowires by dc electrodeposition into porous anodic aluminum oxide (AAO) templates. At different annealing temperatures, the structure has been verified by conventional X-ray diffraction (XRD) and transmission electron microscopy (TEM). On the other hand, the chemical composition was characterized by energy-dispersive X-ray spectroscopy (EDS) of scanning electron microscopy (SEM), while the magnetic properties were measured using a superconducting quantum interference device (SQUID) magnetometer.
Porous anodic aluminum oxide (AAO) templates with a pore length of about 40 mm and a uniformity diameter of about 60 nm were prepared under a two-step anodizing process [11]. Aluminum foils with purity as high as 99.99% were anodized in 0.3 M oxalic acid solution under a constant voltage of 40 V at 12 1C. Following anodization, the remaining aluminum was removed by a saturated CuCl2 solution. Etching treatment was then carried out in 6 wt% H3PO4 at 30 1C for 60 min to remove the barrier layer on the bottom side of the AAO and obtain the through holes template. Afterwards, a 200 nm Cu layer was sputter-deposited (using DC magnetron sputtering) onto one side of the AAO; this served as the working electrode for the following DC electrodeposition. An aqueous bath containing 0.2 M FeCl2+0.004 M PdCl2 was adjusted to pH ¼ 3 by adding appropriate dilute HCl. Next, electrodeposition of FexPd1x nanowires was carried out at a potential of 0.9V1.2V relative to the saturated calomel reference electrode (SCE); this was done through a three-electrode system with a graphite rod as the reference electrode. A series of FexPd1x nanowires with different Fe compositions in the range of x ¼ 0.2–0.95 was then obtained. Pure Fe nanowires were deposited with a solution of 0.2 M FeCl2 with dilute HCl under a potential of 1.2 V relative to the SCE.
Corresponding author at: Department of Mathematics and Physics, Shanghai University of Electric Power, Shanghai 200090, PR China. Tel.: +86 21 6802 0533. E-mail address: [email protected] (H. Hu).
0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.04.157
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The composition of the FexPd1x alloy nanowire array was determined by EDS in SEM [1,5]. All of the magnetic measurements were performed on a SQUID magnetometer (Quantum Design, MPMS-5 s). The coercivity (Hc) and squareness (Mr/Ms) of the nanowire arrays embedded in the AAO template meanwhile were obtained from hysteresis loops. All measurements were carried out at 5 and 300 K. After electrodeposition, Fe0.95Pd0.05 nanowire arrays were annealed at different annealing temperatures, Ta (Ta ¼ 300, 400, 500, 600 and 700 1C), for 1 h under the protection of Ar atmosphere. Before TEM observation, a piece of AAO template with Fe0.95Pd0.05 nanowires was dissolved by 5 wt% NaOH, and washed with alternating distilled water and ethanol for 3–5 times. Afterwards, the nanowires were detached from the substrate by ultrasonic dispersion in 2–3 ml ethanol. A drop of the solution was consequently placed on a Cu grid with carbon film and air dried prior to electron microscope analysis. The structure and morphology of Fe0.95Pd0.05 nanowires at different annealing temperatures were investigated by means of TEM and selected area electron diffraction (SAED).
3. Results and discussion Fig. 1 illustrates the coercivity of the as-deposited FexPd1x alloy nanowires measured at 5 K. Compared with the pure Fe nanowires that have a coercivity of 540 Oe at a similar diameter, the coercivity of these alloy nanowires improved with a Pd component lower than 50 at%. With decreasing Fe content, however, the coercivity of the FexPd1x alloy nanowires rapidly
800
HC (Oe)
600
400
200
0 20
40 60 Fe concentration (%)
80
100
Intensity (cps)
(211)
Pd (111)
5000
(110)
Fig. 1. The coercivity of FexPd1x alloy versus the composition of Fe element x in alloy nanowires.
as-deposite 2500
300°C 400°C 500°C 600°C 700°C 40
60 2θ (deg.)
80
Fig. 2. XRD of as-deposited Fe0.95Pd0.05 nanowire arrays and after annealing at different temperatures Ta.
Fig. 3. TEM image of the Fe0.95Pd0.05 nanowire and its selected-area electron diffraction (inset): (a) as-deposited, (b) Ta ¼ 500 1C and (c) Ta ¼ 600 1C.
ARTICLE IN PRESS H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 2305–2309
2307
1.0 1000
Hc (Oe)
M/MS
0.5
0.0
500 5K 300K
-0.5
-1.0 1.0 0.8
M/MS
M/MS
0.5
0.0
0.4 5K 300K
-0.5
0.0
-1.0
0
-2500
0 H (Oe)
2500
Fig. 4. Hysteresis loops of the as-deposited Fe (a) and Fe0.95Pd0.05 alloy (b) nanowire arrays, with the magnetic field applied parallel (— —) or perpendicular (— —) to the wire’s axis at 5 K.
decreased. The highest coercivity was found in Fe0.95Pd0.05. Therefore, this paper investigated the Fe0.95Pd0.05 nanowires in detail. The XRD spectra of the as-deposited and post-annealed Fe0.95Pd0.05 nanowire arrays embedded in templates are demonstrated in Fig. 2. For the as-deposited FePd nanowire arrays, bcc Fe (110), (2 11) and fcc Pd (111) were evident. Apparently, the Pd element existed in the as-deposited nanowires. Moreover, after annealing at Ta ¼ 300, 400 and 500 1C, these three peaks became weaker with increasing annealing temperatures. At Ta ¼ 600 1C only the peak of Fe (110) could be seen, but very weak, while at 700 1C all peaks disappeared from the XRD spectra. It was also determined that with increasing Ta the Fe (110) peak shifted to the lower angle. This signifies the formation of some FePd alloys following annealing [8]. In comparison, pure Fe nanowire arrays, when electrodeposited, showed a (110) preferred orientation [12]. Fig. 3 shows the TEM image of an isolated Fe0.95Pd0.05 nanowire in the as-deposition and post-annealing states at Ta ¼ 500 and 600 1C, respectively. From all three pictures, one can find that the nanowire was continuous with a uniform diameter of about 60 nm, corresponding to the AAO template used. The selected area electron diffraction (SAED) in Fig. 3a specifically illustrates the as-deposited Fe0.95Pd0.05 nanowire exhibiting a polycrystalline structure. After anneal treatment at Ta ¼ 500 1C, the nanowire maintained and, to some extent, even improved its crystalline structures. In contrast, a higher annealing temperature of Ta ¼ 600 1C produced many defects. The magnetic hysteresis loops of both as-deposited pure Fe (a) and Fe0.95Pd0.05 alloy (b) nanowire arrays were measured by SQUID, as shown in Fig. 4. It can be seen that both Fe and Fe0.95Pd0.05
200
400 Ta (°C)
600
800
Fig. 5. The coercivity (a) and squareness (b) of Fe0.95Pd0.05 nanowire arrays embedded in AAO template versus annealing temperature Ta, with the field applied parallel to the wire at 5 and 300 K.
nanowires have parallel to the wire easy magnetization. The coercivity and squareness of the Fe nanowires were also recorded at 540 Oe and 29%, respectively, while those of the Fe0.95Pd0.05 nanowires corresponded to 870 Oe and 65%. These hysteresis loops clearly show that trace Pd elements doped in the Fe0.95Pd0.05 alloy nanowires improved their coercivity and squareness. To identify the influence of heat treatment, the magnetic properties of annealed Fe0.95Pd0.05 alloy nanowires were measured at 5 and 300 K, as demonstrated by the coercivity (a) and squareness (b) versus annealing temperatures, Ta, in Fig. 5. It can be observed that the coercivity and squareness had similar variation trends at 5 and 300 K. For coercivity, this slightly increased with increasing Ta before 500 1C, reached the maximum point at Ta ¼ 600 1C, then decreased at Ta ¼ 700 1C. As for the squareness, it peaked its maximum at the annealing temperature of 500 1C, decreased at Ta ¼ 600 1C, and reached its nadir at Ta ¼ 700 1C. In this regard, we suggest that the change of magnetic properties after annealing is related to microstructural changes during the annealing process, as further discussed below. In demonstrating the influence of heat treatment clearly, hysteresis loops for the as-deposited and annealed nanowires at Ta ¼ 500, 600, and 700 1C were measured at 5 K, as shown in Fig. 6. For the as-deposited sample, the nanowires exhibited a parallel to the wire easy magnetization axis with a coercivity of 870 Oe and squareness of 74%. After annealing at 500 1C, the coercivity increased to 980 Oe and the squareness equaled 81.6%. However, when increased to Ta ¼ 600 1C, the largest coercivity of 1140 Oe was achieved, but the squareness decreased to about 64.1%. At the highest annealing temperature of Ta ¼ 700 1C, both the coercivity
ARTICLE IN PRESS 2308
H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 2305–2309
1.0
M/MS
0.5 0.0 -0.5 -1.0 1.0
M/MS
0.5 0.0 -0.5 -1.0 -2500
0 H (Oe)
2500
-2500
0 H (Oe)
2500
Fig. 6. Hysteresis loops of the Fe0.95Pd0.05 nanowire arrays under different annealing temperature, where (a) as-deposited, (b) Ta ¼ 500 1C, (c) Ta ¼ 600 1C and (d) Ta ¼ 700 1C. The magnetic field was applied parallel (— —) or perpendicular (— —) to the wire at 5 K.
and squareness decreased sharply, showing magnetic properties similar to superparamagnetic materials. It is well known that anodic aluminum oxide template forms g-Al2O3 and can only be stabilized under low temperatures. After heating to 750 1C, g-Al2O3 changes to a-Al2O3, accompanied with volume shrinkage. In Fig. 6, with an annealing temperature of 500 1C, clearly much lower than 750 1C, the hysteresis loops showed a more square shape than other samples. This may be partly due to the alloy of FePd nanowire, which is also consistent with the improvement of SEAD in TEM, after 500 1C annealing. It was also observed that, while samples embedded in the AAO were annealed at temperatures as high as 600 1C, internal stress that increased in the AAO brought about by the annealing temperature distorted the alumina [13]. On the basis of the TEM with XRD results, after 600 1C annealing, the internal stress in the AAO template induced defects in the alloy nanowires shown as reflection spots in the SAED. These defects block the domain wall movement and increase the proportion of magnetization rotation, thereby improving the coercivity and decreasing the squareness. It can likewise be seen in Fig. 6 that both perpendicularity and parallelism to the wire coercivity were increased with an annealing temperature of 600 1C. With the highest annealing temperature of 700 1C, closer to the transition phase of g to a-Al2O3, the nanowires would be crushed by the shrinkage of the template and then react with the accompanying release of O2 [13,14]. These two effects would decompose the FePd nanowires and reduce its grain size down to below singledomain scale to approach superparamagnetic level. In this experiment, we saw the magnetization decreased by an order following annealing treatment at 700 1C; this also validates the decomposition of the nanowire to very small granules [15].
4. Conclusions Fabrication of Fe0.95Pd0.05 alloy nanowires was successfully done through electrodeposition into AAO templates. TEM observations reveal that isolated nanowires have polycrystalline structures, while magnetic measurements show that both coercivity and
squareness have improved with the addition of 5 at% Pd in Fe nanowires at proper annealing temperatures. The annealing process, on the other hand, maintained the crystalline structure of the Fe0.95Pd0.05 alloy nanowires, if not improved to some degree; this was up to an annealing temperature of about 500 1C. An annealing temperature increasing up to 600 1C was found to cause an increase in internal stress in the AAO, thereby inducing defects in the nanowires. The highest annealing temperature of 7001C was perceived to cause crushing of the nanowires brought about by the shrinkage of the template as well as reacting to the resultant O2 during the transition phase of the AAO template. These two effects produce decomposition of the nanowires into very small granules nearly the scale of superparamagnetic FePd, which sharply reduces their coercivity and squareness. With all these into consideration, the sample annealed at Ta ¼ 500 1C has the largest coercivity and longitudinal anisotropy, therefore generating an optimal heat treatment condition of around 500 1C.
Acknowledgment This work is supported partly by the Shanghai Educational Foundation for General Science (Grant No. 06LZ004), and partly by the Shanghai Educational Science Foundation for Excellent Young Scholars (Grant no. Z-2006-84). References [1] Y.H. Huang, H. Okumura, G.C. Hadjipanayis, D. Weller, J. Appl. Phys. 91 (2002) 6869. [2] S.Z. Chu, S. Inoue, K. Wada, Y. Kanke, K. Kurashima, J. Electrochem. Soc. 152 (2005) 1. [3] Y.K. Su, D.H. Qin, H.L. Zhang, H. Li, H.L. Li, Chem. Phys. Lett. 388 (2004) 406. [4] Y.C. Sui, R. Skomski, K.D. Sorge, D.J. Sellmyer, Appl. Phys. Lett. 84 (2004) 1525. [5] W.X. Li, Y. Peng, G.A. Jones, T.H. Shen, G. Hill, J. Appl. Phys. 97 (2005) 034308. [6] V. Gehanno, Y. Samson, A. Marty, B. Gilles, A. Chamberod, J. Magn. Magn. Mater. 172 (1997) 26. [7] K. Sato, Y. Hirotsu, J. Appl. Phys. 93 (2003) 6291. [8] M. Birsan, B. Fultz, L. Anthony, Phys. Rev. B 55 (1997) 11502. [9] H.N. Hu, J.L. Chen, G.H. Wu, L.J. Chen, H.Y. Liu, Y.X. Li, J.P. Qu, Acta Phys. Sin. 54 (2005) 4370 (in Chinese).
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[10] L.J. Chen, Y.X. Li, G.F. Chen, H.Y. Liu, X.X. Liu, G.H. Wu, Acta Phys. Sin. 55 (2006) 5516 (in Chinese). [11] H. Masuda, K. Fukuda, Science 268 (1995) 1466. [12] H.N. Hu, H.Y. Chen, J.L. Chen, G.H. Wu, Physica. B 368 (2005) 100.
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[13] J.B. Ding, D.H. Qin, Y. Huang, L. Cao, H.L. Li, J. Mater. Eng. Perf. 12 (2003) 584. [14] D.H. Qin, L. Cao, Q.Y. Sun, Y. Huang, H.L. Li, Chem. Phys. Lett. 358 (2002) 484. [15] G. Herzer, IEEE Trans. Magn. 26 (1990) 1397.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties of Fe0.95Pd0.05 nanowire arrays Haining Hu a,b,, Chunhu Yang a, Jinglan Chen c, Guangheng Wu c a b c
Department of Mathematics and Physics, Shanghai University of Electric Power, Shanghai 200090, PR China Applied Surface Physics Laboratory and Department of Physics, Fudan University, Shanghai 200433, PR China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, PR China
a r t i c l e in fo
abstract
Article history: Received 3 August 2007 Received in revised form 11 April 2008 Available online 2 May 2008
Fe0.95Pd0.05 nanowires were fabricated by the electrodeposition in porous anodic aluminum oxide templates and post-annealed at 300–700 1C. Transmission electron microscopy observations demonstrated the isolated nanowires to have polycrystalline structure. Magnetic measurements, however, showed improvement of both coercivity and squareness with the addition of 5 at% Pd in the Fe nanowires as well as proper annealing temperatures of about 500 1C. & 2008 Elsevier B.V. All rights reserved.
PACS: 75.75 81.07 81.07.B Keywords: Nanowire Electrodeposition Magnetic property Structure
1. Introduction
2. Experimental procedure
The FePt, CoPt, and FePd binary alloy systems have recently attracted considerable interest because of their applications in high-density perpendicular magnetic recording media. In fact, there are many reports on FePt and CoPt nanowires [1–5] as well as numerous investigations on FePd alloys utilized in bulk as well as thin film forms [6–8]. These binary alloys, in particular, can form a chemically ordered L10 phase that exhibits large uniaxial magnetocrystalline anisotropy. However, with regard to FePd nanowires, excluding this group’s previous study [9,10], these have been significantly underreported. In this paper, we investigated the array of highly ordered FePd nanowires by dc electrodeposition into porous anodic aluminum oxide (AAO) templates. At different annealing temperatures, the structure has been verified by conventional X-ray diffraction (XRD) and transmission electron microscopy (TEM). On the other hand, the chemical composition was characterized by energy-dispersive X-ray spectroscopy (EDS) of scanning electron microscopy (SEM), while the magnetic properties were measured using a superconducting quantum interference device (SQUID) magnetometer.
Porous anodic aluminum oxide (AAO) templates with a pore length of about 40 mm and a uniformity diameter of about 60 nm were prepared under a two-step anodizing process [11]. Aluminum foils with purity as high as 99.99% were anodized in 0.3 M oxalic acid solution under a constant voltage of 40 V at 12 1C. Following anodization, the remaining aluminum was removed by a saturated CuCl2 solution. Etching treatment was then carried out in 6 wt% H3PO4 at 30 1C for 60 min to remove the barrier layer on the bottom side of the AAO and obtain the through holes template. Afterwards, a 200 nm Cu layer was sputter-deposited (using DC magnetron sputtering) onto one side of the AAO; this served as the working electrode for the following DC electrodeposition. An aqueous bath containing 0.2 M FeCl2+0.004 M PdCl2 was adjusted to pH ¼ 3 by adding appropriate dilute HCl. Next, electrodeposition of FexPd1x nanowires was carried out at a potential of 0.9V1.2V relative to the saturated calomel reference electrode (SCE); this was done through a three-electrode system with a graphite rod as the reference electrode. A series of FexPd1x nanowires with different Fe compositions in the range of x ¼ 0.2–0.95 was then obtained. Pure Fe nanowires were deposited with a solution of 0.2 M FeCl2 with dilute HCl under a potential of 1.2 V relative to the SCE.
Corresponding author at: Department of Mathematics and Physics, Shanghai University of Electric Power, Shanghai 200090, PR China. Tel.: +86 21 6802 0533. E-mail address: [email protected] (H. Hu).
0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.04.157
ARTICLE IN PRESS 2306
H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 2305–2309
The composition of the FexPd1x alloy nanowire array was determined by EDS in SEM [1,5]. All of the magnetic measurements were performed on a SQUID magnetometer (Quantum Design, MPMS-5 s). The coercivity (Hc) and squareness (Mr/Ms) of the nanowire arrays embedded in the AAO template meanwhile were obtained from hysteresis loops. All measurements were carried out at 5 and 300 K. After electrodeposition, Fe0.95Pd0.05 nanowire arrays were annealed at different annealing temperatures, Ta (Ta ¼ 300, 400, 500, 600 and 700 1C), for 1 h under the protection of Ar atmosphere. Before TEM observation, a piece of AAO template with Fe0.95Pd0.05 nanowires was dissolved by 5 wt% NaOH, and washed with alternating distilled water and ethanol for 3–5 times. Afterwards, the nanowires were detached from the substrate by ultrasonic dispersion in 2–3 ml ethanol. A drop of the solution was consequently placed on a Cu grid with carbon film and air dried prior to electron microscope analysis. The structure and morphology of Fe0.95Pd0.05 nanowires at different annealing temperatures were investigated by means of TEM and selected area electron diffraction (SAED).
3. Results and discussion Fig. 1 illustrates the coercivity of the as-deposited FexPd1x alloy nanowires measured at 5 K. Compared with the pure Fe nanowires that have a coercivity of 540 Oe at a similar diameter, the coercivity of these alloy nanowires improved with a Pd component lower than 50 at%. With decreasing Fe content, however, the coercivity of the FexPd1x alloy nanowires rapidly
800
HC (Oe)
600
400
200
0 20
40 60 Fe concentration (%)
80
100
Intensity (cps)
(211)
Pd (111)
5000
(110)
Fig. 1. The coercivity of FexPd1x alloy versus the composition of Fe element x in alloy nanowires.
as-deposite 2500
300°C 400°C 500°C 600°C 700°C 40
60 2θ (deg.)
80
Fig. 2. XRD of as-deposited Fe0.95Pd0.05 nanowire arrays and after annealing at different temperatures Ta.
Fig. 3. TEM image of the Fe0.95Pd0.05 nanowire and its selected-area electron diffraction (inset): (a) as-deposited, (b) Ta ¼ 500 1C and (c) Ta ¼ 600 1C.
ARTICLE IN PRESS H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 2305–2309
2307
1.0 1000
Hc (Oe)
M/MS
0.5
0.0
500 5K 300K
-0.5
-1.0 1.0 0.8
M/MS
M/MS
0.5
0.0
0.4 5K 300K
-0.5
0.0
-1.0
0
-2500
0 H (Oe)
2500
Fig. 4. Hysteresis loops of the as-deposited Fe (a) and Fe0.95Pd0.05 alloy (b) nanowire arrays, with the magnetic field applied parallel (— —) or perpendicular (— —) to the wire’s axis at 5 K.
decreased. The highest coercivity was found in Fe0.95Pd0.05. Therefore, this paper investigated the Fe0.95Pd0.05 nanowires in detail. The XRD spectra of the as-deposited and post-annealed Fe0.95Pd0.05 nanowire arrays embedded in templates are demonstrated in Fig. 2. For the as-deposited FePd nanowire arrays, bcc Fe (110), (2 11) and fcc Pd (111) were evident. Apparently, the Pd element existed in the as-deposited nanowires. Moreover, after annealing at Ta ¼ 300, 400 and 500 1C, these three peaks became weaker with increasing annealing temperatures. At Ta ¼ 600 1C only the peak of Fe (110) could be seen, but very weak, while at 700 1C all peaks disappeared from the XRD spectra. It was also determined that with increasing Ta the Fe (110) peak shifted to the lower angle. This signifies the formation of some FePd alloys following annealing [8]. In comparison, pure Fe nanowire arrays, when electrodeposited, showed a (110) preferred orientation [12]. Fig. 3 shows the TEM image of an isolated Fe0.95Pd0.05 nanowire in the as-deposition and post-annealing states at Ta ¼ 500 and 600 1C, respectively. From all three pictures, one can find that the nanowire was continuous with a uniform diameter of about 60 nm, corresponding to the AAO template used. The selected area electron diffraction (SAED) in Fig. 3a specifically illustrates the as-deposited Fe0.95Pd0.05 nanowire exhibiting a polycrystalline structure. After anneal treatment at Ta ¼ 500 1C, the nanowire maintained and, to some extent, even improved its crystalline structures. In contrast, a higher annealing temperature of Ta ¼ 600 1C produced many defects. The magnetic hysteresis loops of both as-deposited pure Fe (a) and Fe0.95Pd0.05 alloy (b) nanowire arrays were measured by SQUID, as shown in Fig. 4. It can be seen that both Fe and Fe0.95Pd0.05
200
400 Ta (°C)
600
800
Fig. 5. The coercivity (a) and squareness (b) of Fe0.95Pd0.05 nanowire arrays embedded in AAO template versus annealing temperature Ta, with the field applied parallel to the wire at 5 and 300 K.
nanowires have parallel to the wire easy magnetization. The coercivity and squareness of the Fe nanowires were also recorded at 540 Oe and 29%, respectively, while those of the Fe0.95Pd0.05 nanowires corresponded to 870 Oe and 65%. These hysteresis loops clearly show that trace Pd elements doped in the Fe0.95Pd0.05 alloy nanowires improved their coercivity and squareness. To identify the influence of heat treatment, the magnetic properties of annealed Fe0.95Pd0.05 alloy nanowires were measured at 5 and 300 K, as demonstrated by the coercivity (a) and squareness (b) versus annealing temperatures, Ta, in Fig. 5. It can be observed that the coercivity and squareness had similar variation trends at 5 and 300 K. For coercivity, this slightly increased with increasing Ta before 500 1C, reached the maximum point at Ta ¼ 600 1C, then decreased at Ta ¼ 700 1C. As for the squareness, it peaked its maximum at the annealing temperature of 500 1C, decreased at Ta ¼ 600 1C, and reached its nadir at Ta ¼ 700 1C. In this regard, we suggest that the change of magnetic properties after annealing is related to microstructural changes during the annealing process, as further discussed below. In demonstrating the influence of heat treatment clearly, hysteresis loops for the as-deposited and annealed nanowires at Ta ¼ 500, 600, and 700 1C were measured at 5 K, as shown in Fig. 6. For the as-deposited sample, the nanowires exhibited a parallel to the wire easy magnetization axis with a coercivity of 870 Oe and squareness of 74%. After annealing at 500 1C, the coercivity increased to 980 Oe and the squareness equaled 81.6%. However, when increased to Ta ¼ 600 1C, the largest coercivity of 1140 Oe was achieved, but the squareness decreased to about 64.1%. At the highest annealing temperature of Ta ¼ 700 1C, both the coercivity
ARTICLE IN PRESS 2308
H. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 2305–2309
1.0
M/MS
0.5 0.0 -0.5 -1.0 1.0
M/MS
0.5 0.0 -0.5 -1.0 -2500
0 H (Oe)
2500
-2500
0 H (Oe)
2500
Fig. 6. Hysteresis loops of the Fe0.95Pd0.05 nanowire arrays under different annealing temperature, where (a) as-deposited, (b) Ta ¼ 500 1C, (c) Ta ¼ 600 1C and (d) Ta ¼ 700 1C. The magnetic field was applied parallel (— —) or perpendicular (— —) to the wire at 5 K.
and squareness decreased sharply, showing magnetic properties similar to superparamagnetic materials. It is well known that anodic aluminum oxide template forms g-Al2O3 and can only be stabilized under low temperatures. After heating to 750 1C, g-Al2O3 changes to a-Al2O3, accompanied with volume shrinkage. In Fig. 6, with an annealing temperature of 500 1C, clearly much lower than 750 1C, the hysteresis loops showed a more square shape than other samples. This may be partly due to the alloy of FePd nanowire, which is also consistent with the improvement of SEAD in TEM, after 500 1C annealing. It was also observed that, while samples embedded in the AAO were annealed at temperatures as high as 600 1C, internal stress that increased in the AAO brought about by the annealing temperature distorted the alumina [13]. On the basis of the TEM with XRD results, after 600 1C annealing, the internal stress in the AAO template induced defects in the alloy nanowires shown as reflection spots in the SAED. These defects block the domain wall movement and increase the proportion of magnetization rotation, thereby improving the coercivity and decreasing the squareness. It can likewise be seen in Fig. 6 that both perpendicularity and parallelism to the wire coercivity were increased with an annealing temperature of 600 1C. With the highest annealing temperature of 700 1C, closer to the transition phase of g to a-Al2O3, the nanowires would be crushed by the shrinkage of the template and then react with the accompanying release of O2 [13,14]. These two effects would decompose the FePd nanowires and reduce its grain size down to below singledomain scale to approach superparamagnetic level. In this experiment, we saw the magnetization decreased by an order following annealing treatment at 700 1C; this also validates the decomposition of the nanowire to very small granules [15].
4. Conclusions Fabrication of Fe0.95Pd0.05 alloy nanowires was successfully done through electrodeposition into AAO templates. TEM observations reveal that isolated nanowires have polycrystalline structures, while magnetic measurements show that both coercivity and
squareness have improved with the addition of 5 at% Pd in Fe nanowires at proper annealing temperatures. The annealing process, on the other hand, maintained the crystalline structure of the Fe0.95Pd0.05 alloy nanowires, if not improved to some degree; this was up to an annealing temperature of about 500 1C. An annealing temperature increasing up to 600 1C was found to cause an increase in internal stress in the AAO, thereby inducing defects in the nanowires. The highest annealing temperature of 7001C was perceived to cause crushing of the nanowires brought about by the shrinkage of the template as well as reacting to the resultant O2 during the transition phase of the AAO template. These two effects produce decomposition of the nanowires into very small granules nearly the scale of superparamagnetic FePd, which sharply reduces their coercivity and squareness. With all these into consideration, the sample annealed at Ta ¼ 500 1C has the largest coercivity and longitudinal anisotropy, therefore generating an optimal heat treatment condition of around 500 1C.
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