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Core-Shell Cylindrical Magnetic Domains in Nickel Wires Prepared under Magnetic Fields Lixia Sun and Qianwang Chen* Hefei National Laboratory for Physical Science at Microscale and Department of Materials Science & Engineering, UniVersity of Science and Technology of China, Hefei, 230026, P. R. China. ReceiVed: October 14, 2008; ReVised Manuscript ReceiVed: December 16, 2008
An experimental study of the magnetic domains of nickel wires prepared under magnetic fields was carried out by using magnetic force microscopy. Nickel wires with a diameter of approximately 250 nm had a singledomain structure, while unique core-shell cylindrical domain was found in 2-µm-diameter wires. It is suggested that this novel domain structure was formed because of the presence of a magnetic field during the formation of wires and strongly depended on the diameter of the wires. Introduction The magnetic domain structure providing fundamental and applied information is the physical base of magnetic materials. Therefore, the control and observation of magnetic domain structures in ferromagnetic nanostructures are important.1,2 Among the many different varieties of magnetic nanostructures, the study of the magnetic domains of one-dimensional (1D) nanostructures is especially attracting a great deal of attention because of their distinctive properties and potential application as magnetic memory devices and high density magnetic recoding media. In recent studies, various domain structures of magnetic nanowires have been gradually reported. For example, O’Barr et al. found dipolar contrast in all investigated nickel wires, even for those with 1 µm diameter.3 Belliard et al. showed a 90 nm diameter wire exhibiting a multidomain bamboo-type configuration.4 Also for about 90 nm wires, Garcia et al. showed another multidomain structure exhibiting dark and bright regions along the wire’s axis.5 It is believed that novel domain structures could be induced to form in nanowires prepared in special environments, such as an applied electric field and magnetic field, etc. A magnetic field has been introduced as an efficient tool to control materials synthesis in several reaction systems.6-10 Recently, magnetic field-assisted growth of 1D magnetic nanostructured materials has been the subject of fruitful research in our group.11,12 However, so far research has been only focused on the morphology and magnetic properties of the materials prepared under a magnetic field; little attention has been paid to the change of their domain structures. In our previous study, 1D nickel wires with stable structure and improved magnetic properties were prepared under an applied magnetic field, and it is deduced that the applied field in a chemical reaction had changed the magnetic domain structure in wires.13 Herein, we further report the unique core-shell cylindrical domain configuration of nickel wires, and the result is analyzed, taking into account the role of the magnetic field played in the domain formation. Experimental Section Synthesis of the Nickel Wires. A stable green homogeneous solution was prepared by dissolving 0.50 mmol of NiCl2 · 6H2O * To whom correspondence should be addressed. E-mail:
[email protected]; fax: (+86) -551-3607292; tel: (+86) -551-3607292.
and 2.0 g of PVP (K-30) in the mixed solvents of 30 mL of distilled H2O and 5 mL of ethanol. Then 1.0 mL of 85 wt % hydrazine hydrate was added dropwise into the solution. After being vigorously stirred for 40 min, the solution was transferred into a Teflon-lined stainless steel autoclave (60 mL capacity) with a 0.25 T magnetic field, which was heated to and maintained at 170 °C. After 10 h, the autoclave was naturally cooled to room temperature. The black product was separated and washed with ethanol several times and then dried in a vacuum oven at 40 °C for 6 h. Synthesis of the ZF and AF Samples. Amounts of 1.00 mmol of NiCl2 · 6H2O and 4.0 g of PVP (K-30) were dissolved in the mixed solvents of 60 mL of distilled H2O and 10 mL of ethanol. Then 2.0 mL of 85 wt % hydrazine hydrate was added dropwise into the solution. After being vigorously stirred for 40 min, the as-prepared solution was transferred into two Teflonlined stainless steel autoclaves with a capacity of 60 mL (one without an external magnetic field, and the other with one permanent rare earth Nd-Fe-B magnet under the Teflon vessel. The magnetic field strength on the inner surface of the Teflon vessel was about 0.25 T at reaction temperature), where the pretreated silicon wafers (boron-doped, p-type (100)) were stuck on the bottom. Both autoclaves were maintained at 170 °C for 10 h and then cooled to room temperature naturally. It was found that the silicon coated with the shiny-silver film was still stuck on the bottom of each autoclave (samples labeled ZF without an external magnetic field, AF with an external magnetic field, respectively). Characterization. The samples obtained were characterized by X-ray diffraction (XRD) with a Rigaku (Japan) D/max-γ A X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.5418 Å). The transmission electron microscope image and selected area electron diffraction pattern were obtained on JEOL-2010 transmission electron microscope with an accelerating voltage of 200 kV. Field emission scanning electron microscopy (FESEM) was performed with a JEOL JSM-6700F apparatus. Magnetic force microscopy was performed with a Digital Instrument Nanoscope IIIa. The magnetic properties of samples were measured using a superconducting quantum interference device (SQUID, Quantum Design MPMS) magnetometer at room temperature. 3. Results and Discussion The composition and phase purity of the nickel wires was examined by X-ray diffraction (XRD) (Figure 1). All the
10.1021/jp809087d CCC: $40.75 2009 American Chemical Society Published on Web 01/23/2009
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Figure 1. The XRD pattern of nickel wires.
Figure 3. MFM images of nickel wires with a diameter about 250 nm. (a) A topographic AFM image of the wire; (b) corresponding MFM phase image of the wire; (c, d) section analysis of the longitudinal part of the wire; (e, f) section analysis of different transverse parts of the wire.
Figure 2. (a) Low- and (b) high-magnification SEM images of the nickel wires.
reflection peaks can be well indexed as face-centered cubic Ni (PDF standard cards, JCPDS 01-1260, space group Fm3m). No impurities such as NiO or Ni(OH)2 were detected in this pattern, indicating that phase-pure cubic Ni can be obtained under the current synthetic route. The field emission scanning electron microscopy (FESEM) images of the nickel sample are shown in Figure 2. A panoramic image of Figure 2a demonstrates that the sample consists of a large number of nickel wires, and an enlarged image shows that the sample consists of a large number of wires with a diameter of about 250 nm and a few thicker ones in a diameter about 2 µm (Figure 2b), and these wires have a length of tens of microns up to hundreds of microns. The microstructure of 2-µm diameter wires was also investigated by transmission electron microscopy (TEM), and the results show that the wire with width fluctuation is not the simple assembly of particles (see Figure S1 in the Supporting Information). The selected area electron diffraction pattern (SAED) of the wire consists of diffraction rings of low intensity and spots of strong intensity simultaneously, revealing that there
would be a preferential orientation of some nickel grains in the wire (Figure S1, insert). To further examine in more detail the magnetic structure of the sample, magnetic force microscopy (MFM) investigations were carried out on the sample to study the domain structure of nickel wires. MFM has proved to be an efficient observation technique, despite the fact that finding a nanowire suitable for observation too often resembles looking for a needle in a haystack. For our MFM measurements, the samples were imaged in the absence of applied field (in a remanent state). Images were obtained with a two-step “tapping-lift” mode process. As the probes were magnetized up along their pyramidal axis, contrasts on the MFM image indicate the magnetization direction. Dark parts in the magnetic image represent the attractive force, while the bright parts represent the repulsive force between the tip and samples. On the basis of the sample topography (Figure 3a), there is a wire with branches in a diameter of about 250 nm, and the node of the wire is out of the plane. As shown in Figure 3b, a uniform dark contrast is seen on each wire, which indicates that the wires are obviously in a single domain state. In order to show the details of the magnetic configuration in the wires, section analysis of longitudinal and transverse parts of the wires were also carried out. Uniform negative signals in Figure 3c,d further demonstrate that the wires have single-domain structure. However, as the node of the wire is out of the plane, a white spot and its corresponding positive signals are found in Figure 3e,f. As the wire diameter is increased to 2 µm, a unique core-shell cylindrical domain with a dark area in the middle part and bright areas in the shell is seen on the wire (Figure 4a,b). According to the section analysis of the longitudinal and transverse parts of the wires (Figure 4c-h), the corresponding positive signals of the shell indicate that there is repulsive force between the tip and shell,
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Figure 5. Schematic illustration of the domain structures of 2 µm nickel wires. (a) Core-shell cylindrical domain structure of the wires; (b) a cross-sectional view of the core-shell cylindrical domain structure; (c) corresponding concentric annular zone domain structure of the AF sample which can be deduced from b; (d) corresponding magnetic domain wall between the opposite domains in b.
Figure 4. MFM images of nickel wires with a diameter about 2 µm. (a) A topographic AFM image of the wire; (b) corresponding MFM phase image of the wire; (c, d) cection analysis of the middle part of the wire; (e, f) section analysis of the right side of the wire; (g, h) section analysis of the transverse part of the wire.
while the corresponding negative signals of the core indicate attractive force. As for our wires with 2 µm in diameter, the unique core-shell cylindrical multidomain structure was different from those reported,3-5 so it is suggested that the unique multidomain may be aroused by the applied field in the chemical reaction. As reported in our previous study,13 when a magnetic field is applied to the reaction, chemical reduction can easily occur along the magnetic line of force, so nickel crystallites nucleate and grow along the magnetic lines of force, leading to the formation of 1D nickel wires. Therefore, the overall magnetic moments of the nickel crystallites in a wire are aligned in the same direction because of the effect of the applied field. As a result, a singledomain structure can be found in the wires with a diameter about 250 nm. Normally, for any ferromagnetic material, of any size, the magnetostatic energy will be reduced by subdividing it into multidomains.14 In multidomain configurations, exchange energy will increase since the spins inside the domain wall are not parallel to each other, and magnetic anisotropy energy will also increase since the magnetization inside domain walls deviate from the magnetic easy axes. As the wire diameter is increased, the decrease in magnetostatic energy will be larger than the increase in exchange and anisotropy energy, so this makes a
large wire multidomain. In the case of wires with a diameter of 2 µm, the overall magnetic moments of the nickel crystallites in a wire would be also aligned along the magnetic line of force because of the effect of the applied field. However, in order to minimize the total energy of nickel wires, the magnetic moments of the core of the wire have the same magnetization and the shell has opposite magnetization, so the unique core-shell cylindrical multidomain structure is formed. Then a schematic domain structure was used to tentatively explain the unique multidomain structure. As shown in Figure 5a, the core and shell of wires have opposite magnetization, consisting of an inner core magnetized down, and an outer shell magnetized up. The cross-section analysis of the wire and the resulting concentric annular zone domains are schematically deduced in Figure 5b,c. In a wire, the magnetic moments of the core are aligned in the down direction, while the moments of the shell are gradually converted into the opposite direction, and the corresponding magnetic domain wall between the opposite domains is shown in Figure 5d. Furthermore, the two opposite magnetizations would influence each other during the MFM observation. Despite the counteraction of the shell with up magnetization, the core of the wire still has down magnetization (Figure 4c,d). Therefore, it is deduced that the shell is much thinner than the core, i.e., the influence of shell on total magnetization is less than that of the core along the diameter. To further support the core-shell cylindrical model, crosssection observation of nickel wires should be carried out. Then thin film coated on silicon wafer was prepared under the same conditions as that used for the wires, which was labeled as AF sample. The composition and phase purity of the AF sample was examined by XRD (see Figure S2 in the Supporting Information). Combined with the fact that the powder sample consisted of nickel wires, it can be deduced that the AF sample prepared in the same system was composed of wires arrayed on the silicon wafer. As shown in Figure 6a, many wires were found not vertically arrayed on the wafer because nickel crystallites nucleated and grew along the curved magnetic lines
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Figure 7. Hysteresis loops of the AF sample and ZF sample measured at room temperature with the applied field parallel and perpendicular to the film surface. The upper left inset shows the hysteresis loops of AF and ZF samples between -100 and 100 Oe. Magnetic properties of the two samples are summarized in the lower right inset.
Figure 6. (a) A SEM image of the surface of the AF sample. (b) A topographic AFM image of the AF sample. (c) Corresponding MFM phase image of the AF sample and the typical annular domains marked by a rectangle.
of force when a magnetic field was applied in the preparation of the AF sample. MFM images of the AF sample were also taken, which can provide cross-sectional information of magnetization. The tops of the wires are shown in the topography of AF sample (Figure 6b). Figure 6c shows many dark circular domains, corresponding to the wires in a diameter of about 250 nm. Additionally, there are also some typical concentric annular zone domains (labeled by a rectangle) with a dark inner core and bright outer shell, indicating that the inner core and outer shell of the 2 µm wires have opposite magnetization, respectively. Furthermore, it can be seen that the shell is much thinner
than the core, which well explains why the magnetization state of inner core can be detected by MFM (Figure 4b). Consequently, the concentric annular zone domains on the surface of the AF sample further confirm the existence of core-shell cylindrical domain structures in nickel wires, as schematically shown in Figure 5b,c. Similarly, a nickel thin film labeled as ZF sample was obtained by the same synthesis procedure as that used for the wires except without an applied field, and SEM observation reveals that the ZF sample was actually composed of small particles with a distinguishable microstructure (see Figure S3 in the Supporting Information). Surface topography AFM image (see Figure S4a in the Supporting Information) shows that the ZF sample is composed of nanoparticles, which agrees with the result of the SEM image in Figure S3. The corresponding MFM image (Figure S4b) gives the magnetic domain structures of the film. Compared with the concentric annular zone domains of the AF sample (Figure 6c), the magnetic domains of ZF are dark spot domains, corresponding to the nanoparticles in the sample. The magnetic properties of materials have been believed to be highly dependent on the sample morphology, crystallinity, magnetization direction, etc. Thus, hysteresis loops of the AF sample and ZF sample, with the applied magnetic field parallel and perpendicular to the film surface, were measured at room temperature, respectively (Figure 7). For the AF sample, the coercivity field and squareness are distinct for different magnetic field directions. However, it is noted that for the ZF sample, the magnetic properties are not influenced by the direction of applied magnetic filed, especially the coercivity of ZF|(41 Oe) remains almost the same as that of ZF⊥(40 Oe). In addition, magnetic properties of AF sample are significantly enhanced compared with those of ZF sample, as shown in the lower right inset of Figure 7. It is suggested that the improved magnetic properties of AF compared to those of ZF can be ascribed to the special magnetic domain structure in AF. Most of the cases reported in the literature show that preferred magnetization is perpendicular to the film plane with the enhanced coercivities and high squareness.15,16 However, in our study the coercivity and squareness of AF were higher when the magnetic field was parallel to the film plane, i.e., perpendicular to the nickel wires, which may be due to the unique domain wall structure between the opposite domains, as shown in Figure 5d. It is well-known that coercivity is the amount of reverse magnetic field which must be applied to a magnetic material to make the magnetic
2714 J. Phys. Chem. C, Vol. 113, No. 7, 2009 flux return to zero. When the external magnetic field is perpendicular to the film plane, the magnetic moments in either the core or the shell of the core-shell cylindrical domains are identical with the external field, producing a stronger magnetic field. Therefore, a weaker reverse field was required to make the magnetization return to zero. While the field was parallel to film plane, there was not any identical magnetic moments in the domain with the external field, which resulted in the higher coercivity of AF| Therefore, the magnetic measurements show a good agreement with our MFM results, further confirming the magnetic domain model proposed. 4. Conclusion In summary, novel magnetic domain configuration of nickel wires formed under a magnetic field was observed. Nickel wires with a diameter of approximately 250 nm had single-domain structure, while unique core-shell cylindrical domain structures were found in wires with a diameter of 2 µm. The observation of the magnetization state of the AF sample exhibits concentric annular zone domains, which further confirms the existence of core-shell cylindrical domain structures in the as-prepared nickel wires. The unusual magnetization behavior perpendicular and parallel to the wire axis was discussed in terms of the unique core-shell cylindrical domain structures. Acknowledgment. Financial support by the National Natural Science Foundation of China (No. 10774138) is gratefully acknowledged.
Sun and Chen Supporting Information Available: Additional SEM, TEM, and MFM images, and XRD pattern. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Song, H. Z.; Li, Y. X.; Zeng, J. T.; Li, G. R.; Yin, Q. R. J. Magn. Magn. Mater. 2008, 320, 978. (2) Watanabe, K.; Takemura, Y.; Shimazu, Y.; Shirakashi, J. Nanotechnology 2004, 15, S566. (3) OBarr, R.; Lederman, M.; Schultz, S.; Xu, W. H.; Scherer, A.; Tonucci, R. J. J. Appl. Phys. 1996, 79, 5303. (4) Belliard, L.; Miltat, J.; Thiaville, A.; Dubois, S.; Duvail, J. L.; Piraux, L. J. Magn. Magn. Mater. 1998, 190, 1. (5) Garcia, J. M.; Asenjo, A.; Vazquez, M.; Aranda, P.; Ruiz-Hitzky, E. IEEE Trans. Magn. 2000, 36, 2981. (6) Bodea, S.; Ballou, R.; Pontonnier, L.; Molho, P. Phys. ReV. B 2002, 66, 224104. (7) Affleck, L.; Aguas, M. D.; Pankhurst, Q. A.; Parkin, I. P.; Steer, W. A. AdV. Mater. 2000, 12, 1359. (8) Korneva, G.; Ye, H. H.; Gogotsi, Y.; Halverson, D.; Friedman, G.; Bradley, J. C.; Kornev, K. G. Nano Lett. 2005, 5, 879. (9) Platt, M.; Muthukrishnan, G.; Hancock, W. O.; Williams, M. E. J. Am. Chem. Soc. 2005, 127, 15686. (10) Sahoo, Y.; Cheon, M.; Wang, S.; Luo, H.; Furlani, E. P.; Prasad, P. N. J. Phys. Chem. B 2004, 108, 3380. (11) Niu, H. L.; Chen, Q. W.; Ning, M.; Jia, Y. S.; Wang, X. J. J. Phys. Chem. B 2004, 108, 3996. (12) Wang, J.; Chen, Q. W.; Zeng, C.; Hou, B. Y. AdV. Mater. 2004, 16, 137. (13) Sun, L. X.; Chen, Q. W.; Tang, Y.; Xiong, Y. Chem. Commun. 2007, 2844. (14) Liu, X. X.; Itoh, F. J. Appl. Phys. 2003, 93, 7423. (15) Cao, H. Q.; Tie, C. Y.; Xu, Z.; Hong, J. M.; Sang, H. Appl. Phys. Lett. 2001, 78, 1592. (16) Whitney, T. M.; Searson, P. C.; Jiang, J. S.; Chien, C. L. Science 1993, 261, 1316.
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