J. Phys. Chem. C 2008, 112, 1837-1841
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Aging-Induced Strong Anomalous Hall Effect at Room Temperature for Cu(Co) Nanoparticle Film Zhigang Li, Weiping Cai,* Shikuan Yang, Guotao Duan, and Ran Ang Key Laboratory of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P. R. China ReceiVed: September 11, 2007; In Final Form: NoVember 6, 2007
The Co0.12Cu0.88 nanoparticle film with two-dimensional hexagonally arranged microsized pores is fabricated by electrochemical deposition. Subsequent aging at room temperature leads to change of the film in carrier type from n-type to p-type and appearance of anomalous Hall effect. The anomalous Hall effect is well controlled by aging time at room temperature and increases to a maximum (the Hall slope > 40 mΩ cm/T in the field below 40 Oe) with further aging almost 80 times as high as the best value reported previously. Such results are attributed to the formation of the alloy/oxide core-shell nanoparticles during aging. This study could help in the design and fabrication of transparent spintronic devices operable at room temperature.
1. Introduction Electron charge and spins can carry information. This could induce the new generation of devices, namely spintronic devices, combining standard microelectronics with spin-dependent effects1 that arise from the interaction between spin of the carriers and the magnetic properties of the material. One of the most important characteristics in the spin-dependent effects is anomalous Hall effect (AHE). This is because the ferromagnetic spinpolarized carriers can be probed and controlled electrically,2,3 leading to direct application in electronics.4 Group III-V ferromagnetic semiconductors5,6 have emerged as the most popular materials for this new technology. However, the Curie temperatures for ferromagnetic semiconductors usually are low,5,7 leading to limitation in future applications. Some metal (or alloy) materials exhibit large AHE, such as rare-earth-based alloying films,8 ferromagnetic metal-insulator granular films,9,10 and Pt-based ferromagnetic alloy thin film,11-13 due to the highly effective spin-orbit scattering for electron transportion around the interfaces.13 Thus far, the largest Hall slope in metal materials at room temperature is 545 µΩ cm/T.13 These metal materials are of low resistivity (compared with that of semiconductor materials), high sensitivity, and wide operating temperature range. The corresponding AHE sensors can overcome some demerits of ferromagnetic semiconductors, such as low Curie temperature, high resistivity, low operation frequency, and so forth. However, the metal materials do not possess semiconductor properties, which restrict the application of AHE in spintronic devices. Recently, we found that combination of the metal and semiconductor oxide could exhibit the large AHE, high Curie temperature, and semiconductor properties. Two-dimensional (2D) nanostructured ordered pore arrays have attracted much attention because of their potential applications in catalysis,14 sensors,15 photonic and optoelectronic devices,16,17 surface-enhanced Raman scattering active substrates,18 and thermal insulation materials19 for high specific surface area and orderly arrangement of pores. Here, we report the Co0.12Cu0.88 nanoparticle film, with 2D hexagonally arranged microsized pores, fabricated by electrochemical deposition on * Corresponding author. E-mail:
[email protected].
polystyrene sphere (PS) colloidal monolayer. Subsequent aging at room temperature leads to change of the film in carrier type from n-type to p-type and to AHE. Interestingly, the AHE increases to a maximum (the Hall slope higher than 40 mΩ cm/T in the field below 40 Oe) with further aging almost 80 times as high as the best value reported previously.13 This work could help in the design and fabrication of transparent spintronic devices operable at room temperature, combined with the 2D ordered structure of this film.18,20,21 2. Experimental Section A large area (about 1 cm2) of colloidal monolayer composed of 2-µm PSs was first synthesized on a clean glass substrate by spin-coating.22 This monolayer PS was then transferred onto a conducting indium tin oxide (ITO, an n-type semiconductor)coated glass, by a floating transfer method, as described previously.22,23 The substrate covered with the monolayer was used as the working electrode in a three-electrode electrolytic cell, with a graphite plate as the auxiliary electrode, and a saturated calomel electrode as the reference electrode. The electrolyte, containing CoSO4 (20 g/L), CuSo4 (2 g/L), and Na3C6H5O7 (10 g/L), was prepared with distilled water. Electrochemical deposition was carried out for 1 h, at room temperature, under the constant current density of 0.2 mA/cm2. Finally, the samples were heated at 400 °C in hydrogen for 2 h to burn away the PSs.25 The samples were separately characterized by field emission scanning electronic microscope (Sirion 200), high-resolution transmission electronic microscope (HRTEM), X-ray diffraction (XRD), and energy dispersive X-ray spectra (EDS). Hall resistivity and magnetic properties of the sample were measured by a physical property measurement system with a perpendicular external field. 3. Results 3.1. As-Prepared Sample. Figure 1a shows the XRD of the as-prepared sample, corresponding to the face-centered cubic structure with lattice constants close to the Cu crystal. EDS has confirmed that the sample composition is Co0.12Cu0.88 (Figure 1b). Figure 2a shows the corresponding morphology of the as-
10.1021/jp0773132 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/19/2008
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Figure 2. (a) Surface morphology of the as-prepared sample. Inset: local magnification of (a). (b) Magnetic field dependence of Hall resistivity for the sample aged for 1 day. Figure 1. X-ray diffraction (a) and energy dispersive X-ray analysis (b) for the sample aged for 1 day.
prepared sample. The film exhibits hexagonally arranged ordered pore structure due to the PS template.22-25 The pore size at the film surface is about 1200 nm, and the film thickness is thus estimated to be about 800 nm on the basis of the truncated hollow spherical geometry. The film skeleton consists of nearly spherical particles with size about 100 nm, as seen in the inset of Figure 2a. The magnetic field (H) dependence of Hall resistivity Fxy is illustrated in Figure 2b for the sample after aging for 1 day at room temperature, showing n-type carriers. The carrier concentration determined by the ordinary Hall effect is about 1021 cm-3 (in this article, the substrate effect has been removed according to ref 26). Such results reflect a typical metal property. 3.2. Aging Effect. After aging at room temperature for 1 week, however, the corresponding H dependence of Fxy shows a typical semiconductor property, as illustrated in Figure 3a. The carrier concentration is about 3 × 1018 cm-3. The type of carrier has transited from n-type to p-type. The corresponding Hall coefficient (absolute value) is about 3 orders of magnitude higher than that of the sample aged for 1 day. This indicated that the Hall coefficient is strongly aging time dependent. Additionally, in the region of low field, from -2000 to 2000 Oe, a small AHE can be observed (inset of Figure 3a). The hysteresis loop measurement shows that the remanent ratio is about 0.05, coercivity is less than 30 Oe, and the saturation field is about 2000 Oe for vertical field, as shown in Figure 3b. XRD has revealed that there exists a small amount of alloying oxides Cu(Co)O and [Cu(Co)]2O in addition to the dominant alloy Co0.12Cu0.88, as shown in Figure 4. HRTEM examination
has confirmed formation of Cu(Co) alloy/oxide core-shell nanoparticles (inset of Figure 4). Further aging (say, 10 days) leads to the more significant dependence of Fxy on H, as illustrated in Figure 5a. If aging time is for 2 weeks, such dependence is the strongest. The carrier concentration is about 2 × 1018 cm-3. Interestingly, a very strong AHE and high Fxy (about 660 µΩ cm at 1000 Oe) were observed in the low H region. For magnetic sensing, the Hall slope dFxy/ dH is an important parameter, which is directly associated with sensitivity of AHE sensor.9,13 To more clearly illustrate the AHE, a magnetic field-dependent Hall slope can be obtained from Figure 5a. We can see that the Hall slope is higher than 40 mΩ cm/T when the H is below 40 Oe, as illustrated in Figure 5b. This is almost 80 times as high as the best value (545 µΩ cm/ T) reported previously in low field ((10 Oe).13 Because of the high sensitivity to the external field, low Ohmic resistivity (compared with that of semiconductor material), and a wide range ((2000 Oe) of operation field, this nanostructured film consisting of alloy/oxide core-shell nanoparticles could be a very promising candidate for the AHE sensor. The corresponding hysteresis loop is illustrated in Figure 3b. The remanent ratio increases to 0.2, and the coercivity is more than 90 Oe for vertical field. HRTEM examination shows that two-week-long aging induces almost complete oxidation of the alloying particles. But alloying cores a few nanometers in size in the oxidized particles still exist, and there are several alloying cores in each particle, as typically shown in Figure 5c, which means the polycrystalline structure of the particles. After aging for 17 days, however, the AHE decreases significantly and almost disappears. The corresponding ferromagnetic property is hardly detected. The carrier concentration
Hall Effect for Cu(Co) Nanoparticle Film
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Figure 3. (a) Magnetic field dependence of Hall resistivity for the sample aged for 1 week. Inset: results in the low field region. (b) Magnetic hysteresis loops for the samples aged for 1 and 2 weeks.
Figure 5. Magnetic field dependence of Hall resistivity for the samples with different aging time (a), the magnetic field dependence of the Hall slope for the sample aged for 2 weeks (data from the corresponding curve in (a)) (b), and HRTEM image of a single particle for the sample aged for 2 weeks (c).
Figure 4. X-ray diffraction of the sample aged for 1 week. Inset: corresponding HRTEM image of a single particle.
is about 2 × 1018 cm-3 closer to that of the sample aged for 2 weeks (Figure 5a). The Fxy vs H is similar to that of the sample aged for 1 week. XRD measurements confirmed that aging for 17 days at room temperature induced complete oxidation of the film to Cu(Co)O, as shown in Figure 6. Subsequent further aging leads to insignificant change (Figure 5a). 4. Discussion The formation of the hexagonally arranged pore Cu-Co film is easily understood because of the electrodeposition on the PS
colloidal monolayer, as previously described.23,24 In our deposition conditions, homogeneous nucleation would occur and CuCo alloying nanoparticles can be formed in the solution near the substrate. The formed nanoparticles deposit on the interstitials in the PS monolayer, building into the porous skeleton and forming the final film (as shown in the inset of Figure 2a). After the skeleton has aged at room temperature, oxygen in air will enter the porous skeleton and interact with the Cu-Co alloying nanoparticle surface, and hence oxidation will occur. Obviously, the microsized pores in the film are also helpful to the surface oxidation of the metal particles. After a proper time (say, 1 week), alloy/oxide core-shell nanoparticles in the skeleton will be formed as shown in Figure 4. Further aging leads to reduction of the core size. Because of polycrystalline structure, the oxygen can diffuse along the grain boundaries within the particles during aging, forming several alloy cores in single oxide particles. Full
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Li et al. changed greatly during the transition. The other is the AHE, which occurs after formation of the alloy/oxide core-shell structure and disappears when the alloying particles are fully oxidized. The AHE shows strong oxidization dependence and hence can be easily controlled by aging time. 5. Conclusion
Figure 6. X-ray diffraction of the sample aged for 17 days.
oxidation will occur after aging for 17 days. The physical phenomena in this article should be associated with the aginginduced oxidation and formation of core-shell structured nanoparticles. 4.1. Transition of the Carrier Type. The transition of carrier type can be attributed to formation of core-shell nanoparticles. When aging is not long (say, 1 day), the alloy particles just begin to be oxidized but still connect together. In this case, the sample shows metal properties (or n-type carrier) and high carrier concentration. Further aging leads to formation of coreshell structured particles in the film. The alloying cores are isolated to each other. The electrons in alloy cores cannot transport into the oxide freely. Since the oxide is a p-type semiconductor,27 the Hall coefficient shows that the carrier is p-type. 4.2. Anomalous Hall Effect. The AHE is conventionally attributed to spin-orbit coupling and the spin polarization of carriers, which result in asymmetry in orbital angular momentum,28 or to the asymmetric skew scattering of conduction electrons induced by the fluctuation of localized moments.9,30 In this study, although the origin of the AHE in Co0.12Cu0.88 nanoparticle film is not clear, it should be related to aginginduced formation of core-shell structured nanoparticles, which could induce asymmetric carrier scattering by ferromagnetic cores.9,13,29 For the sample aged for 1 week, the ferromagnetic alloy cores were isolated from each other and confined in the local areas because of formation of the core-shell structure of the particles (Figure 4b). In this case, the AHE is due to asymmetric carrier scattering by ferromagnetic cores. The strong AHE, for the sample aged for 2 weeks, corresponds to significant reduction of the core’s size and formation of several ultrafine cores in single particles. Corresponding magnetic measurement has shown that both coercivity and remanent ratio are much (about 3 times) higher than those of the sample aged for 1 week (Figure 3b). Therefore, the strong ferromagnetic exchange interaction between these ultrafine cores could take place and thus possibly lead to strong AHE. Deep and systematic study of the strong AHE is in progress. Such strong AHE could be in favor of application in the spintronic devices at room temperature.30,31 Too-long aging (>17days) induces full oxidation and disappearance of the ferromagnetic alloy cores, and hence the AHE cannot be detected at room temperature. Obviously, there exist two special phenomena during aging for such porous particle film. One is the transition of carrier types, which occurs during formation of core-shell nanostructured particles. The carrier concentration of the sample is
In summary, we have demonstrated the synthesis of a novel material, a hexagonally arranged pore film consisting of Co0.12Cu0.88 alloy/oxide core-shell nanoparticles, on the basis of electrochemical deposition on PS colloidal monolayer and subsequent aging. Such film shows that Hall coefficient at room temperature strongly depends on aging time. Aging induces formation of core-shell structured particles because of surface oxidation and hence leads to change of carrier type from n-type to p-type accompanied with a small AHE in the low field region at room temperature. Such AHE is well controlled by aging time at room temperature and increases to a maximum with further aging, corresponding to formation of ultrafine alloying cores. Too-long aging results in full oxidation of the particles and disappearance of AHE. Such properties imply potential applications in spintronic devices. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 50671100), the Major State Research Program of China “Fundamental Investigation on Micro-Nano Sensors and Systems based on BNI Fusion” (Grant No. 2006CB300402), and the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KJCX2-SW-W31). We thank Prof. L. J. Zou for his helpful discussions. References and Notes (1) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (2) Kent, A. D. Nature 2006, 442, 143. (3) Balakirev, F. F.; Betts, J. B.; Migliori, A.; Ono, S.; Ando, Y.; Boebinger, G. S. Nature 2003, 424, 912. (4) Manyala, N.; Sidis, Y.; Ditusa, J. F.; Aeppli, G.; Young, D. P.; Fisk, Z. Nat. Mater. 2004, 3, 255. (5) Ohno, H.; Chiba, D.; Matsukura, F.; Omiya, T.; Abe, E.; Dietl, T.; Ohno, Y.; Ohtani, K. Nature 2000, 408, 944. (6) Chiba, D.; Yamanouchi, M.; Matsukura, F.; Ohno, H. Science 2003, 301, 943. (7) Chiba, D.; Takamura, K.; Matsukura, F.; Ohno, H. Appl. Phys. Lett. 2003, 82, 3020. (8) McGuire, T. R.; Gambino, R. J.; Taylor, R. C. J. Appl. Phys. 1977, 48, 2965. (9) Pakhomov, A. B.; Yan, X.; Zhao, B. Appl. Phys. Lett. 1995, 67, 3497. (10) Denardin, J. C.; Pakhomov, A. B.; Knobel, M.; Liu, H.; Zhang, X. X. J. Phys.: Condens. Matter 2000, 12, 3397. (11) Miao, G. X.; Xiao, G. Appl. Phys. Lett. 2004, 85, 73. (12) Watanabe, M.; Masumoto, T. Thin Solid Films 2002, 405, 92. (13) Zhu, Y.; Cai, J. W. Appl. Phys. Lett. 2007, 90, 012104. (14) Matsushita, S. I.; Miwa, T.; Fujishima, D. A. Langmuir 1998, 14, 6441. (15) Elizabeth, C. D.; Oomman, K. V.; Keat, G. O. Sensors 2002, 2, 91. (16) Yablonovitch, E.; Gmitter, T. J.; Meade, R. D.; Rappe, A. M. Phys. ReV. Lett. 1991, 67, 80. (17) Imada, M.; Noda, S.; Chutinan, A.; Tlkuda, T.; Murata, M.; Sasaki, G. Appl. Phys. Lett. 1999, 75, 316. (18) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Lenhoff, A. M.; Rabolt, J. F.; Kaler, E. W. AdV. Mater. 2001, 13, 396. (19) Bitzer, T. Honeycomb Technology; Chapman & Hall: London, 1997; pp 30-65. (20) Elizabeth, C. D.; Oomman, K. V.; Keat, G. O. Sensors 2002, 2, 91. (21) Yablonovitch, E.; Gmitter, T. J.; Meade, R. D.; Rappe, A. M. Phys. ReV. Lett. 1991, 67, 3380. (22) Sun, F.; Cai, W.; Li, Y.; Cao, B.; Lei, Y.; Zhang, L. AdV. Funct. Mater. 2004, 14, 283.
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