Resistance Reduction Induced by Small Electric Current in CoCu

The investigation of electrics discovered that resistance reduction could be induced by a small electric current in these ferromagnetic porous films. ...
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J. Phys. Chem. C 2010, 114, 2300–2304

Resistance Reduction Induced by Small Electric Current in CoCu Porous Films Zhigang Li,†,‡ Weiping Cai,*,† Peisheng Liu,*,†,§ Qintao Li,‡ and Liangjian Zou† Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China, Department of Physics, Taizhou UniVersity, Taizhou 31700, People’s Republic of China, and Jiangsu Key Laboratory of ASCI Design, Nantong UniVersity, Nantong 226019, People’s Republic of China ReceiVed: NoVember 11, 2009; ReVised Manuscript ReceiVed: December 31, 2009

The ferromagnetic CoCu porous films were prepared by electrochemical deposition based on the double layered colloidal template on indium tin oxide-coated glass substrates. The investigation of electrics discovered that resistance reduction could be induced by a small electric current in these ferromagnetic porous films. For fixed current density, the resistance decreased: from initial state value to steady state one, it was smaller at high magnetic field than that at low field. For a fixed magnetic field, a smaller current density led to longer relaxation time. The resistance reduction value and relaxation time can be controlled by the external magnetic field and current density. A possible model of the current-driven movement of domain walls in low current density is proposed to explain reduction behaviors in such CoCu porous film and this will have highly valuable applications in magnetic delay switches. I. Introduction CoCu magnetic film has stimulated a great deal of research activities due to its fundamental scientific interest and its potential application in magnetic recording and magnetic sensors. CoCu granular film and Co/Cu multilayer film are famous for their giant magnetic resistance (GMR).1-3 Multilayered Co/Cu nanowire arrays have been investigated for recording media and magnetoresistive reading heads.4 Recently, the current-driven resistance change in Co/Cu or Co/Cu/Co nanopillar5-7 received much attention due to the effect of the current switch domain wall, in which the resistance change is current-density dependent. This brings a new way for manipulating the magnetization and magnetic domain wall by the use of electric current. To date, most of those studies only focus on high current density (more than 107 A/m2).8,9 However, large current density may exceed the threshold values tolerated by the metal interconnects of integrated circuits,10 which limits the future applications in ultradense storage devices. To overcome this shortcoming, it is necessary to investigate the resistance behavior of magnetic film at low current density in detail. Unfortunately, such researches are seldom reported. In this paper, we report small current-driven resistance change under various magnetic fields and current density in CoCu porous films at room temperature. The resistance reduction value and relaxation time can be controlled by the external magnetic field and current density. Under a fixed external magnetic field, the resistance decreased, from the initial value to the final steady one, the resistance induced by a large current density is larger than that induced by a small current density, and the corresponding relaxation time is shorter than that in small current density. In constant current density, under a low magnetic field, the resistance change is larger than that under high magnetic field, and the relaxation time is also much longer than that under * To whom correspondence should be addressed. E-mail: [email protected] and [email protected]. † Chinese Academy of Sciences. ‡ Taizhou University. § Nantong University.

high field. We propose a possible model to explain such resistance reduction based on the current-driven movement of domain walls in low current density. Besides, the resistance reduction value and relaxation time can be controlled by the external magnetic field and current density. Such behavior of resistance reduction induced by small electric current will be highly valuable in its further application in magnetic delay switches. II. Experimental Section A monolayer colloidal crystal with a large area (about 1 cm2) composed of polystyrene spheres (PSs) (Alfa Aesar, PSs size: 2000 nm) was first synthesized on a cleaned glass substrate by spin-coating,11 and then transferred onto a conducting substrate (indium tin oxide (ITO)-coated glass) by a floating-transfer method.12 A double layered colloidal crystal would be obtained by subsequently floating-transfer another colloidal monolayer on the original one. Such a substrate with PSs 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 (SCE) 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 2 h at the constant current density (J) of 0.2 mA/cm2. Finally, the sample was heated at 400 °C in hydrogen for 12 h to burn PSs away and form CoCu porous film. For two references, one is a Co-Cu granular film fabricated under the same condition without PSs template, the other is a porous Cu film obtained by electrodeposition using the same colloidal template. The samples were characterized by field emission scanning electronic microscope (FESEM) (Sirion 200), X-ray diffractions (XRD), and energy dispersive X-ray analysis (EDXA), respectively. The resistance of sample was measured on a physical property measurement system (PPMS) with a perpendicular external field. The magnetic properties of samples were characterized, at room temperature and below the Curie temperature, by the superconducting quantum interference device (SQUID) magnetometer with a perpendicular external field.

10.1021/jp910749b  2010 American Chemical Society Published on Web 01/20/2010

Resistance Reduction in CoCu Porous Films

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Figure 1. (a) FESEM image of an as-prepared sample with macropores of ordered hexagonal arrangement; (b) the corresponding X-ray diffraction pattern; (c) the further spectrum of energy dispersive X-ray analysis; and (d) FESEM of the sample obtained without PSs template.

III. Results The prepared porous film consists of hexagonal arrangement of macrosized pores and the skeleton is composed of nanoparticles. Figure 1a shows the morphology of the as-prepared sample, exhibiting ordered hexagonal arrangement of the porous array. The pore size, from the top view, is about 1600 nm. The protuberance among three closed packed macropores was composed of nanoparticles and its size was about 1 µm. The XRD showed that the sample displayed face centered cubic structure with lattice constants close to a Cu crystal, as illustrated in Figure 1b. EDXA (Figure 1c) confirmed that it consisted of Co, Cu elements and the ratio of Co/Cu was 12:88; the other elements such as carbon and sulfur come from the substrate. XRD and EDXA indicated that the prepared porous array was composed of Cu-Co alloy nanoparticles. Additionally, the morphology of the sample fabricated without PSs is shown in Figure 1d, exhibiting granule film without pores. The magnetization curves on the magnetic field are shown in Figure 2; both CoCu granule film and CoCu porous film displayed ferromagnetic properties. Figure 2a shows the hysteresis loop of the CoCu granule film; the coercivity field is about 130 Oe and the remanence ratio is 0.26. Figure 2b shows the hysteresis loop of the CoCu porous film; the coercivity field is about 150 Oe and the remanence ratio is about 0.26. The coercivity field of the porous film is larger than that of the granule film due to the different morphology of the samples. Resistance measurement showed that the resistance of such porous film could be decreased even at a low current density. Figure 3a presents the behavior of the resistance reduction, under a density of ∼1 A/cm2 for the as-prepared sample shown in Figure 1a, at room temperature. The resistance decreased from an initial value, labeled as R0 (R0 ) 36.204 Ω), to a constant one, labeled as RLow (RLow ) 36.03 Ω), within 3 h. There was a total drop of 170 mΩ. The decreasing resistance evolves in the form of the following relaxation equation:

Rt ) ∆Re-t/τ + RLow Here τ is the relaxation time; ∆R is the resistance change from initial value to final steady value (R0 - RLow). From Figure 3a, one can obtain the relaxation time of about 2300 s (∼38 min).

Figure 2. Hysteresis loops for the CoCu film (a) and the CoCu porous film (b).

A further experiment revealed that such current-induced resistance reduction was recoverable after switching off the current for sufficient time. When the resistance relaxation was completed and the resistance was measured again after an interval, the “initial resistance” showed strong time-interval dependence, as demonstrated in Figure 3b, and more clearly “initial resistance”’ vs. interval time is shown in the inset of Figure 3b, the longer the interval was, the higher the “initial resistance” value was. If the interval was >1 h, the relaxed resistance could be recovered almost completely. Further, the resistance reduction was also current-density dependent: larger current density led to a faster relaxation rate of resistance, as shown in Figure 3c. The corresponding relaxation times for different current densities, e.g., 0.1, 1, and 10 µΑ cm-2, are about 3000, 2300, and 1200 s, respectively. It can be seen that the larger the applied current is, the shorter the resistance relaxation time is. It should be mentioned that such resistance reduction was not observed in the two reference samples: one was CoCu granule film prepared without PSs template, as shown in Figure 3d, the other was Cu porous film with the same porous structure as the CoCu porous film, see the inset of Figure 3d. This indicated that the reduction behavior originated from the special ferromagnetic domain structure of the present porous film. Additionally, the external magnetic field could significantly influence the reduction behaviors of the resistance. Figure 4 shows the results for the as-prepared sample under external magnetic fields 0 Oe, 200 Oe, and 2 T perpendicular to the film surface with current density 10 A/cm2. The resistance change ∆R () R0 - RLow) showed strong magnetic field dependence. ∆R values were about 0.2, 100, and 170 mΩ, for applied constant fields 2 T, 200 Oe, and zero, respectively. If a changing field, from 0 to 5000 Oe, was applied on the CoCu porous film, a significant resistance reduction (or magnetoresistance) was larger than that under zero field, as shown in Figure 4. The corresponding ∆R for changing field increased

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Figure 3. (a) Resistance reduction for porous film at room temperature; (b) the time dependence on original the resistance restore under zero current after the resistance relaxation to a steady value, here “0”, “1”, “2”, “3”, and “4” correspond to interval times of 0, 10, 300, 1800, and 3000 s; (c) resistance reduction for different current densities; and (d) resistance relaxation for the CoCu film at room temperature, the inset is the resistance relaxation for the Cu porous film obtained by the same colloidal template.

Figure 4. Magnetic dependent resistance reduction. From top to bottom, the resistance reduction curves correspond to 2T, 200 Oe, 0 Oe, and 0-5000 Oe, respectively.

to 500 mΩ, which was larger than 2 times that under zero field. The trend of curves for resistance reduction in different fields was close to that of the magnetoresistance curve, the decreased resistance value was about 3 times of that of 0 Oe field. This indicated that the mechanisms between magnetoresistance and the resistance reduction were close, but not identical. However, the magnetic dependent resistance reductions also were not observed in reference samples. IV. Discussion The formation of the CoCu film with hexagonally arranged pores is easily understood due to the electrodeposition on the PS colloidal crystals, as previously described.11,12 In our deposition conditions, homogeneous nucleation occurs and

Cu-Co alloying nanoparticles are formed in the solution near the substrate. The nanoparticles grow on the interstitials in the PS’s double layered template, building into the porous skeleton and forming the resulting film shown in Figure 1a after the removal of PSs. As for the current and magnetic dependent resistance reduction, there coexist two mechanisms: one is based on the spin glass-like state at some of the grain boundaries (GB) in the conduction path,13-15 or relates to interface magnetic scattering;14 the other comes from the current-driven switching of the domains in magnetic materials.12 The former usually describes the material resistance change in a high field,14,15 and is current independent. Besides this, the former is also grain boundary dependent and morphology independent.14 However, in our case, the resistance reduction showed strong magnetic and current dependence. Moreover, the corresponding resistance reduction was not observed in the referenced CoCu granule film and Cu porous film. This reflects that the present resistance reduction arises from the motion of the magnetic micro/nanostructures of the porous films, rather than the simple GB mechanism. Generally, the dependence of the resistance change on magnetics and current can be explained by the current-driven switching mechanism, namely, spin-polarized currents reverse the magnetization direction of micro/nanometer sized metallic structures through torque.17-24 In present mechanism, the very high current density is needed to switch the domain wall (above 107 A/m2).9,10 However, in our case, the resistance reduction occurs when the current density is as low as 103 A/m2. Obviously, the current-driven switching mechanism cannot well explain our observations and possibly one should be further proposed.18 Note that the resistance reduction property dependence on current and magnetic properties in the CoCu porous film is not

Resistance Reduction in CoCu Porous Films SCHEME 1: Schematic Illustration of the Interaction between the Current and Magnetic Domainsa

a

Panel a: Without current, the direction of the domains is random. Panel b: Under a low current density, the direction of the domains will turn to a certain direction. The black arrows show the direction of domains.

observed in the referenced CoCu granular films and Cu porous films, which indicates that the origins of resistance reduction behavior could form the special ferromagnetic domain of the present porous film. It can be seen that there exists protuberance in the intersection of the hexagonal pore array from Figure 1a. Considering the protuberance size (diameter about 1 µm and height about 1-2 µm), each ferromagnetic protuberance thus probably forms a single magnetic domain according to micromagnetics.24 Surely, there may be two or three pillar-like protuberances forming a magnetic domain, but this does not influence our discussion. To better understand the mechanism, we view each protuberance as a single magnetic domain, and the whole sample will form a network or a domain array, as seen in Scheme 1. Only the bottom part of domains are connected to each other. The distance between the two neighboring domains is very large, about 1 µm, see Figure 1a, leading to the weak interactions between domains. Combining these facts and previous reports about current-driven switching in ferromagnetic materials,16-22 a model of the current-driven movement of domain walls in low current density is thus proposed to explain the resistance reduction, as addressed in what follows: In the absence of external magnetic field and electric current, as shown in Figure 3a, the directions of the ferromagnetic domains in as-prepared samples are almost random. The conduction electrons in each domain are partially polarized along the polarized direction of the domain. As the electric current is switched on, the current is partially polarized due to the net internal molecular field in the ferromagnetic sample. When the polarized current passes through the magnetic domains, it exerts a torque17-23 on each domain, as sketched in Scheme 1. Under the spin polarized current, these domains overcome the surrounding molecular field barrier, and are dragged to the direction shown in Scheme 1, thus the ordering degree of the domains increases with increasing time. On the other hand, with increase of ordering degree of magnetic domains, the demagnetization energy increases greatly and blocks the degree to further increase. Finally, after the characteristic time, τ, under the competition between current effect and demagnetization energy, the ordering degree of the magnetic domains reaches a saturated value then does not change. This change also leads the decrease of the spin scattering to a steady value, hence the corresponding resistance also decreases to a low saturated value. Under different current densities, the torque of different current densities exerted on the domains is also different. Obviously, the small torque on the domains of samples costs more time to saturation, so the higher the current is, and the larger the torque

J. Phys. Chem. C, Vol. 114, No. 5, 2010 2303 is. Since the dissipation in the relaxation is torque dependent, the relaxation time τ is current dependent. Thus the resistance change also shows a current density dependence, as shown in Figure 3c. If the current is switched off when the resistance relaxes to RLow, the domain walls will return to the initial state due to the interaction between domains. In the high magnetic field, for example B ) 2 T, see Figure 4, all the polarized directions of the domains are parallel to the external field and the ordering degree of magnetic domains is very high, so the current hardly switches the directions and the resistance reduction effect is almost not observed. In the low field, the directions of domains are not as consistent as those of high field, so the resistance reduction effect is more obvious than that in high field. In zero field, the polarized degree of the domains is the lowest, so the resistance reduction is the most obvious. Thereby, a considerable dependence on resistance reduction of magnetic properties can be observed, namely, ∆R is small in a high field, and ∆R is large in a low field. However, in a changing field, the directions of domains were influenced not only by current density but also by the external field.25 Hence, the resistance reduction in the varying field is very large, in comparison with that of zero field, as seen in Figure 3a. The low density current-driven resistance reduction in the present sample arises from the current drag effect overcoming the interaction between the domains.26 In the porous film, each protuberance forms a single domain due to micromagnetics.24 However, each domain only partially connects with the neighborhood domains, so the coupling between domains is far smaller than that in the CoCu film. In the CoCu porous film, the current drag effect is dominant and the resistance reduction can be easily observed. In contrast, in the CoCu granule film, the interaction between the domains may be stronger than the current effect, and the reduction is hard to observe in a low current density. So, to investigate the current switch domain wall in the Co/Cu/Co nanopillar, the Cu layer should be thick enough to reduce the orange peer effect and the RKKY coupling, see ref 25. For the reference sample Cu porous film, although having the same morphology with the CoCu porous film, there is no magnetic domain in it, so the current-driven resistance reduction is not observable. The investigation on the current induced resistance reduction in ferromagnetic material can gain much information about the coupling between the current and domain walls, which may be in favor of understanding the mechanism of current induced magnetic domain wall motion. This also could be helpful for the investigation of current induced switching in magnetic tunnel junctions.13 Such current and magnetic dependent resistance reduction properties provide an opportunity for the development of technologically important devices based on this effect, such as magnetic delay switches. V. Conclusions In summary, the resistance reductions in 2-dimensional CoCu porous films, fabricated by electrodeposition using double layered colloidal crystal as a template, were investigated in various magnetic fields and low current densities. By comparing with CoCu and Cu porous films, the resistance of CoCu porous film exhibited a distinct time relaxation effect, which showed strong current and magnetic field dependencies at room temperature. We ascribe this to the current-driven domain wall drag effect. These results are in favor of understanding the mechanism of the current-driven motion of domain walls in low current density and highly valuable in its further application in magnetic delay switches.

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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), the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KJCX2-SW-W31), and the Scientific Research Foundation of Nantong University (Grant No. 09R23 and 09Z051), the Scientific Research Foundation of Taizhou University (Grant No. 09ZD10 and 09ND11). References and Notes (1) Wang, W. D.; Zhu, F. W.; Weng, J.; Xiao, J. M. Appl. Phys. Lett. 1998, 72, 1118. (2) Darques, M.; Bogaert, A. S.; Elhoussine, F.; Michotte, S.; Medina, J. T.; Encinas, A.; Piraux, L. J. Phys. D: Appl. Phys. 2006, 39, 5025. (3) Parkin, S. S. P.; Bhadra, R.; Roche, K. P. Phys. ReV. Lett. 1991, 66, 2152. (4) Pirota, K. R.; Vazquez, M. AdV. Eng. Mater. 2005, 7, 1111. (5) Albert, F. J.; Katin, J. A.; Buhrman, R. A.; Ralph, D. C. Appl. Phys. Lett. 2000, 77, 3809. (6) Katine, J. A.; Albert, F. J.; Buhrman, R. A.; Myers, E. B.; Ralph, D. C. Phys. ReV. Lett. 2000, 84, 3149. (7) Tsoi, M.; Jansen, A. G. M.; Bass, J.; Chiang, W. C.; Tsoi, V.; Wyder, P. Nature 2000, 406, 46. (8) Sauret, O.; Feinberg, D. Phys. ReV. Lett. 2004, 92, 106601. (9) Ruster, C.; Borzenko, T.; Gould, C.; Schmidt, G.; Molenkamp, L. W.; Liu, X.; Wojtowicz, T. J.; Furdyna, J. K.; Yu, Z. G.; Flatte, M. E. Phys. ReV. Lett. 2003, 91, 216602.

Li et al. (10) Yamanouchi, M.; Chiba1, D.; Matsukura, F.; Ohno, H. Nature 2004, 428, 539. (11) (a) Li, Y.; Cai, W. P.; Duan, G. T. Chem. Mater. 2008, 20, 615. (b) Sun, F. Q.; Cai, W. P.; Li, Y.; Cao, B. Q.; Lei, Y.; Zhang, L. D. AdV. Funct. Mater. 2004, 14, 283. (c) Sun, F. Q.; Cai, W. P.; Li, Y.; Jia, L. C.; Lu, F. AdV. Mater. 2005, 17, 2872. (12) (a) Li, Z.; Cai, W.; Duan, G.; Liu, P. Phys. E (Amsterdam, Neth.) 2008, 40, 680. (b) Li, Z.; Cai, W.; Duan, G.; Zeng, H.; Liu, P. J. Nanosci. Nanotechnol. 2009, 9, 2970. (13) Ventura, J.; Araujo, J. P.; Carpinteiro, F.; Sousa, J. B.; Liu, Y.; Zhang, Z.; Freitas, P. P. J. Magn. Magn. Mater. 2005, 290-291, 1067. (14) Kozlova, N.; Walter, T.; Do¨rr, K.; Eckert, D. Phys. B (Amsterdam, Neth.) 2004, 346-347, 74. (15) Kozlova, N.; Do¨rr, K.; Eckert, D.; Handstein, A. Appl. Phys. 2003, 93, 8325. (16) Zeng, Z. Y.; Yao, X. X.; Qin, M. J.; Ge, Y. Phys. C (Amsterdam, Neth.) 1997, 291, 229. (17) Slonczewski, J. C. J. Magn. Magn. Mater. 1996, 159, L1. (18) Tserkovnyak, Y.; Brataas, A.; Bauer, G. E. W. Phys. ReV. B 2002, 66, 224403. (19) Grollier, J.; Boulenc, P.; Cros, V.; Hamzic´, A.; Vaure`s, A.; Fert, A.; Faini, G. Appl. Phys. Lett. 2003, 83, 509. (20) Vernier, N.; Allwood, D. A.; Atkinson, D.; Cooke, M. D.; Cowburn, R. P. Europhys. Lett. 2004, 65, 526. (21) Yamaguchi, A.; Ono, T.; Nasu, S.; Miyake, K.; Mibu, K.; Shinjo, T. Phys. ReV. Lett. 2004, 92, 077205. (22) Berger, L. J. Appl. Phys. 2001, 89, 5521. (23) Slonczewski, J. C. J. Magn. Magn. Mater. 2002, 247, 324. (24) Fidler, J.; Schrefl, T. J. Phys. D: Appl. Phys. 2000, 33, R135. (25) Gerber, A.; Milner, A.; Groisman, B.; Karpovsky, M.; Gladkikh, A.; Sulpice, A. Phys. ReV. B 1997, 55, 6446. (26) Ravelosona, D.; Lacour, D.; Katine, J. A.; Terris, B. D.; Chappert, C. Phys. ReV. Lett. 2005, 95, 117203.

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