Measurement of the Giant Magnetoresistance ... - ACS Publications

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Measurement of the Giant Magnetoresistance Effect in Cobalt−Silver Magnetic Nanostructures: Nanowires Jose Garcia-Torres,* Elvira Gómez, and Elisa Vallés Electrodep., Physical Chemistry Department and Nanoscience and Nanotechnology Institute (IN2UB), University of Barcelona, Martí i Franquès, 1, 08028 Barcelona, Spain ABSTRACT: Arrays of cobalt (Co) and cobalt−silver (Co− Ag) nanowires have been prepared by electrodeposition using polycarbonate membranes as templates. Scanning electron microscopy characterization revealed a parallel growth of both types of nanowires with constant length. X-ray and electron diffraction patterns showed that cobalt nanowires were grown with hcp (hexagonal close-packed) structures, but different preferred orientations were obtained depending on the diameter: (110), (100), and (002) directions for nanowires with diameters 200, 100, and 50 nm, respectively. The structural differences were reflected on the magnetic response of the nanowires. On the other hand, the structural characterization of Co−Ag nanowires revealed the hcp and fcc (face-centered cubic) structure for cobalt and silver, respectively. The heterogeneous nature of the Co−Ag nanowires was evidenced by transmission electron microscopy. Giant magnetoresistance values as high as 0.5% were measured at room temperature. is one with the highest GMR values reported,15,16 is very interesting as the phase diagram indicates practically total immiscibility.17 Thus, sharp magnetic/nonmagnetic interfaces are expected to be obtained, characteristics required to have high GMR values. On the other hand, the electrodeposition of Co−Ag nanowires is a challenge because just a few papers deals with the electrodeposition of Co−Ag nanowires,18−20 and just one reports on the GMR of these nanowires.18 In this sense, the objective of the present work is the preparation of Co−Ag nanowires by template-assisted electrodeposition as well as their characterization: morphology, structure, and magnetotransport properties. However, because of the restricted geometry in which electrodeposition must be performed, the first step proposed was to test the viability of electrodeposition to prepare single metal nanowires (cobalt nanowires). Morphology, structure, and magnetic properties will also be examined.

1. INTRODUCTION A clear example of the breakthrough in nanoscience and nanotechnology in the last decades has been the discovery of the giant magnetoresistance (GMR) effect in 1998 in magnetic multilayers.1,2 What is called GMR is the change in the electrical resistance of a material upon the application of an external magnetic field. A huge scientific interest was focused on the GMR effect immediately after its discovery and arose because of the considerable industrial interest in the area of spintronics,3,4 an industry of broad future perspective. Although the GMR effect was discovered in magnetic multilayers, soon after, it was observed by Berkowitz et al.5 and Xiao et al.6 in the so-called granular films. Moreover, some years later, the GMR effect was also detected in other nanostructured materials: nanowires and nanoparticles.7,8 In particular, magnetic nanowire arrays have attracted considerable attention because of the unique properties that these 1D magnetic materials display.9−11 In this line, the GMR effect has been improved in magnetic nanowires if compared to either granular or multilayered films.3 The fabrication of nanowires is normally based on the use of templates.12,13 The most appropriate technique to grow the nanowires into the template is electrodeposition, which has been revealed as the most successful method in comparison to more sophisticated techniques such as molecular beam epitaxy or microlithography.14 Template synthesis by electrochemical deposition is a versatile and particularly simple approach. Arrays of nanowires are obtained by filling the channels or pores of the template with the desired material. Moreover, template-assisted synthesis allows the tailoring of magnetic properties by tunning the length and diameter of the porous material. In this work, the selected system to study the GMR effect is the cobalt−silver (Co−Ag) system. The Co−Ag system, which © 2012 American Chemical Society

2. EXPERIMENTAL SECTION Co−Ag nanowires were prepared by electrodeposition from an electrolyte whose composition was 0.002 M AgNO3 + 0.2 M CoCl2 + 3.5 M NaCl, reagents being of analytical grade. Co nanowires were prepared from a similar solution but in the absence of silver nitrate. The solutions were freshly prepared with water first double-distilled and then treated with a Millipore Milli-Q system. The pH was kept around 2.7, and the temperature was maintained at 25 °C. Solutions were deaerated Received: January 4, 2012 Revised: May 5, 2012 Published: May 14, 2012 12250

dx.doi.org/10.1021/jp300119w | J. Phys. Chem. C 2012, 116, 12250−12257

The Journal of Physical Chemistry C

Article

by argon bubbling before each experiment and maintained under an argon atmosphere during it. Electrochemical experiments were performed in a threeelectrode cylindrical cell with an upward facing cathode at the bottom of the cell using an Autolab with PGSTAT30 equipment and GPES software. Working electrodes were polycarbonate membranes 20 μm in thickness and with a pore density of 5.8 × 108 pores cm−2. Membranes with pore diameters of 200, 100, and 50 nm were employed to grow the nanowires. Membranes were coated by vacuum evaporation with around a 100 nm thick gold layer to make the membrane conductor. Prior to the electrodeposition, the porous template was kept in distilled water for several hours to make the pores hydrophilic for uniform filling of the pores. After that, the distilled water was replaced with the prepared electrolyte baths for depositing either cobalt or cobalt−silver nanowires. This additional step was crucial for obtaining homogeneous growth over the entire membrane. The reference electrode was an Ag/ AgCl/1 M NaCl electrode. All potentials were referred to this electrode. The counter electrode was a platinum spiral. Deposits were prepared by chronopotentiometric technique. Nanowires were observed using Hitachi H-4100 FE field emission scanning electron microscope (FE-SEM) and JEOL 2100 transmission electron microscopy (TEM). The structure was determined by X-ray diffraction (XRD) and selected area electron diffraction (SAED). The composition was determined by energy dispersive spectroscopy (EDS) coupled to the SEM microscope. Magnetic measurements were taken in a SQUID magnetometer at room temperature in helium atmosphere. The magnetization−magnetic field (M−H) curves were recorded maintaining the samples both parallel and perpendicular to the applied magnetic field. The magnetoresistance (MR) measurements were performed at room temperature with the four-point probe method in magnetic fields between −8 kOe and +8 kOe in the currentperpendicular-to-plane configuration (CCP). Both the longitudinal and the transverse MR (field parallel to current and field perpendicular to current, respectively) components were recorded. The MR was defined as follows:

Figure 1. Cyclic voltammetry recorded from the solution 0.2 M CoCl2 + 3.5 mol dm−3 NaCl using (A) vitreous carbon and (B) a gold-coated membrane as the working electrode. Scan rate, 50 mV s−1.

the other hand, the lower j/E slope attributed to the difficulty of deposit growth into the channels. From this voltammetric study, information about the adequate current density range to apply and the associated potential for cobalt reduction was obtained. A current density of −10 mA cm−2 was selected to grow cobalt nanowires. Figure 2A shows the potential−time (E−t) profile in which the potential initially reaches the most negative value for initial nucleation of metallic cobalt into the pores. As electrodeposition proceeds, E goes toward more positive potentials. Finally, a constant E value is reached at short times. This potential value falls in the potential range estimated from the previous voltammetric study. 3.2. Characterization of Co Nanowires. 3.2.1. Morphology. A cross-sectional view of the electrodeposited membrane is shown in Figure 2B. As it can be observed, nanowires were well grown parallel to each other throughout the entire membrane. Moreover, nanowires fully continuous with a constant length were obtained, a length that can be controlled in the range from several nanometers to around 20 μm by varying the deposition time. On the other hand, a high proportion of the pores filled with cobalt metal was obtained (Figure 2C). The same results were observed in all of the electrodeposited membranes with different pore sizes. 3.2.2. Structure and Magnetic Properties. Figure 3 shows the XRD pattern of the nanowires with different diameter. As it can be observed, in all three cases, nanowires are highly crystalline (polycrystalline) in nature with hcp structure. However, different preferred crystallographic orientations were obtained depending on the diameter. Preferential growth along (110), (100), and (002) was noticed from the patterns

MR(H ) = 100 × [R(H ) − R 0]/R 0

where R(H) is the resistance in the magnetic field H and R0 is the resistance when H = 0.

3. RESULTS AND DISCUSSION 3.1. Preparation of Co Nanowires. The electrodeposition into the pores of polycarbonate membranes is not an easy process because of the rather restricted geometry. Because of this, the first step proposed in this study was to test the viability of single metal nanowire electrodeposition as well as to have a good control of the electrodeposition process. First, the general trends of the electrochemical behavior of Co nanowires electrodeposition in the selected electrolyte were analyzed by cyclic voltammetry (Figure 1). Scanning toward negative potentials, reduction begins over vitreous carbon substrate at around −850 mV, and one reduction peak centered around −1000 mV was recorded meanwhile; the oxidation of the deposit was detected during the backward scan (Figure 1A). The same general behavior was detected when the gold-coated membrane was the cathode; however, some differences were detected: on one hand, the ease of the cobalt reduction on metallic seed layer, starting at −700 mV (Figure 1B), and on 12251



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Figure 3. XRD pattern of cobalt nanowires with different diameters: 200, 100, and 50 nm. Figure 2. (A) Potential−time curve for electrodeposition of cobalt at a constant current density of 10 mA cm−2. FE-SEM image of (B) the cross-section and (C) on-top view of an electrodeposited membrane with Ø = 200 nm (scale bar = 7.5 μm).

The high stability of the grown nanowires is evident from TEM analyses after being liberated from the membrane (Figure 4A). Moreover, it can also be observed that the nanowires are continuous along its length, robust, and homogeneous in diameter. Moreover, the SAED patterns confirm the hcp crystalline structure along the entire nanowire (Figure 4B). Figure 5 shows the normalized hysteresis loops of the prepared Co nanowires arrays with the magnetic field applied parallel and perpendicular to the nanowire axis. Important differences can be observed depending on the nanowire diameter. Whereas the easy magnetization axis of the nanowires with Ø = 50 nm is parallel to the long wire axis, there is no obvious easy magnetization axis for the nanowires with Ø = 100 and Ø = 200 nm. On the other hand, a clear variation of the

for nanowires with diameters 200, 100, and 50 nm, respectively. All of the peaks appeared at the expected positions. TEMs and SAED are shown in Figure 4. It must be noted here that the nanowires were subjected to TEM studies after removing them from the template. First, the gold layer was removed using a saturated I2/I− solution, and then, the polycarbonate membrane was dissolved with chloroform. The residue was magnetically separated and cleaned with ethanol. Finally, a drop of the nanowires was casted over a copper grid. 12252



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Figure 4. (A) TEM image of individual nanowires (scale bar = 200 nm). (B) SAED pattern along a nanowire.

respectively. This means that the (002) direction is perpendicular to the nanowire axis; hence, magnetocrystalline anisotropy tends to align magnetic moments in this direction. For nanowires with Ø = 200 and Ø = 100 nm, shape anisotropy and magnetocrystalline anisotropy tend to align the magnetic moments in different directions. It is also worthy to mention that shape anisotropy contributes less as the nanowire diameter increases because the nanowire length/diameter ratio decreases. Summarizing, when the easy magnetization axis of the magnetocrystalline anisotropy is in the same direction as that of the shape anisotropy, the effective anisotropy will be augmented through synergic action of the magnetocrystalline anisotropy and the shape anisotropy energy, which is the case for nanowires with Ø = 50 nm. On the other hand, if they are in different directions as in Ø = 200 and Ø = 100 nm nanowires, their competition will weaken anisotropy; hence, M−H loops both longitudinal and perpendicular will be practically coincident. Another difference among the hysteresis loops with the magnetic field applied parallel is the coercivity. When the applied field is parallel to the axis of the nanowires, the coercivity (Hc) of the cobalt wires with Ø = 50 nm is smaller (around 65 Oe) than the Hc values for higher diameters (around 110 Oe). Moreover, for these 50 nm diameter nanowires, the Hc values are smaller in parallel than in the perpendicular hysteresis loop. Although these results are in contrast with the vast majority of the published papers,21−23 the magnetic dipolar interactions among nanowires could explain the decrease of the coercivity along the easy axis for the 50 nm diameter nanowires as Garciá et al. suggest.24 According to them, the dipolar interaction is enhanced when the applied magnetic field is parallel to the easy axis, and the field emanating from each wire favors the magnetization reversal. 3.3. Preparation of Co−Ag Nanowires. Once it was observed that single metal nanowires electrodeposition was successful, it was decided to prepare Co−Ag nanowires into the pores of polycarbonate membranes. The nanowires were grown into membranes with pore diameters of 200 nm. Again, a basic electrochemical study was first performed to select the appropriate current density range to prepare Co−Ag nanowires with modulated composition. The cyclic voltammogram recorded using a gold-coated polymer membrane as the substrate shows two reduction peaks and two oxidation features related to the independent reduction/oxidation of both metals (Figure 6A). These results were in agreement with the codeposition of a heterogeneous material. According to the previous voltammetric experiment, the electrodepositon of Co−Ag nanowires was carried out at a constant current density in the range (−1,−3) mA cm−2 to

Figure 5. Magnetic hysteresis loops of cobalt nanowires with different diameters: 200 and 50 nm.

remanent magnetization with the nanowire's diameter was observed in the hysteresis loops when the field is applied parallel to the wire axis. The squareness (Mr/Ms) decreased from a value close to 0.5 for the smallest diameter to 0.1 for a Ø = 200 nm. The origin of this difference could be attributed to two different contributions. On one hand, the shape anisotropy tends to force the magnetization to be along the nanowire axis. On the other hand, the magnetocrystalline anisotropy could also reinforce the shape anisotropy contribution if the easy axis of crystal anisotropy [the (002) direction for the hcp structure] lies parallel to the cylinder axis. This is the situation of the nanowire with the smallest diameter (50 nm). Thus, both shape and magnetocrystalline anisotropy reinforce the anisotropy of the 50 nm diameter nanowires, leading to a square hysteresis loops in the nanowire axis. On the other hand, as the nanowires grew in diameter, the preferred orientation along the wire axis changed to (110) and (100) for Ø = 200 and Ø = 100, 12253



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structural characterization was performed by SAED. The SAED pattern (Figure 8C) of the granular nanowires is a complex ring pattern characteristic of polycrystalline materials. The d spacing values calculated from the positions of the most intense spots in the electron diffraction pattern agreed well with the d values reported for hcp-Co (100) and hcp-Co (101) planes and for the fcc-Ag (111) plane, confirming again the heterogeneous nature of the prepared nanostructures. 3.4.2. Magnetotransport Properties. Although nanowires are ideal for the study of CPP-GMR effect, there remains a technical difficulty in making an electrical contact with individual nanowires for the CPP-GMR measurements. In this sense, the magnetotransport properties of nanowires have been mainly studied by measurement on arrays of nanowires. However, the number of nanowires used for the CPP-GMR measurements should be minimized to maintain a relatively large electrical resistance. In this sense, the conventional fourpoint probe method was employed but only using two mechanical contacts, one on top of the membrane and the other on the gold rear layer of the membrane. The procedure to carry out the MR measurements is shown in Figure 9. After the electrodeposition of the nanowires (Figure 9A), a square of 200 μm side of a few nanometer-thick gold layer is sputtered on top of the membrane by means of a mask (Figure 9B). Mainly, the nanowires that had emerged from the membrane will contact with the gold layer, and they will be the only contributing to the CPP-GMR measurement. After that, both current and voltage copper wires were attached to the gold square on top (Figure 9C) and the gold layer on bottom (Figure 9D) using small amounts of metallic indium. A typical room-temperature MR curve measured in the Co− Ag granular nanowires is shown in Figure 10A. Both the longitudinal and the transverse MR curves were negative for the as-prepared Co−Ag granular nanowires indicative of GMR. MR(H) curves shared some characteristics. On one hand, the saturation of the MR can not be achieved even at the highest applied magnetic field, and on the other hand, there is no splitting of the curves around 0 kOe. Both facts are indicative of the main presence of superparamagnetic particles. To quantitatively decompose the total GMR into its superparamagnetic (GMRSPM) and ferromagnetic (GMRFM) contributions, the numerical analysis of the MR field dependence curves [MR(H)] suggested by Bakonyi et al.25 was applied as follows:

Figure 6. (A) Cyclic voltammetry recorded from the solution 0.002 mol dm−3 AgNO3 + 0.2 mol dm−3 CoCl2 + 3.5 mol dm−3 NaCl using a gold-coated membrane as the working electrode. (B) Typical E−t transient recorded during the Co−Ag nanowire electrodeposition process. Scan rate, 50 mV s−1.

obtain nanowires with variable cobalt content. Several hours were required to completely fill the 20 μm thick membranes. Figure 6B shows a typical E−t transient recorded during the electrodeposition process. Initially, the potential reached the most negative value for the initial filling of the pores. After a short period of time, the E value increased up to adopt a constant value, potential value that fell in the Co−Ag codeposition potential range previously observed by cyclic voltammetry. 3.4. Characterization of Co−Ag Nanowires. 3.4.1. Morphology, Composition, and Structure. The cross-section FESEM image of Co−Ag nanowires (Figure 7A) demonstrates that with the developed solution, the deposited nanostructures do indeed fill the nanopores uniformly and that the nanowires are apparently continuous and parallel with constant length. Once the nanowires were extracted from the polycarbonate membrane, their stability and continuity were checked (Figure 7B−D). Moreover, these images show compact and uniform nanowires and no damage after etching. Energy dispersive Xray spectroscopy analysis corroborated not only the presence of both cobalt and silver but also the increased cobalt content in the fabricated nanowires as the current density was made more negative (Figure 7B−D). Figure 8A shows a representative TEM image of the asdeposited nanowires having a diameter of about 200 nm, a value that completely fits with the pore diameter of the polycarbonate membranes used. Moreover, a random distribution of nanometric cobalt particles (darker areas) into the silver matrix (lighter background) can also be observed, confirming the granular nature of the nanowires prepared (Figure 8B). The

MR(H ) = MR FM + GMR SPML(x)

(1)

for magnetic fields H > Hs. Whereby MRFM = AMR + GMRFM is a constant term, Hs is the saturation field of the ferromagnetic region and L(x) is the Langevin function. Briefly, the numerical analysis consists on fitting the MR(H) curves with eq 1 for magnetic fields beyond Hs ≈ 1.7 kOe, which provides with the GMRSPM(H) contribution. The MRFM contribution is obtained when subtracting the GMRSPM(H) term from the experimental data. The result of the fitting analysis gives a superparamagnetic contribution slightly higher than the total MR. This indicates that the relative weight of the superparamagnetic contribution (% SPM) is approximately 100%, which is in accordance with the only presence of superparamagnetic particles. On the other hand, the calculated % SPM in Co−Ag nanowires is higher than that observed in Co−Ag granular films in which % SPM was around 90%,26,27 probably due to the restricted geometry of the pores. 12254



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Figure 7. (A) Cross-section view of an electrodeposited membrane (scale bar = 15 μm). (B−D) FE-SEM images of the nanowires released from the membrane. Insets show the corresponding EDS spectra.

Figure 8. (A) TEM image showing Co−Ag nanowires released from the membrane (scale bar = 1 μm). (B) Image in which nanometric cobalt clusters are observed (scale bar = 50 nm). (C) Corresponding SAED pattern (scale bar = 10 1/nm).

Figure 9. Procedure followed to perform the MR measurement. After the electrodeposition of the nanowires (A), a 200 μm side square gold layer is sputtered on top of the membrane (B). After that, electrical contacts are attached to the square gold layer on top (C) and the gold layer on bottom (D) using metallic indium.

trend than that previously observed in Co−Ag granular films is obtained: GMR values increased with the ferromagnetic metal

A clear dependence of the MR on the nanowire's cobalt content was also observed (Figure 10B). The same general 12255



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Figure 10. (A) Typical room-temperature magnetoresitance curve for the granular nanowires. (B) Dependence of the GMR on the nanowire's cobalt content.

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content up to a maximum and then dropped off at higher cobalt contents, this variation being attributed to the modification of parameters that governs GMR.26,27 The maximum MR value measured at room temperature was 0.5%. It is also important to mention that the values obtained here are higher than the values reported in the unique work dealing with the GMR of granular Co−Ag nanowires (0.2%),18 which indicates that more progress can be made.

(1) Baibich, M. N.; Broto, J. M.; Fert, A.; Nguyen Van Dau, F.; Petroff, F.; Eitenne, P.; Creuzet, G.; Friederich, A.; Chazelas, J. Phys. Rev. Lett. 1988, 61, 2472. (2) Binasch, G.; Grünberg, P.; Saurenbach, F.; Zinn, W. Phys. Rev. B 1989, 39, 4828. (3) Bakonyi, I.; Péter, L. Prog. Mater. Sci. 2010, 55 (3), 107. (4) Gregg, J. F.; Pretej, I.; Jouguelet, E.; Dennis, C. J. Phys. D: Appl. Phys. 2002, 35, R121. (5) Berkowitz, A. E.; Carey, M. J.; Michell, J. R.; Young, A. P.; Zhang, S.; Spada, F. E.; Parker, F. T.; Hutten, A.; Thomas, G. Phys. Rev. Lett. 1992, 68, 3745. (6) Xiao, J. Q.; Jiang, J. S.; Chien, C. L. Phys. Rev. Lett. 1992, 68, 3749. (7) Blondel, A.; Meier, J. P.; Doudin, B.; Ansermet, J. Appl. Phys. Lett. 1994, 65 (23), 3019. (8) Ram, S.; Frankwicz, P. S. Phys. Status Solidi A 2001, 3, 1129. (9) Yu, C. Y.; Yu, Y. L.; Sun, H. Y.; Xu, T.; Li, X. H.; Li, W.; Gao, Z. S.; Zhang, X. Y. Mater. Lett. 2007, 61, 1859. (10) Whitney, T. M.; Searson, P. C.; Jiang, J. S.; Chien, C. L. Science 1993, 261, 1316. (11) Meng, G.; Cao, A.; Cheng, J. Y.; Vijayaraghavan, A.; Yung, Y. J.; Shima, M.; Ajayan, P. M. J. Appl. Phys. 2005, 97, 064303. (12) Talapatra, S.; Tang, X.; Padi, M.; Kim, T.; Vatjai, R.; Sastry, G. V. S.; Shima, M.; Deevi, S. C.; Ajayan, P. M. J. Mater. Sci. 2009, 44, 2271. (13) Fert, A.; Piraux, L. J. Magn. Magn. Mater. 1999, 200, 338. (14) Gijs, M. A. M.; Lenczowski, S. K. L.; Giesbers, J. B. Phys. Rev. Lett. 1993, 70, 3343. (15) Barnard, J. A.; Hossain, S.; Parker, M. R.; Waknis, A.; Watson, M. L. J. Appl. Phys. 1993, 73, 6372. (16) Berkowitz, A. E.; Michell, J. R.; Carey, M. J.; Young, A. P.; Rao, D.; Starr, A.; Zhang, S.; Spada, F. E.; Parker, F. T.; Hutten, A.; Thomas, G. J. Appl. Phys. 1993, 73 (10), 5320. (17) Hugh, B., Ed. Alloy phase diagram. ASM Handbook; ASM International: Cleveland, OH, 1992; Vol. 3, p 27. (18) Fedosyuk, V. M.; Kasyutich, O. I.; Schwarzacher, W. J. Magn. Magn. Mater. 1999, 198−199, 246. (19) Wang, Y. W.; Wang, G. Z.; Wang, S. X.; Gao, T.; Sang, H.; Zhang, L. D. Appl. Phys. A: Mater. Sci. Process. 2002, 74, 577. (20) Wang, Y. W.; Zhang, L. D.; Meng, G. W.; Peng, X. S.; Jin, Y. X.; Zhang, J. J. Phys. Chem. B 2002, 106, 2502. (21) Han, X.; Liu, Q.; Wang, J.; Li, S.; Ren, Y.; Liu, R.; Li, F. J. Phys. D: Appl. Phys. 2009, 42, 095005. (22) Ferré, R.; Ounadjela, K.; George, J. M.; Piraux, L.; Dubois, S. Phys. Rev. B 1997, 56 (21), 14066. (23) Narayanan, T. N.; Shaijumon, M. M.; Ci, L.; Ajayan, P. M.; Anantharaman, M. R. Nano Res. 2008, 1, 465.

4. CONCLUSIONS Magnetic nanowires of different diameters have been successfully prepared by chronopotentiometry technique into the pores of polycarbonate membranes. Fully continuous nanowires with nearly constant length were obtained. The structural characterization revealed that all of the cobalt nanowires grown showed the typical hcp structure; however, different preferred orientations were detected. Magnetic anisotropy was clearly detected in nanowires with Ø = 50 nm due to the synergic effect between shape and magnetocrystalline anisotropy, which tended to align the magnetic moments along the nanowire axis. Co−Ag granular nanowires with nearly uniform length were also successfully prepared by chronopotentiometry. TEM studies shed light on the heterogeneous nature of the grown nanowires as well as on the fcc-Ag and hcp-Co structure. The experimental method allowed performance of the GMR measurement. Magnetotransport properties studies indicated a clear dependence of the GMR values on the cobalt content. MR values as high as 0.5% were obtained.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel: +34934021234. Fax: +34934021231. E-mail: garcia. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper was supported by contract CTQ2010-20726 (subprogram BQU) from the Comisión Interministerial de Ciencia y Tecnologiá (CICYT). We thank the Centres ́ Cientifics i Tecnològics de la Universitat de Barcelona (CCiTUB) for the use of their equipment. 12256



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(24) García, J. M.; Asenjo, A.; Velázquez, J.; García, D.; Vázquez, M.; Aranda, P.; Ruiz-Hitzky, E. J. Appl. Phys. 1999, 85 (8), 5480. (25) Bakonyi, I.; Péter, L.; Rolik, Z.; Kiss-Szabó, K.; Kupay, Z.; Tóth, J.; Kiss, L. F.; Pádár, J. Phys. Rev. B 2004, 70, 054427. (26) García-Torres, J.; Vallés, E.; Gómez, E. J. Phys. Chem. C 2010, 114 (28), 12346. (27) García-Torres, J.; Vallés, E.; Gómez, E. J. Phys. Chem. C 2010, 114 (19), 9146.

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Measurement of the Giant Magnetoresistance ... - ACS Publications

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