Enhanced Electrocatalytic Activity of Copper–Cobalt Nanostructures

ARTICLE pubs.acs.org/JPCC

Enhanced Electrocatalytic Activity of Copper
Cobalt Nanostructures Jahangeer Ahmed,† Aparna Ganguly,† Soumen Saha,† Govind Gupta,‡ Phong Trinh,§ Amos M. Mugweru,*,§ Samuel E. Lofland,^ Kandalam V. Ramanujachary,§ and Ashok K. Ganguli*,† †

Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Surface Physics Division, National Physical Laboratory (CSIR), New Delhi, 110012, India § Department of Chemistry and Biochemistry and ^Department of Physics and Astronomy, Rowan University, 201 Mullica Hill Road, Glassboro, New Jersey 08028, United States ‡

bS Supporting Information ABSTRACT: Novel core
shell nanostructures containing Cu and Co have been synthesized using the microemulsion method at 700 C. The core consists of Cu
Co composite particles, whereas the shell is composed of Cu
Co alloy particles (shell thickness ∼12 nm). It is to be noted that in bulk Cu
Co binary system there is practically no miscibility. TEM studies show formation of spherical-shaped nanoparticles of core
shell structures. The composition of the core (Cu
Co composite) and shell (Cu
Co alloy) were confirmed by XPS studies. The formation of the Cu
Co alloy as the shell is mainly driven by surface energy considerations. We have also obtained Cu
Co nanocomposites (by controlling the concentration of reducing agent) with particle size in the range of 40
200 nm. These Cu
Co nanostructures show ferromagnetic behavior at 4 K. The saturation magnetization of the core
shell (Cu
Co composite @ Cu
Co alloy) nanostructure (125 emu/g) is found to be higher than that of pure Cu
Co nanocomposite or alloy, which may be useful for applications as a soft magnet. Electrochemical studies of these nanocrystalline Cu
Co particles show higher hydrogen evolution efficiencies (∼5 times) compared to bulk (micrometer-sized) Cu
Co alloy particles.

1. INTRODUCTION Nanoscale particles of metals, alloys, and core
shell structures have attracted much attention due to their unique electronic, optical, biological, catalytic, and magnetic properties.1
5 Metallic and bimetallic nanoparticles have applications in drug delivery, in hyperthermia, as electrocatalysts, and in magnetic devices.6
8 Cu
Co alloy particles show giant magnetoresistance (GMR) behavior originating mainly from spin-dependent scattering of conduction electrons at the interface of the ferromagnetic particles.9
11 The microstructural factors controlling GMR effect in granular materials are mainly the size distribution and the volume fraction of the ferromagnetic Co particles embedded in the nonferromagnetic matrix as well as the roughness of the interfaces.12 Ferromagnetic behavior was observed in cobalt-rich bulk CuxCo100
x systems (x = 20
100) at room temperature while the Cu90Co10 system showed paramagnetic behavior.10 Cu
Co alloy-based catalysts are selective for higher alcohol (C3 and higher) synthesis from syngas and also in dehydrogenation reactions.13 The electrocatalytic behavior of Cu
Co, Co
Ni, Fe
Co alloy particles for hydrogen and oxygen evolution reaction is also of importance.14
16 A number of physical techniques have been used to produce metastable solid solutions of Cu
Co such as arc melting,11,12 mechanical alloying,10 electrochemical deposition,17 melt-spinning method,18
20 and ball milling,21 which show limited solubility of Cu and Co. The copper
cobalt binary system is r 2011 American Chemical Society

an immiscible system (no solubility of Cu in Co or Co in Cu) under ambient conditions. The phase diagram of Cu
Co binary system22 shows that the maximum solid solubility of Co in Cu is 7 wt % at 1112 C, and the solubility decreases sharply with decreasing temperatures becoming negligible below 500 C. Among the known low-temperature chemical routes used for the synthesis of nanoparticles, the microemulsion method (reverse micelle) allows good control of size and homogeneity. Reverse micelles are water in oil microemulsions consisting of nanometer-sized water droplets that are dispersed in an oil medium and stabilized by surfactants. These water droplets are suitable as reaction media for the synthesis of nanoparticles where the size of particles can be controlled by varying the molar ratio of water to surfactant. Earlier, we exploited the microemulsion method for synthesis of Co,23,15 Co
Ni,15 Fe
Co,16 and oxide nanoparticles23
25 and also designed several core
shell nanostructures.26,27 There are also other reports on synthesis of bimetallic nanoparticles of Au
Ag,28 Au
Pd,29 and Au
Pt30 by microemulsions. In this article, we report the synthesis of novel core
shell nanostructured materials (Cu
Co composite @ Cu
Co alloy) and nanocrystalline Cu
Co composites using the microemulsion method. The formation of Cu
Co alloy-based shell by this Received: March 14, 2011 Revised: June 17, 2011 Published: June 23, 2011 14526

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method is significant as it gives a possibility to design the entire solid solution of the binary Cu
Co alloy phase. Electrocatalytic studies (hydrogen evolution reaction) show nearly five times enhancement using the Cu
Co nanostructures compared to Cu
Co bulk alloys.

2. EXPERIMENTAL SECTION Cu(NO3)2.3H2O (CDH, 98%), Co(CH3COO)2.4H2O (Qualigens, AR, 98%), NaOH (Qualigens, LR, 97%), N2H4. H2O (Qualigens, 99%), cetyl trimethylammonium bromide (CTAB) (Spectrochem, AR, 99%), 1-butanol (Qualigens, 99.5%), and isooctane (spectrochem, 99%) were used in the synthesis of the core
shell nanoparticles of Cu
Co nanostructures. These nanoparticles were synthesized by the microemulsion method with CTAB (cetyltrimethyl ammonium bromide) as the surfactant, 1-butanol as the cosurfactant, and isooctane as the oil phase. Four microemulsions with different aqueous phases containing 0.1 M Cu (NO3)2.3H2O, 0.1 M cobalt acetate, 20 M N2H4.H2O, and 0.1 M NaOH were prepared. The weight fractions of various constituents in these microemulsions were as follows: 16.76% of CTAB, 13.90% of 1-butanol, 59.29% of isooctane, and 10.05% of the aqueous phase.15 These microemulsions were mixed slowly on a magnetic stirrer and a green precipitate was obtained. The green precipitate was washed with 1:1 chloroform/methanol mixture and dried in air and subsequently reduced in hydrogen atmosphere at 700 C for 6 h to form the core
shell nanostructures. The same procedure was used for the synthesis of Cu
Co nanocomposite except that a lower concentration of hydrazine (1.0 M N2H4.H2O) was employed in the initial microemulsion. Powder X-ray diffraction (PXRD) studies were carried out on a Bruker D8 Advance diffractometer with Ni-filtered Cu
KR radiation with a scan speed of 1 s and scan step of 0.02. TEM studies were carried out with a Technai G2 20 electron microscope operated at 200 kV. The specimens for TEM were prepared by dispersing the powder in acetone by ultrasonic treatment, dropping onto a porous carbon film supported on a copper grid, and then drying in air. Cyclic voltammetry (CV) was carried out with a electrochemical workstation (CHI 660c, USA) with ohmic drop 98% compensated. The reference electrode was Ag/AgCl while Pt wire was used as counter electrode. Glassy carbon electrodes (0.07 cm2) used as a working electrode were first polished with 1 μm diamond polishing paste then ultrasonicated in distilled water for 1 min before immobilizing the different materials for hydrogen evolution reactions (HER). Five milligrams of nanoparticle sample was mixed with 2 μL of PVDF. The mixture was placed on glassy carbon electrode and air-dried for about 1 h. The electrode was then placed in the cell containing about 5 mL of 0.5 M KOH solution. For each experiment, freshly prepared electrode and solutions were used. The experiments were carried out at room temperature (22
23 C). Cyclic voltammetry was carried out at a scan rate of 40 mV/s with a peak window between 0 and
1.5 V versus Ag/AgCl electrode. Amperometry was carried out for 200 s for each electrode with the potential fixed at
1.4 V versus Ag/AgCl. X-ray photoelectron spectroscopy measurements were performed in an ultrahigh vacuum chamber (P H1257) with base pressure of 4  10
10 Torr. The XPS spectrometer was equipped with a high-resolution hemispherical electron energy analyzer (279.4 mm diameter) with 25 meV resolution, and a dual anode

Figure 1. Powder X-ray diffraction pattern of Cu
Co system (Cu
Co composite @ Cu
Co alloy) obtained using 20 M of hydrazine.

Mg/Al KR X-ray source. The source used for this study was the Al (KR) X-ray with excitation energy of 1486.6 eV. The general scan showed very small amount of C and O impurities. Ar+ ions with an energy of 4 keV was used for bombardment for different time periods with simultaneous XPS studies, to determine the elemental composition along the interior of the nanoparticles. Because the incident X-ray beam size was 2
3 mm in diameter, the results provide an average composition for the nanoparticles. The magnetization studies were carried out at temperatures ranging from 4 to 300 K, in applied fields of up to 5 kOe with a Quantum Design Physical Properties Measurement System.

3. RESULTS AND DISCUSSION The core
shell and composite type of nanoparticles of Cu
Co were obtained at 700 C from the precursors synthesized using the microemulsion method. PXRD patterns show the green colored powder obtained at room temperature to be amorphous. The IR study confirms that there is no surfactant left after washing with chloroform: methanol mixture (Figure S1 of the Supporting Information). These powders after annealing in hydrogen at 700 C for 6 h turned black in color. PXRD shows formation of a nanocrystalline Cu
Co alloy (Figure 1) where all reflections could be indexed based on a single phase. From the line broadening studies, the crystallite size was found to be ∼31 nm (Figure S2 of the Supporting Information). TEM studies showed the formation of core
shell nanostructures with a diameter of core ∼80 nm and shell of ∼12 nm, respectively (parts a and b of Figure 2). Part c of Figure 2 shows the EDAX pattern for the core
shell nanostructures confirming the presence of cobalt in the sample. The presence of copper in the sample has also been confirmed from the SEM EDAX given in Figure S3 of the Supporting Information. Core
shell nanoparticles of Cu
Co appeared to form a regular arrangement of particles (part a of Figure 2), which may be due to magnetic interaction between the particles. Yang et al. have earlier reported nanoscale Cu88Co12 alloy particles with an average size of ∼10 nm from arc melting,12 whereas nanorods of Cu90Co10 alloy (diameter = 200 nm, length = 5 μm) have been reported by electrochemical deposition method using polycarbonate membrane as a template.17 Hydrazine hydrate 14527

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Figure 2. (a, b) TEM and (c) EDAX measurement of Cu
Co system (Cu
Co composite @ Cu
Co alloy).

works as good reducing agent under alkaline conditions as shown by the following reactions: 2Cu2þ þ N2 H4 :H2 O þ 4OH
f 2Cu þ N2 þ 5H2 O 2Co2þ þ N2 H4 :H2 O þ 4OH
f 2Co þ N2 þ 5H2 O To understand the compositional and structural difference between the core and shell, detailed XPS studies were carried out.

The changes in the relative composition of Cu
Co alloy and Cu
Co composite observed in the core
shell nanostructures by removal of the surface layers of the nanoparticles by 4 keV Ar+ ion bombardment provides valuable information regarding the distribution of the nature of Cu
Co phases along the interior of the nanoparticles. Figure 3 shows the core level XPS spectra of Cu2p3/2 for unsputtered and 2 keVAr+ ion sputtered Cu
Co alloy nanoparticles. The Cu 2p3/2 core level spectra of unsputtered samples exhibits a broad peak that has been deconvoluted into two peaks at 936.9 eV (peak a1) and 935.2 eV (peak a2). The low energy peak at binding energy of 935.2 eV corresponds to 14528

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Figure 3. XPS spectra of Cu
Co system (Cu
Co composite @ Cu
Co alloy) showing the Cu2p3/2 region after sputtering for different time periods with respect to Cu (a
e) for 0, 30, 60, 90, and 120 min; and (f
i) with respect to Co after sputtering for 0, 30, 60, 90, and 120 min.

the Cu
Co alloy phase while the appearance of a small peak at 936.9 eV is a signature of surface layer of Cu
Co oxide. These results obtained from XPS are in good agreement with those reported elsewhere.31 A significant shift in binding energy was observed as expected for the formation of solid solution of Cu
Co. The Cu 2p3/2 peaks broadened after 30 min of sputtering which was deconvoluted into three peaks a1, a2, and a3 as shown in part b of Figure 3. The evolution of a3 peak at binding energy of 932.7 eV indicates presence of Cu
Co composite in the core of the

nanoparticles. On further sputtering the nanoparticles, a1 phase disappears completely and only two peaks (a2 and a3) remained in the Cu core level spectra corresponding to Cu
Co alloy and Cu
Co composite (3c, 60 min sputtering). On further sputtering the surface (90 min, part d of Figure 3 and 120 min, and part e of Figure 3), the contribution of the Cu
Co composite phase (a3) increases and that of the Cu
Co alloy decrease (a2), which suggests that the core is made up of Cu
Co composite and covered with Cu
Co alloy. 14529

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Figure 5. Powder X-ray diffraction patterns of Cu
Co nanocomposite (obtained using 1 M hydrazine).

Figure 4. Depth profile curve obtained using X-ray photoelectron spectroscopy of (a) core
shell and (b) elemental composition of Cu
Co nanoparticles.

Similarly when deconvolution of the unsputtered and sputtered sample is carried out we observe that the peak at binding energy of 780.2 eV (b2) that corresponds to Cu
Co alloy nanoparticles diminishes while the peak at 778.1 eV (peak b3) which corresponds to Cu
Co composite increases as the sputtering time is increased (parts f
i of Figure 3). A detailed discussion on the XPS sputtering and the percentage area covered by various peaks in the deconvoluted spectra of Cu
Co nanoparticles after different sputtering time has been given in the Supporting Information (Table SII of the Supporting Information). The percentage composition of Cu
Co alloy and Cu
Co composite in the core
shell nanoparticles with Ar+ ion depth profiling is shown in part a of Figure 4. Using geometrical consideration and ignoring preferential sputtering and inelastic mean free path of the core electrons, we quantitatively estimate the composition by assuming a radial dependence of the density of each element.32 From the above studies we observe that the shell consists of Cu
Co alloy, whereas the core comprises of Cu
Co composite particles and we estimate the shell thickness to be ∼15 nm, which matches closely with TEM studies.

Note that surface energies of cobalt, copper and copper
cobalt alloy are 2.709, 1.934, and 0.75 J m
2, respectively.33
35 On the basis of the surface energies, we may conclude that copper
cobalt alloy would prefer to be on the surface as is observed. The composition was found to be 2:1 for Cu/Co from the depth profile curve (part b of Figure 4). Lowering the hydrazine concentration (1 M) during the synthesis of Cu
Co nanoparticles leads to formation of a composite phase containing Cu and Co (no core
shell structures). PXRD patterns clearly show the formation of biphasic mixture of copper and cobalt in the nanocomposites (Figure 5). All the reflections in the pattern could be indexed on the basis of face centered cubic cell reported for copper (JCPDS # 85
1326) and cobalt (JCPDS # 15
0806). TEM micrographs of these nanocomposites show particles (part a of Figure 6) with size ranging from 40
200 nm. High-resolution TEM studies (part b of Figure 6) confirm the presence of copper and the lattice spacing is consistent with that of the (111) plane of Cu. Part c of Figure 6 shows the EDAX analysis of the sample, which confirms the presence of Co in the composite nanoparticle. With 1 M hydrazine, copper
cobalt composite nanoparticles are formed. However with a higher concentration (∼20 M) of the reducing agent, core
shell nanostructures were formed with composite (Cu/Co) as core and Cu
Co alloy as shell. It appears that with higher concentration of the reducing agent, smaller particles of the precursor are formed.36,37 And these smaller sized precursor particles favor the formation of metastable phases (Cu
Co alloy). It is well-known that some metastable phases can be obtained only by decreasing the size of the material.38 Thus a minor phase of alloy formation takes place. The amorphous precursor (formed at room temperature) contains a mixture of Cu and Co, which on reduction leads to alloy formation when a higher concentration of hydrazine is employed. Annealing the above at high temperature causes the phase separation of the material. It appears that the phase separation is incomplete under our conditions and hence we are left with Cu, Co, and alloy of Cu
Co and due to surface energy effects the alloy phase migrates preferentially to form a shell on the Cu
Co composite core. CV was used to characterize the Cu
Co nanostructures for application as electrode materials. Figure 7 shows cyclic voltammograms of the nanocomposite (Cu
Co) and core
shell-based nanostructures. Four irreversible peaks were observed for both 14530

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Figure 7. Cyclic voltammograms of Cu
Co nanocomposite and core
shell nanoparticles on glassy carbon electrode.

Figure 6. (a) TEM and (b) HRTEM corresponding to the (111) plane of copper in the Cu
Co nanocomposite and (c) TEM EDAX for the nanocomposite.

core
shell and Cu
Co nanocomposite structures. The peaks may be understood as due to formation of four couples (in basic medium): Cu/Cu+ and Cu+/Cu2+ for copper and Co/Co2+ and Co2+/Co3+. At a potential of
1.2 V, Cu
Co nanocompositebased cell leads to higher current density (Figure 7) than core
shell nanostructures. However, at about
1.5 V these two materials have almost the same hydrogen evolution efficiency. The nanocomposite may therefore be slightly better for HER activity at lower potentials than
1.4 V. The chronoamperometric method was used to compare the HER properties of the different electrode materials at
1.4 V versus Ag/AgCl electrode. The performance of the electrode material was also determined by the current densities and stability, determined from 10 cycles, with

Figure 8. Chronoamperometric voltammograms of Cu
Co nanocomposite and core
shell nanoparticles on glassy carbon electrode and at
1.4 V.

each cycle lasting approximately 20 seconds. These two nanostructures as electrode materials were found to be stable for HER during the time of the experiment. Figure 8 shows the chronoamperometric studies comparing the HER for the Cu
Co nanocomposites and core
shell (Cu
Co composite @ Cu
Co alloy) electrode materials. The current generated in each cycle was nearly the same for the two types of nanostructures as electrodes. The minor variation in the currents may indicate slightly different 14531

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particles indicating that the materials were also good electrocatalysts for hydrogen evolution reactions.

’ ASSOCIATED CONTENT

bS

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], Tel: 91-11-26591511, Fax: 91-11-26854715 (A.K.G.); E-mail: [email protected], Tel.: 856-256-5454, Fax: 1-856-256-4478 (A.M.M.).

Figure 9. Hysteresis loops of core
shell (Cu
Co composite @ Cu
Co alloy) and composite nanoparticles of Cu
Co at 4 K.

composition of nanostructures. The nanocomposite shows slightly higher current, which may be due to slightly higher oxidation and reduction rates as a result of higher Cu content than the core
shell nanostructures. The current density (∼15 mA/cm2) for nanocrystalline Cu
Co particles was found to be much higher (5 times) than the current generated (∼3 mA/cm2) for bulk Cu
Co alloy.14 The magnetization and the hysteresis loops of both core
shell nanostructures (Cu
Co composite @ Cu
Co alloy) and Cu
Co nanocomposite have been studied at 4 K with an applied magnetic field up to 7 T (Figure 9). The saturation magnetization values of core
shell and composite nanoparticles of Cu
Co were found to be 124 and 72 emu/g, respectively (Figure 8). Wen et al. have earlier reported the saturation magnetization of 5 emu/g for nanocrystalline Cu
Co particles (10 nm).39 Note that the reported value of saturation magnetization for bulk Cu
Co alloy (1:4) is 120 emu/g at 4 K.10 This value decreases with increase in the amount of copper in the Cu
Co alloy which finally shows paramagnetic behavior for Cu
Co (9:1) alloy.10 Note that bulk Cu and Co metals are paramagnetic and ferromagnetic, respectively. The core
shell (composite @ alloy) nanostructures prepared by us have higher saturation magnetization value compared to Cu
Co nanocomposite particles and bulk Cu
Co alloy. The core
shell nanoparticles may be useful as soft magnetic materials due to their high saturation magnetization.

4. CONCLUSIONS The microemulsion method has been used to obtain a novel core
shell nanostructured material (Cu
Co composite @ Cu
Co alloy). This method gives the possibilities of obtaining pure metastable alloys which are normally immiscible and thus has scope for stabilizing other binary alloys (e.g., Mn
Ni) which are unstable in the bulk. In addition, by control of the reduction process we have also obtained Cu
Co composite nanoparticles. The core
shell and composite nanoparticles show ferromagnetic behavior and the saturation magnetization of the core
shell nanoparticles is enhanced significantly compared to the bulk Cu
Co alloy particles. Electrochemical studies of Cu
Co nanostructures showed much higher current density (∼15 mA/cm2) compared to bulk Cu
Co alloy

’ ACKNOWLEDGMENT A.K.G. thanks DST and CSIR, Govt. of India, for financial assistance. S.E.L. acknowledges support of National Science Foundation, MRSEC DMR 0520471. J. Ahmed and A.G. thank CSIR for their senior research fellowship. K.V.R. acknowledges the CP-STIO award from the Department of Science and Technology, Govt. of India. ’ REFERENCES (1) Jo, C.; Lee, J.; Jang, Y. Chem. Mater. 2005, 17, 2667. (2) Mattei, G.; Fernandez, C. D. J.; Mazzoldi, P.; Sada, C.; De, G.; Battaglin, G.; Sangregorio, C.; Gatteschi, D. Chem. Mater. 2002, 14, 3440. (3) Lian, K.; Thorpe, S. J.; Kirk, D. W. Electrochim. Acta 1992, 37, 169. (4) Chen, C. W.; Chen, M. Q.; Serizawa, T.; Akashi, M. Chem. Commun. 1998, 831. (5) Caruso, F.; Fiedler, H.; Haage, K. Colloids Surf., A 2000, 169, 287. (6) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D.: Appl. Phys. 2003, 36, 167. (7) Connolly, J.; St Pierre, T. G.; Rutnakornpituk, M.; Riffle, J. S. J. Phys. D: Appl. Phys. 2004, 37, 2475. (8) Namdeo, M.; Saxena, S.; Tankhiwale, R.; Bajpai, M.; Mohan, Y. M.; Bajpai, S. K. J. Nanosci. Nanotech. 2008, 8, 3247. (9) Wang, W.; Zhu, F.; Weng, J.; Xiao, J.; Lai, W. Appl. Phys. Lett. 1998, 72, 1118. (10) Fan, X.; Mashimo, T.; Huang, X.; Kagayama, T.; Chiba, A.; Koyama, K.; Motokawa, M. Phys. Rev. B 2004, 69, 094432. (11) Wang, W.; Zhu, F.; Lai, W.; Wang, J. Q.; Yang, G.; Zhu, J.; Zhang, Z. J. Phys. D: Appl. Phys. 1999, 32, 1990. (12) Yang, G. Y.; Zhu, J.; Wang, W. D.; Zhang, Z.; Zhu, F. W. Mater. Res. Bull. 2000, 35, 875. (13) Nguyen, T. T.; Zahedi-Niaki, M. H.; Alamdari, H.; Kaliaguine, S. Int. J. Chem. Reactor Eng. 2007, 5, A82. (14) Brossard, L.; Marouis, B. Int. J. Hydro. Energy 1994, 19, 231. (15) Ahmed, J.; Sharma, S.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K. J. Colloid Interface Sci. 2009, 336, 814. (16) Ahmed, J.; Kumar, B.; Mugweru, A. M.; Trinh, P.; Ramanujachary, K. V.; Lofland, S. E.; Govind; Ganguli, A. K. J. Phys. Chem. C 2010, 114, 18779. (17) Xue, S.; Cao, C.; Ji, F.; Xu, Y.; Wang, D.; Zhu, H. Mater. Lett. 2005, 59, 3173. (18) Allia, P.; Coy sson, M.; Tiberto, P.; Vinai, F. J. Alloys Compd. 2007, 434, 601. (19) Cezara, J. C.; Tolentino, H. C. N.; Knobel, M. J. Magn. Magn. Mater. 2001, 233, 103. (20) Juarez, G.; Villafuerte, M.; Heluani, S.; Fabietti, L. M.; Urreta, S. E. J. Magn. Magn. Mater. 2008, 320, 22. 14532

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