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Influence of the preparation method on the properties of Cu–Co heterogeneous alloys
Journal of Non-Crystalline Solids 287 (2001) 26±30
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In¯uence of the preparation method on the properties of Cu±Co heterogeneous alloys R. L opez Ant on a,*, M.L. Fdez-Gubieda a, M. Insausti b, A. Garcõa-Arribas a, J. Herreros c a
Depto. de Electricidad y Electr onica, Facultad de Ciencias (U.P.V.), Univ. del Paõs Vasco. Apdo. 644, 48080 Bilbao, Spain b Depto. de Quõmica Inorg anica, Aplicada II. Univ. del Paõs Vasco. Apdo. 644, 48080 Bilbao, Spain c Depto. de Fõsica Aplicada II. Univ. del Paõs Vasco. Apdo. 644, 48080 Bilbao, Spain
Abstract Cu88 Co12 samples have been obtained by electrodeposition and laser ablation and have been annealed at increasing temperatures (450°C, 500°C and 550°C) to compare the in¯uence of the fabrication method on the magnetotransport properties of the samples. Magnetoresistance (MR) and X-ray diraction (XRD) measurements show dierent properties between the samples prepared by the two techniques. It was found that the electrodeposited (ED) samples had a larger MR. The properties of the laser-ablated samples depend on the deposition process, showing an anisotropy when prepared with a smaller target rotation speed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 75.70.P; 73.50.J
1. Introduction Granular Cu±Co alloys are among the systems that present giant magnetoresistance (MR) [1,2]. At room temperature, the solubility of Co in Cu is practically negligible [1]. However, some preparation methods make it possible to obtain a metastable solid solution of a small quantity of Co in Cu [3±7]. In this situation, the energetically more favourable segregation of small Co precipitates takes place whenever possible (after annealing or sometimes at preparation), giving rise to the socalled granular alloys [1±7]. Berkowitz et al. [1] and Xiao et al. [2] measured a considerable GMR eect in heterogeneous Cu±Co alloys and ®lms * Corresponding author. Tel.: +34-94 601 5369; fax: +34-94 464 8500, 601 3071. E-mail address: [email protected] (R. L opez Ant on).
produced by sputtering. Granular Cu±Co alloys have been produced by dierent techniques, including mechanical alloying, sputtering, vapour deposition, melt-spinning and laser ablation [3±7]. In particular, laser ablation and electrodeposition techniques have been shown to be very convenient because of their simplicity of use, and samples of dierent compositions are prepared with ease [6,8±11]. In this work, our aim is to study the in¯uence of the preparation method on the magnetotransport properties of Cu±Co heterogeneous alloys. In this sense, samples with composition Cu88 Co12 have been obtained by electrodeposition and laser ablation techniques, and thermal treatments have been applied to the samples so as to obtain MR. Scanning electron microscopy and X-ray diraction (XRD) and MR measurements have been performed to compare their properties.
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 5 3 5 - X
R. Lopez Anton et al. / Journal of Non-Crystalline Solids 287 (2001) 26±30
2. Experimental The electrodeposited (ED) samples were deposited at room temperature from the following bath: CoSO4 7H2 O 67:45 g=l, CuSO4 25.54 g/l, NaCl 4 g/l and Na3 C5 H6 O7 173:52 g=l . The deposition process was controlled potentiostatically using a potentiostat (AMEL model 2501) with a calomel reference electrode. The samples were deposited onto borosilicate glass covered by a copper layer 10 nm thick. The thickness of the samples obtained was about 1 lm. The laser-ablated samples were deposited using a laser Lambda Physik Compex 102 (248 nm, 300 mJ/pulse). The target used was a disc with cobalt and copper sectors in proportions appropriate to the desired ®lm composition of about 10% Co. The target was rotated by a step-by-step drive. Two dierent series of samples were obtained by laser ablation, the ®rst one (series 1) with a laser repetition rate of 19 Hz and a 150 rpm target angular speed and the other (series 2) with 15 Hz and 15 rpm. The ®lms were deposited onto Si substrates at room temperature. The pressure in the deposition chamber was about 10 5 mbar. The thickness of the ®lms obtained was about 250 nm. After growth, small pieces of all the samples (ED and laser-ablated) were annealed for 30 min at 450°C, 500°C and 550°C under a constant Ar ¯ux to reduce oxidation. These temperatures were chosen as the optimum annealing temperatures for obtaining good MR samples, as found in the literature [1,2,6]. Energy dispersive X-ray analysis (EDX) was used to determine the overall composition of the ®lms within 2%. XRD spectra of all the samples were obtained using Cu Ka radiation in a h±2h powder diractometer. MR measurements were performed at room temperature using the wellknown four-probe technique with a maximum applied ®eld of 2 kOe.
27
were found, neither on the as-deposited nor annealed samples, which we suggest is due to the Co clusters being too small to produce re¯ections [12] and only produce a distortion of the Cu-matrix structure. The lattice parameters obtained from peak positions are shown in Fig. 1. On the ED samples, the lattice parameter of the Cu±Co fcc mixed phase changes with the annealing temperature from an initial value of 0:3601 0:0004 nm for the as-prepared sample to 0:3614 0:0004 nm for the sample annealed at 550°. This better parameter is quite close to that of the Cu fcc phase (0.3615 nm) [12], which is due to the fact that initially we have a Cu±Co solid solution and the Co segregates from the Cu during the annealing of the sample. On the other hand, in the laser-ablated samples only the peak of the (1 1 1) re¯ection of the Cu±Co fcc phase is observed. The absence of the other re¯ections is an indication of a texture with the (1 1 1) planes parallel to the plane of the ®lm. This texture may be due to the growing process, since (1 1 1) planes are the most densely packed planes in the fcc structure [8,12]. Noteworthily, the changes of the lattice parameter dier in the two laser-deposited series. For the series 1 (rotation speed of the target: 150 rpm, frequency of the laser: 19 Hz), the lattice parameter for the as-deposited sample is 0:3621 0:0004 nm, quite larger than that of the Cu fcc phase, indicating the presence of stress and defects within the ®lm. In
3. Results and discussion The XRD measurements showed re¯ections of a mixed Cu±Co fcc phase, with peaks quite close to those of the Cu fcc. No peaks of the Co fcc phase
Fig. 1. Changes of the lattice parameters with the annealing temperature, Tann , for: ED, laser-ablated series 1 (150 rpm, 19 Hz) and laser-ablated series 2 (15 rpm, 15 Hz). Lines are drawn as guides to the eye.
28
R. Lopez Anton et al. / Journal of Non-Crystalline Solids 287 (2001) 26±30
fact, after annealing at 450°C, the lattice parameter decreases to 0:3601 0:0004 nm due to relaxation of the stress. Larger annealing temperatures do not change the lattice parameter. Meanwhile, in series 2 (15 rpm, 15 Hz), all the samples have lattice parameters close to those of the pure Cu fcc, with changes within the experimental error after annealing. With regard to the magnetotransport properties, we have found that they depend on the preparation process. All the samples require annealing for MR but their MR curves found afterwards dier. The MR curves presented for the ED samples are just those obtained with the applied ®eld parallel to the plane of the sample given the negligible anisotropy existing in these type of samples [10,11]. These ED samples have the largest MR of all the samples. Additionally, the shape of their curves depends on the annealing temperature. Hence, the sample annealed at 450°C has an MR of 2.8% at 20 kOe but the curve does not saturate at this ®eld and has an approximately linear trend. Meanwhile, the sample annealed at 500°C has the largest MR maximum of all the samples considered (about 4%) yet the MR seems to begin to saturate at 2 kOe. Finally, the 550°C annealed sample shows a saturated MR, with an almost invariant curve for ®elds larger than 10 kOe (Fig. 2). On the other hand, the laser-ablated samples have quite a dierent MR that depends on the
Fig. 2. Room temperature MR, as a function of the magnetic ®eld (applied parallel to the plane of the sample) for the ED samples. Lines are drawn as guides to the eye.
deposition condition. Thus, for the series 1, no dierences are found in the MR measurements whether the magnetic ®eld is applied perpendicular or parallel to the plane of the sample and the MRs obtained are about 0.5%. All MRs have the same trend: a peak at ®elds up to 4 kOe and a slope at larger ®elds, as can be seen in Fig. 3. On the contrary, the series 2 samples present an important anisotropy (Fig. 4): when the ®eld is applied parallel to the sample, a peak is observed for low ®elds and a big decrease of the slope at about 4 kOe. When the applied ®eld is perpendicular, the MR peak is wider (more than the widening due to the demagnetization ®eld), decreasing the slope at applied ®elds of about 1 kOe. With regard to the in¯uence of the annealing temperature, the maximum of the MR is found after annealing at 450°C (2%) whereas the samples annealed at 500°C and 550°C have an MR of 1% (see Fig. 4). The great dierence in the magnetotransport and the XRD spectra found between the two laserablated series has its origin in the dierent microstructures of these samples. For the series 1 (150 rpm), the as-deposited sample likely has very small Co clusters (<5 nm) and isolated Co atoms embedded in a distorted Cu matrix [13], in such a way that when we anneal the sample the only eect is a relaxation of the stress, giving place to a decrease of the lattice parameter from 0.3621 to 0.3601 nm.
Fig. 3. Room temperature MR, as a function of the magnetic ®eld (applied parallel to the plane of the sample) for the laserdeposited series 1 (150 rpm, 19 Hz). Lines are drawn as guides to the eye.
R. Lopez Anton et al. / Journal of Non-Crystalline Solids 287 (2001) 26±30
(a)
29
(b)
Fig. 4. Room temperature MR, as a function of the magnetic ®eld for the laser-deposited series 2 (15 rpm, 15 Hz), with the magnetic ®eld applied (a) parallel and (b) perpendicular to the plane of the sample. Lines are drawn as guides to the eye.
This lattice parameter is equal to the one of the asED samples (hence corresponding to the parameter expected for a solid solution). In contrast, the series 2 (15 rpm) has Co clusters with disc shape and thickness <1 nm, whose growth is also favoured by the application of annealing temperatures larger than 400°C [13,14]. Thus, the anisotropy found in the MR curves for this series is an evidence for the existence of an easy-magnetization axis in the sample plane linked to planar growth of the Co clusters [11]. The MR changes with the annealing temperature would be related to the growth of the Co clusters and the changes induced in their distribution. Nevertheless, the fact that the lattice parameter of the series 2 changes does not change (within error of measurements) with the annealing and that it is very close to that of the Cu fcc indicates the existence of larger Co clusters even in the as-deposited sample. On the other hand, the ED samples have MRs more similar to those found in melt-spinning samples (lack of anisotropy, larger MR) [6]. In particular, the as-deposited sample is a solid solution, with no GMR and a lattice parameter smaller than that of the Cu fcc and even smaller than that following Vegaard's law [15] (probably owing to the existence of stress). As we anneal the segregation of the Co approaches the lattice parameter of the Cu fcc and the GMR phenomenon appears, increasing initially and decreasing after 500°C as the Co clusters become too large.
4. Conclusions We have compared the magnetotransport properties of Cu±Co heterogeneous alloys obtained by laser ablation and electrodeposition. We have found that the laser-deposited samples are textured in the (1 1 1) direction and have stress and defects within the ®lm, whereas the ED ones have no texture. In addition, the changes of the lattice parameter with the annealing temperature are dierent for all the series considered, which evidences the dierent microstructures induced. The MRs are dierent except for the lack of MR without annealing. First of all, the ED samples have larger GMRs at all the annealing temperatures. In addition, these samples change from non-saturating at 450°C to saturating at larger temperatures (incipient at 500°C, evident at 550°C). In contrast, the ablated ones obtained with the smaller rotation speed of the target (15 rpm) saturate for all the annealing temperatures considered and present an anisotropy, due to, we assume, a planar growth of the Co clusters, whereas the ones fabricated with 150 rpm are not anisotropic. Acknowledgements This work has been partially ®nanced by the Spanish CICYT under project MAT99-0667. One
30
R. Lopez Anton et al. / Journal of Non-Crystalline Solids 287 (2001) 26±30
of the authors (R.L.A.) thanks the Basque Government for the ®nancial support. We also thank the help provided by A. Svalov, from the Institute of Physics and Applied Mathematics (Ural State University), and G. Kourlyandskaya, from the University of the Basque Country, in the sample preparation. References [1] A.E. Berkovitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett. 68 (1992) 3745. [2] J.Q. Xiao, J.S. Jiang, C.L. Chien, Phys. Rev. Lett. 68 (1992) 3749. [3] J. Wecker, R. Von Helmolt, L. Schultz, K. Samwer, IEEE Trans. Magn. 29 (1993) 3087. [4] N. Kataoka, H. Endo, K. Fukamichi, Y. Shimada, Jpn. J. Appl. Phys. 32 (1993) 1969.
[5] P. Allia, M. Knobel, P. Tiberto, F. Vinai, Phys. Rev. B 52 (1995) 15045. [6] Y. Huai, M. Chaker, H. Pepin, S. Boily, X. Bian, R.W. Cochrane, J. Magn. Magn. Mater. 136 (1994) 204. [7] Y. Ueda, S. Ikeda, Mater. Trans. JIM 36 (1995) 384. [8] T.J. Jackson, S.B. Palmer, H.J. Blythe, A.S. Halim, J. Magn. Magn. Mater. 159 (1996) 269. [9] V. Madurga, J. Vergara, R.J. Ortega, S. Palacios, E. Azcoiti, K.V. Rao, J. Magn. Magn. Mater. 177±181 (1998) 945. [10] V.M. Fedosyuk, O.I. Kasyutich, D. Ravinder, H.J. Blythe, J. Magn. Magn. Mater. 156 (1996) 345. [11] H. Zaman, A. Yamada, H. Fukuda, Y. Ueda, J. Electrochem. Soc. 145 (1998) 565. [12] A. Maeda, M. Kume, S. Oikawa, Y. Shimizu, M. Doi, J. Phys. (1993) 4641. [13] J. Vergara, PhD thesis, University of Zaragoza, 1999, p. 117. [14] J. Vergara, V. Madurga, J. Magn. Magn. Mater. 196&197 (1999) 91. [15] B.D. Cullity, Elements of X-ray Diraction, AddisonWesley, New York, 1967, pp. 99&376.
www.elsevier.com/locate/jnoncrysol
In¯uence of the preparation method on the properties of Cu±Co heterogeneous alloys R. L opez Ant on a,*, M.L. Fdez-Gubieda a, M. Insausti b, A. Garcõa-Arribas a, J. Herreros c a
Depto. de Electricidad y Electr onica, Facultad de Ciencias (U.P.V.), Univ. del Paõs Vasco. Apdo. 644, 48080 Bilbao, Spain b Depto. de Quõmica Inorg anica, Aplicada II. Univ. del Paõs Vasco. Apdo. 644, 48080 Bilbao, Spain c Depto. de Fõsica Aplicada II. Univ. del Paõs Vasco. Apdo. 644, 48080 Bilbao, Spain
Abstract Cu88 Co12 samples have been obtained by electrodeposition and laser ablation and have been annealed at increasing temperatures (450°C, 500°C and 550°C) to compare the in¯uence of the fabrication method on the magnetotransport properties of the samples. Magnetoresistance (MR) and X-ray diraction (XRD) measurements show dierent properties between the samples prepared by the two techniques. It was found that the electrodeposited (ED) samples had a larger MR. The properties of the laser-ablated samples depend on the deposition process, showing an anisotropy when prepared with a smaller target rotation speed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 75.70.P; 73.50.J
1. Introduction Granular Cu±Co alloys are among the systems that present giant magnetoresistance (MR) [1,2]. At room temperature, the solubility of Co in Cu is practically negligible [1]. However, some preparation methods make it possible to obtain a metastable solid solution of a small quantity of Co in Cu [3±7]. In this situation, the energetically more favourable segregation of small Co precipitates takes place whenever possible (after annealing or sometimes at preparation), giving rise to the socalled granular alloys [1±7]. Berkowitz et al. [1] and Xiao et al. [2] measured a considerable GMR eect in heterogeneous Cu±Co alloys and ®lms * Corresponding author. Tel.: +34-94 601 5369; fax: +34-94 464 8500, 601 3071. E-mail address: [email protected] (R. L opez Ant on).
produced by sputtering. Granular Cu±Co alloys have been produced by dierent techniques, including mechanical alloying, sputtering, vapour deposition, melt-spinning and laser ablation [3±7]. In particular, laser ablation and electrodeposition techniques have been shown to be very convenient because of their simplicity of use, and samples of dierent compositions are prepared with ease [6,8±11]. In this work, our aim is to study the in¯uence of the preparation method on the magnetotransport properties of Cu±Co heterogeneous alloys. In this sense, samples with composition Cu88 Co12 have been obtained by electrodeposition and laser ablation techniques, and thermal treatments have been applied to the samples so as to obtain MR. Scanning electron microscopy and X-ray diraction (XRD) and MR measurements have been performed to compare their properties.
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 5 3 5 - X
R. Lopez Anton et al. / Journal of Non-Crystalline Solids 287 (2001) 26±30
2. Experimental The electrodeposited (ED) samples were deposited at room temperature from the following bath: CoSO4 7H2 O 67:45 g=l, CuSO4 25.54 g/l, NaCl 4 g/l and Na3 C5 H6 O7 173:52 g=l . The deposition process was controlled potentiostatically using a potentiostat (AMEL model 2501) with a calomel reference electrode. The samples were deposited onto borosilicate glass covered by a copper layer 10 nm thick. The thickness of the samples obtained was about 1 lm. The laser-ablated samples were deposited using a laser Lambda Physik Compex 102 (248 nm, 300 mJ/pulse). The target used was a disc with cobalt and copper sectors in proportions appropriate to the desired ®lm composition of about 10% Co. The target was rotated by a step-by-step drive. Two dierent series of samples were obtained by laser ablation, the ®rst one (series 1) with a laser repetition rate of 19 Hz and a 150 rpm target angular speed and the other (series 2) with 15 Hz and 15 rpm. The ®lms were deposited onto Si substrates at room temperature. The pressure in the deposition chamber was about 10 5 mbar. The thickness of the ®lms obtained was about 250 nm. After growth, small pieces of all the samples (ED and laser-ablated) were annealed for 30 min at 450°C, 500°C and 550°C under a constant Ar ¯ux to reduce oxidation. These temperatures were chosen as the optimum annealing temperatures for obtaining good MR samples, as found in the literature [1,2,6]. Energy dispersive X-ray analysis (EDX) was used to determine the overall composition of the ®lms within 2%. XRD spectra of all the samples were obtained using Cu Ka radiation in a h±2h powder diractometer. MR measurements were performed at room temperature using the wellknown four-probe technique with a maximum applied ®eld of 2 kOe.
27
were found, neither on the as-deposited nor annealed samples, which we suggest is due to the Co clusters being too small to produce re¯ections [12] and only produce a distortion of the Cu-matrix structure. The lattice parameters obtained from peak positions are shown in Fig. 1. On the ED samples, the lattice parameter of the Cu±Co fcc mixed phase changes with the annealing temperature from an initial value of 0:3601 0:0004 nm for the as-prepared sample to 0:3614 0:0004 nm for the sample annealed at 550°. This better parameter is quite close to that of the Cu fcc phase (0.3615 nm) [12], which is due to the fact that initially we have a Cu±Co solid solution and the Co segregates from the Cu during the annealing of the sample. On the other hand, in the laser-ablated samples only the peak of the (1 1 1) re¯ection of the Cu±Co fcc phase is observed. The absence of the other re¯ections is an indication of a texture with the (1 1 1) planes parallel to the plane of the ®lm. This texture may be due to the growing process, since (1 1 1) planes are the most densely packed planes in the fcc structure [8,12]. Noteworthily, the changes of the lattice parameter dier in the two laser-deposited series. For the series 1 (rotation speed of the target: 150 rpm, frequency of the laser: 19 Hz), the lattice parameter for the as-deposited sample is 0:3621 0:0004 nm, quite larger than that of the Cu fcc phase, indicating the presence of stress and defects within the ®lm. In
3. Results and discussion The XRD measurements showed re¯ections of a mixed Cu±Co fcc phase, with peaks quite close to those of the Cu fcc. No peaks of the Co fcc phase
Fig. 1. Changes of the lattice parameters with the annealing temperature, Tann , for: ED, laser-ablated series 1 (150 rpm, 19 Hz) and laser-ablated series 2 (15 rpm, 15 Hz). Lines are drawn as guides to the eye.
28
R. Lopez Anton et al. / Journal of Non-Crystalline Solids 287 (2001) 26±30
fact, after annealing at 450°C, the lattice parameter decreases to 0:3601 0:0004 nm due to relaxation of the stress. Larger annealing temperatures do not change the lattice parameter. Meanwhile, in series 2 (15 rpm, 15 Hz), all the samples have lattice parameters close to those of the pure Cu fcc, with changes within the experimental error after annealing. With regard to the magnetotransport properties, we have found that they depend on the preparation process. All the samples require annealing for MR but their MR curves found afterwards dier. The MR curves presented for the ED samples are just those obtained with the applied ®eld parallel to the plane of the sample given the negligible anisotropy existing in these type of samples [10,11]. These ED samples have the largest MR of all the samples. Additionally, the shape of their curves depends on the annealing temperature. Hence, the sample annealed at 450°C has an MR of 2.8% at 20 kOe but the curve does not saturate at this ®eld and has an approximately linear trend. Meanwhile, the sample annealed at 500°C has the largest MR maximum of all the samples considered (about 4%) yet the MR seems to begin to saturate at 2 kOe. Finally, the 550°C annealed sample shows a saturated MR, with an almost invariant curve for ®elds larger than 10 kOe (Fig. 2). On the other hand, the laser-ablated samples have quite a dierent MR that depends on the
Fig. 2. Room temperature MR, as a function of the magnetic ®eld (applied parallel to the plane of the sample) for the ED samples. Lines are drawn as guides to the eye.
deposition condition. Thus, for the series 1, no dierences are found in the MR measurements whether the magnetic ®eld is applied perpendicular or parallel to the plane of the sample and the MRs obtained are about 0.5%. All MRs have the same trend: a peak at ®elds up to 4 kOe and a slope at larger ®elds, as can be seen in Fig. 3. On the contrary, the series 2 samples present an important anisotropy (Fig. 4): when the ®eld is applied parallel to the sample, a peak is observed for low ®elds and a big decrease of the slope at about 4 kOe. When the applied ®eld is perpendicular, the MR peak is wider (more than the widening due to the demagnetization ®eld), decreasing the slope at applied ®elds of about 1 kOe. With regard to the in¯uence of the annealing temperature, the maximum of the MR is found after annealing at 450°C (2%) whereas the samples annealed at 500°C and 550°C have an MR of 1% (see Fig. 4). The great dierence in the magnetotransport and the XRD spectra found between the two laserablated series has its origin in the dierent microstructures of these samples. For the series 1 (150 rpm), the as-deposited sample likely has very small Co clusters (<5 nm) and isolated Co atoms embedded in a distorted Cu matrix [13], in such a way that when we anneal the sample the only eect is a relaxation of the stress, giving place to a decrease of the lattice parameter from 0.3621 to 0.3601 nm.
Fig. 3. Room temperature MR, as a function of the magnetic ®eld (applied parallel to the plane of the sample) for the laserdeposited series 1 (150 rpm, 19 Hz). Lines are drawn as guides to the eye.
R. Lopez Anton et al. / Journal of Non-Crystalline Solids 287 (2001) 26±30
(a)
29
(b)
Fig. 4. Room temperature MR, as a function of the magnetic ®eld for the laser-deposited series 2 (15 rpm, 15 Hz), with the magnetic ®eld applied (a) parallel and (b) perpendicular to the plane of the sample. Lines are drawn as guides to the eye.
This lattice parameter is equal to the one of the asED samples (hence corresponding to the parameter expected for a solid solution). In contrast, the series 2 (15 rpm) has Co clusters with disc shape and thickness <1 nm, whose growth is also favoured by the application of annealing temperatures larger than 400°C [13,14]. Thus, the anisotropy found in the MR curves for this series is an evidence for the existence of an easy-magnetization axis in the sample plane linked to planar growth of the Co clusters [11]. The MR changes with the annealing temperature would be related to the growth of the Co clusters and the changes induced in their distribution. Nevertheless, the fact that the lattice parameter of the series 2 changes does not change (within error of measurements) with the annealing and that it is very close to that of the Cu fcc indicates the existence of larger Co clusters even in the as-deposited sample. On the other hand, the ED samples have MRs more similar to those found in melt-spinning samples (lack of anisotropy, larger MR) [6]. In particular, the as-deposited sample is a solid solution, with no GMR and a lattice parameter smaller than that of the Cu fcc and even smaller than that following Vegaard's law [15] (probably owing to the existence of stress). As we anneal the segregation of the Co approaches the lattice parameter of the Cu fcc and the GMR phenomenon appears, increasing initially and decreasing after 500°C as the Co clusters become too large.
4. Conclusions We have compared the magnetotransport properties of Cu±Co heterogeneous alloys obtained by laser ablation and electrodeposition. We have found that the laser-deposited samples are textured in the (1 1 1) direction and have stress and defects within the ®lm, whereas the ED ones have no texture. In addition, the changes of the lattice parameter with the annealing temperature are dierent for all the series considered, which evidences the dierent microstructures induced. The MRs are dierent except for the lack of MR without annealing. First of all, the ED samples have larger GMRs at all the annealing temperatures. In addition, these samples change from non-saturating at 450°C to saturating at larger temperatures (incipient at 500°C, evident at 550°C). In contrast, the ablated ones obtained with the smaller rotation speed of the target (15 rpm) saturate for all the annealing temperatures considered and present an anisotropy, due to, we assume, a planar growth of the Co clusters, whereas the ones fabricated with 150 rpm are not anisotropic. Acknowledgements This work has been partially ®nanced by the Spanish CICYT under project MAT99-0667. One
30
R. Lopez Anton et al. / Journal of Non-Crystalline Solids 287 (2001) 26±30
of the authors (R.L.A.) thanks the Basque Government for the ®nancial support. We also thank the help provided by A. Svalov, from the Institute of Physics and Applied Mathematics (Ural State University), and G. Kourlyandskaya, from the University of the Basque Country, in the sample preparation. References [1] A.E. Berkovitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett. 68 (1992) 3745. [2] J.Q. Xiao, J.S. Jiang, C.L. Chien, Phys. Rev. Lett. 68 (1992) 3749. [3] J. Wecker, R. Von Helmolt, L. Schultz, K. Samwer, IEEE Trans. Magn. 29 (1993) 3087. [4] N. Kataoka, H. Endo, K. Fukamichi, Y. Shimada, Jpn. J. Appl. Phys. 32 (1993) 1969.
[5] P. Allia, M. Knobel, P. Tiberto, F. Vinai, Phys. Rev. B 52 (1995) 15045. [6] Y. Huai, M. Chaker, H. Pepin, S. Boily, X. Bian, R.W. Cochrane, J. Magn. Magn. Mater. 136 (1994) 204. [7] Y. Ueda, S. Ikeda, Mater. Trans. JIM 36 (1995) 384. [8] T.J. Jackson, S.B. Palmer, H.J. Blythe, A.S. Halim, J. Magn. Magn. Mater. 159 (1996) 269. [9] V. Madurga, J. Vergara, R.J. Ortega, S. Palacios, E. Azcoiti, K.V. Rao, J. Magn. Magn. Mater. 177±181 (1998) 945. [10] V.M. Fedosyuk, O.I. Kasyutich, D. Ravinder, H.J. Blythe, J. Magn. Magn. Mater. 156 (1996) 345. [11] H. Zaman, A. Yamada, H. Fukuda, Y. Ueda, J. Electrochem. Soc. 145 (1998) 565. [12] A. Maeda, M. Kume, S. Oikawa, Y. Shimizu, M. Doi, J. Phys. (1993) 4641. [13] J. Vergara, PhD thesis, University of Zaragoza, 1999, p. 117. [14] J. Vergara, V. Madurga, J. Magn. Magn. Mater. 196&197 (1999) 91. [15] B.D. Cullity, Elements of X-ray Diraction, AddisonWesley, New York, 1967, pp. 99&376.