Kondo-like effect in magnetoresistive CuCo alloys L. M. Fabietti, 1,2 J. Ferreyra, 3 M. Villafuerte, 3,4 S. E. Urreta, 1 and S. P. Heluani 3 1 Facultad de Matemática, Astronomía y Física, Universidad Nacional de Córdoba, Ciudad Universitaria, 5000 Córdoba, Argentina 2 IFEG-CONICET, Facultad de Matemática, Astronomía y Física, Universidad Nacional de Córdoba, Ciudad Universitaria, 5000 Córdoba, Argentina 3 Laboratorio de Física del Sólido, Dpto. de Física, Facultad de Ciencias Exactas y Tecnología, Universidad Nacional de Tucumán, 4000 San Miguel de Tucumán, Argentina 4 CONICET, Argentina Received 12 September 2010; published 22 November 2010 Electronic transport properties of twin roller melt spun Cu 100-x Co x x =10,15 alloys, are investigated in the temperature range between 10 and 300 K. Negative magnetoresistance is observed up to 0.85 T in the as-cast state, which further increases with a treatment of 1 h at 923 K. Resistance exhibits a metallic behavior below room temperature and draws a minimum near 30 K in all the as-cast microstructures; this minimum diminishes when a magnetic field is applied and completely disappears after high-temperature annealing. A logarithmic dependence of the electric resistance on temperature is found below 30 K. A Kondo-like scattering mechanism involving small Co spin clusters is considered to explain the minimum. DOI: 10.1103/PhysRevB.82.172410 PACS numbers: 81.40.Rs, 75.47.-m, 72.10.Fk I. INTRODUCTION After the discovery of giant magnetoresistance GMR in magnetic Fe/Cr multilayer, 1 a similar magnetoelectronic be- havior was also found in different granular magnetic sys- tems. For many years, Cu 1-x Co x x  0.3 alloys have been investigated as model granular systems 2–4 to gain insight into the spin-dependent transport processes in metallic alloys containing a fine dispersion of nanometric magnetic par- ticles. Since the first works in CuCo alloys, the main issue has been the description of the actual mechanisms leading to the magnetotransport properties observed and the microstructure associated with their optimum values. At first, small mag- netic Co clusters, embedded in the nonmagnetic Cu metallic matrix were considered as the main source for the electronic spin-selective scattering observed. 2–5 More recently, 6,7 GMR effects have been correlated with a lamellar spinodal decom- position in the form of parallel nanometric stripes, forming within the matrix grains. This self-organized microstructure, 8 appearing as a consequence of the natural segregation taking place during the melt spinning process, is proposed to be responsible for GMR in these materials. However, because of the microstructure complexity—a modulated Co compo- sition leading to periodic strips and small Co nanoparticles dispersed in the matrix and grain boundaries—the predomi- nant mechanism leading to the observed magnetoresistance remains still uncertain. We have previously reported 9,10 the low-temperature mag- netic, resistive, and magnetoresistive properties of Cu 90 Co 10 twin roller melt spun ribbons, solidified at tangential wheel speeds of 5 m/s and 23 m/s, in both, the as-cast condition and after a heat treatment of 1 h at 923 K, respectively. This solidification method is known to promote a more uniform microstructure as compared to those obtained with single wheel devices, being possible to explore lower cooling rates. The x-ray patterns of the alloys, even in the as-cast state, were found to exhibit double peaks, corresponding to two different concentrations of Co in a Cu fcc matrix. These two CuCo peaks, better resolved in ribbons processed at low speeds 5 m/s, are consistent with the spinodal-like modu- lated composition revealed by Miranda et al. 6,7 in similar alloys. In the as-cast state, the mean sizes of these composi- tion strips formed at different quenching rates were estimated using the Scherrer formula; for samples cooled at 5 m/s, values of 51  4 nm and 55  12 nm were estimated for the Co-rich and the Co-poor strips, respectively. For samples quenched at 23 m/s these sizes were 44  4 nm Co rich and 85  20 nm Co poor, respectively. These values are comparable to those reported by other authors 6,7 for similar melt spun CoCu alloys, on the basis of transmission electron microscopy observations. The hysteresis curves of these twin roller melt spun rib- bons were all well fitted 9,11 by the sum of a small ferromag- netic contribution and a superparamagnetic one, the latter arising in Co clusters of about 3.5 nm and 2.3 nm in mean size, for ribbons quenched at 5 m/s and 23 m/s, respectively. Time dependence of the magnetization 11 was consistent with a mean fluctuations field of only 2 mT and an activation length of about 20 nm, certainly larger than the mean volume of small Co particles but comparable with the mean stripe width. Then, it is concluded that the samples processed in the twin roller contain both: a fine dispersion of Co nanoparticles in the fcc Cu matrix exhibiting alternating paramagnetic and ferromagnetic stripes, of about 50 nm mean width. The low- temperature electric resistance of these CuCo as-cast melt spun ribbons is found to decreases 10 almost linearly during cooling from 300 K and passes through a minimum at about 30 K. A similar resistance minimum has been previously reported in very diluted CuCo alloys 12,13 and also in more concentrated 0.5–2 % alloys. 14 In the present work we re- port further results concerning this minimum in the electrical resistance measured in nanostructured Cu 90 Co 10 and Cu 85 Co 15 twin roller melt spun alloys. This minimum, found near 30 K in all the as-cast samples, is examined in connec- tion with the microestructure and the observed magnetoresis- tance. PHYSICAL REVIEW B 82, 172410 2010 1098-0121/2010/8217/1724104 ©2010 The American Physical Society 172410-1