* Corresponding author: Tel.: #1-619-594-6274; fax: #1- 619-594-5485. E-mail address: milton@sciences.sdsu.edu (M.S. Torikachvili). Journal of Magnetism and Magnetic Materials 226 } 230 (2001) 42 } 44 Magnetoresistance measurements in UCu Al compounds M.S. Torikachvili*, R.F. Jardim, S. Chang, A. Christianson, R. Muccillo, H. Nakotte, A.H. Lacerda Department of Physics, San Diego State University, San Diego, CA 92182-1233, USA Instituto de Fisica, Universitdad Sa J o Paulo, 05315-970 Sa J o Paulo, SP, Brazil Department of Physics, New Mexico State University, Las Cruces, NM 88003-8001, USA National High Magnetic Field Laboratory, Los Alamos, NM 87545, USA Instituto de Pesquisas Energeticas e Nucleares, 05422-970 Sa J o Paulo, SP, Brazil Abstract The intermetallic UCu Al compounds (0.1)x)1.95) have remarkable electronic and magnetic properties. The Cu-poor compositions within this x-range are antiferromagnetic (AF). The Ne H el temperature (¹ ) drops gradually from +36 K for x"0.1 until AF is completely suppressed near x"1.1, while the Sommerfeld coe$cient () increases dramatically with x, reaching a maximum value of 750 mJ/mol K for x"1.75. Reported in this investigation are magnetoresistance (MR) and magnetic susceptibility () measurements at low temperatures (¹) and high magnetic "elds (B). The isothermal MR data below 40 K crosses over from positive in the low-x to negative in the high-x range. These results are interpreted in the context of spin #uctuations, hybridization, and non-magnetic atomic disorder. 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnetoresistance; Antiferromagnetism; Heavy fermions The UCu Al compounds crystallize in the tet- ragonal ThMn -type structure for 0.1)x)1.95 [1] and their electronic and magnetic properties depend very strongly on the Cu/Al content ratio. The Cu-poor com- pounds within this x-range are antiferromagnetic (AF) with reported ¹ near 35 K. However, the substitution of Cu for Al is accompanied by (1) a rapid suppression of AF in the 0.1(x(1.15 range; and (2) an enormous increase from about 100mJ/molK (x+0.25) to 750mJ/molK (x"1.75) [2,3]. Neutron di!raction (ND) [3] and NMR [4] results con"rm the suppression of AF near x"1.15. ND also indicates that the substitu- tion of Cu for Al favors one of two inequivalent Al sites, and that the U-ordered moments lie along the tetragonal c-axis. Since the ionic size of Cu is smaller than Al, it has been suggested that the suppression of AF when x in- creases is due to an enhancement of c}f hybridization, favoring the intra-site Kondo over the inter-site RKKY groundstate [1]. The electrical resistivity () vs. ¹ data for the Cu-rich compounds [1] as well as the MR [5] are consistent with the Kondo picture. However, an inter- pretation in terms of hybridization alone is oversimpli- "ed, and changes in the Fermi level need to be considered [2]. For example, the crossover from heavy fermion (HF) behavior to AF in the CeCu Al system occurs for increasing Cu concentration [6], contrary to the UCu Al system. It has been pointed out by Gschneidner et al. that false indications of HF behavior can occur as a result of non-magnetic atomic disorder, as for example in the Cu}Al compounds CeCu Al (NaZn -type structure), CeCu Al, and CeCu Al [7]. The origin of the false HF behavior is the formation of low-lying crystal "eld (CF) levels due to disorder, giving rise to a large Schottky anomaly contribution [7]. In order to probe further the e!ect of the substitution of Cu 0304-8853/01/$- see front matter 2001 Elsevier Science B.V. All rights reserved. PII:S0304-8853(00)00744-7