Reverse resistance anomaly in (La, Gd)Al₂

We report resistivity measurements performed between 0.4 K and 10 K, in magnetic fields, 0≦B≦5T, on (La1−x Gd x )Al2 alloys with 0.23 at%≦x≦2 at %. In zero field, the incremental resistivityρ i due to the Gd atoms increases approximately as ln(T/1 K). The anomalous temperature dependence ofρ i wich is the “reverse” of the Kondo effect is theoretically expected since the conduction electron-impurity exchange interaction is ferromagnetic. Using our data of the magnetoresistivity, a semiquantitative analysis of the zero field results can be made. Comparison with other physical quantities, e.g. EPR andT c depression, taken from the literature, allows us to determine (i) the Heisenberg exchange integral which agrees with that one estimated from the spin glass properties of (La, Gd)Al2 and (ii) the local density of states at the Gd site