TYPE-II SUPERCONDUCTIVITY IN A DILUTE MAGNETIC SYSTEM - LA1-XTMXRU3SI2

Superconducting and magnetic properties of La1-xTmxRu3Si2 (x = 0, 0.08 , and 0.16) have been investigated through measurements of both electr ic and magnetic properties. Tm is in a trivalent state and carries a m agnetic moment almost-equal-to 8 mu(B). We find that the magnetic meas urements are not suited to the determination of the critical line H(c) (T) for the onset of type-II superconductivity in materials where one has at the same time superconducting diamagnetic shielding and paramag netic contribution due to the localized spins. On the other band, tran sport experiments are much less ambiguous, and allow us to determine a ccurately the critical temperature in a given applied magnetic field. In particular, they allow us to suggest a more appropriate interpretat ion of the maximum in the magnetic-susceptibility curve already observ ed in materials of the same family: this maximum results from a compet ition between the incomplete diamagnetic shielding and the paramagneti c contribution from the magnetic ions in the mixed phase, rather than from a spin-glass freezing of the paramagnetic ions. In zero field, th e rate of depression of the critical temperature upon substituting non magnetic La ions by magnetic Tm ions is dT/dx = - 8+/-1 K/at. % Tm.