Kondo Screening Effect and Ferromagnetic Order in $UCu_2Si_2$

Electrical resistivity and low temperature magnetoresistivity measurements made on a single crystal of $UCu_2Si_2$ are reported. By using as a phonon reference the temperature dependence of the electrical resistivity of $ThCu_2Si_2$ we could establish that $UCu_2Si_2$ has both a ferromagnetic and a Kondo behaviour. Such a phenomenon can be described by the underscreened Kondo lattice model. The magnetoresistivity revealed the presence of magnetic fluctuations within the ferromagnetic order as it was reported previously for $UGe_2$. Also one of the calculated Fermi surface sheets exhibits nesting properties, being in perfect agreement with the previous neutron diffraction data, supporting the possibility of a presence of the spin density wave phase. In this ternary silicide, where the strong ferromagnetic behaviour exists, this phase is signalised by magnetic fluctuations.