  A spin-flip potential $V$ is introduced in a BCS Hamiltonian in order to
study a possible thermodynamic effect that might be caused by the addition of a
magnetic impurity in a superconductor. We found that the competition between
the pairing and the spin-flip effect, caused by the interaction between the
itinerant electrons and the spin of the impurity, gives rise to a first-order
phase transition for certain values of $V$. The phase diagram shows a
tricritical point in a frontier line which is topologically similar to those
found in the phase diagrams of some Ising-like models.
