  We consider the process of quantum tunneling between the superconducting and
paramagnetic states of a nanometer-scale superconducting grain placed in a
magnetic field. The grain is supposed to be coupled via tunneling junction to a
normal metallic contact that plays a role of the spin reservoir. Using the
instanton method we find the probability of the quantum tunneling process and
express it in terms of the applied magnetic field, order parameter of the
superconducting grain and conductance of the tunneling junction between the
grain and metallic contact.
