  The magnetic moment of a quantum dot can be screened by its coupling to a
superconducting reservoir, depending on the hierarchy of the superconducting
gap and the relevant Kondo scale. This screening-unscreening transition can be
driven by electrostatic gating, tunnel coupling, and, as we demonstrate here,
magnetic field. We perform high-resolution spectroscopy of subgap excitations
near the screening-unscreening transition of asymmetric superconductor -
quantum dot - superconductor (S-QD-S) junctions formed by the electromigration
technique. Our measurements reveal a re-entrant phase boundary determined by
the competition between Zeeman energy and gap reduction with magnetic field. We
further track the evolution of the phase transition with increasing
temperature, which is also evidenced by thermal replicas of subgap states.
