  We investigate the triplet-singlet relaxation in a double quantum dot defined
by top-gates in an InAs nanowire. In the Pauli spin blockade regime, the
leakage current can be mainly attributed to spin relaxation. While at weak and
strong inter-dot coupling relaxation is dominated by two individual mechanisms,
the relaxation is strongly reduced at intermediate coupling and finite magnetic
field. In addition we observe a charateristic bistability of the spin-non
conserving current as a function of magnetic field. We propose a model where
these features are explained by the polarization of nuclear spins enabled by
the interplay between hyperfine and spin-orbit mediated relaxation.
