  We detect in real time inter-dot tunneling events in a weakly coupled two
electron double quantum dot in GaAs. At finite magnetic fields, we observe two
characteristic tunneling times, T_d and T_b, belonging to, respectively, a
direct and a blocked (spin-flip-assisted) tunneling. The latter corresponds to
lifting of a Pauli spin blockade and the tunneling times ratio eta=T_b/T_d
characterizes the blockade efficiency. We find pronounced changes in the
behavior of eta upon increasing the magnetic field, with eta increasing,
saturating and increasing again. We explain this behavior as due to the
crossover of the dominant blockade lifting mechanism from the hyperfine to
spin-orbit interactions and due to a change in the contribution of the charge
decoherence.
