  By nonperturbatively solving the nonequilibrium Anderson two-impurity model
with the hierarchical equations of motion approach, we report a robust
ferromagnetic (FM) phase in series-coupled double quantum dots, which can
suppress the antiferromagnetic (AFM) phase and dominate the phase diagram at
finite bias and detuning energy in the strongly correlated limit. The FM
exchange interaction origins from the passive parallel spin arrangement caused
by the Pauli exclusion principle during the electrons transport. At very low
temperature, the Kondo screening of the magnetic moment in the FM phase induces
some nonequilibrium Kondo effects in magnetic susceptibility, spectral
functions and current. In the weakly correlated limit, the AFM phase is found
still stable, therefore, a magnetic-field-free internal control of spin states
can be expected through the continuous FM--AFM phase transition.
