  We have observed photocarrier transport behaviors in
BiFeO$_3$/La$_{1-x}$Sr$_x$MnO$_3$~(BFO/LSMO) heterostructures by using
time-resolved synchrotron x-ray magnetic circular dichroism in reflectivity.
The magnetization of LSMO layers was used as a probe of photo-induced carrier
dynamics in the photovoltaic BFO layers. During the photo-induced
demagnetization process, the decay time of LSMO~($x$=0.2) magnetization
strongly depends on the ferroelectric polarization direction of the BFO layer.
The variation of decay time should be attributed to the different sign of
accumulated photocarriers at the BFO/LSMO interface induced by the photovoltaic
effect of the BFO layer. The photocarriers can reach the BFO/LSMO interface and
influence the magnetization distribution in the LSMO layers within the
timescale of $\sim$100~ps. Our results provide a novel strategy to investigate
carrier dynamics and mechanisms of optical control of magnetization in thin
film heterostructures.
