  Using ultrafast optical spectroscopy, we show that polaronic behavior
associated with interfacial antiferromagnetic order is likely the origin of
tunable magnetotransport upon switching the ferroelectric polarity in a
La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/BiFeO$_{3}$ (LCMO/BFO) heterostructure. This is
revealed through the difference in dynamic spectral weight transfer between
LCMO and LCMO/BFO at low temperatures, which indicates that transport in
LCMO/BFO is polaronic in nature. This polaronic feature in LCMO/BFO decreases
in relatively high magnetic fields due to the increased spin alignment, while
no discernible change is found in the LCMO film at low temperatures. These
results thus shed new light on the intrinsic mechanisms governing
magnetoelectric coupling in this heterostructure, potentially offering a new
route to enhancing multiferroic functionality.
