  The effect of low magnetic fields on the transport properties of a manganite
compound with magnetic phase separation is studied. The different behavior
obtained in samples of La0.5Ca0.5MnO3 related to the way in which the low field
is applied is consistent with a picture of changes in the metallic fraction
induced by the magnetic field in a field-cooled cycle. Using a simple model of
conduction through a binary mixture, the interrelation between
magnetoresistance, metallic fraction and percolation temperature is accounted
for. A new physical coefficient relating magnetic field and metallic fraction
emerges as the relevant parameter in the description of phase separated
manganites.
