  The magnetic transport properties have been measured for La0.67-xYxCa0.33MnO3
(0 <= x <= 0.14) system. It was found that the transition temperature Tp almost
linearly moves to higher temperature as H increases. Electron spin resonance
confirms that above Tp, there exist ferromagnetic clusters. From the magnetic
polaron point of view, the shift of Tp vs. H was understood, and it was
estimated that the size of the magnetic polaron is of 9.7~15.4 angstrom which
is consistent with the magnetic correlation length revealed by the small-angle
neutron-scattering technique. The transport properties at temperatures higher
than Tp conform to the variable-range hopping mechanism.
