Abstract:
To gain insight into the origin of the colossal reduction of resistance in response to magnetic field
in colossal magnetoresistance manganite, the magnetic field induced transition in ferromagnetic
La0.7Ca0.3MnO3 was studied using a high-resolution magneto-optical imaging MOI technique. The
MO images were captured in various magnetic fields over a wide temperature range for both highly
dense samples with strong-link grain boundaries and porous samples with weak-link boundaries.
Formation and evolution of magnetic domains as a function of field or temperature were clearly
observed around and far below the Curie temperature TC=240 K. Ferromagnetic areas tend to grow
to large sizes and finally join together at the expense of paramagnetic areas as the field increases or
temperature decreases for strong-link samples. A sharp magnetoresistive transition is observed when
the sample changes from a paramagnetic insulator to a metallic ferromagnetic phase in the vicinity
of TC. In contrast, the porous samples showed magnetoresistance over a wide temperature range and
exhibited a remarkable grain boundary related magnetization process in addition to magnetization
within grains. A close correlation is found between the magnetization process observed by MOI and
magnetoresistance measurements. Our MOI results indicate that the strong-link or weak-link grain
boundaries are responsible, respectively, for magnetoresistance occurring either only in the vicinity
of the ferromagnetic transition or over a very wide temperature range.
