Rare-earth manganites of the composition R_1-xA_xMnO₃, where R stands
for a trivalent rare earth ion such as La, Nd, Pr or Gd and A for a divalent
alkaline ion, exhibit the colossal magnetoresistance effect (CMR) resulting from
a subtle balance between spin, orbital, charge and lattice degrees of freedom.
CMR manganites have a complex phase diagram in which metal-insulator transition
accompanies the magnetic ordering transition. CMR is found on a magnetic field
scale of several Tesla, which is not very appealing for applications. This
stimulated investigations of extrinsic magnetoresistance effects such as;
grain-boundary magnetoresistance, spin-polarized tunnelling and domain-wall
magnetoresistance, promising a large magnetoresistance ratio in low magnetic
fields.

The extrinsic effects may dominates transport properties of classical and CMR
ferromagnets. Resistivity increase at low temperatures in lightly doped
La_1-xCa_xMnO₃ (LCMO) manganites has been associated with
spin-dependent tunnelling between adjacent ferromagnetic domains. A serial
arrangement of intrinsic tunnel junctions leads to pronounced resistance
oscillations. It has been speculated that intrinsic tunnel junctions are located
within the domain walls. Surprisingly, little is known about the magnetic domain
structures in CMR perovskite single crystals. In this presentation we concentrate
on spontaneous magnetic domains visualized and investigated by means of the
magneto-optical technique and their relation to the low temperature transport
properties of LCMO manganites

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