  Colossal magnetoresistance and field-induced ferromagnetism are well
documented in manganite compounds. Since domain wall resistance contributes to
magnetoresistance, data on the temperature and magnetic field dependence of the
ferromagnetic domain structure are required for a full understanding of the
magnetoresistive effect. Here we show, using cryogenic Magnetic Force
Microscopy, domain structures for the layered manganite
La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ as a function of temperature and magnetic
field. Domain walls are suppressed close to the Curie temperature T$_C$, and
appear either via the application of a c-axis magnetic field, or by decreasing
the temperature further. At temperatures well below T$_C$, new domain walls,
stable at zero field, can be formed by the application of a c-axis field.
Magnetic structures are seen also at temperatures above T$_C$: these features
are attributed to inclusions of additional Ruddleston-Popper manganite phases.
Low-temperature domain walls are nucleated by these ferromagnetic inclusions.
