  Magnetic field induced transitions in the antiferromagnetic layered manganite
La1.4Sr1.6Mn2O7 were studied using magnetization measurements and a
high-resolution magneto-optical imaging technique. We report the first direct
observation of the formation of ferromagnetic domains appearing at the first
order spin-flop transition. The magnetization process proceeds through
nucleation of polarized domains at crystal defect sites and not by domain wall
motion. A small magnetic hysteresis is caused by the nucleation and
annihilation of domains in the mixed state. These results establish a direct
link betweeen the magnetic structure on the atomic scale as seen in neutron
scattering and the macroscopic properties of the sample as seen in
magnetization and conductivity measurements.
