Direct imaging of the first-order spin-flop transition in the layered manganite La1.4Sr1.6Mn2O7

Magnetic field induced transitions in aritiferromagnetic La1.4Sr1.6Mn2O7 we re studied using magnetization measurements and a high-resolution magneto-o ptical imaging technique. We report the direct observation of the formation of ferromagnetic domains appearing at the first-order spin-flop transition . The magnetization proceeds through nucleation of polarized domains at cry stal defect sites and not by domain wall motion. A small magnetic hysteresi s is caused by the nucleation and annihilation of domains in the mixed stat e. The results establish a direct link between the magnetic structure on th e atomic scale and the macroscopic transport and magnetic properties of the sample.