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

The spin-flop transition in the antiferromagnetic layered manganite La1.4Sr 1.6Mn2O7 was studied using magnetization measurements and a high-resolution magneto-optical imaging technique. We report the direct observation of the formation of ferromagnetic domains appearing at the first order spin-flop transition. The magnetization process proceeds through nucleation of polari zed domains at crystal defect sites and not through the expansion of polari zed domains due to domain wall motion. A small magnetic hysteresis is cause d by the difference between the mechanisms of nucleation and annihilation o f domains in the mixed state. These results establish a direct link between the magnetic structure on the atomic scale as seen in neutron scattering a nd the macroscopic properties of the sample as seen in magnetization and co nductivity measurements. (C) 2000 American Institute of Physics. [S0021-897 9(00)54008-0].