We have performed small-angle neutron scattering (SANS) of the flux line lattice (FLL) in a Fe doped NbSe_2 sample which presents a large peak effect in the critical current. The scattered intensity and the width of the Bragg peaks of the equilibrium
FLL indicate an ordered structure in the peak effect zone. The history dependence in the FLL structure has been studied using field cooled and zero field cooled procedures, and each state shows the same intensity of Bragg scattering and good orientational order. These results strongly suggest that the peak effect is unrelated to a bulk disordering transition, and confirm the role of a heterogeneous distribution of screening current.
Small angle neutron scattering (SANS) magnetic and electrical transport measurements were performed to study a single crystal of Pr0.7Ca0.3MnO3, a colossal magnetoresistive (CMR) material. While the magnetic field induced transformation of this phase
separated compound consisting of an antiferromagnetic insulating phase (AFI) and a ferromagnetic insulating phase (FI), is continuous at high temperature (above 5K), at lower temperature a step like transformation is observed (around 5T at 2K). Macroscopic magnetization measurements and SANS indicate that this transformation occurs by the formation of mesoscopic ferromagnetic metallic (FM) domains in the AFI phase, and, eventually, in the FI phase. Although above 5K this transformation is continuous, below 5K a magnetization step marks the abrupt transition from a large scale FI/AFI phase separation to a large scale phase separation between AFI, FI and FM phases. Our results suggest that relaxation of elastic strains inherent to the coexistence of these different phases plays a crucial role in the mechanism of these transformations. The occurrence of magnetization steps could result from an intrinsic behavior of the AFI phase at low temperature.
