FIELD-DEPENDENCE OF THE MAGNETIC-ORDERING OF CU IN R2CUO4 (R=ND,SM)

We have used neutron diffraction techniques to study the field dependence of the magnetic ordering of Cu spins in R2CuO4 (R=Nd,Sm) in order to distinguish between the proposed collinear and noncollinear spin structures. In the proposed collinear spin structure, there are two separate domains with the spins either along the (110) or along the (110BAR) directions, while in the noncollinear model there is a single domain with the alternate-layer spins along the (100) and (010) directions, respectively. If a magnetic field is applied along the (110BAR), strong hysteresis effects are anticipated for the collinear spin structure due to domain repopulation, while such effects are not expected for the noncollinear spin structure. Our field dependent data do not show any hysteresis effects associated with the pure Cu ordering, which strongly suggest that the noncollinear spin structure is correct for the magnetic spin configuration of the Cu spins in both compounds. Hysteresis effects in a field are observed in Sm2CuO4 near and below the Sm ordering temperature, and these are most likely caused by the interaction between Sm and Cu sublattices.