Magnetic structure of $La_{8}$$Cu_{7}$$O_{19}$

International audienceThe magnetic structure of the five-leg ladder compound $La_{8}$$Cu_{7}$$O_{19}$ has been determined using single crystalline neutron diffraction. The material orders antiferromagnetically with the propagation vector q=(12,12,0) below TN=102.5(3)K. Above this temperature the magnetic susceptibility corresponds indeed to that of a five-leg S=1/2 spin ladder, assuming isotropic couplings along the rungs and the legs [J/kB=J′/kB=194(3)K]. Crystallographically, the Cu magnetic moments divide into two subsystems, depending on the environment. Moments situated at centers of octahedra built up by oxygens orient along the b axis and couple to neighboring moments antiferromagnetically, whereas those creating a complicated ribbon structure possess all three Cartesian components. All Cu moments are found to have magnitudes between 0.69 and 1.32μB. Thus, it is seen that the five-leg ladder $La_{8}$$Cu_{7}$$O_{19}$ exhibits long-range antiferromagnetic ordering and possesses a rather complicated magnetic structure which has a noncollinear character