Magnetic structure of La8Cu7O19

The magnetic structure of the five-leg ladder compound La₈Cu₇O₁₉ has been determined using single crystalline neutron diffraction. The material orders antiferromagnetically with the propagation vector q =(1/2 ,1/2 ,0 ) below T_N=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 /k_B=J^'/k_B=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 1.32 μ_B . Thus, it is seen that the five-leg ladder La₈Cu₇O₁₉ exhibits long-range antiferromagnetic ordering and possesses a rather complicated magnetic structure which has a noncollinear character.