Crystal and magnetic structures of the superconductor CeNi _0.8Bi₂

We have performed powder neutron diffraction on the new superconductor, CeNi_0.8Bi₂ with a superconducting transtion temperature T_c~ 4.2 K. The structural parameters of this compound at room temperature are determined by Rietveld analysis. Below about 5 K, the clear magnetic Bragg peaks with propagation vector q = (0 0 0) are observed. The observed intensities of magnetic Bragg peaks can be explained by the magnetic structure that the two Ce moments in the unit cell are antiparallel along the c axis. The magnetic Bragg peaks are observed in the superconducting state, indicating the coexistence of the antiferromagnetic ordering and the superconductivity in this compound. The intensity of magnetic Bragg peak monotonously increases with decreasing temperature below T_N and does not exhibit apparent anomaly at T_c, obviously different from cases of heavy fermion superconductors in which the magnetic ordering and the superconductivity coexist, for example, Cd-doped CeCoIn₅. These results suggest that the 4f electron of the Ce atom is not coupled with the superconducting carrier, and the magnetic ordering is almost independent of the superconductivity in CeNi_0.8Bi₂.