MAGNETIC PHASE-DIAGRAM AND MAGNETIC-STRUCTURES OF CE4BI3

The (H,T) magnetic phase diagram of Ce4Bi3 (cubic, a = 9.672 angstrom space group 14BAR3d) has been determined from neutron diffraction and specific heat measurements. There exist two magnetically ordered phases at low temperature. The phase II, incommensurate, is characterized by a single-k wavevector, k = <0, 0, 0.506>, and has a domain of existence between 4.4 and 3.3 K (H = 0), for magnetic fields lower than 0.2 T. The phase III, non-collinear ferromagnetic, with k = 0, develops at temperatures lower than 3.3 K (H = 0), and the transition temperature strongly increases with the applied field. A group theory analysis based on the symmetries of the crystal and the Landau theory of phase transition allowed to determine the possible couplings between the magnetic moments of the 8 Ce atoms per unit cell. Both structures consist of noncollinear ferromagnetic layers of Ce atoms, with the moments lying along the <111> local axes, slightly tilted of 12-14-degrees towards the common [001] axis. The incommensurate-commensurate transition towards a ferromagnetic state is foreseeable from the ANNNI model.