Magnetic phase diagram and magnetic structures of Ce 4Bi 3

Abstract

The ( H,T) magnetic phase diagram of Ce 4Bi 3 (cubic, a = 9.672 A ̊ space group 1 4 3d) ahs been betermined 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° towards the common [001] axis. The incommensurate-commensurate transition towards a ferromagnetic state is foreseeable from the ANNNI model.