Neutron scattering investigation of magnetic phase transitions and crystal-field splitting in UX3 (X identical to Br, Cl)

Abstract

Neutron scattering experiments were performed in the temperature range from 1.5 to 293 K on polycrystalline samples of the non-metallic 5f3 electron systems UBr3 and UCl3. The hexagonal crystal structure P63/m was refined at T=293 and 10 K. UBr3 orders at TN=5.4(1) K with a second-order transition to a 0+-configuration of magnetic moments of magnitude mu U(3.9K)=2.08(2) mu B oriented parallel to the c axis. At Tt=3.0(5) K occurs a second transition to an ordered antiferromagnetic ground state, corresponding to the lattice constants am= square root 3a, bm=a, cm=2a. This structure is anticentered in the basal plane. The magnetic moments of magnitude mu U(1.6K)=0.82(4) mu B are oriented parallel to the b axis. In both magnetic phases the antitranslation (0, 0, 1/2) suggests predominantly antiferromagnetic exchange within the chains of uranium ions along the c axis. The situation is similar for UCl3: the first transition at TN=6.5(1) K with an ordered magnetic moment mu U(4.0K) of 1.8(1) mu B, and the second transition at Tt=3.8(5) K with a magnetic moment mu U(1.6K) of 0.8(1) mu B. By means of inelastic neutron scattering the authors have measured the crystal-field splitting of UBr3 and UCl3 and determined the crystal-field parameters. The assumption of anisotropic magnetic interactions reasonably predicts the value of the zero-field magnetisation and provides an explanation for the magnetic phase transitions.