CVM study of BCC Ising ferromagnets with antiferromagnetic second neighbour interactions: implications to the magnetism of the heavy rare earths

Calculations using the cluster variation method (CVM) in the tetrahedron approximation have been applied to BCC Ising systems with spin number up to s=15/2. Ferromagnetic first neighbour interactions ( J⁽¹⁾<0) and antiferromagnetic second neighbour interactions ( J⁽²⁾>0) of variable strength α=- J⁽²⁾/ J⁽¹⁾ have been analysed. For small α values the system is completely ferromagnetic, showing a second order transition from the ferromagnetic to the paramagnetic state. The Curie temperature T_C depends on the value of α. For a critical value α_C a multicritical point appears at T_C in the α× T phase diagram. For α values higher than α_C, two transitions occur: at low temperatures, a first order transition from the ferromagnetic ground state to the antiferromagnetic state, followed by a second order transition from the antiferromagnetic to the paramagnetic state. The specific heat capacities calculated for these model systems present a remarkable resemblance to experimental data for HCP Tb ( s=6) and Dy ( s=15/2). These results suggest that the complex magnetic structures observed in these metals should be interpreted as an effect of the high spin number statistics.