CVM calculation of the magnetic specific heat capacity of gadolinium

The magnetic specific heat capacity of Gadolinium does not fit the normal scheme due to a pronounced convex hump in the low temperature wing (T ∼ Tc/2) of cpmag while the 3d-elements show two concave wings on either side of Tc. In earlier work the low temperature part of this hump at T < Tc/4 was discussed in terms of a spin wave contribution. The present study tries to investigate whether or not the observed hump in cpmag has to do with the statistics of this high spin number. For this purpose the Cluster Variation Method (CVM) has been used in the regular tetrahedron approximation. This approximation can be used in hexagonal close packed structures (HCP) if only first neighbor interactions are considered. The magnetic interactions are usually considered as short range, so that ferromagnetism can be treated in the first neighbor approximation. The c/a-ratio of Gd is about 1.59 at room temperature rather than the ideal HCP ratio of 1.633. This may imply a difference between the interactions within or out of the basal plane. Such an effect, if present, cannot be taken into account by the present treatment