Curie temperatures and modified de Gennes factors of rare earth nitrides

The Curie temperatures T_C of the nitrides of the rare earths (Gd, Tb, Dy, Ho, and Er), including binary systems, were investigated. T_C was found to be approximately proportional to the de Gennes factor, ξ=(g−1)²J(J+1), where g is the Landé g-factor and J is the total angular momentum quantum number of a trivalent rare earth (RE). This proportionality was significantly improved by introducing a modified de Gennes factor, ξ_bi. The conventional de Gennes factor ξ indicates the exchange interaction given by the inner product of the effective spin components of ions of the same kind, whereas our modified de Gennes factor ξ_bi also considers interactions between different kinds of ions and statistical factors calculated on the basis of the binomial distribution. The good proportionality obtained between T_C and ξ_bi indicates that the spin components of RE ions interact with each other. This interaction is considered to be responsible for the ferromagnetism of these nitrides (including binary systems). These considerations were supported by the synthesis of and magnetization measurements on Gd_xEr_1−xN (x=0, 0.25, 0.5, 0.75, 1) samples.