Chemical and structural disorder effects on the Curie temperature

The disorder effects on the Curie temperature of ferromagnetic and ferroelectric systems are studied by factorizing the spin–spin (or dipole–dipole) interaction into a chemical (on-site) and a structural (off-site) part. Assuming the statistical independence of the two contributions, the Curie temperature T_c is calculated in the limit of small disorder and in the mean-field approximation. The chemical disorder always enhances T_c . In the absence of spin waves (Ising-like systems), the structural disorder enhances T_c in turn. The only negative contribution to T_c is found in Heisenberg-like systems, and is ascribed to the interplay between structural disorder and spin waves. A comparison is made with other mean-field theories that adopt a different representation of the disorder. The application of the results obtained to real systems is considered, with special reference to recent experimental data on ferroelectric perovskites. An approximate expression, consistent with the mean-field approach, is suggested to estimate the relative weight of the chemical and structural disorder effects, even when an exact factorization is impossible, as is the case of the exchange interactions.