SPECIFIC-HEAT IN SOME GADOLINIUM COMPOUNDS .2. THEORETICAL-MODEL

A detailed thermodynamical analysis of commensurate and incommensurate magnetic phases in localized magnetic systems within a mean-field theory is presented. Interesting results are obtained concerning the specific heat of amplitude-modulated magnetic structures: In particular, the specific-heat discontinuity at the ordering temperature T(N) is reduced by a factor of 2/3 relative to that expected in ferromagnetic or helimagnetic structures. In addition, the exact shape of the specific-heat curves strongly depends on the relative magnitude of the successive exchange couplings J(nQ). Under certain conditions, the maximum of specific heat at T(N) is shifted to a temperature lower than T(N). Recent experimental data on GdCu2Si2, GdNi2Si2, GdGa2, and GdCu5 compounds support very well these peculiarities.