Specific heat in some gadolinium compounds. II. 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 ${\mathit{T}}_{\mathit{N}}$ 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 ${\mathit{T}}_{\mathit{N}}$ is shifted to a temperature lower than ${\mathit{T}}_{\mathit{N}}$. Recent experimental data on ${\mathrm{GdCu}}_{2}$${\mathrm{Si}}_{2}$, ${\mathrm{GdNi}}_{2}$${\mathrm{Si}}_{2}$, ${\mathrm{GdGa}}_{2}$, and ${\mathrm{GdCu}}_{5}$ compounds support very well these peculiarities.