Strong fluctuations near the frustration point in cubic lattice ferromagnets with localized moments

Thermodynamic properties of cubic Heisenberg ferromagnets with competing exchange interactions are considered near the frustration point where the coefficient $D$ in the spin-wave spectrum $E_{\mathbf{k}}\sim D k^{2}$ vanishes. Within the Dyson-Maleev formalism, it is found that, at low temperatures, thermal fluctuations stabilize ferromagnetism by increasing the value of $D$. For not overly strong frustration, this leads to an unusual “concave” shape of the temperature dependence of magnetization, which is in agreement with experimental data on europium chalcogenides. The phase diagram is constructed by means of Monte Carlo simulation, and suppression of the magnetization and Curie temperature is found in comparison with the results of the spin-wave theory. This effect is explained by the presence of nonanalytical corrections to the spin-wave spectrum which are represented in the lowest order by the term ${\sim}\,(T/S)^{2} k^{2}\ln{k}$.