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

TL;DR

This paper investigates how thermal fluctuations near the frustration point in cubic lattice ferromagnets stabilize ferromagnetism and alter magnetization behavior, supported by theoretical and simulation results.

Contribution

It reveals the stabilizing effect of thermal fluctuations on ferromagnetism near the frustration point and introduces non-analytical corrections to the spin-wave spectrum.

Findings

Thermal fluctuations increase the spin-wave coefficient D at low temperatures.

Magnetization exhibits a concave temperature dependence near the frustration point.

Monte Carlo simulations show suppression of Curie temperature compared to spin-wave theory.

Abstract

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 too strong frustration this leads to an unusual "concave" shape of the temperature dependence of magnetization, which is in agreement with experimental data on the europium chalcogenides. The phase diagram is constructed by means of Monte Carlo simulation, and suppression of magnetization and Curie temperature is found in comparison with the results of the spin-wave theory. This effect is explained by the the presence of non-analytical corrections to the spin-wave spectrum which are represented in the lowest order by the term $\sim (T/S)^{2} k^{2}\log{k}$.