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 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 $\sim (T/S)^{2} k^{2}\log{k}$.Comment: 5 pages, 5 figures; updated versio

The Neel temperature and sublattice magnetization for the stacked triangular-lattice antiferromagnet with a weak interlayer coupling

The quantum Heisenberg antiferromagnet on the stacked triangular lattice with the intralayer nearest-neighbor exchange interaction J and interlayer exchange J' is considered within the non-linear $\sigma$-model with the use of the renormalization group (RG) approach. For J' << J the asymptotic formula for the Neel temperature $T_{Neel}$ and sublattice magnetization are obtained. RG turns out to be insufficient to describe experimental data since it does not take into account the $\mathcal{Z}_2$-vortices. Therefore $T_{Neel}$ is estimated using the Monte-Carlo result for the 2D correlation length [10] which has a Kosterlitz-type behavior near the temperature $T_{KT}$ where the vortices are activated.Comment: 4 pages, 1 figur

Detoxification analysis of aquatic environment with biological testing method

The article is devoted to the problem of waste water clearing from heavy metals. For analyses of water detoxification it was examined a method of microalgae E. gracilismovement testing at light and dark conditions. It was shown that cells E. graciliswere the most active in organic-mineral media at light and more sensitive to heavy metals in dark. Velocity of cells motion and their life ability were decreased with increasing of heavy metals Fe 2+ , Cu 2+ , Zn 2+ , Co 2+ , Ni 2+ , Cd 2+ , Сr 6+ concentration from 10 –7 to 10 –2 М and time of testing in water medium. Biological testing of cells movement makes it possible to appreciate in for 15–30 min a level of waste water detoxification at different stages of technological water cleaning