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

The experience of formation of schoo-lchildren’s healthy lifestyle in the conditions of activities of the regional innovative platform in Belgorod region

The article is devoted to an important problem of modern Russian education -saving the pupils’ health and forming a healthy lifestyle of children and teenagers. The authors examine the results of the regional innovation platforms which united six schools of the Belgorod regio

Strong short-range magnetic order in a frustrated FCC lattice and its possible role in the iron structural transformation

We investigate magnetic properties of a frustrated Heisenberg antiferromagnet with a face-centered cubic (FCC) lattice and exchange interactions between the nearest- and next-nearest neighbours, J1 and J2. In a collinear phase with the wave vector Q = (pi,pi,pi) the equations of the self-consistent spin-wave theory for the sublattice magnetization and the average short range order parameter are obtained and numerically solved. The dependence of the Neel temperature T_N on the ratio J2/J1 is obtained. It is shown, that at strong enough frustration there is a wide temperature region above T_N with strong short range magnetic order. Application of this result to description of structural phase transition between alpha and gamma-phase of Fe is considered

Scaling picture of magnetism formation in the anomalous f-systems: interplay of the Kondo effect and spin dynamics

Formation of magnetically ordered state in the Kondo lattices is treated within the degenerate $s-f$ exchange and Coqblin-Schrieffer models. The Kondo renormalizations of the effective coupling parameter, magnetic moment and spin excitation frequencies are calculated within perturbation theory. The results of one-loop scaling consideration of the magnetic state in Kondo lattices are analyzed. The dependence of the critical values of the bare model parameters on the type of the magnetic phase and space dimensionality is investigated. Renormalization of the effective Kondo temperature by the interatomic exchange interactions is calculated. An important role of the character of spin dynamics (existence of well-defined magnon excitations, presence of magnetic anisotropy etc.) is demonstrated. The regime of strongly suppressed magnetic moments, which corresponds to magnetic heavy-fermion system, may occur in a rather narrow parameter region only. At the same time, in the magnetically ordered phases the renormalized Kondo temperature depends weakly on the bare coupling parameter in some interval. The critical behavior, corresponding to the magnetic transition with changing the bare $s-f$ coupling parameter, is investigated. In the vicinity of the strong coupling regime, the spectrum of the Bose excitations becomes softened. Thus on the borderline of magnetic instability the Fermi-liquid picture is violated in some temerature interval due to scattering of electrons by these bosons. This may explain the fact that a non-Fermi-liquid behavior often takes place in the heavy-fermion systems near the onset of magnetic ordering.Comment: 20 pages, RevTeX, 13 figure