Papers by Author: Valentin Yu. Irkhin

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_k ~ Dk² 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. Anomalous temperature behavior of magnetization is confirmed by Monte Carlo simulation. Strong field dependence of magnetization (paraprocess) at finite temperature is found near the frustration point.

Abstract: The Hubbard model with strong correlations is treated in the many-electron representation of the Hubbard's operators. The regions of stability of saturated and non-saturated ferromagnetism in the n–U plane for the square and simple cubic lattices are calculated. The role of the singularities of bare density of states for the magnetic phase diagram is discussed. A comparison with the results of previous works is performed.

Abstract: A review of fundamental works by Shubin and Vonsovsky on the formulation of the polar and s–d(f) exchange models is given. Their ideas are compared with subsequent developments in the theory of magnetism in d- and f-metals and their compounds. Modern approaches including various slave-boson and slave-fermion representations, formation of exotic quasiparticles etc. are discussed. Internal connections between different many-electron models (the Heisenberg, Hubbard, t–J, Anderson Hamiltonians) are presented. Description of anomalous rare-earth and actinide compounds (Kondo lattices, systems with heavy fermions and non-Fermi-liquid behavior) within the framework of the s–d(f) exchange model and related models is considered.

Abstract: 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 -model with the use of the renormalization group (RG) approach. For J'  J the asymptotic formula for the Neel temperature TNeel and sublattice magnetization are obtained. RG turns out to be insufficient to describe experimental data since it does not take into account the 2-vortices. Therefore TNeel is estimated using the Monte-Carlo result for the 2D correlation length [10] which has a Kosterlitz-type behavior near the temperature TKT where the vortices are activated.