On September 29, 2025, Boris Ivanovich Kochelaev, an outstanding theoretical physicist, professor-consultant of the Department of Theoretical Physics at Kazan University, and Doctor of Physical and Mathematical Sciences, passed away at the age of 91.

In LaF₃ crystals the EPR spectra of Ho³⁺ ion replacing La³⁺ ion in a C₂ symmetry position were recorded and studied in the subterahertz frequency range. The parameters of splitting in the zero field, hyperfine structure constant and the value of g-factor are determined.

Mixed-valence manganese complex oxide LiMn₂TeO₆ has dimeric, chain and planar structure motifs. Three Mn positions form an antiferromagnetic framework, and the fourth one ferromagnetically couples with it. The studies by nuclear magnetic resonance (NMR), AC and DC magnetization, specific heat methods show a non-trivial scenario of spin system transformation as temperature decrease. A change in correlation regimes with slow dynamics in high magnetic fields is found, as well as an unusual magnetic static phase at T < 6K and B < 5T. All obtained results are summarized on the magnetic phase diagram.

This work presents the first comprehensive study of the magnetic properties of the novel perovskite-type high-entropy oxide Ca_0.25Sr_0.5La_0.25Mn_0.5Ti_0.5O₃. A polycrystalline powder of the compound was synthesized using the solid-state reaction method. The compound crystallizes in the tetragonal system with space group I4/mcm and lattice parameters a = b = 5.48320 Å, c = 7.74480 Å, and α = β = γ = 90°. The homogeneity and uniform distribution of the constituent elements were confirmed by X-ray fluorescence analysis. The emergence of magnetic ordering at ≈ 40 K is evidenced by a maximum in the temperature-dependent ESR integrated intensity, coinciding with the divergence of the ZFC/FC magnetization curves. The ESR spectrum itself is well fitted by four lines. A second, distinct phase transition is observed at approximately 300 K. Magnetization hysteresis loops measured below room temperature exhibit weak ferromagnetic behavior, with the coercive field increasing to 750 Oe at 5 K. The sign reversal of the Seebeck coefficient at T ≈ 475 K indicates a change in the conductivity type of the compound. The temperature dependence of the sample's conductivity was described within the small polariton hopping model, and the activation energy was 269 meV.