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.

Thermal decomposition of graphene oxide (GO), often referred to as “thermal reduction” is broadly used to obtain so-called “thermally reduced GO”. At the same time, chemical and structural transformations, accompanying this process remain largely unexplored. In this work, using the combination of electron spin resonance spectroscopy, thermogravimetry, IR spectroscopy, and X–ray powder diffraction, we investigate the early stages of the GO thermal decomposition, which occur in the 80^◦C–190^◦C temperature range. Massive decomposition of the oxygen-containing groups begins at ∼130^◦C. At this temperature we observe formation of C-H bonds and a sharp increase in the content of paramagnetic centers. The highest content of the radicals 1.3 × 10¹⁸ spin/g is registered in the samples, annealed at 150^◦C. This is 3.5 times higher than that in original GO (3.8 × 10¹⁷ spin/g). At the same temperature we observe the loss of the interlayer registry in the material due to crumpling of the partially decomposed GO layers, and the C–H bonds are no longer observed. At 190^◦C, the content of the paramagnetic centers sharply decreases down to 1.0 × 10¹⁷ spin/g, being 3.8 times smaller than that in original GO. This suggests that electrons are largely delocalized due to the enlargement and percolation of graphenic domains, and/or dangling bonds, formed at 130–150^◦C largely recombine. Our new findings add critical details to understanding the fine chemical structure and chemistry of GO.

The study presents results of first-principles modeling of spin dynamics in disordered ensembles of impurity-induced magnetic moments in a carbon spherical structure near 1% concentration of magnetic centers. Averaged signals of Rabi oscillations and their spectra are evaluated using a numerical algorithm based on direct calculation of quantum eigenstates. The Fourier spectrum includes a strong peak around the Rabi frequency and an additional rise in the low-frequency interval. Both peaks demonstrate the standard broadening proportional to the dipole interaction energy. Low-frequency oscillations are observed only for random spin clusters, while regular structures do not produce such dynamics. This effect results from quasi-periodic spin dynamics caused by random distances between particles and, correspondingly, the realization of a set of incommensurate eigenfrequencies in the spin dynamics. Thus, the low-frequency part of the spectrum can be used to characterize spatial disorder in ensembles of spin clusters.

The negatively charged boron vacancy in hexagonal boron nitride is one of the most prominent representatives of an optically active qubit in two-dimensional van der Waals materials. In this case, the electron-nuclear interactions of the vacancy with the magnetic moments of the hBN lattice atoms are of particular interest. In this paper, we investigated the nuclear quadrupole interactions of the boron vacancy with the removed nuclear spins of nitrogen ¹⁴N (I = 1) using the method of electron-nuclear double resonance. The constant of the corresponding quadrupole interaction is determined and the type of the corresponding tensor is determined. A comparative analysis of the obtained parameters of the nuclear quadrupole interaction with the parameters for the nitrogen atoms closest to the vacancy is carried out. Based on the data presented, it is proposed to use the electron spin of the boron vacancy as a spin probe to study the fundamental properties of boron nitride, such as the constants of the nuclear quadrupole interaction.  

Using the example of a study on a solid natural porous material, such as the core of dolomite rock, the effectiveness of three complementary NMR techniques is demonstrated. These techniques allow for the determination of the characteristics of the porous structure with sufficient accuracy, including the pore size distribution. Based on the combination of classical NMR relaxation and DDIF techniques, a method has been proposed for determining the surface relaxation parameter of the studied object without the need for additional physico-chemical research or reference to literature data.

GdTiO₃ is a prototypical Mott-insulating perovskite that exhibits a rich interplay between spin, orbital, and lattice degrees of freedom. Here we report a systematic study of the magnetic anisotropy of high-quality GdTiO₃ single crystal grown by the optical floating-zone technique. Comprehensive magnetization measurements performed with a vibrating-sample magnetometer (5K ≤ T ≤ 70K, |µ₀H| ≤ 9T) reveal a pronounced magnetic anisotropy. The ordering temperature is isotropic (T_N ≈ 32K), but both the magnetic susceptibility and the spin-flop fields depend strongly on the direction of the applied field (H ∥ a,b,c). For each crystallographic orientation three distinct magnetic regimes are identified: ferromagnetic-like low-field phase, ferrimagnetic high-field phase and paramagnetic phase. These findings provide a solid experimental benchmark for theoretical treatments of spin-orbit coupling and anisotropic exchange in rare-earth titanates. Moreover, the sizable magnetic entropy change associated with the field-driven transitions suggests that GdTiO₃ could be exploited in solid-state cryogenic refrigeration based on the magnetocaloric effect, offering a potential route to more efficient hydrogen liquefaction.

In this article, the capabilities of using low-field NMR of liquid ¹²⁹Xe near triple point in the study of porous media were demonstrated. The possibility of detecting the NMR signal of Xenon with natural isotopic composition and Boltzmann equilibrium spin polarization in the liquid and solid phases was explored. The problems encountered during the planning and conducting the experiments, along with their solutions, are presented. Key issues included low sensitivity, particularly in magnetic fields up to 1T; difficulties in filling the experimental cell with liquid Xenon; the critical need to control the temperature gradient along the NMR probe axis and prevent capillary blockage; control of the solid-to-liquid transition; RF-noise from the electrical leads. As a result, spin-lattice relaxation times (T₁) of bulk Xenon were measured and the spin-spin relaxation times (T₂) of bulk Xenon were estimated. Some of preliminary results on ¹²⁹Xe NMR in porous media are presented.