Alexey Alfonsov

Is a metallic ferromagnet with a layered crystal structure demonstrating intriguing electronic and magnetic properties, such as large anomalous Hall effect, strong canting of the spin lattice, and a peculiar spin‐reorientation transition. Here, we report the results of the temperature‐dependent X‐ray diffraction, static magnetization, and in particular electron spin resonance (ESR) and ferromagnetic resonance (FMR) experiments carried out over a wide range of frequencies, magnetic fields, and temperatures. Though X‐ray data reveals no structural transition down to 15 K, a strong magneto‐elastic coupling is detected across the ferromagnetic transition at K. ESR results indicate a presence of the quasi‐static short‐range correlations extending far above , which is a typical fingerprint of the low‐dimensional magnetism. The frequency‐field diagram of the FMR modes mapped below strongly suggests presence of two magnetic sublattices in . A quantitative understanding of the FMR excitations was achieved within a phenomenological model of interacting orthogonal ferro‐ and antiferromagnetic sublattices, which was earlier proposed to explain unusually strong spin canting observed by neutron diffraction [E. Granado et al., Phys. Rev.Lett. 89 , 107204 (2002)]. The FMR results corroborate this scenario and call for the development of the underlying microscopic model of unconventional magnetism in .

Transition‐metal dichalcogenides provide a versatile platform for the intercalation of magnetic ions in their van der Waals gaps, enabling tuning of correlated electronic and magnetic states. Despite extensive studies on sulfide‐based systems, Cr _x NbSe ₂ remains poorly understood due to scarcity of single‐crystals and uncertainties in structural analyses leading to contradictory reports on its magnetic behavior. Here, high‐quality single crystals of Cr _1/4 NbSe ₂ via chemical vapor transport were synthesized, along with comprehensive investigation of chemical, structural, electronic, optical, electrical, and magnetic properties, supported by density‐functional theory calculations, presented for the first time. Single crystal x‐ray diffraction, scanning transmission electron microscopy, and selected area electron diffraction reveal ordered Cr intercalation forming a centrosymmetric 2 × 2 superlattice with hexagonal symmetry P 6 ₃ / mmc . Optical reflectivity indicates an enhanced electron scattering rate. DFT calculations predict antiferromagnetic ground state induced by Cr intercalation. Consistently, SQUID magnetometry and specific heat measurements hint a correlated antiferromagnetic state at T _N ≈ 107 K, while resistivity measurements show metallic transport, with no signatures of superconductivity. Electron spin resonance suggests weak internal field which possibly arise from subtle magnetic anisotropy, undetectable in angle‐resolved magnetometry. These findings resolve longstanding inconsistencies and establish Cr _1/4 NbSe ₂ as a benchmark vdW antiferromagnet for next‐generation spintronic and quantum materials.

The use of antiferromagnets in magnetoelectronic devices as counterparts of ferromagnets is a new, rapidly developing trend in spintronics that leverages antiferromagnetic (AFM) magnons for transmitting of spin currents. Van der Waals (vdW) antiferromagnets are particularly attractive in this respect as they possess tunable magnetic properties and can be easily integrated into spintronic devices. In this work, electron spin resonance (ESR) spectroscopy is used to assess the potential of the vdW AFM compound CuCrP ₂ S ₆ for magnonic applications by exploring the magnetic field ( H ) dependence of the spectrum of magnon excitations below its AFM ordering temperature T _N ≈ 30 K and the correlated spin dynamics above T _N . ESR reveals prominent ferromagnetic (FM) spin correlations that persist far above T _N suggesting an intrinsically 2D character of the spin dynamics in CuCrP ₂ S ₆ . Most interestingly, at T < T _N , CuCrP ₂ S ₆ features two non‐degenerate, i.e., distinct in energy AFM magnon modes at H = 0 which can be tuned to the FM type of collective spin excitations with increasing H . These remarkable properties are favorable for the induction and control of unidirectional spin current in CuCrP ₂ S ₆ and suggest it as a new functional material for magnetoelectronics.

We present the design of a continuous-wave, highly sensitive optical spectrometer for millimeter-wave frequencies between 50 and 1000 GHz, with optimal performance at cryogenic temperatures. The spectrometer uses photomixing of near-infrared light to generate radiation over a wide frequency range, and the optical power absorbed by the sample is determined directly by measuring the sample temperature. This enables a dynamic range of up to 106 for the absorption coefficient below liquid-helium temperatures, making it suitable for measurements on highly reflective samples. The instrument can also be used in combination with high magnetic fields. In order to validate its performance, we measure the ferromagnetic resonance in the Mott insulator YTiO3, the electron spin resonance in a standard free-radical reference compound, and the antiferromagnetic resonance in a van der Waals magnetic material.

Our knowledge about endohedral metallofullerenes (EMFs) is restricted to the structures with sufficient kinetic stability to be extracted from the arc-discharge soot and processed by chromatographic and structural techniques. For the most abundant rare-earth monometallofullerene M^III@C₈₂, experimental studies repeatedly demonstrated C_2v(9) and C_s(6) carbon cage isomers, while computations predicted equal stability of the "missing" C_3v(8) isomer. Here we report that this isomer is indeed formed but has not been recovered from soot using standard protocols. Using a combination of redox extraction and subsequent benzylation and trifluoromethylation with single-crystal XRD analysis of CF₃ adduct, we prove that Dy@C_3v(8)-C₈₂ is one of the most abundantly produced metallofullerenes, which was not identified in earlier studies because of the low kinetic stability. Further, using the Dy@C_3v(8)-C₈₂(CF₃) and Dy@C_3v(8)-C₈₂(CH₂Ph) monoadducts for the case study, we analyzed the role of metal-fullerene bonding on the single-ion magnetic anisotropy of Dy in EMFs. The multitechnique approach, combining ab initio calculations, EPR spectroscopy, and SQUID magnetometry, demonstrated that coordination of the Dy ion to the fullerene cage induces moderate, nonaxial, and very fluid magnetic anisotropy, which strongly varies with small alterations in the Dy-fullerene coordination geometry. As a result, Dy@C_3v(8)-C₈₂(CH₂Ph) is a weak field-induced single-molecule magnet (SMM), whose signatures of magnetic relaxation are detectable only below 3 K. Our results demonstrate that metal-cage interactions should have a detrimental effect on the SMM performance of EMFs. At the same time, the strong variability of the magnetic anisotropy with metal position suggests tunability and offers strategies for future progress.

The field of research on magnetic van der Waals compounds -- a special subclass of quasi-two-dimensional materials -- is currently rapidly expanding due to the relevance of these compounds to fundamental research where they serve as a playground for the investigation of different models of quantum magnetism and also in view of their unique magneto-electronic and magneto-optical properties pertinent to novel technological applications. The Electron Spin Resonance (ESR) spectroscopy plays an important role in the exploration of the rich magnetic behavior of van der Waals compounds due to its high sensitivity to magnetic anisotropies and unprecedentedly high energy resolution that altogether enable one to obtain thorough insights into the details of the spin structure in the magnetically ordered state and the low-energy spin dynamics in the ordered and paramagnetic phases. This article provides an overview of the recent achievements in this field made by the ESR spectroscopic techniques encompassing representatives of antiferro- and ferromagnetic van der Waals compounds of different crystal structures and chemical composition as well as of a special category of these materials termed magnetic topological insulators.