arXiv subjects

M. V. Magnitskaya

Publications and source records attributed to M. V. Magnitskaya.

13 recordsLinked to original sources

arXiv2026

The New High-entropy Compound RhMnFeCoGe4 with Cubic Non-centrosymmetric B20 Structure

A novel high-entropy compound, RhMnFeCoGe$_4$, with a cubic non-centrosymmetric B20 struc- ture, has been synthesized under conditions of high pressure and temperature. The electrical transport and magnetic properties of the obtained compound at both ambient and elevated pres- sures have been investigated. In addition, nuclear magnetic resonance (NMR) spectra were obtained at 4.2 K and ab initio calculations were performed. The new material exhibits ferromag- netic behavior with a critical temperature of $T_C$ = 146 K and a spontaneous moment of 2.5 $μ_B$ per formula unit. The magnetization data obtained at the critical region yielded the critical temperature and exponents, which were found to be $T_C$ = 146(1) K, $β$ = 0.337(1), $γ$ = 1.121(1), and $δ$ = 4.326(1). The magnetic moments of Mn and Co were determined from NMR spectra to be 2.2 $μ_B$ and 0.5 $μ_B$, respectively. Ab initio calculations yielded reasonable values for the lattice parameter and the magnetic moments of all constituents. The density of states and band structure are determined for both paramagnetic and ferromagnetic states. Lattice compression results in the increase in the $T_C$.

cond-mat.mtrl-sci↗

arXiv2026

Adaptive fine-tuning of foundation models for crystal structure prediction: Discovery of high-pressure phases in the CaFeNi system

The prediction of crystal structures is a key challenge in chemistry and materials science, but evolutionary crystal structure prediction (CSP) remains computationally expensive because it relies on repeated \textit{ab initio} relaxations and energy ranking. Machine learning interatomic potentials (MLIPs) can accelerate CSP, yet their use is limited by the need for large training sets and by the difficulty of choosing which candidate structures should be labeled by density functional theory (DFT). Here we introduce a self-consistent, foundation-model-assisted CSP workflow that combines evolutionary search with adaptive data selection and fine-tuning. Starting from a pretrained MLIP, the algorithm rapidly explores configuration space while iteratively selecting compact, representative, and physically relevant subsets of structures for DFT labeling, thereby reducing redundant calculations and improving a system-specific potential. We apply the method to the chemically complex Ca--Fe--Ni ternary system. The workflow reproduces the known low-pressure convex hull and enables efficient high-pressure exploration. It predicts a previously unreported compound, Ca$_6$FeNi, which becomes thermodynamically stable above 100~GPa. These results show that foundation-model-based, data-efficient CSP can greatly reduce computational cost while preserving accuracy and enabling the discovery of new materials in complex multicomponent systems.

cond-mat.mtrl-sci↗

arXiv2023

Local structure, thermodynamics, and melting curve of boron phosphide at high pressures by deep learning-driven ab initio simulations

Boron phosphide (BP) is a (super)hard semiconductor constituted of light elements, which is promising for high demand applications at extreme conditions. The behavior of BP at high temperatures and pressures is of special interest but is also poorly understood because both experimental and conventional ab initio methods are restricted to studying refractory covalent materials. The use of machine learning interatomic potentials is a revolutionary trend that gives a unique opportunity for high-temperature study of materials with ab initio accuracy. We develop a deep machine learning potential (DP) for accurate atomistic simulations of solid and liquid phases of BP as well as their transformations near the melting line. Our DP provides quantitative agreement with experimental and ab initio molecular dynamics data for structural and dynamic properties. DP-based simulations reveal that at ambient pressure tetrahedrally bonded cubic BP crystal melts into an open structure consisting of two interpenetrating sub-networks of boron and phosphorous with different structures. Structure transformations of BP melts under compressing are reflected by the evolution of low-pressure tetrahedral coordination to high-pressure octahedral coordination. The main contributions to structural changes at low pressures are made by the evolution of medium-range order in B-subnetwork and at high pressures by the change of short-range order in P-sub-network. Such transformations exhibit an anomalous behavior of structural characteristics in the range of 12--15 GPa. Analysis of the results obtained raise open issues in developing machine learning potentials for covalent materials and stimulate further experimental and theoretical studies of melting behavior in BP.

cond-mat.mtrl-sci↗

arXiv2022

Split of the magnetic and crystallographic states in Fe$_{1-x}$Rh$_{x}$Ge

We report on a comprehensive experimental and theoretical study of Fe$_{1-x}$Rh$_{x}$Ge compounds, within the entire concentration range $x \in \left[0.0 - 1.0\right]$, using X-Ray diffraction, small-angle neutron scattering, magnetometry and theoretical calculations. While FeGe and RhGe are single phase helimagnet and unconventional superconductor, respectively, an internal splitting of the crystallographic and magnetic states is found for intermediate compositions $x \in \left[0.2 - 0.9\right]$. A theoretical analysis of the stability of the two detected phases, together with the experimental data, indicate that this splitting preserves a common space group and occurs within single crystallites. Despite their apparent similarity, these two phases however display different magnetic structures, with distinct ferro- and helimagnetic character.

cond-mat.mtrl-sci↗

arXiv2020

Dualism of the 4f electrons and high-temperature antiferromagnetism of the heavy-fermion compound YbCoC$_{2}$

We report on the first study of the noncentrosymmetric ternary carbide YbCoC$_{2}$. Our magnetization, specific heat, resistivity and neutron diffraction measurements consistently show that the system behaves as a heavy-fermion compound, displaying an amplitude-modulated magnetic structure below the Néel temperature reaching $T_{N}$ = 33 K under pressure. Such a large value, being the highest among the Yb-based systems, is explained in the light of our ab initio calculations, which show that the 4f electronic states of Yb have a dual nature -- i.e., due to their strong hybridization with the 3d states of Co, 4f states expose both localized and itinerant properties.

cond-mat.str-el↗

arXiv2019

Ab initio study of lattice dynamics of dodecaborides ZrB12 and LuB12

We performed ab initio lattice-dynamics calculations of frame-cluster dodecaborides ZrB12 and LuB12. As a whole, our calculated phonon frequencies and atom-projected density of states are consistent with the results of available first-principles calculations and experimental measurements. So we conclude that the ab initio DFT approach is quite appropriate to study the sufficiently subtle physics of these compounds. Our experiment-independent calculations provide an explicit quantitative confirmation of mixing the eigenvectors of boron and metal vibrations, which was previously observed in experiments.

physics.chem-ph↗

arXiv2019

Stability of vacancy-free crystalline phases of titanium monoxide at high pressure and temperature

There have existed for a long time a paradigm that TiO phases at ambient conditions are stable only if structural vacancies are available. Using an evolutionary algorithm, we perform an ab initio search of possible zero-temperature polymorphs of TiO in wide pressure interval. We obtain the Gibbs energy of the competing phases taking into account entropy via quasiharmonic approximation and build the pressure-temperature diagram of the system. We reveal that two vacancy-free hexagonal phases are the most stable at relatively low temperatures in a wide range of pressures. The transition between these phases takes place at 28 GPa. Only above 1290 K at ambient pressure the phases with vacancies (B1-derived) become stable. In particular, the high-pressure hexagonal phase is shown to have unusual electronic properties, with a pronounced pseudo-gap in the electronic spectrum. The comparison of DFT-GGA and GW calculations demonstrates that the account for many-body corrections significantly changes the electronic spectrum near the Fermi energy.

physics.chem-ph↗

arXiv2019

Ab initio study of noncentrosymmetric transition-metal monogermanide B20-RhGe synthesized at high temperature and pressure

We present ab initio density-functional study of the noncentrosymmetric B20-type phase of RhGe, which is not found in nature and can be synthesized only at extreme pressures and temperatures. The structural, thermodynamic, electronic, lattice-dynamical, and transport properties of B20-RhGe are calculated, and their evolution with increasing pressure is traced. The temperature dependence of the charge and heat transport properties is evaluated within the semi-classical Boltzmann approach. Using the quasi-harmonic approximation, we determine the range of pressures and temperatures, in which B20-RhGe is stable, and make recommendations for optimizing the synthesis conditions in order to reduce the number of defects that occur in a sample during solidification.

physics.chem-ph↗

arXiv2019

Theoretical and experimental study of high-pressure synthesized B20-type compounds Mn$_{1-x}$(Co,Rh)$_x$Ge

The search and exploration of new materials not found in nature is one of modern trends in pure and applied chemistry. In the present work, we report on experimental and \textit{ab initio} density-functional study of the high-pressure-synthesized series of compounds Mn$_{1-x}$(Co,Rh)$_x$Ge. These high-pressure phases remain metastable at normal conditions, therewith they preserve their inherent noncentrosymmetric B20-type structure and chiral magnetism. Of particular interest in these two isovalent systems is the comparative analysis of the effect of $3d$ (Co) and $4d$ (Rh) substitution for Mn, since the $3d$ orbitals are characterized by higher localization and electron interaction than the $4d$ orbitals. The behavior of Mn$_{1-x}$(Co,Rh)$_x$Ge systems is traced as the concentration changes in the range $0 \leq x \leq 1$. We applied a sensitive experimental and theoretical technique which allowed to refine the shape of the temperature dependencies of magnetic susceptibility $χ(T)$ and thereby provide a new and detailed magnetic phase diagram of Mn$_{1-x}$Co$_x$Ge. It is shown that both systems exhibit a helical magnetic ordering that very strongly depends on the composition $x$. However, the phase diagram of Mn$_{1-x}$Co$_x$Ge differs from that of Mn$_{1-x}$Rh$_x$Ge in that it is characterized by coexistence of two helices in particular regions of concentrations and temperatures.

physics.chem-ph↗

arXiv2018

Doping evolution of the gap structure and spin-fluctuation pairing in $Ba(Fe_{1-x}Co_{x})_{2}As_{2}$ superconductors

Doping dependence of the superconducting state structure and spin-fluctuation pairing mechanism in the $Ba(Fe_{1-x}Co_{x})_{2}As_{2}$ family is studied. BCS-like analysis of experimental data shows that in the overdoped regime, away from the AFM transition, the spin-fluctuation interaction between the electron and hole gaps is weak, and $Ba(Fe_{1-x}Co_{x})_{2}As_{2}$ is characterized by three essentially different gaps. In the three-gap state an anisotropic (nodeless) electron gap $Δ_e (x, ϕ)$ has an intermediate value between the dominant inner $Δ_{2h}(x)$ and outer $Δ_{1h}(x)$ hole gaps. Close to the AFM transition the electron gap $Δ_e (x, ϕ)$ increases sharply and becomes closer in magnitude to the dominant inner hole gap $Δ_{2h}(x)$. The same two-gap state with close electron and inner hole gaps $Δ_{2h}(x) \approx Δ_e (x, ϕ)$ is also preserved in the phase of coexisting antiferromagnetism and superconductivity. The doping dependence of the electron gap $Δ_e (x, ϕ)$ is associated with the strong doping dependence of the spin-fluctuation interaction in the AFM transition region. In contrast to the electron gap $Δ_e (x, ϕ)$, the doping dependence of the hole gaps $Δ_{1,2h}(x)$ and the critical temperature $T_{c}(x)$, both before and after the AFM transition, are associated with a change of the density of states $γ_{nh}(x)$ and the intraband electron-phonon interaction in the hole bands. The non-phonon spin-fluctuation interaction in the hole bands in the entire Co concentration range is small compared with the intraband electron-phonon interaction and is not dominant in the $Ba(Fe_{1-x}Co_{x})_{2}As_{2}$ family.

cond-mat.supr-con↗

arXiv2018

Magnetic, electronic and transport properties of high-pressure-synthesized chiralmagnets Mn$_{1-x}$Rh$_x$Ge (B20)

We report on structural, magnetic and transport properties of a new set of the high-pressuresynthesized compounds Mn$_{1-x}$Rh$x$Ge ($0 \leq x \leq 1$) with the chiral magnetic ordering. The magnetic and transport properties depend substantially on the concentration of rhodium (x) and the pressure. The saturation magnetic moment corresponds to a known high-spin value for pristine MnGe (x = 0) and decreases almost linearly with increasing concentration $x$. In addition, XMCD spectra taken at 10 K and 2 T indicate magnetic polarization of the Rh 4d electron states and Ge $4p$ states, which decreases with $x$, too. In rhodium rich compounds ($x \geq 0.5$) the temperature of the magnetic ordering increases significantly with pressure, whereas in manganese rich compounds ($x < 0.5$) the temperature decreases. Three different tendencies are also found for several structural and transport properties. In the intermediate range ($0.3 \leq x \leq 0.7$) samples are semiconducting in the paramagnetic phase, but become metallic in the magnetically ordered state. We carried out ab initio density-functional calculations of Mn$_{1-x}$Rh$_x$Ge at various concentrations $x$ and traced the evolution of electronic and magnetic properties. The calculation results are in good agreement with the measured magnetic moments and qualitatively explain the observed trends in transport properties.

cond-mat.str-el↗

arXiv2006

Ab initio calculations of the physical properties of transition metal carbides and nitrides and possible routes to high-Tc

Ab initio linear-response calculations are reported of the phonon spectra and the electron-phonon interaction for several transition metal carbides and nitrides in a NaCl-type structure. For NbC, the kinetic, optical, and superconducting properties are calculated in detail at various pressures and the normal-pressure results are found to well agree with the experiment. Factors accounting for the relatively low critical temperatures Tc in transition metal compounds with light elements are considered and the possible ways of increasing Tc are discussed.

cond-mat.supr-con↗

arXiv2001

Electric field gradients in MgB$_2$ synthesized at high pressure: $^111$Cd TDPAC study and ab initio calculation

We report the high-pressure synthesis of novel superconductor MgB$_2$ and some related compounds. The superconducting transition temperature of our samples of MgB$_2$ is equal to 36.6 K. The MgB$_2$ lattice parameters determined via X-ray diffraction are in excellent agreement with results of our ab initio calculations. The time-differential perturbed angular correlation (TDPAC) experiments demonstrate a small increase in quadrupole frequency of $^111$Cd probe with decreasing temperature from 293 to 4.2 K. The electric field gradient (EFG) at the B site calculated from first principles is in fair agreement with EFG obtained from $^11$B NMR spectra of MgB$_2$ reported in the literature. It is also very close to EFG found in our $^111$Cd TDPAC measurements, which suggests that the $^111$Cd probe substitutes for boron in the MgB$_2$ lattice.

cond-mat.mtrl-sci↗