arXiv subjects

A. E. Petrova

Publications and source records attributed to A. E. Petrova.

At least 19 recordsLinked to original sources

arXiv2026

Magnetoresistance in the helical itinerant magnets MnSi and Mn$_{1-x}$Co$_x$Si

We studied the longitudinal and transverse magnetoresistance of helical magnets, MnSi and Mn$_{1-x}$Co$_x$Si, at temperatures between 1.8 and 100~K and in magnetic fields up to 9 Tesla. All substances exhibited negative longitudinal and transverse magnetoresistance at temperatures above 4~K, which is most likely related to the suppression of spin fluctuations by the magnetic field. Note that in contrast to our finding, the longitudinal magnetoresistance of ferromagnetic metals was found to be positive. The unique positive and anisotropic magnetoresistance of pure MnSi at low temperatures (1.8 and 4~K) in the induced ferromagnetic phase shows effective suppression of fluctuations by the magnetic field. The significant difference in behavior between pure MnSi and doped MnSi lies in the specifics of the latter material, which forms a sort of helical fluctuation cloud and reveals quantum critical properties at low temperatures. The observed isotropic magnetoresistance in MnSi and Mn$_{1-x}$Co$_x$Si at higher temperatures can tentatively be attributed to the shortening of the mean free path of electrical carriers due to scattering on magnetic fluctuations and impurities, which results in a suppression of Lorentz force effects.

cond-mat.str-el↗

arXiv2025

Linear and isotropic magnetoresistance of Co$_{1-x}$Fe$_x$Si at x=0.2; 0.4; 0.65

We studied the magnetoresistance (MR) of well-characterized samples of Co$_{1-x}$Fe$_x$Si at x=0.2, 0.4, and 0.65 at temperatures between 1.8 and 100~K and magnetic fields of 9~T. The quasilinear dependence of MR on the magnetic field at low temperatures and the practically isotropic properties of MR in these compounds are tentatively attributed to the specifics of Weyl electron spectra and general disorder of the materials.

cond-mat.str-el↗

arXiv2025

On the Topological Features of the Helical Phase Transition in MnSi

Many decades of study have revealed very unusual properties of the helical phase transition in MnSi. This situation is briefly described and illustrated in the present note. As one will be able to see, one peculiarity is that the phase transition point in MnSi is accompanied by extremes of different thermodynamic and kinetic quantities on the high temperature side of the transition, which look similar to a property of 2D systems. The whole situation can be tentatively described as a phase transformation of the helical phase of MnSi to the paramagnetic state, which occurs in two steps, first as a first order phase transition and then following a breakdown of topological objects such as spin vortices.

cond-mat.str-el↗

arXiv2024

Magnetoresistance of a bulk sample of FeSi

The magnetoresistance, MR of a well-characterized bulk FeSi sample was studied. It is shown that after a chaotic behavior at temperatures below 6 K, the magnetoresistance of FeSi becomes regular functions of temperature and magnetic field. The observations suggest that the mean free path of carriers defines along with the unknown negative component the negative values of magnetoresistance of FeSi then approaching zero values at high temperatures.

cond-mat.str-el↗

arXiv2023

Spin-fluctuation heat capacity at magnetic phase transition in the Co,Fe doped MnSi

An universal line revealing an independence of spin fuctuation contributions to the heat capacity on impurity content and its nature is discovered in the helical phase of Mn(Co,Fe)Si. This situation declares an invariance of the heat capacity of spin subsystem under doping, which probably arises as a result of relative stiffness of the helical spin structure in respect to the impurity spins. On the other hand the situation drastically changes at the helical fuctuation region when no long range spin order exists.

cond-mat.str-el↗

arXiv2022

Quantum criticality features in the Co doped MnSi

The mysterious universal line revealing an independence of spin fluctuation contributions to the heat capacity of (Mn,Co)Si on impurity contents and its nature is discovered. This situation probably declares an invariance of the spin subsystem energy that may provide by the response of itinerant electron system on the volume change at doping.

cond-mat.str-el↗

arXiv2022

Magnetoresistance and Kohler rule in the topological chiral semimetals CoSi

The transverse and longitudinal magnetoresistance (MR) of two samples of the topological chiral semimetal CoSi with different RRR was studied. It is shown that the Kohler rule works for the transverse MR. The Kohler rule is also fulfilled in the case of longitudinal MR at a low reduced magnetic field. A sharp deviation of longitudinal MR curve for sample with low RRR from the Kohler prediction at high fields reveals its tendency to a sign change at higher magnetic fields. The Shubnikov de Haas quantum oscillations were observed and analyzed in both perpendicular and parallel configurations of the current and magnetic field in sample CoSi 1 with RRR 9.33 at low temperatures.

cond-mat.str-el↗

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↗

arXiv2022

Physical properties of MnSi at extreme doping with Co: Quantum criticality

The samples of (Mn$_{1-x}$Co$_x$)Si with $x=0.15$ and $x=0.17$ were grown and their physical properties: magnetization and magnetic susceptibility, resistivity and heat capacity were studied. The data analysis included also the previous results at $x=0.057, 0.063, 0.09$. The indicated doping MnSi with Co completely destroys the helical phase transition whereas basically saves the helical fluctuation area normally situated slightly above the phase transition temperature. This area spreading from $\sim$5 to 0 K is not changed much with doping and forms some sort of helical fluctuation cloud revealing the quantum critical properties: $C_p/T\rightarrow\infty$ at $T\rightarrow0$.

cond-mat.str-el↗

arXiv2020

Physical properties of (Mn,Co)Si

We have grown and characterized three samples of Co doped MnSi and studied their physical properties (magnetization and magnetic susceptibility, heat capacity and electrical resistance). All three samples show non-Fermi liquid physical properties. From literature data and current results follow that impurities (Co and Fe) eliminate the first order phase transition peaks and spread the fluctuation maxima in such a way that its low temperature part effectively reaches the zero temperature, where the fluctuations inevitably become quantum. The behavior of low temperature branches of the heat capacity of the samples suggests that a gradual transition from classical to quantum fluctuations can be described by a simple power function of temperature with the exponent less than one. The $dρ/dT$ data generally support this suggestion. The values of the heat capacity exponents immediately lead to the diverging ratio $C_p/T$ and hence to the diverging effective electron mass. We found out that at large concentration of the dopant there are no distinct phase transition points. What we observe is a cloud of the helical fluctuations spreading over a significant range of concentrations and temperatures, which become quantum close to 0~K.

cond-mat.str-el↗

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

Physical properties of (Mn 0.85 Fe 0.15) Si along the critical trajectory

We report results of studying the magnetization, specific heat and thermal expansion of a single crystal with nominal composition Mn 0.85 Fe 0.15 Si . We found no thermodynamic evidences in favor a second order phase transition in the 15 % Fe substituted MnSi. The trajectory corresponding to the present composition of (MnFe)Si is a critical one, i.e. approaching quantum critical point at lowering temperature, but some properties may feel the cloud of helical fluctuations bordering the phase transition line.

cond-mat.str-el↗

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

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↗

arXiv2018

The Vollhardt crossing point at high magnetic field

Direct measurements of the Vollhardt crossing point coordinates were performed making use data of dilatometric and ultrasound experiments on MnSi. As is shown the crossing points are not invariant in the extended range of magnetic field and should be viewed as a side effects of flattening and spreading out the fluctuation maxima or minima by magnetic field~\cite{Sti16}. Correspondingly the crossing point can not be identified with a characteristic feature controlling the phase transition. So further studies are needed to understand intriguing features of the phase diagram of helical itinerant magnet MnSi.

cond-mat.str-el↗

arXiv2016

Notes on the Vollhardt "invariant" and phase transition in the helical itinerant magnet MnSi

In this paper we argue that rounded "hills" or "valleys" demonstrated by the heat capacity, thermal expansion coefficient, and elastic module are indications of a smeared second order phase transition, which is flattened and spread out by the application of a magnetic field. As a result, some of the curves which display a temperature dependence of the corresponding quantities cross almost at a single point. Thus, the Vollhardt crossing point should not be identified with any specific energy scale. The smeared phase transition in MnSi preceding the helical first order transition most probably corresponds to the planar ferromagnetic ordering, with a small or negligible correlation between planes. At lower temperatures, the system of ferromagnetic planes becomes correlated, acquiring a helical twist.

cond-mat.str-el↗

arXiv2016

Thermal expansion and magneto-volume studies of the itinerant helical magnet MnSi

Thermal expansion and forced magnetostriction of MnSi were measured as a function of temperature down to 5 K and magnetic field to 3 T. The small length (volume) discontinuity at the magnetic phase transition in MnSi decreases with application of magnetic field to a value $ΔL/L \sim 10^{-7}$, and then suddenly the discontinuity seemingly jumps to zero. Thermal expansivity peaks strongly deteriorate with magnetic fields. No specific features identifying a tricritical point were observed. We propose that the Frenkel concept of heterophase fluctuations may be relevant in the current case. Therefore, we suggest that the magnetic phase transition in MnSi always remains first order at any temperature and magnetic field, but the transition is progressively smoothed by heterophase fluctuations. These results question the applicability of a model of a fluctuation-induced first order phase transition for MnSi. Probably a model of coupling of an order parameter with other degrees of freedom is more appropriate.

cond-mat.str-el↗

arXiv2015

Superconductivity and Magnetism in Noncentrosymmetric RhGe

RhGe synthesized at high pressure is crystallized in noncentrosymmetric cubic structure of the B20 type. Measurements of the electrical resistivity and magnetization demonstrate a superconducting state below T$_c$ ~ 4.5 K and a weak ferromagnetism below T$_m$ ~ 140 K. Specific heat data confirm the bulk nature of superconductivity in this ferromagnetic superconductor. The superconducting region forms a dome on the P-T diagram with a maximum of T$_c$ near 4 GPa. Ab initio simulations suggest that the observed weak magnetization emerges from the pronounced spin polarization with magnetic quadrupole and toroidal moments located at Rh and Ge sites.

cond-mat.supr-con↗