arXiv subjects

S. V. Grigoriev

Publications and source records attributed to S. V. Grigoriev.

16 recordsLinked to original sources

arXiv2024

Stability boundaries of the skyrmion phase in non-centrosymmetric ferromagnets with Dzyaloshinskii-Moriya interaction

Stability boundaries of the skyrmion lattice in non-centrosymmetric bulk ferromagnets with the Dzyaloshinskii-Moriya interaction in external magnetic field are discussed. We compare the classical energies of the spin configuration of the conical helix and skyrmion lattice within the framework of the stereographic projection approach. It is well known that at low temperatures the skyrmion lattice loses energetically to the conical helix in the entire range of fields, $0 < H < H_{c2}$, where $H_{c2}$ is the transition field to the polarized collinear phase, and $gμ_B H_{c2} \ll T_c$. We show that taking into account the dipole interaction does not qualitatively change the situation. However, the possibility of fluctuations in the absolute value of the equilibrium local magnetization in the Ginzburg-Landau functional leads, with increasing temperature, $T$, to the skyrmion lattice becoming energetically more favorable than the conical helix in a certain range of fields. We show that it occurs already in the first order of small parameter, $\propto gμ_B H_{c2}/|T-T_c|$, at the level of mean field theory.

cond-mat.str-el↗

arXiv2022

Spin wave stiffness and damping in a frustrated chiral helimagnet Co$_8$Zn$_8$Mn$_4$ as measured by small-angle neutron scattering

Multiple intriguing low temperature phenomena have recently been discovered in the family of chiral cubic Co-Zn-Mn compounds with $β-$Mn-type structure. In particular, Co$_8$Zn$_8$Mn$_4$ displays a reduction of the helical spiral pitch on cooling, along with lattice shape transformations of metastable skyrmions and the manifestation of peculiar magnetic textures due to strong magnetocrystalline anisotropy. Here we report on temperature-dependent measurements of helimagnon excitations in the field polarized regime Co$_8$Zn$_8$Mn$_4$ using the spin wave small-angle neutron scattering (SWSANS) technique. By applying a new analytical expression to interpret the data, quantitative estimates for both spin wave stiffness and damping are extracted across a wide temperature range between 70 K and 250 K. We speculate that their non-trivial temperature-dependencies arise due to the effects of magnetic frustration arising from Mn magnetic moments, which is further reflected in continuous variations of both exchange and Dzyaloshinskii-Moriya interactions.

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↗

arXiv2019

Coherent charge and magnetic ordering in Ho/Y superlattice revealed by element-selective x-ray scattering

Magnetic rare-earth / non-magnetic metal superlattices are well-known to display chiral spin helices in the rare-earth layers that propagate coherently across the non-magnetic layers. However, the underlying mechanism that preserves the magnetic phase and chirality coherence across the non-magnetic layers has remained elusive. In this Letter, we use resonant and element-specific x-ray scattering to evidence directly the formation of two fundamentally different long-range modulations in a Holmium/Yttrium (Ho/Y) multilayer: the known Ho chiral spin helix with periodicity 25 Å, and a newly observed charge density wave with periodicity 16 Å that propagates through both the Ho and non-magnetic Y layer. With x-ray circular magnetic dichroism measurements ruling out the existence of a magnetic proximity effect induced moment in the non-magnetic Y layers, we propose that the charge density wave is also chiral, thus providing the means for the transmittance of magnetic chirality coherence between Ho layers.

cond-mat.mes-hall↗

arXiv2019

Morphology and magnetic properties of nanocomposite magnetic multilayers {[(Co$_{40}$Fe$_{40}$B$_{20}$)$_{34}$(SiO$_2$)$_{66}$]/[C]}$_{47}$

We report on the investigation of morphology, magnetic and conductive properties of the mutilayered nanostructures [(Co$_{40}$Fe$_{40}$B$_{20}$)$_{34}$(SiO$_2$)$_{66}$]/[C]$_{47}$ consisting of the contacting magnetic (Co$_{40}$Fe$_{40}$B$_{20}$)$_{34}$(SiO$_2$)$_{66}$ nanocomposite and amorphous semiconductor carbon C layers. It is shown by Grazing-Incidence Small-Angle X-ray Scattering method that the ordering and the size of nanoparticles in the magnetic layers do not change profoundly with increasing of carbon layer thickness. Meanwhile, the electrical conductance and the magnetic properties are significantly varied: resistance of the samples changes by four orders of magnitude and superparamagnetic blocking temperature changes from 15 K to 7 K with the increment of carbon layer thickness $h_c$ from 0.4 nm to 1.8 nm. We assume that the formation of the homogeneous semiconductor interlayer leads to modification of the metal-insulator growth process that drives the changes in the magnetic and conductive properties.

cond-mat.mtrl-sci↗

arXiv2018

Magnetic structure of the promising candidate for three-dimensional artificial spin ice: small angle neutron diffraction and micromagnetic simulations

Geometrical frustration arised in spin ices leads to fascinating emergent physical properties. Nowadays there is a wide diversity of the artificial structures, mimicking spin ice at the nanoscale and demonstrating some new effects. Most of the nanoscaled spin ices are two dimensional. Ferromagnetic inverse opal-like structures (IOLS) are among inspiring examples of the three-dimensional system exhibiting spin ice behaviour. However detailed examination of its properties is not straightforward. Experimental technique which is able to unambiguously recover magnetization distribution in 3D mesoscaled structures is lacking. In this work we used an approach based on complementary exploiting of small-angle neutron diffraction technique and micromagnetic simulations. External magnetic field was applied along three main directions of the IOLS mesostructure. Comparison of the calculated and measured data allowed us to determine IOLS magnetic state. The results are in good agreement with the spin ice model. Moreover influence of the demagnetizing field and vortex states on the magnetizing process were revealed. Additionally, we speculate that this approach can be also applied to other 3D magnetic mesostructures.

cond-mat.mes-hall↗

arXiv2016

Spin-wave dynamics in FeGe helimagnet: studied by small-angle neutron scattering

We have studied the spin-wave stiffness of the Dzyaloshinskii-Moriya helimagnet FeGe in a temperature range from 225~K up to $T_C \approx$~278.7~K by small-angle neutron scattering. The method we have used is based on [S. V. Grigoriev et al. Phys. Rev. B \textbf{92} 220415(R) (2015)] and was extended here for the application in polycrystalline samples. We confirm the validity of the anisotropic spin-wave dispersion for FeGe caused by the Dzyaloshinskii-Moriya interaction. We have shown that the spin-wave stiffness $A$ for FeGe helimagnet decreases with a temperature as $A(T) = 194(1-0.7(T/T_C)^{4.2})$ meVÅ$^2$. The finite value of the spin-wave stiffness $A = 58$ meVÅ$^2$ at $T_C$ classifies the order-disorder phase transition in FeGe as being the first order one.

cond-mat.str-el↗

arXiv2016

Magnetization of the Mn$_{1-x}$Fe$_x$Si in high magnetic field up to 50 T: possible evidence of a field-induced Griffiths phase

Magnetic properties of single crystals of Mn$_{1-x}$Fe$_x$Si solid solutions with $x < 0.2$ are investigated by pulsed field technique in magnetic fields up to 50 T. It is shown that magnetization of Mn$_{1-x}$Fe$_x$Si in the paramagnetic phase follows power law $M(B) \sim B^α$ with the exponents $α\sim 0.33-0.5$, which starts above characteristic fields $B_c \sim 1.5-7$ T depending on the sample composition and lasts up to highest used magnetic field. Analysis of magnetization data including SQUID measurements in magnetic fields below 5 T suggests that this anomalous behavior may be likely attributed to the formation of a field-induced Griffiths phase in the presence of spin-polaron effects.

cond-mat.str-el↗

arXiv2016

Spin-ice behavior of three-dimensional inverse opal-like magnetic structures: micromagnetic simulations

We perform micromagnetic simulations of the magnetization distribution in inverse opal-like structures (IOLS) made from ferromagnetic materials (nickel and cobalt). It is shown that the unit cell of these complex structures, whose characteristic length is approximately 700 nm, can be divided into a set of structural elements some of which behave like Ising-like objects. A spin-ice behavior of IOLS is observed in a broad range of external magnetic fields. Numerical results describe successfully the experimental hysteresis curves of the magnetization in Ni- and Co-based IOLS. We conclude that ferromagnetic IOLS can be considered as the first realization of three-dimensional artificial spin ice.

cond-mat.mes-hall↗

arXiv2015

Thermal expansion and compressibility of monogermanides of 3d-metals

Synchrotron diffraction as a function of temperature and pressure, specific heat, magnetic susceptibility and small-angle neutron scattering experiments have revealed an anomalous response of MnGe. Similar but less pronounced behavior has also been observed in Mn$_{1-x}$Co$_x$Ge and Mn$_{1-x}$Fe$_x$Ge solid solutions. Spin density fluctuations and Mn spin state instability are discussed as possible candidates for the observed effects.

cond-mat.mtrl-sci↗

arXiv2015

Scrutinizing Hall effect in Mn$_{1-x}$Fe$_{x}$Si: Fermi surface evolution and hidden quantum criticality

Separating between ordinary (OHE) and anomalous (AHE) Hall effect in the paramagnetic phase of Mn$_{1-x}$Fe$_{x}$Si reveals OHE sign inversion associated with the hidden quantum critical (QC) point $x^*\sim0.11$. The semimetallic behavior at intermediate Fe content leads to verifiable predictions in the field of fermiology, magnetic interactions and QC in Mn$_{1-x}$Fe$_{x}$Si. The change of electron and hole concentrations is considered as a driving force for tuning the QC regime in Mn$_{1-x}$Fe$_{x}$Si via modifying of RKKY exchange interaction within the Heisenberg model of magnetism.

cond-mat.str-el↗

arXiv2015

The Dzyaloshinskii-Moriya interaction is under control: an orchestrated flip of the chiral link between structure and magnetism for Fe$_{1-x}$Co$_x$Si

Monosilicides of 3d-metals frequently show a chiral magnetic ordering with the absolute configuration defined by the chirality of the crystal structure and the sign of the Dzyaloshinskii-Moriya interaction (DMI). Structural and magnetic chiralities are probed here for Fe$_{1-x}$Co$_x$Si series and their mutual relationship is found to be dependent on the chemical composition. The chirality of crystal structure was previously shown to be governed by crystal growth, and the value of the DMI is nearly the same for all monosilicides of Fe, Co and Mn. Our findings indicate that the sign of the DMI in Fe$_{1-x}$Co$_x$Si is controlled by the Co composition $x$, thus, opening a route towards controlled design of chiral spintronics devices.

cond-mat.mtrl-sci↗

arXiv2014

Universal Ratio of Intrinsic Resistivities of Spin Helix in B20 (Fe-Co)Si Magnets

The B20 magnets with the Dzyaloshinskii-Moriya (D-M) interaction exhibit spin helix and Skyrmion spin textures unattainable in traditional Heisenberg ferromagnets. We have determined the intrinsic resistivity of the spin helix, which is a macroscopic Bloch domain wall, in B20 (Fe-Co)Si magnets. We found a universal resistance ratio of gamma = 1.35 with current parallel and perpendicular to the helix, independent of composition and temperature. This gamma value is much smaller than 3, the well-known minimum value for domain wall resistivity in traditional ferromagnets, due to the significant spin-orbit coupling in the B20 magnets.

cond-mat.mtrl-sci↗

arXiv2014

Magnetization scaling in the paramagnetic phase of Mn1-xFexSi solid solutions

The magnetization field and temperature dependences in the paramagnetic phase of Mn1-xFexSi solid solutions with x<0.3 are investigated in the range B<5 T and T<60 K. It is found that field dependences of the magnetization M(B,T=const) exhibit scaling behavior of the form B\partial M/\partial B-M=F(B/(T-Ts)), where Ts denotes an empirically determined temperature of the transition into the magnetic phase with fluctuation driven short-range magnetic order and F(\c{hi}) is a universal scaling function for given composition. The scaling relation allowed concluding that the magnetization in the paramagnetic phase of Mn1-xFexSi is represented by the sum of two terms. The first term is saturated by the scaling variable \c{hi}=B/(T-Ts), whereas the second is linearly dependent on the magnetic field. A simple analytical formula describing the magnetization is derived and applied to estimates of the parameters characterizing localized magnetic moments in the studied system. The obtained data may be qualitatively interpreted assuming magnetic inhomogeneity of the paramagnetic phase on the nanoscale.

cond-mat.str-el↗

arXiv2011

Chiral criticality in doped Mn$_{1-y}$Fe$_y$Si compounds

The critical spin fluctuations in doped compounds Mn$_{1-y}$Fe$_y$Si have been studied by means of ac-susceptibility measurements, polarized neutron small angle scattering and spin echo spectroscopy. It is shown that these compounds undergo the transition from the paramagnetic to helimagnetic phase through continuous, yet well distinguishable crossovers: (i) from paramagnetic to partially chiral, (ii) from partially chiral to highly chiral fluctuating state. The crossover points are identified on the basis of combined analysis of the temperature dependence of ac-susceptibility and polarized SANS data. The whole transition is marked by two inflection point of the temperature dependence of ac-susceptibility: the upper one corresponds to the crossover to partially chiral state at $T^*$, where the inverse correlation length $κ\approx 2 k$, the lower one corresponds to the transition to the spin helix structure. The intermediate crossover to the highly chiral phase is observed at the inflection point $T_k$ of the first derivative of ac-susceptibility, where $κ\approx k$. The temperature crossovers to the highly chiral fluctuating state is associated with the enhancing influence of the Dzyaloshinskii-Moria interaction close to $T_c$.

cond-mat.mtrl-sci↗

arXiv2006

Magnon Bose condensation in symmetry breaking magnetic field

Magnon Bose condensation (BC)in the symmetry breaking magnetic field is a result of unusual form of the Zeeman energy, which has terms linear in the spin-wave operators and terms mixing excitations differ in the Wave-vector of the magnetic structure. The following examples are considered: simple easy-plane tetragonal antiferromagnets (AF), frustrated AF family$R_2Cu O_4$ where $R=Pr,Nd$ etc. and cubic magnets with the Dzyaloshinskii-Moriya interaction ($Mn Si$ etc.). In all cases the BC becomes important when the magnetic field becomes comparable with the spin-wave gap. The theory is illustrated by existing experimental results.

cond-mat.str-el↗