arXiv subjects

V. Yu. Irkhin

Publications and source records attributed to V. Yu. Irkhin.

At least 73 records · Page 4Linked to original sources

arXiv2015

Laser spectroscopy of finite size and covering effects in magnetite nanoparticles

The experiments on the impact of the size of magnetite clusters on various magnetic properties (magnetic moment, Curie temperature, blocking temperature etc.) have been carried out. The methods of magnetic separation, centrifuging of water suspensions of biocompatible iron oxide nanoparticles (NPs) allow producing fractions with diameter of nanoparticles in the range of 4÷22 nm. The size of NPs are controlled by the methods of dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). For the first time the DLS method is applied in real time to control the size during the process of the separation of the NPs in aqueous suspensions. The changes of the size of NPs cause a shift in the Curie temperature and in the changes in the specific magnetic properties of the iron NPs. The experimental data is interpreted on the basis of Monte Carlo simulations for the classical Heisenberg model with different bulk and surface magnetic moments. It is demonstrated experimentally and by theoretical modeling that magnetic properties of magnetite NPs are determined not only by their sizes, but also by the their surface spin states, while both growing and falling dependences of the magnetic moment (per Fe3O4 formula unit) being possible, depending on the number of magnetic atoms in the nanoparticle. Both NPs clean and covered with a bioresorbable layer clusters have been investigated.

cond-mat.mtrl-sci↗

arXiv2015

Spiral magnetism in the single-band Hubbard model: the Hartree-Fock and slave-boson approaches

The ground-state magnetic phase diagram is investigated within the single-band Hubbard model for square and different cubic lattices. The results of employing the generalized non-correlated mean-field (Hartree-Fock) approximation and generalized slave-boson approach by Kotliar and Ruckenstein with correlation effects included are compared. We take into account commensurate ferromagnetic, antiferromagnetic, and incommensurate (spiral) magnetic phases, as well as phase separation into magnetic phases of different types, which was often lacking in previous investigations. It is found that the spiral states and especially ferromagnetism are generally strongly suppressed up to non-realistically large Hubbard $U$ by the correlation effects if nesting is absent and van Hove singularities are well away from the paramagnetic phase Fermi level. The magnetic phase separation plays an important role in the formation of magnetic states, the corresponding phase regions being especially wide in the vicinity of half-filling. The details of non-collinear and collinear magnetic ordering for different cubic lattices are discussed.

cond-mat.str-el↗

arXiv2015

Correlation Effects and Non-Collinear Magnetism in the Doped Hubbard Model

The ground--state magnetic phase diagram is investigated for the two-- and three--dimensional $t$--$t'$ Hubbard model. We take into account commensurate ferro--, antiferromagnetic, and incommensurate (spiral) magnetic phases, as well as phase separation into magnetic phases of different types, which was often missed in previous investigations. We trace the influence of correlation effects on the stability of both spiral and collinear magnetic order by comparing the results of employing both the generalized non-correlated mean--field (Hartree--Fock) approximation and generalized slave boson approach by Kotliar and Ruckenstein with correlation effects included. We found that the spiral states and especially ferromagnetism are generally strongly suppressed up to non-realistic large Hubbard $U$, if the correlation effects are taken into account. The electronic phase separation plays an important role in the formation of magnetic states and corresponding regions are wide, especially in the vicinity of half--filling. The details of magnetic ordering for different cubic lattices are discussed.

cond-mat.str-el↗

arXiv2014

Anomalous temperature and field behaviors of magnetization in cubic lattice frustrated ferromagnets

Thermodynamic properties of cubic Heisenberg ferromagnets with competing exchange interactions are considered near the frustration point where the coefficient $D$ in the spin-wave spectrum $E_{\mathbf{k}}\sim D k^{2}$ vanishes. Within the Dyson-Maleev formalism it is found that at low temperatures thermal fluctuations stabilize ferromagnetism by increasing the value of $D$. For not too strong frustration this leads to an unusual "concave" shape of the temperature dependence of magnetization, which is in agreement with experimental data on the europium chalcogenides. Anomalous temperature behavior of magnetization is confirmed by Monte Carlo simulation. Strong field dependence of magnetization (paraprocess) at finite temperature is found near the frustration point.

cond-mat.str-el↗

arXiv2014

Molecular magnetism and crystal field effects in the Kondo system Ce$_3$Pd$_{20}$(Si,Ge)$_6$ with two Ce sublattices

The unusual electronic and magnetic properties of the systems Ce$_3$Pd$_{20}$T$_6$ (T = Ge, Si) with two non-equivalent Ce positions are discussed. The logarithmic growth of resistance for both the systems confirms the presence of the Kondo effect in the two respective temperature ranges. The two-scale behavior is explained by consecutive splitting of Ce ion levels in the crystal field. The effects of the frustration caused by the coexistence of the different positions of cerium are treated, which may also significantly enhance the observed values of specific heat. A model of "molecular magnetism" with Ce2 cubes is developed.

cond-mat.str-el↗

arXiv2014

Pseudofermion ferromagnetism in the Kondo lattices: a mean-field approach

Ground state ferromagnetism of the Kondo lattices is investigated within slave fermion approach by Coleman and Andrei within a mean-field approximation in the effective hybridization model. Conditions for formation of both saturated (half-metallic) and non-saturated magnetic state are obtained for various lattices. A description in terms of universal functions which depend only on bare electron density of states (DOS) is presented. A crucial role of the energy dependence of the bare DOS (especially, of DOS peaks) for the small-moment ferromagnetism formation is demonstrated.

cond-mat.str-el↗

arXiv2014

Ferromagnetism and transport properties of the Kondo system Ce4Sb1.5Ge1.5

Ferromagnetic ordering with a small magnetic moment is found below 14 K from SQUID measurements for the compound Ce4Sb1.5Ge1.5. The transport characteristics of a number of Ce4Sb3-xTx (T = Ge, Si, Sn, Pb, Al) systems were measured at room temperature, Ce4Sb1.5Ge1.5 demonstrating the highest Seebeck coefficient. Its transport properties (resistivity, thermal conductivity, thermoelectric power) are experimentally investigated in detail up to low temperatures. The Kondo-lattice behavior of resistivity and its anomaly at the ordering point are found. A comparison with the Wiedemann-Franz law is performed.

cond-mat.str-el↗

arXiv2014

Anomalies of transport properties in antiferromagnetic YbMn2Sb2 compound

The low-temperature transport properties (resistivity, thermal conductivity, thermoelectric power) are experimentally investigated for the compound YbMn2Sb2. Large Seebeck coefficient and nearly linear temperature dependence of resistivity are obtained. The thermal conductivity is large and has mainly phonon origin. Appreciable anomalies of transport characteristics near the antiferromagnetic transition point are found. Possible influence of pseudo-Kondo scattering is discussed.

cond-mat.str-el↗

arXiv2013

Exchange Interactions in a Dinuclear Manganese (II) Complex with Cyanopyridine-N-oxide Bridging Ligands

The structure and magnetic properties of oligonuclear manganese complex [Mn(hfa)2cpo]2 (where hfa is hexafluoroacetylacetonate dehydrate and cpo is 4-cyanopyridine-N-oxide) are presented. In the unit cell, the dinuclear molecules are well isolated from each other. The non-monotonous dependence of magnetic susceptibility is explained in terms of the hierarchy of exchange parameters by using exact diagonalization. The thermodynamic behavior of pure cpo and [Mn(hfa)2(cpo)]2 is simulated numerically in a dimer approximation by an extrapolation to spin S = 5/2. The Mn-Mn exchange integral is evaluated.

cond-mat.mtrl-sci↗

arXiv2013

Magnetic properties of Gd3+ ions in the spatially distributed DNA molecules

Magnetic properties of DNA liquid-crystal dispersions are experimentally investigated by SQUID magnetometer. The magnetic susceptibility of pure DNA and DNA doped by La, and Gd is measured in the region 4.2-300 K. The total magnetic moment is represented as a sum of: the paramagnetic part and negative diamagnetic part. The number of paramagnetic Gd3+ ions is calculated in a good agreement with the number of phosphate complexes. The temperature dependence of magnetic susceptibility indicates the presence of interaction between Gd3+ magnetic moments, which is discussed in terms of long-range RKKY-type exchange in one-dimensional metals.

cond-mat.dis-nn↗

arXiv2013

Strong fluctuations near the frustration point in cubic lattice ferromagnets with localized moments

Thermodynamic properties of cubic Heisenberg ferromagnets with competing exchange interactions are considered near the frustration point where the coefficient $D$ in the spin-wave spectrum $E_{\mathbf{k}}\sim D k^{2}$ vanishes. Within the Dyson-Maleev formalism it is found that at low temperatures thermal fluctuations stabilize ferromagnetism by increasing the value of $D$. For not too strong frustration this leads to an unusual "concave" shape of the temperature dependence of magnetization, which is in agreement with experimental data on the europium chalcogenides. The phase diagram is constructed by means of Monte Carlo simulation, and suppression of magnetization and Curie temperature is found in comparison with the results of the spin-wave theory. This effect is explained by the the presence of non-analytical corrections to the spin-wave spectrum which are represented in the lowest order by the term $\sim (T/S)^{2} k^{2}\log{k}$.

cond-mat.str-el↗

arXiv2013

Anomalous ferromagnetism and non-Fermi-liquid behavior in the Kondo lattice CeRuSi2

The structural, electronic and magnetic properties of the Kondo-lattice system CeRuSi2 are experimentally investigated and analyzed in the series of other ternary cerium compounds. This system is shown to be an excellent model system demonstrating coexistence of the Kondo effect and anomalous ferromagnetism with a small magnetic moment which is confirmed by magnetic and μSR measurements. Data on specific heat, resistivity and Seebeck coefficient are presented. Being deduced from the resistivity and specific heat data, the non-Fermi-liquid behavior is observed at low temperatures, which is unusual for a ferromagnetic Kondo system. A comparison with other magnetic Kondo lattices is performed.

cond-mat.str-el↗

arXiv2013

Magnetism and Verwey transition in magnetite nanoparticles in thin polymer film

Magnetic and structural properties of magnetite nanoparticles stabilized in polyvinyl-alcohol thin films are investigated by using X-ray diffraction (XRD), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) and static magnetometry techniques. The nanoparticles have well-defined crystallinity, and are superparamagnetic at room temperature. Their size distribution is characterized by the distinct log-normal law (with average diameters near 5-7 nm) and slight maximum near 70-80 nm. The EPR spectra and static magnetization data demonstrated pronounced anomalies in the interval between 130 K (corresponding to Verwey transition) and 200 K. The experimental data obtained can be understood on the basis of the half-metallic electronic structure, complex temperature behavior of the magnetic anisotropy, along with effects of "weak magnetic-electron" sublattice of magnetite.

cond-mat.mtrl-sci↗

arXiv2012

Thermoelectricity in ternary rare-earth systems

Crystallographic data, Seebeck coefficient, electrical resistance and thermal conductivity are reported for a large number of rare-earth compounds, manifestations of the Kondo effect being discussed. In more detail, thermoelectric properties of Yb3Co4Ge13 and Yb3Co4Sn13 compounds and Yb2CeCo4Ge13 and Yb2.3La0.7Co4Ge13 solid solutions are presented.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic fluctuations and itinerant ferromagnetism in two-dimensional systems with van Hove singularities

We use the quasistatic approach to analyze the criterion of ferromagnetism for two-dimensional (2D) systems with the Fermi level near Van Hove singularities (VHS) of the electronic spectrum. It is shown that the spectrum of spin excitations (paramagnons) is positively defined when the interaction I between electrons and paramagnons, which corresponds to the Hubbard on-site repulsion U, is sufficiently large. The critical interactions I_c and U_c remain finite at Van Hove filling and exceed considerably their values obtained from the Stoner criterion due to incommensurate spin fluctuations which are important near the ferromagnetic quantum phase transition. Combining the quasistatic approximation and the equation-of -motion method for the Green functions we obtain the results for the electronic self-energy to first order in the inverse number of spin components.

cond-mat.str-el↗

arXiv2012

Thermoelectric properties of Pr3Rh4Sn13-type Yb3Co4Ge13 and Yb3Co4Sn13 compounds

Crystallographic data and thermoelectric properties (from 240 K up to 380 K) of Yb3Co4Ge13, Yb3Co4Sn13 compounds and Yb2CeCo4Ge13 and Yb2.3La0.7Co4Ge13 solid solutions are reported. The Seebeck coefficients, electric resistance and thermal conductivity increase monotonically with increasing temperature from 240 to 380 K for all the compounds. The Seebeck coefficient is S = 14 - 27 μV//K for Yb3Co4Sn13, and S = -21 -- -12 μV/K for Yb3Co4Ge13. The substitution of Yb for cerium or lanthanum in Yb3Co4Ge13 shifts the Seeebeck coefficient to positive values. The Yb3Co4Sn13 has a maximal ZT parameter from available Pr3Rh4Sn13-type compounds. The ZT parameter of Yb3Co4Sn13 compound increases from 0.006 up to 0.017 with increasing temperature from 240 K to 380 K.

cond-mat.mtrl-sci↗

arXiv2011

The Kondo-lattice state and non-Fermi-liquid behavior in the presence of Van Hove singularities

A scaling consideration of the Kondo lattices is performed with account of logarithmic Van Hove singularities (VHS) in the electron density of states. The scaling trajectories are presented for different magnetic phases. It is demonstrated that VHS lead to a considerable increase of the non-Fermi-liquid behavior region owing to softening of magnon branches during the renormalization process. Although the effective coupling constant remains moderate, renormalized magnetic moment and spin-fluctuation frequency can decrease by several orders of magnitude. A possible application to f-systems and weak itinerant magnets is discussed.

cond-mat.str-el↗

arXiv2011

Evolution of the spectrum and the metal-insulator transition in local approximations for many-electron models

In the framework of the many-electron s-d exchange model and Hubbard model, self-consistent equations are derived for the one-particle retarded Green's function in the many-electron Hubbard X-operator representation. We analyze the general structure of the single-site approximations and their connection with the coherent potential approximation (CPA) and dynamic effective field theory (DMFT). Using the self-consistent approximation, we examine in detail the picture of the evolution of the electron spectrum with the model parameters (coupling constants, the concentration of charge carriers). The influence of various factors (Kondo many-electron scattering, smearing due to damping, dynamics of localized moment subsystem) on the shape of the density of states N(E) in the interacting system is investigated. It is shown that the use of the locator representation allows to avoid in some cases the non-analyticity in approximate expressions for the Green's functions. Our approach enables one to reproduce, at certain values of the parameters, three-peak structure of N(E) near the metal-insulator transition.

cond-mat.str-el↗