arXiv subjects

V. N. Narozhnyi

Publications and source records attributed to V. N. Narozhnyi.

17 recordsLinked to original sources

arXiv2015

Magnetic Single Domain State of the Monochiral Helimagnet MnSi in Zero Field Limit: Magnetic Properties Study

Attention is drawn to a possibility to obtain the monochiral helimagnet MnSi in a magnetic single domain state (SDS) in zero magnetic field limit. It is shown that this metastable zero field magnetic SDS can be achieved by gradual decrease of the field down to zero after initial transformation of MnSi to a spin-polarized state in high fields $H$. This can be achieved only for $\bf{H}\parallel$ [111]. Investigations of MnSi in magnetic SDS give us a possibility to determine the two components of low field magnetic susceptibility of this compound, $χ_{\perp}(T)$ and $χ_{\parallel}(T)$ [$χ_{\perp}$ and $χ_{\parallel}$ correspond to $\bf{H}\perp$, and $\bf{H}\parallel$ (111)-plane containing magnetic moments in helically ordered state]. These results are compared with macroscopic magnetic susceptibility of MnSi earlier determined for this compound only in magnetic multi domain state (MDS). In addition our results are compared with the data reported for some non-monochiral helimagnets. Characteristic features of monochiral helimagnets are elucidated.

cond-mat.str-el↗

arXiv2014

Specific Features of Magnetic Phase Diagram of an MnSi Helimagnet

Magnetization curves of MnSi single crystals have been measured in a range of temperatures $T = 5.5 - 35$ K and magnetic field strengths $H \leq 11$ kOe for $H \parallel$ [111], [001], and [110]. Special attention has been paid to the temperature interval near $T_N = 28.8$K, where MnSi exhibits a transition to the state with a long-period helical magnetic structure. Some new features in the magnetic behavior of MnSi have been found. In particular, in an intermediate temperature region above the transition (28.8 K $= T_N \leq T < 31.5$ K), the $dM(H)/dH$ curves exhibit anomalies that are not characteristic of the typical paramagnetic state. It is established that the line of the characteristic field $H^ \ast (T)$ of this anomaly is a natural extrapolation of the temperature dependence of the field of the transition from a conical phase to an induced ferromagnetic phase observed at $T < T_N$. It is concluded that the properties of MnSi in the indicated intermediate region are related to and governed by those of the conical phase (rather than of the A phase). Based on these data, magnetic phase diagrams of MnSi for $H \parallel$ [111], [001], and [110] are plotted and compared to diagrams obtained earlier by other methods.

cond-mat.str-el↗

arXiv2014

Studying the Magnetic Properties of CoSi Single Crystals

The magnetic properties of CoSi single crystals have been measured in a range of temperatures $T = 5.5 - 450$ K and magnetic field strengths $H \leq 11$ kOe. A comparison of the results for crystals grown in various laboratories allowed the temperature dependence of magnetic susceptibility $χ(T) = M(T)/H$ to be determined for a hypothetical "ideal" (free of magnetic impurities and defects) CoSi crystal. The susceptibility of this ideal crystal in the entire temperature range exhibits a diamagnetic character. The $χ(T)$ value significantly increases in absolute value with decreasing temperature and exhibits saturation at the lowest temperatures studied. For real CoSi crystals of four types, paramagnetic contributions to the susceptibility have been evaluated and nonlinear (with respect to the field) contributions to the magnetization have been separated and taken into account in the calculations of $χ(T)$.

cond-mat.str-el↗

arXiv2013

Comment on "Self-doping effects in cobalt silicide CoSi: Electrical, magnetic, elastic, and thermodynamic properties"

In this Comment it is argued that Stishov et al. [Phys. Rev. B 86, 064433 (2012)] incorrectly estimated concentrations of (supposed) paramagnetic centers with μ_eff = 4.8μ_B in the investigated CoSi crystals. Correct estimation gives concentrations of such centers from 25 to 50 times smaller than reported (appr. 0.04 to 0.16% instead of appr. 2 to 4%). Also the reported data on temperature dependence of resistivity ρ(T) of four CoSi crystals prepared in different Labs are so close to each other at T = 250 to 300K that it is extremely unlikely to be reproducible for any reasonable accuracy of resistivity measurements. These and some other problems of the paper are related to the key points of the authors argumentation. As a result their main conclusions become unjustified.

cond-mat.str-el↗

arXiv2004

Evidence for strong electron-phonon coupling in MgCNi_3

The title compound is investigated by specific heat measurements in the normal and superconducting state supplemented by upper critical field transport, susceptibility and magnetization measurements. From a detailed analysis including also full potential electronic structure calculations for the Fermi surface sheets, Fermi velocities and partial densities of states the presence of both strong electron-phonon interactions and considerable pair-breaking has been revealed. The specific heat and the upper critical field data can be described to first approximation by an effective single band model close to the clean limit derived from a strongly coupled predominant hole subsystem with small Fermi velocities. However, in order to account also for Hall-conductivity and thermopower data in the literature, an effective general two-band model is proposed. This two-band model provides a flexible enough frame to describe consistently all available data within a scenario of phonon mediated s-wave superconductivity somewhat suppressed by sizeable electron-paramagnon or electron-electron Coulomb interaction. For quantitative details the relevance of soft phonons and of a van Hove type singularity in the electronic density of states near the Fermi energy is suggested.

cond-mat.supr-con↗

arXiv2002

Anisotropic magnetic behavior of GdBa_2Cu_3O_{6+y} single crystals

Magnetic properties of high-quality Al-free nonsuperconducting GdBa_2Cu_3O_{6+y} single crystals grown by flux method have been studied. The magnetic anisotropy below the Néel temperature T_N~2.3K corresponds to the direction of Gd^{3+} magnetic moments along the tetragonal c-axis. At T < T_N clear indications of spin-flop transitions for H||c have been observed on magnetization curves at H_{sf}~10kOe. Magnetic phase diagrams have been obtained for H||c as well as for H||ab. A pronounced anisotropy in the magnetic susceptibility (unexpected for Gd-based compounds) has been found above T_N.

cond-mat.mtrl-sci↗

arXiv2002

MgCNi3 : Complex behavior in a strongly coupled superconductor

Polycrystalline samples of the recently discovered MgCNi3 superconductor were investigated by transport, ac susceptibility, dc magnetization and specific heat measurements in magnetic fields up to 16 T. Consistent results were obtained for the temperature dependence of the upper critical field H_c2(T) from resistance, ac susceptibility and specific heat measurements. A WHH like temperature dependence of H_c2(T) and the quadratic relationship H_c2(0) ~ T_c^2 point to an effective predominant single band behavior near the quasi clean limit. Evidence for strong electron-phonon and electron-paramagnon coupling was found analyzing the specific heat data. The corresponding s- and p-wave scenarios are briefly discussed using calculated densities of states of different Fermi surface sheets.

cond-mat.supr-con↗

arXiv2002

Comparative Study of Dense Bulk MgB$_2$ Materials Prepared by Different Methods

We report on the results of a comparative investigation of highly dense bulk MgB$_2$ samples prepared by three methods: (i) hot deformation; (ii) high pressure sintering; and (iii) mechanical alloying of Mg and B powders with subsequent hot compaction. All types of samples were studied by ac-susceptibility, dc-magnetization and resistivity measurements in magnetic fields up to $μ_0H=160$ kOe. A small but distinct anisotropy of the upper critical field $H_{c2}^{a,b}/H_{c2}^{c}\sim1.2$ connected with some texture of MgB$_2$ grains was found for the hot deformed samples. The samples prepared by high pressure sintering as well as by mechanical alloying show improved superconducting properties, including high upper critical fields $H_{c2}$ ($μ_0H_{c2}(0)\sim23$ T), irreversibility fields $H_{irr}$ which are strongly shifted towards higher values $H_{irr}(T)\sim0.8H_{c2}(T)$ and high critical current $J_c$ ($J_c=10^5$ A/cm$^2$ at 20 K and 1 T).

cond-mat.supr-con↗

arXiv2001

Fully dense MgB_2 superconductor textured by hot deformation

Bulk textured MgB_2 material of nearly full density showing a weak c-axis alignment of the hexagonal MgB_2 grains parallel to the pressure direction was obtained by hot deformation of a stoichiometric MgB_2 pellet prepared by a gas-solid reaction. The texture of the material was verified by comparing the x-ray diffraction patterns of the hot deformed material with isotropic MgB_2 powder. A small, but distinct anisotropy of the upper critical field up to Hc2^{a,b}/Hc2^{c}~1.2 depending on degree of texture was found by resistance and susceptibility measurements. No anisotropy of the critical current density determined from magnetization measurements was found for the textured material.

cond-mat.supr-con↗

arXiv2001

Upper critical field and irreversibility line in superconducting MgB_2

The upper critical field Hc2(T) of sintered pellets of the recently discovered MgB_2 superconductor was investigated by transport, ac susceptibility and dc magnetization measurements in magnetic fields up to 16 T covering a temperature range between Tc ~ 39 K and T = 3 K ~ 0.1Tc. The temperature dependence of the upper critical field, Hc2(T), shows a positive curvature near Tc similar to that found for the borocarbides YNi_2B_2C and LuNi_2B_2C indicating that MgB_2 is in the clean limit. The irreversibility line was consistently determined from dc magnetization measurements and from the imaginary component of ac susceptibility. The irreversibility field was found to increase up to 8.5 T at 10 K.

cond-mat.supr-con↗

arXiv2001

The upper critical field in superconducting MgB_2

The upper critical field Hc2(T) of sintered pellets of the recently discovered MgB_2 superconductor was investigated in magnetic fields up to 16 T. The upper critical field of the major fraction of the investigated sample was determined from ac susceptibility and resistance data and was found to increase up to Hc2(0) = 13 T at T = 0 corresponding to a coherence length of 5.0 nm. A small fraction of the sample exhibits higher upper critical fields which were measured both resistively and by dc magnetization measurements. The temperature dependence of the upper critical field, Hc2(T), shows a positive curvature near Tc and at intermediate temperatures. This positive curvature of Hc2(T) is similar to that found for the borocarbides YNi_2B_2C and LuNi_2B_2C indicating that MgB_2 is in the clean limit.

cond-mat.supr-con↗

arXiv1999

Magnetic properties of Gd_{1-x}Pr_xBa_2Cu_3O_{7-y} single crystals

Magnetic properties were studied for the high quality Al-free orthorhombic Gd_{1-x}Pr_xBa_2Cu_3O_{7-y} single crystals (0<=x<=1.0) grown by the flux method. An indication on the interaction between the Pr and Cu(2) magnetic sublattices was found for Pr123. Different signs of magnetic anisotropy were established for the Pr and Gd ions at low temperatures. It was also shown that superconductivity reported by Zou et al. [Phys. Rev. Lett. 80 (1998) 1074] for Pr123 single crystals grown by TSZF method seems to be connected with partial substitution of Ba for the Pr-sites.

cond-mat.supr-con↗

arXiv1999

PrBa_2Cu_3O_{7-y}: Superconducting or Anomalously Magnetic?

In PrBa_2Cu_3O_{7-y} (Pr123) single crystals grown by the flux method the kink in the magnetic susceptibility chi_{ab}(T), connected with AFM ordering of Pr, disappears after field cooling (FC) in a field H||ab-plane whereas the kink in chi_c(T) remains unchanged after FC in H||c-axis. This seems to be connected with the coupling between the Pr and Cu(2) sublattices. The Curie constant C determined from the data reported for superconducting Pr123 crystals grown by traveling-solvent floating zone (TSFZ) method (Zou et al, Phys. Rev. Lett., 80, 1074 (1998)) is about one half of that for our flux crystals. Thus, the concentration of Pr in TSFZ crystal seems to be about one half of the nominal for Pr123. Therefore, superconductivity in TSFZ samples is connected most probably with the partial substitution of Pr by nonmagnetic Ba.

cond-mat.supr-con↗

arXiv1999

Hall-effect in LuNi_2B_2C and YNi_2B_2C borocarbides: a comparative study

The Hall effect in LuNi_2B_2C and YNi_2B_2C borocarbides has been investigated in normal and superconducting mixed states. The Hall resistivity rho_{xy} for both compounds is negative in the normal as well as in the mixed state and has no sign reversal below T_c typical for high-T_c superconductors. In the mixed state the behavior of both systems is quite similar. The scaling relation rho_{xy}\simρ_{xx}^β(ρ_{xx} is the longitudinal resistivity) was found with β=2.0 and 2.1 for annealed Lu- and Y-based compounds, respectively. The scaling exponent βdecreases with increasing degree of disorder and can be varied by annealing. This is attributed to a variation of the strength of flux pinning. In the normal state weakly temperature dependent Hall coefficients were observed for both compounds. A distinct nonlinearity in the ρ_{xy} dependence on field H was found for LuNi_2B_2C in the normal state below 40K, accompanied by a large magnetoresistance (MR) reaching +90% for H=160kOe at T=20K. At the same time for YNi_2B_2C only linear ρ_{xy}(H) dependences were observed in the normal state with an approximately three times lower MR value. This difference in the normal state behavior of the very similar Lu- and Y-based borocarbides seems to be connected with the difference in the topology of the Fermi surface of these compounds.

cond-mat.supr-con↗

arXiv1998

Hall-effect in LuNi_2B_2C in normal and superconducting mixed states

The Hall resistivity rho_{xy} of LuNi_2B_2C is negative in the normal as well as in the mixed state and has no sign reversal typical for high-T_c superconductors. A distinct nonlinearity in the rho_{xy} dependence on field H was found in the normal state for T < 40K, accompanied by a large magnetoresistance reaching +90% for mu_0H=16T at T=20K. The scaling relation rho_{xy} ~ ρ_{xx}^β(ρ_{xx} is the longitudinal resistivity) was found in the mixed state, the value of βbeing dependent on the degree of disorder.

cond-mat.supr-con↗

arXiv1998

Anomalous magnetic ordering in PrBa_2Cu_3O_{7-y} single crystals: Evidence for magnetic coupling between the Cu and Pr sublattices

In Al-free PrBa_2Cu_3O_{7-y} single crystals the kink in the temperature dependence of magnetic susceptibility chi_{ab}(T), connected with Pr antiferromagnetic ordering, disappears after field cooling (FC) in a field H || ab-plane. The kink in chi_c(T) remains unchanged after FC in H || c-axis. As a possible explanation, freezing of the Cu magnetic moments, lying in the ab-plane, caused by FC in H || ab, hinders their reorientation and, due to coupling between the Pr and Cu(2) sublattices, ordering of the Pr^{3+} moments. A field induced phase transition and a field dependence of the Pr^{3+} ordering temperature have been found for both H || c and H || ab.

cond-mat.supr-con↗

arXiv1998

Comment on ``Superconducting PrBa_2Cu_3O_x''

Recently, Zou et al. (Phys. Rev. Lett. 80, 1074, 1998) reported the observation of bulk superconductivity (SC) for a PrBa_2Cu_3O_x (Pr123) single crystal grown by the traveling-solvent floating zone (TSFZ) method. The aim of this Comment is to show the inconsistency of the value of effective magnetic moment μ_{eff} reported by Zou et al. (2.92μ_B) with their magnetic susceptibility data. The estimation made directly from their data points gives a considerably smaller value of μ_{eff}=2.09μ_B. At the same time the values of mu_{eff}=2.9μ_B and 3.1μ_B were obtained for our Pr123 single crystals grown by flux method for H||ab-plane and H||c-axis, respectively. This suggests that Pr occupies only about a half of the RE sites in TSFZ crystal. The other half of the RE sites is occupied most probably by the nonmagnetic Ba. Noteworthy, SC with T_c=43 K was observed earlier for Pr_{0.5}Ca_{0.5}Ba_2Cu_3O_{7-y} thin films. Ba^{2+} has a larger ionic radius than Pr^{3+} and so the substitution of Ba for Pr could give a natural explanation not only for the SC in TSFZ Pr123 but also for the elongation of the distance between the CuO_2 planes observed by Zou et al.

cond-mat.supr-con↗