arXiv subjects

R. Pelka

Publications and source records attributed to R. Pelka.

8 recordsLinked to original sources

arXiv2016

Magnetism of a sigma-phase Fe60V40 alloy: magnetic susceptibilities and magnetocaloric effect studies

Magnetic properties of a sigma-phase Fe60V40 intermetallic compound were studied by means of ac and dc magnetic susceptibility and magnetocaloric effect measurements. The compound is a soft magnet yet it was found to behave like a re-entrant spin-glass system. The magnetic ordering temperature was found to be T_C ca.170 K, while the spin-freezing temperature was ca.164 K. Its relative shift per decade of ac frequency was 0.002, a value smaller than that typical of canonical spin-glasses. Magnetic entropy change, DeltaS, in the vicinity of T_C was determined for magnetic field, H, ranging between 5 and 50 kOe. Analysis of DeltaS in terms of the power law yielded the critical exponent, n, vs. temperature with the minimum value of 0.75 at T_C, while from the analysis of a relative shift of the maximum value of DeltaS with the field a critical exponent Delta=1.7 was obtained. Based on scaling laws relationships values of other two exponents viz. betha=0.6 and gamma=1 were determined.

cond-mat.str-el↗

arXiv2013

Full susceptibility tensor for localized spin models with S=1, 3/2, 2, 5/2 and with rhombic anisotropy

A general discussion of the simulation procedure of the full susceptibility tensor and isothermal magnetization pseudovector for compounds comprising weakly-interacting magnetic centers is presented. A single-crystal-sample as well as a powder-sample case are considered. The procedure is used to obtain explicit expressions for the full susceptibility tensor for spins S=1, 3/2, 2, and 5/2 for non-vanishing rhombic local anisotropy and any form of spectroscopic tensor.

cond-mat.other↗

arXiv2013

Magnetocaloric effect in {[Fe(pyrazole)$_4$]$_2$[Nb(CN)$_8$]$\cdot$4H$_2$O}$_n$ molecular magnet

Magnetocaloric effect in {[Fe(pyrazole)$_4$]$_2$[Nb(CN)$_8$]$\cdot$4H$_2$O}$_n$ molecular magnet is reported. It crystallizes in tetragonal I4$_1$/a space group. The compound exhibits a phase transition to a long range magnetically ordered state at $T_\mathrm{c}\approx$8.3 K. The magnetic entropy change $ΔS_\mathrm{M}$ as well as the adiabatic temperature change $ΔT_\mathrm{ad}$ due to applied field change $μ_0ΔH$=0.1, 0.2, 0.5, 1, 2, 5, 9 T as a function of temperature have been determined by the relaxation calorimetry measurements. The maximum value of $ΔS_\mathrm{M}$ for $μ_0ΔH=5$ T is 4.9 J mol$^{-1}$ K$^{-1}$ (4.8 J kg$^{-1}$ K$^{-1}$) at 10.3 K. The corresponding maximum value of $ΔT_\mathrm{ad}$ is 2.0 K at 8.9 K. The temperature dependence of the exponent $n$ characterizing the field dependence of $ΔS_\mathrm{M}$ has been estimated. It attains the value of 0.64 at the transition temperature, which is consistent with the 3D Heisenberg universality class.

cond-mat.mtrl-sci↗

arXiv2013

Magnetic systems at criticality: different signatures of scaling

Different aspects of critical behaviour of magnetic materials are presented and discussed. The scaling ideas are shown to arise in the context of purely magnetic properties as well as in that of thermal properties as demonstrated by magnetocaloric effect or combined scaling of excess entropy and order parameter. Two non-standard approaches to scaling phenomena are described. The presented concepts are exemplified by experimental data gathered on four representatives of molecular magnets.

cond-mat.str-el↗

arXiv2003

Perturbative expansion for the half-integer rectilinear disclination line in the Landau-de Gennes theory

The structure of the half-integer rectilinear disclination line within the framework of the Landau-de Gennes effective theory of nematic liquid crystals is investigated. The consistent perturbative expansion is constructed for the case of $L_2\neq 0$. It turns out that such expansion can be performed around only a discrete subset of an infinite set of the degenerate zeroth order solutions. These solutions correspond to the positive and negative wedge disclination lines and to four configurations of the twist disclination line. The first order corrections to both the order parameter field as well as the free energy of the disclination lines have been found. The results for the free energy are compared with the ones obtained in the Frank-Oseen-Zocher director description.

cond-mat.soft↗

arXiv2001

Perturbations of planar interfaces in Ginzburg-Landau models

Certain dissipative Ginzburg-Landau models predict existence of planar interfaces moving with constant velocity. In most cases the interface solutions are hard to obtain because pertinent evolution equations are nonlinear. We present a systematic perturbative expansion which allows us to compute effects of small terms added to the free energy functional of a soluble model. As an example, we take the exactly soluble model with single order parameter $ϕ$ and the potential $V_0(ϕ) = Aϕ^2 + B ϕ^3 + ϕ^4$, and we perturb it by adding $V_1(ϕ) = {1/2} ε_1 ϕ^2 \partial_i ϕ\partial_i ϕ+ 1/5 ε_2 ϕ^5 + 1/6 ε_3 ϕ^6. $ We discuss the corresponding changes of the velocity of the planar interface.

cond-mat.soft↗

arXiv2000

Evolution of interfaces and expansion in width

Interfaces in a model with a single, real nonconserved order parameter and purely dissipative evolution equation are considered. We show that a systematic perturbative approach, called the expansion in width and developed for curved domain walls, can be generalized to the interfaces. Procedure for calculating curvature corrections is described. We also derive formulas for local velocity and local surface tension of the interface. As an example, evolution of spherical interfaces is discussed, including an estimate of critical size of small droplets.

cond-mat.soft↗

arXiv1997

Backreaction of excitations on a domain wall

Backreaction of excitations on a planar domain wall in a real scalar field model is investigated in the cases of homogeneous, plane wave and wave packet type excitations. It is found that the excited domain wall radiates. The method of calculating backreaction for the general forms of excitations is also presented.

hep-th↗