arXiv subjects
Publications and source records attributed to J. Kováč.
We study the behaviour of discrete dynamical systems generated by a continuous map $f$ of a compact real interval into itself where at randomly chosen times a function different from $f$ - so called impulse function is applied. We show that both the splittting property and the average contraction property guarantee the stability of the system. We give a number of examples where the verification of these properties is simple.
(Fe,Co)$_2$B-based compounds with specified 5$d$ substitutions are considered as promising materials for permanent magnets without rare-earth elements. We conducted a combined first-principles and experimental study focused on (Fe$_{0.7}$Co$_{0.3}$)$_2$B alloys doped with W and Re. First, we used full-potential local-orbital scheme to systematically investigate (Fe,Co)$_2$B alloys with 3$d$, 4$d$, and 5$d$ substitutions. Computational analyses showed a significant increase in magnetocrystalline anisotropy only for the Re doped sample. Simultaneously, the structural and magnetic properties of the (Fe$_{0.7-x}$Co$_{0.3-x}$M$_{2x}$)$_2$B ($M$ = W, Re; $x$ = 0, 0.025) alloys were investigated experimentally. The desired (Fe,Co)$_2$B tetragonal phase was synthesized by heat treatment of amorphous precursors. We observed that isothermal annealing increases the coercive field of all samples. However, the obtained values, without further optimization, are well below the threshold for permanent magnet applications. Nevertheless, annealing of substituted samples at 750$^o$C significantly improves saturation magnetization values. Furthermore, Mössbauer spectroscopy revealed a reduction of the hyperfine field due to the presence of Co atoms in the (Fe,Co)$_2$B phase, where additional defect positions are formed by Re and W. Radio-frequency Mössbauer studies showed that (Fe$_{0.7}$Co$_{0.3}$)$_2$B and the W-substituted sample began to crystallize when exposed to an radio frequency field of 12 Oe, indicating that the amorphous phase is stabilized by Re substitution. Improvement of thermal stability of (Fe$_{0.675}$Co$_{0.275}$Re$_{0.05}$)$_2$B alloy is consistent with the results of differential scanning calorimetry and thermomagnetic measurements.
We report the data on magnetization losses and critical current of multifilament copper stabilized coated conductors. Eight centimeters long samples of copper stabilized YBa2Cu3O7-x (YBCO) coated conductors manufactured commercially were subdivided into superconducting filaments by near-IR laser micromachining. The width of the superconducting stripes was varied from 0.2 mm to 0.04 mm. Some of the samples were striated leaving superconducting bridges for current sharing between the filaments. The AC losses were measured at different sweep rates of the magnetic field up to 14 T/s. We will present the results for the hysteresis and coupling losses and discuss the means to reduce the coupling loss by changing the processing parameters of micromachining and by post-ablation treatment.
We report on magnetic measurements carried out in a textured YBa$_2$Cu$_3$O$_{7-δ}$ and YBa$_2$(Cu$_{1-x}$Ag$_x$)$_3$O$_{7-δ}$ (at $x \approx$ 0.02) crystals. The so-called fishtail-effect (FE) or second magnetization peak has been observed in a wide temperature range 0.4~$<T/T_c<$~0.8 for $\textbf{H}\parallel c$. The origin of the FE arises for the competition between surface barrier and bulk pinning. This is confirmed in a non-monotonically behavior of the relaxation rate $R$. The value $H_{max}$ for Ag-doped crystals is larger than for the pure one due to the presence of additional pinning centers, above all on silver atoms.
$M(B)$-curves were experimentally investigated. Fishtail-effect (FE) was observed in MG YBa$_2$Cu$_3$O$_{7-δ}$ and YBa$_2$Cu$_{3-x}$Ag$_x$O$_{7-δ}$ (at $x \approx$ 0.02) crystals in a wide temperature range 40 K $<T<$ 75 K at the orientation of magnetic field $\textbf{H}\parallel c$. It was obtained that the influence of bulk pinning on FE is more effective at low temperatures while the influence of surface barriers is more effective at high temperatures. The value $H_{max}$ for Ag-doped crystals is larger than for a pure one that due to the presence of additional pinning centers, above all on silver atoms.