Reversible Magnetization Data Research Articles

Reversible magnetization, critical fields, and vortex structure in grain-aligned YBa2Cu4O8.

Reversible-magnetization data have been studied for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ in order to determine the thermodynamic properties and the magnetic-field dependence of the superfluid density. By fitting the Hao-Clem variational model to the reversible magnetization data below ${\mathit{T}}_{\mathit{c}}$, superconducting parameters such as the Ginzburg-Landau parameter \ensuremath{\kappa}, the thermodynamic critical field ${\mathit{H}}_{\mathit{c}}$, and the superconducting order parameter are determined. The upper and lower critical fields ${\mathit{H}}_{\mathit{c}2}$,${\mathit{H}}_{\mathit{c}1}$, the penetration depth \ensuremath{\lambda}, and the coherence length \ensuremath{\xi} are derived. Within the model there is a direct connection between the magnetization and the superfluid density, so the vortex structure, the behavior of the average order parameter in (H,T) plane, and the magnetic-field dependence of the superfluid density are obtained. In the high-field region, the evidence of strong vortex fluctuation effects is observed and explained by vortex fluctuation for a three-dimensional superconductor. The mean-field transition temperature ${\mathit{T}}_{\mathit{c}}$(H) and ${\mathit{dH}}_{\mathit{c}2}$/dT near ${\mathit{T}}_{\mathit{c}}$ derived from the analysis of vortex fluctuation effects are comparable with fitting results.

Thermodynamic parameters of HgBa2Ca2Cu3O8-x from high-field magnetization.

We have measured the temperature dependence of reversible magnetization of grain-aligned ${\mathrm{HgBa}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{8\mathrm{\ensuremath{-}}\mathit{x}}$ high-${\mathit{T}}_{\mathit{c}}$ superconductors with external magnetic field parallel to the c axis. Our data show the crossover of the magnetization below the superconducting onset temperature, which indicates a dominant vortex fluctuation. Except this vortex fluctuation region, reversible magnetization data could be described by using Hao and Clem's model. From the analysis magnetization curve based on this model, the thermodynamic critical field ${\mathit{H}}_{\mathit{c}}$(T) and Ginzburg-Landau parameter \ensuremath{\kappa}=118 were extracted. Also, we obtained the superconducting parameters; the penetration depth ${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$(0)=2060 \AA{}, coherence length ${\ensuremath{\xi}}_{\mathit{a}\mathit{b}}$(0)=18 \AA{}, and the zero-temperature upper critical field ${\mathit{H}}_{\mathit{c}2}$(0)=108 T.

The penetration length λ, the bulk critical field H c1, and the geometrical barrier in Tl 2Ba 2CaCu 2O 8 single crystals

Magnetization curves of Tl 2Ba 2CaCu 2O 8 crystal were measured in applied fields up to 8 Tesla. At temperatures above 25 K we observe reversible magnetization at high magnetic fields. Equilibrium magnetization value was derived from these data. The penetration length λ and the bulk critical field H cl were determined by fitting the theoretical curves to the reversible magnetization data. The magnetization corresponding to the first maximum is much higher than the calculated value based on the measured equilibrium magnetization at high fields. We conclude that geometrical barrier effects change the magnetization curves drastically.

Reversible magnetization of Bi 2Sr 2CaCu 2O 8 single crystals at 30 K ≤ T < T c

We report magnetization measurements on high-quality Bi 2Sr 2CaCu 2O 8 single crystals in H ∥ the c-axis. Our reversible magnetization data at 30 K ≤ T < T c show strong deviations from the linear dependence of M on ln( H) in the intermediate-field region. The Ginzburg-Landau parameter κ, obtained by using the Hao-Clem variational model, was found to vary significantly with the field range for the data fitting. This unusual behavior can not be explained by the thermal fluctuations of the vortices in layered superconductors.

Flux-line-lattice melting in Bi2Sr2Ca2Cu3O10.

We have performed dc magnetization studies in the c-axis-oriented bulk ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ compound. The penetration depth ${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$(T) is extracted from reversible magnetization data M(H) taking into account fluctuations of the vortices. An ``irreversibility line'' is quantitatively compared with predictions of flux-line-lattice (FLL) melting models. The results strongly suggest that the ``irreversibility line'' near ${\mathit{T}}_{\mathit{c}}$ is associated with melting of FLL caused by thermal fluctuations.

Thermodynamics of superconducting Bi2Sr2Ca2Cu3O10 in the critical fluctuation region near the Hc2(T) line.

We report a detailed study of the temperature and magnetic-field dependence of free energy, magnetization, and specific heat of superconducting ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ in the critical fluctuation region near ${\mathit{H}}_{\mathit{c}2}$(T) line. Our approach is to apply the recently developed nonperturbative scaling theory (Te\ifmmode \check{s}\else \v{s}\fi{}anovi\ifmmode \acute{c}\else \'{c}\fi{} et al.) in high fields to describe the reversible magnetization data taken on a c-axis-oriented superconducting ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ thin tape. We find good agreement on some of the superconducting parameters obtained in the present study with those derived by applying the Ginzburg-Landau theory to our reversible magnetization data taken outside the critical fluctuation region.

Role of vortex fluctuations in determining superconducting parameters from magnetization data for layered superconductors.

The fluctuations of vortices in Josephson-coupled layered superconductors are shown to have a profound effect on values of the temperature-dependent penetration depth, Ginzburg-Landau parameter, and upper critical field extracted from reversible magnetization data below the critical temperature.

Two-dimensional fluctuations in the magnetization of Bi2Sr2Ca2Cu3O10.

Magnetization measurements on c-axis-oriented bulk ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ with the magnetic field parallel to the c axis are reported. The results show large fluctuation effects, which can be explained by Ginzburg-Landau fluctuation theory for a two-dimensional (2D) system. The magnetization in high fields displays good scaling behavior as a function of [T-${\mathit{T}}_{\mathit{c}}$(H)]/(TH${)}^{1/2}$. The weak-field fluctuation diamagnetic susceptibility \ensuremath{\chi}(T) above ${\mathit{T}}_{\mathit{c}}$ can be fitted well in terms of the 2D Lawrence-Doniach model. Values of ${\ensuremath{\xi}}_{\mathit{a}\mathit{b}}$(0) and ${\mathit{dH}}_{\mathit{c}2}$/dT derived from our fluctuation analysis are consistent with those obtained by fitting the Hao-Clem variational model to the reversible magnetization data below ${\mathit{T}}_{\mathit{c}}$.

Magnetic properties of ultrathin epitaxial films of iron

Ultrathin Fe films epitaxially grown on Cu(100), Au(100), Pd(100) and Ru(0001) substrates are used to study monolayer magnetism, surface magnetic anisotropy and two-dimensional (2D) critical behavior, by means of in-situ surface magneto-optical Kerr-effect measurements. Epitaxial structures and growth modes are characterized by auxiliary low-energy electron diffraction and Auger electron studies. The magnetic properties of the films depend on the growth temperature. For 100 K growth, all films exhibit a universal behavior of perpendicular spin orientations in the monolayer regime, while for 300 K growth, Fe/Pd(100) and Fe/Au(100) have in-plane easy-axis of magnetization for all thickness. Fe/Ru(0001) lacks a ferromagnetic response for thickness less than two monolayers (ML). Fe/Pd(100) has remarkable thermal stability in the monolayer and submonolayer range. Reversible magnetization data are obtained for 0.6 to 3.0 ML thick films, which show second-order phase transitions at thickness-dependent Curie temperatures ( T c ). An effective magnetization exponent β eff of 0.127 ± 0.004 is obtained for a 1.2 ML Fe/Pd (100) film, in agreement with the theoretical value of β c = 0.125 for an ideal 2D Ising model. The T c trend with film thickness for Fe/Pd(100) is also in agreement with Monte Carlo calculations for Ising films.