Frederik Wolff-Fabris

NMR signal averaging in 62T pulsed fields.

Nuclear Magnetic Resonance (NMR) experiments in pulsed high magnetic fields up to 62T at the Dresden High Magnetic Field Laboratory (Hochfeld-Magnetlabor Dresden) are reported. The time dependence of the magnetic field is investigated by observing various free induction decays (FIDs) in the vicinity of the maximum of the field pulse. By analyzing each FIDs phase and its evolution with time the magnetic fields time dependence can be determined with high precision. Assuming a quadratic or cubic dependence on time near the field maximum its confidence is found to be better than ± 0.03ppm at low fields and ± 0.8ppm near 62T. In turn, the thus obtained time dependence of the field can be used to demodulate and phase-correct all FIDs so that they appear phase-locked to each other. As a consequence signal averaging is possible. The increase in signal-to-noise ratio is found to be close to that expected theoretically. This shows that the intrinsic time dependence of the pulsed fields can be removed so that the NMR signals appear to be taken at rather stable static field. This opens up the possibility of performing precise shift measurements and signal averaging also of unknown, weak signals if a reference signal is measured during the same field pulse with a double-resonance probe.

CMR-B-Scalar Sensor Application for High Magnetic Field Measurement in Nondestructive Pulsed Magnets

In this paper, we present the investigation of the axial and radial magnetic-field distribution inside and outside of the bore of a nondestructive pulsed-field coil presently installed at the Dresden High Magnetic Field Laboratory (HLD). The array used for these magnetic-field measurements was made up of three CMR-B-scalar sensors based on nanostructured La-Sr-Mn-O films. The investigations were performed at a temperature of 270 K and at a peak field of 46 T. The experimental results fit well with calculations obtained using ANSYS code based on the finite element method (FEM). It is concluded that the CMR-B-scalar sensors can be successfully used for the investigation of the magnetic-field distribution in pulsed high magnetic field coils.

Electron transport and anisotropy of the upper critical magnetic field in Ba0.68K0.32Fe2As2 single crystals

Early work on the iron-arsenide compounds supported the view, that a reduced dimensionality might be a necessary prerequisite for high-Tc superconductivity. Later, however, it was found that the zero-temperature upper critical magnetic field, Hc2(0), for the 122 iron pnictides is in fact rather isotropic. Here, we report measurements of the temperature dependence of the electrical resistivity, ρ(T), in Ba0.5K0.5Fe2As2 and Ba0.68K0.32Fe2As2 single crystals in zero magnetic field and in Ba0.68K0.32Fe2As2 in static and pulsed magnetic fields up to 60 T. We find that the resistivity of both compounds in zero field is well described by an exponential term due to inter-sheet umklapp electron-phonon scattering between light electrons around the M point to heavy hole sheets at the Γ point in reciprocal space. From our data, we construct an H-T phase diagram for the inter-plane (H | c) and in-plane (H | ab) directions for Ba0.68K0.32Fe2As2. Contrary to published data for 122 underdoped FeAs compounds, we find that Hc2(T) is in fact anisotropic in optimally doped samples down to low temperatures. The anisotropy parameter, γ = Hc2ab/Hc2c, is about 2.2 at Tc. For both field orientations we find a concave curvature of the Hc2 lines with decreasing anisotropy and saturation towards lower temperature. Taking into account Pauli spin paramagnetism, we perfectly can describe Hc2 and its anisotropy.

Nuclear magnetic resonance apparatus for pulsed high magnetic fields

A nuclear magnetic resonance apparatus for experiments in pulsed high magnetic fields is described. The magnetic field pulses created together with various magnet coils determine the requirements such an apparatus has to fulfill to be operated successfully in pulsed fields. Independent of the chosen coil it is desirable to operate the entire experiment at the highest possible bandwidth such that a correspondingly large temporal fraction of the magnetic field pulse can be used to probe a given sample. Our apparatus offers a bandwidth of up to 20 MHz and has been tested successfully at the Hochfeld-Magnetlabor Dresden, even in a very fast dual coil magnet that has produced a peak field of 94.2 T. Using a medium-sized single coil with a significantly slower dependence, it is possible to perform advanced multi-pulse nuclear magnetic resonance experiments. As an example we discuss a Carr-Purcell spin echo sequence at a field of 62 T.

Magnetic irreversibility: An important amendment in the zero-field-cooling and field-cooling method

This work was supported under the Brazilian grant MCTI= CNPq Universal 14=2012 (contract number 477506=2012-7). The experimental work at the HLD was supported by Euromagnet II (contract number 228043).

Functional behavior of the anomalous magnetic relaxation observed in melt-textured YBa2Cu3O7-δ samples showing the paramagnetic Meissner effect

This work was financed by the Brazilian MCTI/CNPq Universal 14/2012 (contract number 477506/2012-7). We acknowledge the support of the HLD-HZDR, member of the European Magnetic Field Laboratory (EMFL).

Functional behavior of the melt-textured YBa2Cu3O7-δ Meissner effectanomalous magnetic relaxation observed in samples showing the paramagnetic

This work was financed by the Brazilian MCTI/CNPq Universal 14/2012 (contract number 477506/2012-7). We acknowledge the support of the HLD-HZDR, member of the European Magnetic Field Laboratory (EMFL).

Comparison Between the Magnetic Irreversibility and Zero Resistance of High-Quality Melt-Processed YBaCuO Superconductors

This paper portrays a detailed study of the magnetic irreversibility limit Tirr (H) and of the zero resistance point Tc0 (H) of three different top-seeding melt-textured YBa2Cu3O7-δ superconducting samples, with well-aligned c-axis and doped with a high density of nonsuperconducting Y2Ba1Cu1O5 (Y211) pinning centers. We have performed measurements for applied magnetic fields up to 140 kOe and for the whole set of the different field-current configurations. The magnetization measurements were performed using an MPMS-XL SQUID magnetometer and a vibrating sample magnetometer, both from Quantum Design. The electric transport measurements were made using a physical properties measurement system from Quantum Design. The goal of this exhaustive study is obtaining precise data about magnetic flux mobility along the various directions in the sample for the different field-current configurations, thereby defining the nature and effects, due to the strength and anisotropy of the pinning mechanisms and disclosing the various physical mechanisms dissipating electric transport in these systems below the superconducting transition temperature. We discuss our results in terms of the anisotropic flux pinning by the Y211 grains dispersed into the superconducting matrix.

The 9th International Conference on Research in High Magnetic Fields in Retrospective View

The 9th International Conference on Research in High Magnetic Fields (RHMF 2009) was held in the historical center of Dresden during July 22–25, 2009. The conference carried on the tradition of previous meetings in Sendai (2006), Toulouse (2003), Porto (2000), Sydney (1997), Nijmegen (1994), Amsterdam (1991), Leuven (1988), and Osaka (1982) as well. These events have established the RHMF conference series as the most important platform to present latest results of research in high magnetic fields and, as a satellite meeting of the International Conferences on Magnetism (ICM), have linked it to the concert of large international conferences. The number of participants followed the trend of the previous RHMF conferences and has further grown reflecting the increasing demand of high magnetic fields in research. 219 official participants from 27 countries with strongest appearance from Europe, Japan, United States, Russia, China, and Israel have contributed 176 posters and 43 talks within a scientific program of three days. Amongst them, 91 participants contributed papers to the proceedings of RHMF 2009 which are published in this special issue of the Journal of Low Temperature Physics. Each of the articles has been carefully evaluated by two referees before it was accepted for publication. According to the dedication of RHMF conferences to all aspects related to recent advances in solid-state physics, a wide spectrum of contributions in magnetism, the physics of strongly correlated electron systems, superconductivity, semiconductor physics, mesoscopic phenomena, the physics of low-dimensional or organic materials, highfield technology as well as reports on high-field facilities are published here. RHMF 2009 clearly reflected that research in high magnetic fields has become even more a driving factor for solid-state physics, material sciences, and technology in the recent