Mapping the flux penetration profile in a 2G-HTS tape at the microscopic scale: deviations from a classical critical state model

Datasets underpinning the 6 Figures for "Mapping the flux penetration profile in a 2G-HTS tape at the microscopic scale: deviations from a classical critical state model" in Superconductor Science and Technology. The data files consist of .txt files for each figure organised in individual folders and presented in labelled columns (tab delimited). The data was acquired by scanning Hall microscopy (SHM) in two operation modes: the 'local' magnetometry mode where ‘local’ magnetic induction is measured, and the rapid 'flying' mode that makes a rapid 2D scan of the maximum field of view. The data files consist of individual graph curves and SHM images. A critical state model was used to fit the experimental data and estimate the superconducting critical temperature at different temperatures.The primary datasets are scanning Hall microscopy (SHM) used to directly image the magnetic field component perpendicular to the surface of a 10 × 14 mm2 piece of 2G-HTS tape at different temperatures ranging from 88 K down to 65 K. The microscope used is a modified commercial low temperature scanning tunnelling microscope (STM) where the tunnelling tip has been replaced by a custom-fabricated GaAs chip. The Hall probe is patterned in the two-dimensional (2D) electron gas of a GaAs/AlGaAs heterostructure, defined by the intersection of two 0.8 μm wide wires situated ∼5 μm from the Au-coated corner of a deep mesa etch that acts as an integrated STM tip. The Hall probe is mounted at an angle of 1°−2° with respect to the sample plane, with the STM tip being the closest point to the sample surface [10]. The Hall probe with ∼0.8 μm spatial resolution and ∼5 mG Hz−1/2 minimum detectable field was approached at a point approximately 1 mm from one of the long edges of the tape and then retracted ∼1 μm for fast data collection. Two operation modes were used; a rapid ‘flying mode’ where the Hall sensor makes a rapid 2D scan of the maximum field of view, and a ‘local’ magnetometry mode whereby the sensor is parked above a desired location and the ‘local’ magnetic induction measured while sweeping an external magnetic field perpendicular to the plane of the sample. By systematically acquiring SHM images at regularly spaced points on a magnetic field cycle starting from the zero field-cooled state, a spatial map can be made of the critical state established around a hysteresis loop.The SHM image data in the archive are raw as-captured data without any post-processing. The SHM data plotted are scaled by the values stated at the figure captions in the published paper.SHM image datasets are formatted as the magnetic induction in Gauss measured at each point on a 64 × 64 array of pixel positions. At the measurement temperatures of 83 K, 77 K and 65 K this corresponds to a scan range of 18 µm×18 µm, 16 µm×16 µm and 14 µm×14 µm, respectively

Mapping the flux penetration profile in a 2G-HTS tape at the microscopic scale: deviations from a classical critical state model

Understanding vortex behaviour at microscopic scales is of extreme importance for the development of higher performance coated conductors with larger critical currents. Here, we study and map the critical state in a YBCO-based coated conductor at different temperatures using two distinct operation modes of scanning Hall microscopy. An analytical Bean critical state model for long superconducting strips is compared with our measurements and used to estimate the critical current density. We find several striking deviations from the model; pronounced flux front roughening is observed as the temperature is reduced below 83 K due to vortex-bundle formation when strong broadening of the flux front profile is also seen. In higher magnetic fields at the lower temperature of 65 K, fishtail-like magnetization peaks observed in local magnetization measurements are attributed to flux-locking due to an increase in the critical current density near the edges of the tape, which we tentatively link to vortex pinning matching effects. Our measurements provide valuable insights into the rich vortex phenomena present in coated conductor tapes at the microscopic scale

Effect of rounded corners on the magnetic properties of pyramidal-shaped shell structures

In recent years, the advance of novel chemical growth techniques has led to the fabrication of complex, three-dimensional magnetic nanostructures. The corners and edges of such realistic geometries are generally not sharp but rounded. In a previous article we have argued that high demagnetization fields in the vicinity of sharp edges lead to the formation of an asymmetric vortex state in pyramidal-shaped magnetic shell structures. The asymmetric vortex state is potentially interesting with respect to future magnetic memory devices. In this work a micromagnetic model is used to investigate the effect of rounded corners and edges on the magnetic reversal process within these pyramidal-shaped magnetic shell structures. In particular, we explore the degree of rounding, which has to be introduced in order to suppress the asymmetric vortex state. Another emphasis is placed on the magnetic reversal of (quasi-)homogeneous states within these structures. We demonstrate that the rounding of corners significantly reduces the coercivity. This complies with former studies on cuboidal structures, which suggest the important effect of corners on the magnetic reversal of homogeneous magnetic states. The present study uses a finite-element discretization for the numerical solution of the micromagnetic equations, which provides flexibility with respect to the modeling of complex shapes. In particular, this method is very accurate with respect to structures with a smooth surface

A simplified model for minor and major loop magnetic hysteresis and its application for inference of temperature in induction heated particle beds

In this work, a LangArc model is presented that successfully fits both major and minor hysteresis loops of a bed of magnetic particles in real time using instruments that detect changes in the magnetic field strength, such as in-situ pick-up coils. A novel temperature measurement application is demonstrated based on a real-time characterisation of a magnetic material, in this case magnetite, as a function of temperature. Magnetic hysteresis can be used to provide useful induction heating in a packed bed of magnetic materials. This can be used for general heating and to provide energy to chemical reactions in chemical processes. Accurate temperature measurement of magnetic particles under induction heating is a well-known challenge: conventional techniques give a single-point measurement, and are subject to inaccuracy due to self-heating of the instrument tip. Thermal lag can be problematic given the rapid heating rates that are characteristic of induction heating. The LangArc inferred temperature measurement technique is shown to detect heating rates in excess of 30 °C·s−1, under which circumstances an in-bed thermocouple was shown to lag by as much as 180 °C. This new method has significant importance for temperature measurement in applications involving the induction heating of magnetic materials as it avoids the location of an instrument inside the magnetic particle bed and is highly responsive under rapid heating where other techniques can give misleading results.</p