Improved system for measurement of the magnetization characteristics of high-temperature superconductors by the compensation method in weak magnetic fields

This article follows the previous publication (1999) of the author where th e theoretical sensitivity limits and the basic analysis of the parameters o f the superconducting quantum interference device systems for measurement o f the magnetization characteristics of high-temperature (HTc) superconducto rs in ac low-frequency magnetic field with the help of a compensation metho d were given. It contains the theoretical base for the optimization of such systems and the results achieved by its application in an experimental mod el of the measuring system. The article deals with the dependence of the sp ectral sensitivity and the signal-to-noise ratio of the electronic unit of superconducting quantum gradiometer (EU SQG) output on the volume and the s usceptibility of the HTc superconducting material sample. It concluded that the chosen dimensions of the pellet shaped sample (diameter 12 mm, thickne ss 2.5 mm) were optimum for the given measuring system in magnetically unsh ielded ambient. A method is presented of how to reduce the magnetization cu rrent source noise component influence on the sensitivity of the measuring system using a resistance current divider. The concept of spectral sensitiv ity components of the measuring system has been introduced. These component s correspond to the noise spectral densities of three considered noise sour ces (magnetization current noise component, magnetic noise of the environme nt, and the EU SQG intrinsic noise). Derived theoretical relations have bee n applied in the computation and optimization of the experimental system wi th the standard EU SQG with the input energy sensitivity 1x10(-28) J/Hz. In the magnetically unshielded Faraday chamber originally intended for biomag netic fields measurement, it achieves the spectral sensitivity 8x10(-10) Am -2 Hz(-1/2). Examples of typical magnetization characteristics measured usi ng the triangle alternating magnetization field with a frequency of 0.1 Hz are given. The method and the system are basically also suitable for nonsup erconducting materials. (C) 2001 American Institute of Physics.