Theoretical sensitivity limits of the compensation method for magnetization characteristics measurement of HTc superconducting materials using LTc SQUID 2nd order gradiometer

This article deals with theoretical analysis of a fast compensation method of measuring magnetization characteristics using superconductive quantum 2n d order gradiometer suitable for measuring material samples with low magnet ic susceptibility in an alternating low-frequency magnetic field. Special a ttention is paid to measuring magnetization characteristics of HTc supercon ductors at a temperature of 78K. Transmission and noise properties of the m easuring system consisting of LTc SQUID 2nd order gradiometer and magnetiza tion coil system with the sample of HTc superconductive material placed in liquid nitrogen in an external case under an LHe cryostat of a gradiometer are studied. The maximum sensitivity of the system given by noise propertie s of a superconductive quantum gradiometer, noise parameters of the magneti zation magnetic field and environmental noise in the measuring environment is determined. It follows from a theoretical analysis that the spectral den sity limit of an input equivalent noise magnetic moment of a system placed in a magnetically shielded room with parameters achieved in rooms designed for measuring biomagnetic fields can reach the order of 10(-13) Am-2 Hz(-1/ 2). Under conditions typical of unshielded magnetic laboratories for measur ing weak magnetic fields the order of (10(-8) divided by 10(-11)) Am-2 Hz(- 1/2),, be achieved theoretically. The measuring process is outlined and par ameters achieved with systems in an unshielded environment and examples of measured characteristics of HTc superconductors in an alternating magnetiza tion magnetic field with the frequency 0.1 Hz are cited. The accessibility of a sample for manipulation, general arrangement and a simple measuring pr ocess allow quick scanning and recording of magnetization characteristics ( a set of five amplitude phased hysteretic curves in a time of similar to 2. 5 min). (C) 1999 Elsevier Science Ltd. All rights reserved.