design and development of a vibrating sample magnetometer for high magnetic field measurements of superconductors

Abstract

A Vibrating Sample Magnetometer (VSM) has been designed, constructed and developed. The design specification was for a VSM to work in magnetic fields up to 15T, and provide a variable temperature environment for the sample from 4.2K to 130K. A probe has been made to protect the detection coils and provide the variable temperature environment. The detection coils have been designed using computer modelling techniques such that a 4mm long sample may be measured. A low-field insert has been used to eliminate problems with flux jumping in the high field magnet system causing extremely high noise at magnetic fields below 2T. The VSM has been calibrated against a nickel standard in fields up to 15T. Considering electrical noise, the sensitivity of the VSM is 10(^-6)A.m(^2). Hopefully, this may be improved upon by modifications made to the VSM that have not yet been fully tested. The temperature measurement is accurate to +200mK from 4.2K to 30K in a liquid helium bath and from 77K to 130K in a liquid nitrogen bath. Outside these ranges it is necessary to use a temperature sensor mounted on the sample holder to ensure accurate temperature measurement. The VSM has been used to study the superconducting properties of two single crystals of YBa(_2)Cu(_3)O(_7-8) grown under identical conditions. This study was done in applied fields up to 220mT and at temperatures from 77K to 95K. It is shown from the transition temperature and the magnetisation critical current densities that the crystals do not exhibit identical superconducting properties. Some high field measurements have also been performed with the VSM. Studies of multifilaraentary niobium-titanium wires have been made and the information has been used to modify and develop the VSM, as have measurements on a tape of Bi(_2)Sr(_2)Ca(_2)Cu(_3)O(_x). Magnetic hysteresis of a multifilamentary Bi(_2)Sr(_2)CaCu(_2)O(_x) wire has been measured at 4.2K and the magnetisation critical current density determined. This agrees well with transport critical current measurements.