The design of a system of adjustable superconducting plates for balancing a gradiometer

The gradiometer of a SQUID-system for measuring very small biomagnetic signals should be made as insensitive as possible to the much larger uniform background fields. The balance of a gradiometer can be improved considerably by modifying the effective area of the gradiometer loops by a system of small adjustable superconducting plates.\ud This paper is intended as an aid for designing such a balancing system. We calculated the effective area of a gradiometer loop perpendicular to a rectangular or parallel to a circular plate. The paper contains graphs showing how large the plates have to be and where they must be placed.\ud The calculations show that for accurate balancing the distance of a plate to the wire of a loop must be large and that the plates must be as small as possible

Partial lattice participation in the spin-lattice relaxation of potassium chromium alum

We developed a SQUID-based frequency sweeping system for a.c. susceptibility measurements. Using this instrument we found that in Potassium Chromium Alum only a part of the lattice system is involved in the spin-lattice relaxation process. This partial lattice participation amounts 60–75% of the total lattice specific heat

The influence of the environment on the dynamic susceptibility of cerium magnesium nitrate in strong magnetic fields

Dynamic susceptibility curves observed as a function of frequency often deviate considerably from curves predicted by the simple Casimir-Du Pré model. In the case of cerium magnesium nitrate these deviations are mainly caused by the poor thermal conductivity of the liquid or gaseous helium that surrounds the sample and becomes extremely large when the external magnetic field becomes larger than approximately 1 T. We found that, even in this extreme situation, the influence of the environment can be accurately described with the thermal conduction model developed by Flokstra et al. We also discovered that the lattice temperature is stabilized much more effectively if the sample is placed in helium gas at its saturation pressure. The thin helium film that is formed on the surface of the sample provides, through condensation and reevaporation, a far better thermal stabilization of the lattice than bulk liquid helium does. The susceptibility curves that are observed under these conditions differ much less from the Casimir-Du Pré curves

An automatic frequency-sweeping SQUID susceptometer

Abstract. A new. sQc1D-based, measuring system has been developed for the investigation of the dynamic properties of magnetic materials. Its main advantages. compared to conventional mutual inductance systems, are its high sensitivity and its nearly frequency independent response. down to extremely low frequencies. With the SQUID system it is now possible to measure directly and rapidly the frequency dependence of the dynamic susceptibility of weakly magnetic materials in the range from 0.002 Hz to 5 kHz at a fixed value of a constant background field of up to 5 T. The frequency-swept operation allows experiments which are beyond the capabilities of conventional systems. for example the investigation of thermally isolated samples or the study of the dynamics at magnetic phase transitions in cases where the magnetic properties change very rapidly in a small field interval. 1

Partial lattice participation in the spin-lattice relaxation of potassium chromium alum

We developed a SQUID-based frequency sweeping system for a.c. susceptibility measurements. Using this instrument we found that in Potassium Chromium Alum only a part of the lattice system is involved in the spin-lattice relaxation process. This partial lattice participation amounts 60–75% of the total lattice specific heat