SIMULTANEOUS MEASUREMENT OF THE VARIATIONS IN THE MAGNETIC-SUSCEPTIBILITY AND REMANENCE OF MATERIALS IN THE TEMPERATURE-RANGE 10-DEGREES-700-DEGREES-C AND AT ELEVATED PRESSURE (16-160 MPA)

A method for simultaneously measuring the magnetic susceptibility and remanence of materials as a function of temperature and elevated pressure is described. A cylindrical sample of diameter 1 cm and length 1.5 cm may be heated up to 700-degrees-C and subjected to pseudo-hydrostatic pressure up to 160 MPa in an evacuated environment. The susceptibility of the sample is measured, using an AC bridge technique at 703 Hz. The typical minimum measurable susceptibility (dipole moment per unit field) is 2 x 10(-9) m3, equivalent to the susceptibility of 4 mg of magnetic recording material such as fine acicular gamma-Fe2O3. The remanence of the sample is measured using two aligned horizontal fluxgate probes, connected in series opposition to combat environmental noise. The minimum measurable sample moment (total dipole moment) is about 5 x 10(-5) A m2, equivalent to the saturation remanence of about 1 mg of randomly orientated gamma-Fe2O3. The magnetic susceptibility, remanence, temperature and pressure signals are multiplexed into a microcomputer. The data is stored on disk, processed and displayed on a visual display unit or printer. Some preliminary results involving the pressure- and temperature-induced phase transition (inversion) of gamma-Fe2O3 are presented.