Magnetic properties of hydrothermally synthesized greigite (F3S4) - II. High- and low-temperature characteristics

The magnetic behaviour of hydrothermally synthesized greigite was analysed in the temperature range from 4 K to 700 degrees C. Below room temperature, hysteresis parameters were determined as a function of temperature, with e mphasis on the temperature range below 50 K. Saturation magnetization and i nitial susceptibility were studied above room temperature, along with X-ray diffraction analysis of material heated to various temperatures. The magne tic behaviour of synthetic greigite on heating is determined by chemical al teration rather than by magnetic unblocking. Heating in air yields more dis criminative behaviour than heating in argon. When heated in air, the amount of oxygen available for reaction with greigite determines the products and magnetic behaviour. In systems open to contact with air, haematite is the final reaction product. When the contact with air is restricted, magnetite is the final reaction product. When air is excluded, pyrrhotite and magneti te are the final reaction products; the amount of magnetite formed is deter mined by the purity of the starting greigite and the degree of its surficia l oxidation. The saturation magnetization of synthetic greigite is virtuall y independent of temperature from room temperature down to 4 K. The saturat ion remanent magnetization increases slowly by 20-30 per cent on cooling fr om room temperature to 4 K. A broad maximum is observed at similar to 10 It which may be diagnostic of greigite. The coercive and remanent coercive fo rce both increase smoothly with decreasing temperature to 4 K. The coercive force increases from similar to 50 mT at room temperature to approximately 100-120 mT at 4 K, and the remanent coercive force increases from approxim ately 50-80 mT at room temperature to approximately 110-180 mT at 4 K.