Magnetic studies on specimens of intergrown maghemite and hematite

Abstract

The direction and intensity of remanent magnetization of intergrown maghemite and hematite have been studied as a function of the hematite content by heating artificial specimens of maghemite under an astatic magnetometer at temperaures of 540 and 580°C in a field of 1.08 oe. The remanence vector during the inversion from maghemite to hematite is directed along the applied field and is not controlled by the magnetization of the host mineral, maghemite. An anomalously high remanence is observed for hematite contents of 92 to 95%. Thermal demagnetization shows this magnetization to be primarily due to the residual maghemite. It is suggested that spin exchange interactions across the interface of intergrown maghemite and hematite are the cause of the high remanence because they are responsible for aligning the moments of the residual maghemite fraction along the direction of magnetization of hematite. The experimental evidence further suggests that the chemical remanence of hematite obtained by inverting maghemite at 540°C is not due to a spin-canted moment but originates from uncompensated antiferromagnetic sublattices. Results from torque measurements show a high-field rotational hysteresis for the maghemite-hematite specimens. The rotational hysteresis increases, however, with the hematite content and attains a maximum for pure hematite, indicating that the magnetic hardness of hematite rather than exchange interactions between the two magnetic minerals is primarily responsible for the rotational hysteresis.