A kinematic model of TRM acquisition in multidomain magnetite

Abstract

Samples containing 10–15 μm, 30–40 μm and 100–150 μm magnetite grains have been used in a series of experiments to study the behaviour of pTRM. pTRMs were induced by cooling each sample through a range of temperature intervals in a field of 0.84 mT, and a vibrating sample magnetometer was used to obtain a continuous record of the change in moment throughout the temperature cycles. The moment was measured during cooling in zero field after induction of a pTRM in a temperature interval T 1 to T 2 (pTRM T2 T1 ). The moment was inferred during acquisition while cooling in 0.84 mT from T 2 to room temperature (pTRM 0 T2 ) from the total moment at temperature T minus the reversible induced moment at T. On cooling from T 2 to room temperature the normalized moment {pTRM T2 T1 (T)/ j s(T)} decreases while {pTRM 0 T2 ( T)/ j s( T)} increases at the same rate. In c total TRM on cooling from the Curie point, T c, the normalized total TRM value is achieved on cooling close below T c and no further increase is observed on further cooling. A new kinematic model of multidomain TRM acquisition is presented where ‘blocking’ of domain walls in potential wells is relatively unimportant. Both total TRM and the demagnetized state are considered to be equilibrium states. The magnetic field has to be maintained during cooling to keep the remanence from decaying, and conversely, zero field has to be maintained to prevent remanence from being acquired. The remanence alters in response to a change in equilibrium conditions during cooling, i.e., the turning on or off of the field, because a temperature dependent change in the number or configuration of domains aids the magnetic system to approach the new equilibrium.