Theory of partial thermoremanent magnetization in multidomain grains: 1. Repeated identical barriers to wall motion (single microcoercivity)

Abstract

We extend the Néel (1955) theory of thermoremanent magnetization (TRM) in multidomain grains to include (1) the acquisition of partial TRM produced by cooling from T₂ to T₁ in an applied field H₀ and (2) the thermal demagnetization of partial and total TRM. If T₂ is ≥1°C below the Curie temperature T_c, the internal demagnetizing field −NM_s (M_s is saturation magnetization and N is demagnetizing factor) is larger than geophysically reasonable values of H₀. The initial state for partial TRM blocking is then close to a demagnetized state and is quite different from the near‐saturation initial state modeled by Néel for TRM blocking. As a result, a partial TRM M_ptr(T₂, T₁,H₀) usually has a considerably lower intensity than total TRM M_tr(T_c,T₀,H₀) or partial TRM M_ptr(T_c,T₁,H₀). However, during thermal demagnetization, M_tr or M_ptr(T_c,T₁,H₀) will begin to disappear at lower temperatures than M_ptr(T₂,T₁,H₀) and exactly the same stable remanence will be isolated at high temperatures in all three cases. Blocking and unblocking do not occur at the same temperature as in the case of single‐domain grains: TRM or partial TRM blocking is a sharp process (apart from possible reequilibration of walls when H₀ → 0), but thermal unblocking proceeds gradually toward a demagnetized state by continuous wall reequilibration and is complete only close to T_c. Above a threshold temperature T_crit which is dependent on the type of partial TRM, the remanence intensity decreases in proportion to H_c (T) for continuous thermal demagnetization or ∝ H_c(T)IM_s(T) for step wise thermal demagnetization, where H_c is average microcoercivity. Because a grain with a single microcoercivity can acquire partial TRMs in many different blocking temperature ranges, each with a different set of wall displacements, Thellier's Law of Additivity of partial TRMs is only very approximately obeyed and the various partial TRMs are not independent.