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 Tc, the internal demagnetizing field −NMs (Ms 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 Mptr(T2, T1,H0) usually has a considerably lower intensity than total TRM Mtr(Tc,T0,H0) or partial TRM Mptr(Tc,T1,H0). However, during thermal demagnetization, Mtr or Mptr(Tc,T1,H0) will begin to disappear at lower temperatures than Mptr(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 Tc. Above a threshold temperature Tcrit which is dependent on the type of partial TRM, the remanence intensity decreases in proportion to Hc (T) for continuous thermal demagnetization or ∝ Hc(T)IMs(T) for step wise thermal demagnetization, where Hc 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.