We extend the N¿el (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 Ho 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 Ho. 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 N¿el for TRM blocking. As a result, a partial TRM M ptr(T2, T1, Ho usually has a considerably lower intensity than total TRM M tr(Tc,To,Ho) or partial TRM M ptr(Tc, T1, Ho). However, during thermal demagnetization, M tr or M ptr(Tc, T1, Ho) will begin to disappear at lower temperatures than M ptr(T2, T1, Ho) 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 Ho→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)/MS(T) for stepwise 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. ¿ American Geophysical Union 1994