THEORY OF PARTIAL THERMOREMANENT MAGNETIZATION IN MULTIDOMAIN GRAINS .2. EFFECT OF MICROCOERCIVITY DISTRIBUTION AND COMPARISON WITH EXPERIMENT

We extend the thermoremanent magnetization (TRM) and pTRM theories developed in paper 1 (Dunlop and Xu, this issue) to grains in which domain walls are pinned by microcoercivities of varying magnitudes. Assuming microcoercivities to be exponentially distributed, we find that the intensity of a total TRM is linearly proportional to the inducing field H(o) for small H(o), to a power of roughly 1-1/n for intermediate H(o), and independent of H(o) for large H(o), similar to the results obtained in paper 1. Here n represents the temperature dependence of microcoercivity that goes as the n th power of the saturation magnetization M(s) (T). The above three field dependent regions correspond to thermally blocked, field-blocked and reequilibrated walls, respectively. When being thermally demagnetized, a TRM induced in a high field has low unblocking temperatures, as observed. For a partial TRM acquired from T2 (< T(c)) to T1, there may be no region in which walls are field blocked if the interval (T2, T1) is not large enough. This will be the case for magnetite when T2 < 565-degrees-C if n = 2 or < 500-degrees-C if n = 4 for T1 = T(o), independent of H(o). If T1 > T(o), an even higher T2 is required. In such cases, the room temperature intensity of pTRM is approximately proportional to H(o)2 when H(o) is small. The resulting thermal demagnetization curve, normalized to the intensity before heating, is independent of both H(o) and the mean value of microcoercivities. Complete demagnetization will not occur at a demagnetizing temperature T2 but only at a temperature close to T(c). The theory is supported by experimental data of thermal demagnetizations of pTRMs measured for various multidomain magnetite samples.