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 dev eloped in paper 1 (Dunlop and Xu, this issue) to grains in which domai n walls are pinned by microcoercivities of varying magnitudes. Assumin g microcoercivities to be exponentially distributed, we find that the intensity of a total TRM is linearly proportional to the inducing fiel d 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 obt ained in paper 1. Here n represents the temperature dependence of micr ocoercivity that goes as the n th power of the saturation magnetizatio n M(s) (T). The above three field dependent regions correspond to ther mally blocked, field-blocked and reequilibrated walls, respectively. W hen being thermally demagnetized, a TRM induced in a high field has lo w unblocking temperatures, as observed. For a partial TRM acquired fro m T2 (< T(c)) to T1, there may be no region in which walls are field b locked 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 hi gher 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 re sulting thermal demagnetization curve, normalized to the intensity bef ore heating, is independent of both H(o) and the mean value of microco ercivities. 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 m easured for various multidomain magnetite samples.