Theory of Stable Multidomain Thermoviscous Remanence Based on Repeated Domain Wall Jumps

We developed a theory of multidomain (MD) thermoviscous remanence that can be solved analytically to yield an expression relating the blocking temperature of a MD grain to its relaxation time. This expression is analogous to Neel's widely used theory of single-domain (SD) thermoviscous remanence but yields a different time-temperature relationship. The theory is based on a two-domain model with a domain wall (DW) that can jump between different pinning sites. In contrast to previous theories of this kind, our theory considers repeated DW jumps rather than a single jump in isolation. It is shown that while SD remanence behavior is fully described by the two quantities (V,H-K0), that is, volume and microscopic coercivity, MD particle remanence depends on three quantities: volume, Barkhausen volume, and DW pinning field, denoted (V,V-Bark,H-K0). Pullaiah nomograms of this new theory show that while small MD grains behave almost identically to SD grains, larger grains show different slopes depending on the above quantities. The theory predicts that MD grains can be highly stable remanence carriers, in particular showing a high thermal stability. Grains with weak pinning fields, however, while thermally stable, are highly unstable under alternating field (AF) demagnetization, being demagnetized under fields of a few millitesla. Our theory also explains (1) why samples dominated by MD grains show curved vector demagnetization plots for both thermal and AF demagnetization, as well as (2) why MD grains affect thermal demagnetization plots even at high temperatures, while their remanence is completely removed within the first few AF demagnetization steps.