Magnetic screening in multidomain material.

Abstract

The screening of domain moments associated with the displacement and pinning of a single domain wall from an initial energy minimum position is investigated . The domain moment acquired by the pinning of a domain wall in this fashion is reduced (or screened) by the subsequent readjustment of the non-pinned, mobile domain walls within the magnetic material. Although only a simple , one-dimensional domain geometry is considered, the screening effects are quite variable depending on the grain size, the number of domains and which wall is pinned. In all cases , however, the screening in grains with a few mobile domain walls is generally very large . The results of these screening calculations have important consequences regarding the grain size dividing stable from unstable remanence in multidomain grains.

Toward a better understanding of magnetic screening in multidomain grains.

“…This case has been previously considered by MOON and MERRILL (1986a). From (8), the screening factor, al, (hereafter the numerical subscript in a refers to the number of pinned walls in a grain) is:…”

“…(The screening factor, a, is called the reduced moment by MOON and MERRILL (1986a). In addition, M0=0 in their calculations because only grains with an even number of domains were considered.)…”

“…As a consistent check, we calculate al from (9) as a function of wall pinned, k, for a 12-domain grain with an aspect ratio of 3 (as was used by MOON and MERRILL (1986a)). …”

“…As a consequence, the magnetization associated with pinned walls in a grain can be masked, or screened, by regions in which walls are free to move. The first rock magnetic study using a microscopic approach to treat magnetic screening was conducted by MOON and MERRILL (1986a), who arbitrarily displaced and pinned a single wall in a grain. They found that screening could be amazingly effective and counter-intuitively that in a few circumstances it was even possible to produce a net moment opposite to the original pinned moment.…”

“…In addition, the approximation enables one to carry out analytical calculations. Screening in a MD grain is generally very complex, and it was argued by MOON and MERRILL (1986a) that it might even lead to a self-reversal of a grain in some cases. However, it is shown in this paper that the particular domain state required for such a self-reversal will not likely ever be realized in practice.…”