The effect of surface roughness on the hysteresis properties of single-domain and pseudo- single-domain grains of magnetite.

Numerical micromagnetic modelling has provided huge advances in our understanding of the stability of paleomagnetic remanences in both single domain (SD) and pseudo-single-domain (PSD) grains. In mineral magnetism, the numerical model has usually employed finite difference (FD) method, which allows simulation of magnetic domain structures in relatively large grains (up to one micron). The FD models are constructed from regularly shaped cubic cells, and so the efficiency of computation is made at the expense of the accuracy with which a grain's geometry can be represented. However, one of the most important factors that affect the domain state, and the stability of the paleomagnetic recording, is the internal demagnetizing field. The demagnetizing field is in turn determined by both the grain size and its geometry. By taking a finite element/boundary element (FEBE) approach where the grain geometry is represented by arbitrary shaped tetrahedral elements, a much better representation of the grain geometry can be achieved. Thus a much more accurate determination of the demagnetizing field is possible. The FEBE approach, therefore, allows us to examine, for the first time, the influence of irregular grains shapes and in particular the grain surface roughness, on the nucleation of domain states. This initial study will look at the effect of surface roughness on spherical grains of magnetite between 30nm to 100nm in diameter, covering the SD to PSD grain size range. Spherical grains are chosen in order to eliminate any influence of grain shape other than the surface roughness. In addition, the effects of magnetocrystalline anisotropy are ignored. The roughness is defined both in terms of the average amplitude of the surface peaks above the mean diameter of the sphere, as well as the mean angular frequency of surface the peaks and troughs. The results demonstrate that, as expected, rough surfaces act to encourage nucleation of domain reversals in the SD grain size range. Thus coercivities will be greatly reduced. However it is observed that in the PSD grain size range the irregular internal demagnetizing field produces non-symmetric vortex domain states, which have a much higher coercivity and saturation remanence. The implication is that that while the truly SD grain size range maybe smaller, the PSD size range is capable of contributing a much larger and stable remanence than previously thought.