Modelling Self-demagnetisation and Body-body Interactions in the Fourier Domain

The assumption of a linearly induced magnetisation begins to break down in bodies with moderate to high susceptibilities. These bodies exhibit self-demagnetisation effects which tend to suppress the observed field and rotate the magnetisation vector into the plane of the body. We present a rapid Fourier method to accurately model these effects, including anisotropy of susceptibility and remanence, in a discretised volume. This method is tested for accuracy against the known analytic demagnetising effect of an ellipsoid. The Fourier method performs as well as other real space numerical schemes with a maximum error of better than 5% and 3% for the coarse and fine discretised grids, respectively. The speed of the method also allows us to model multiple bodies in a single volume and investigate the effect of body-body field interactions. Two vertical sheets are modelled where the self-demagnetisation of each sheet individually is found to suppress the field by around 18%, and to rotate the magnetisation vector into the plane of the body. The two bodies are then modelled simultaneously to demonstrate the body-body interaction effects, and this is found to enhance the demagnetising effect of each sheet by around 3%, and to slightly counteract the magnetisation rotation effect.