Numerical calculation of the stray field

We compute the stray field from our simulation data as it would be read from a magnetic force microscope by simulating several layers of ``empty space'' above the sample and measuring the demagnetising field. This has the advantage that the efficient OOMMF algorithm for the computation of the demagnetising field can be used. However, we need to compute the second derivative numerically (using central differences):

$${\partial^2f \over \partial x^2} = {f(x+h) - 2f(x) + f(x-h) \over h^2} + \mathcal{O}(h^2)$$ (6.6)

where $h$ is the distance between the upper and lower layers of the demagnetising field.