Anisotropy field

$$\mathbf{H}_\mathrm{ani}=\frac{2 K_1}{\mu_0 M_\mathrm{s}^2} \mathbf{a}(\mathbf{M} \cdot \mathbf{a})$$

$K_1$	(J/m$^3$)	first anisotropy constant,
$\mu_0$	$=4\pi 10^{-7}$ Vs/(Am)	magnetic field constant,
$M_\mathrm{s}$	(A/m)	saturation magnetization,
$\mathbf{a}$		unit vector in direction of the easy axis,
$\mathbf{M}$	(A/m)	magnetization vector of the computational cell