Magnetization of ferrofluids with dipolar interactions: A Born-Mayer expansion

For ferrofluids that are described by a system of hard spheres interacting via dipolar forces we evaluate the magnetization as a function of the inter nal magnetic field with a Born-Mayer technique and an expansion in the dipo lar coupling strength. Two different approximations are presented for the m agnetization, considering different contributions to a series expansion in terms of the volume fraction of the particles and the dipolar coupling stre ngth.