Zone center physics in magnetic diffuse neutron scattering

Probing physical quantities using neutron scattering is crucial in many areas of physics. Magnetic neutron scattering measures a generalized magnetic susceptibility, but which susceptibility is actually measured as the scattering vector Q→0? We consider the case of spin ice, which lends itself well to such a study as it obeys both the static approximation of neutron scattering, the classical fluctuation-dissipation theorem and it also has an extremely large paramagnetic susceptibility. By comparing a direct magnetic measurement using a SQUID magnetometer and a neutron scattering measurement near a Brillouin zone center, we show that the external susceptibility is recovered rather than the intrinsic one. Theoretically we investigate this result by Monte Carlo simulations of the spin ice Hamiltonian with different boundary conditions. Our conclusion is that when measuring the paramagnetic structure factor for a Brillouin zone center point in Fourier space, a demagnetizing transformation may be needed in order to recover the intrinsic susceptibility of a magnetic material.