  Taking advantage of recent progress in neutron instrumentation and in the
understanding of magnetic-field-dependent small-angle neutron scattering, here,
we study the three-dimensional magnetization distribution within an isotropic
Nd-Fe-B bulk magnet. The magnetic neutron scattering cross section of this
system features the so-called spike anisotropy, which points towards the
presence of a strong magnetodipolar interaction. This experimental result
combined with a damped oscillatory behavior of the corresponding correlation
function and recent micromagnetic simulation results on spherical nanoparticles
suggest an interpretation of the neutron data in terms of vortex-like
flux-closure patterns. The field-dependent correlation length Lc is well
reproduced by a phenomenological power-law model. While the experimental
neutron data for Lc are described by an exponent close to unity (p = 0.86), the
simulation results yield p = 1.70, posing a challenge to theory to include
vortex-vortex interaction effects.
