Fingerprint of vortexlike flux closure in an isotropic Nd-Fe-B bulk magnet

Taking advantage of recent progress in neutron instrumentation and in the understanding of magnetic-fielddependent small-angle neutron scattering, 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 toward the presence of a strong magnetodipolar interaction. This experimental result combined with the 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 vortexlike 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.