Experimental observation of magnetic poles inside bulk magnets via q≠0 Fourier modes of magnetostatic field

The pole-avoidance principle of magnetostatics results in an angular anisotropy of the magnetic neutron scattering cross section dΣ_M/dΩ. For the case of a sintered Nd–Fe–B permanent magnet, we report the experimental observation of a ‘spike’ in dΣ_M/dΩ along the forward direction. The spike implies the presence of long-wavelength magnetization fluctuations on a length scale of at least 60 nm. Using micromagnetic theory, it is shown that this type of angular anisotropy is the result of the presence of unavoidable magnetic poles in the bulk of the magnet and is related to the q≠0 Fourier modes of the magnetostatic field. Thus, our observation proves the existence of such modes. (paper)