Magnetische Neutronenkleinwinkelstreuung an inhomogenen Ferromagneten

Small-angle neutron scattering (SANS) is a powerful method to investigate the magnetic microstructure in the bulk and on length scales between about 1 − 100 nm. SANS experiments are commonly realized with an unpolarised or a polarised incident neutron beam, however, an analysis of the neutron spin state after the scattering process is usually not performed. As a result of the development of efficient 3He spin filters, a so-called longitudinal (one-dimensional) neutron-polarization analysis (POLARIS) at SANS instruments is now routinely available. The analysis of POLARIS data allows one to extract the Fourier components of the magnetization vector field. Experimental results on Fe-based two-phase nanocrystalline alloys serve as examples in order to demonstrate the capabilities of the POLARIS technique.
In the investigated soft magnetic heterogeneous nanocomposites, the jump of the magnetization at the phase boundary between particles and matrix gives rise to a magnetostatic stray field, which represents a non-negligible source of spin disorder. As one of the central results of this dissertation, an analytical micromagnetic theory is presented, which allows one to analyze the magnetic-field-dependent SANS cross section of such particle-matrix-type ferromagnets. It is shown that the magnetic anisotropy field and the magnetostatic field decisively determine the properties of the magnetic SANS cross section, such as the asymptotic power-law behaviour, the range of spin-misalignment correlations and angular anisotropies. In particular, the theory explains the “clover-leaf”-shaped angular anisotropy, which is observed for several nanostructured magnetic materials and which can be attributed to jumps of the magnetization at internal interfaces. The micromagnetic aproach allows one to specify the field behaviour of the magnetic SANS and to quantitatively assess substantial features of the spin structure, for instance, the average exchange-stiffness constant and the mean magnetic anisotropy and magnetostatic fields.