In conventional ferromagnets, the magnetic moments of each domain are aligned along one of the energetically preferred crystal directions ("easy axes"). However, in nanocrystalline ferromagnets, the direction of the easy axes jumps on the scale of grain size (i.e., a few nanometers). If the ferromagnetic exchange length is greater than this scale, the magnetization cannot follow the jumps. This has significant consequences for the macroscopic magnetic behavior. Magnetic neutron small-angle scattering is often the only way to study the variation in spin (mis-)orientation in volume and with the required resolution of nanometers. In addition, the method of field-dependent magnetic neutron small-angle scattering developed by us allows the quantitative investigation of magnetic interaction terms such as exchange coupling as well as the strength and spatial structure of the anisotropy field. 
 

This work is done in cooperation with  PD Dr. Andreas Michels.

Selected Publications:

Differential scattering cross section of a nanocrystalline nickel sample at different applied magnetic fields. The lines are fits with a micromagnetic model. From the fit parameters, the spatial structure and strength of the anisotropy field can be determined. These quantities are typically inaccessible to other methods.

Independent of model neutron scattering data determined values for the correlation length of the spin misorientation in nanocrystalline cobalt as a function of the magnetic field. The line is the prediction of the "Random Anisotropy" model (without free parameters).