The Interaction Potential

The interaction potentials and their spatial Fourer transforms are derived for nuclear and magnetic scattering, as well as for interactions with atomic electric fields. For the nuclear interaction, the Fermi pseudopotential is introduced and the scattering length operator is defined. The neutron spin dependence of the nuclear and magnetic interaction is calculated, and general expressions for spin-dependent scattering are obtained. The longitudinal and XYZ polarization analysis methods are described, and the technique of spherical neutron polarimetry is explained. The Blume-Maleev equation which gives the final neutron polarization for an arbitrary incident polarization are derived.

Proof of the elusive high-temperature incommensurate phase in CuO by spherical neutron polarimetry

CuO is the only known binary multiferroic compound, and due to its high transition temperature into the multiferroic state, it has been extensively studied. In comparison to other prototype multiferroics, the nature and even the existence of the high-temperature incommensurate paraelectric phase (AF3) were strongly debated—both experimentally and theoretically—since it is stable for only a few tenths of a kelvin just below the Néel temperature. Until now, there is no proof by neutron diffraction techniques owing to its very small ordered Cu magnetic moment. Here, we demonstrate the potential of spherical neutron polarimetry, first, in detecting magnetic structure changes, which are not or weakly manifest in the peak intensity and, second, in deducing the spin arrangement of the so far hypothetic AF3 phase. Our findings suggest two coexisting spin density waves emerging from an accidental degeneracy of the respective states implying a delicate energy balance in the spin Hamiltonian.

Mag2Pol: a program for the analysis of spherical neutron polarimetry, flipping ratio and integrated intensity data

Mag2Pol is a graphical user interface program which is devoted to the treatment of data from polarized neutron diffractometers with spherical polarization analysis. Nuclear and magnetic structure models can be introduced using space-group symbols and individual symmetry operators, respectively, and viewed in an OpenGL widget. The program calculates nuclear/magnetic structure factors, flipping ratios and polarization matrices for magnetic Bragg reflections, taking into account structural twins and magnetic domains. Spherical neutron polarimetry data can be analyzed by refining a magnetic structure model including magnetic domain populations in a least-squares fit and can also be correlated with an integrated intensity data set in a joint refinement. Further features are the simultaneous refinement of nuclear and magnetic structures with integrated intensity data and the analysis of flipping ratios either with tabulated magnetic form factors or using a multipole expansion of the magnetization density.