Magnetoelectric multiferroics exhibit a strong correlation of magnetism and ferroelectricity. Among the multiferroics with a strong magnetoelectric effect many have a chiral antiferromagnetic structure. In these materials it is possible to control the electric polarisation by an applied magnetic field and, inversely, to manipulate the antiferromagnetic domains by an applied electric field. The observation of antiferromagnetic domains requires a microscopic method, which neutron scattering with polarised neutrons is particularly suitable for. This thesis reports on neutron and X-ray measurements on several (chiral) antiferromagnetic multiferroics. Special attention is devoted to the switching of (chiral) antiferromagnetic domains.
Neutron-diffraction data on the magnetic structures of the pyroxenes NaFeSi2O6 and LiFeSi2O6 are presented. LiFeSi2O6 undergoes a single magnetic phase transition below 18 K into a canted antiferromagnetic structure with the magnetic space group P21/c. NaFeSi2O6 undergoes two magnetic phase transitions. Both phases are incommensurate with propagation vector k = (0, 0.77, 0). Below 8 K a transverse spin-density wave with moments in the ac plane sets in and below 6 K a helix with moments remaining in the ac plane evolves. By the use of spherical neutron polarisation analysis it is demonstrated that antiferromagnetic domains in LiFeSi2O6 can be reversed by a combination of electric and magnetic fields. The magnetic structure of LiFeSi2O6 gives rise to a toroidal moment. Therefore, the results are discussed in the context of manipulating toroidal domains.
Furthermore, the magnon dispersion and the spin density of LiFeSi2O6 are presented.
In many chiral multiferroics it is possible to reverse the chirality of the magnetic structure by an applied electric field providing the opportunity of driving hysteresis loops (chiral ratio vs. electric field). Results of the time dependence of this switching process in MnWO4 studied by stroboscopic techniques for polarised neutron scattering reveal a surprisingly slow relaxation process in the time scale of 2 ms to 30 ms and a strong temperature dependence.
Furthermore, static hysteresis loops recorded on TbMnO3 and DyMnO3 are reported. In TbMnO3, the coercive field increases linearly with decreasing temperature. In DyMnO3, driving of hysteresis loops is possible only close to the ferroelectric phase transition. Further investigations on TbMnO3 show that the quasi-lock-in of the magnetic propagation vector takes place at temperatures slightly above the development of the chiral magnetic structure. In addition, the propagation vector increases linearly with isotropic pressure. X-ray diffraction on single crystals of TbMnO3 and YMn2O5 reveals that the deviation of the ions from their centrosymmetric positions in the ferroelectric phase is beyond the resolution limit of the performed diffraction experiments
