Order and disorder in the charge and spin structures of $YFe_{2}O_{4}-delta$ and $Ni_{0.42}Mn_{0.58}TiO_{3}$

YFe2O4-The main part of this thesis is dedicated to the determination of the charge order crystallographicstructures of YFe2O4- at 160K and 200 K. YFe2O4- is above the charge orderingtransition isostructural to LuFe2O4- with the space group R¯3m. The structure is characterizedby an alternating stacking of triangular Yttrium layers and Fe bilayers each triangular,with a mean iron valence of 2.5. The arrangement of the Fe2+ and Fe3+ ions on the triangularlattice leads to frustration which is resolved at lower temperates through charge order.LuFe2O4- was long time the primary example for a charge order driven ferroelectric materialbut was found to be not ferroelectric. The isostructural YbFe2O4- is indicated to showferroelectricity by one recent report, although much weaker than predicted for LuFe2O4-. Inthe series of the RFe2O4- ferrites YFe2O4- is of special interest because it is the upper endmember in regard of the ionic radius of the rare earth ion, which determines the intra andinter Fe bilayer spacing and therefore interaction strengths and ultimately the charge order.The charge order of YFe2O4- at 160K is described by a propagation vector of141434witha symmetry reduction from R¯3m to P¯1. The charge order is found to be bimodal in contrast toprevious results. The charge order at 200K is incomplete with a seven times enlarged supercellbased on a propagation vector of272737again with spacegroup P¯1 in contrast to a previouslysuggested monoclinic cell.Based on this charge ordered cell possible magnetic structures are evaluated using singlecrystal neutron diffraction. In contrast to the observation of well separated phases in X-raydiffraction, in neutron diffraction at 200K and 160K a superposition of the two CO-phases isobserved, although the phases observed in X-ray diffraction are dominant. This is most likelydue to the larger sample volume and different cooling times. The magnetic cell based on the200K charge-order structure is characterized by a propagation vector of5145141814hex. Themagnetic cell based on the 160K charge order cell is based on a propagation vector of01212CO. Both cells have the magnetic spacegroup Ps¯1. The magnetic phases show no metamagnetictransitions up to 24 T in contrast to LuFe2O4-.INi0:42Mn0:58TiO3The smaller part of the thesis is dedicated to another mechanism of multiferroicity, a vortexlike arrangement of spins having a toroidal moment breaks both space and time inversion andis intrinsically magnetoelectric. Such an toroidal moment can be induced in specific materialsby the application of crossed electric and magnetic fields.Ni0:42Mn0:58TiO3 is of particular interest because it is so far the only material in whicha toroidal moment can be induced in absence of long range magnetic order, since it is anXY-spin glass. Single crystals of Ni0:42Mn0:58TiO3 were grown by the optical floating zonemethod and the the Ni/Mn-ratio was confirmed by powder and single crystal X-ray diffraction.The spin glass behavior is proved by a magnetic memory test and the observation ofa frequency shift in the AC-susceptibility. A magnetoelectric effect as reported in the literaturecould not be reproduced after magnetoelectric field cooling. Neutron diffraction showsdiffuse magnetic scattering along (00`) with enhanced intensity at the positions expected forNiTiO3 and MnTiO3. Polarization analysis confirms the spin direction in the chex-plane butan evaluation of the toroidal state could not be performed due to experimental problems