Abstract:
The perovskite structure is one of the most versatile ceramic host, for example for magneto-electrically coupled multiferroics. Recent studies showed that doping rare earth manganites (as EuMnO3) with smaller cations (as yttrium) results in a new family of manganites with properties comparable to rare earth manganites with similar tolerance factor, but unfortunately some interesting rare earth manganites cannot be examined by neutron diffraction, as they are highly neutron absorbing, like Eu or Sm. One of the goals of this thesis is to replace Nd in NdMnO3 with Y to reduce the tolerance factor and therefore to simulate the magneto-electric properties of RMnO3 to investigate the evolution of the multiferroic properties as in TbMnO3 and DyMnO3. Polycrystalline and single crystal Nd1-xYxMnO3 compositions were synthesized with solid state reaction technique and floating zone method. Magnetic properties of the polycrystalline samples were measured as well as the x-ray and neutron diffraction patterns to initially map out a phase separation where the A-type antiferromagnetic order co-exists with a modulated spin order and is visible at approximately x ~ 0.40 to 0.45. Neutron diffraction of single crystals proved this transition with increasing x and the temperature dependence of the modulated spin order from paramagnetic to a collinear spin density wave along the b-direction and at 2K to form a cycloidal structure with the cycloid in the bc plane propagating along the b-direction. This cycloidal order is responsible for the ferroelectric polarization of TbMnO3. Physical property measurements confirmed that with establishment of the spin cycloid in Nd1-xYxMnO3 the material exhibits a spontaneous polarization along the c-direction. This polarization can be flopped from c to a by applying a magnetic field along b or c, what is comparable to TbMnO3 and DyMnO3. Temperature dependent neutron scattering experiments confirmed the co-existence of both magnetic components. The A-type antiferromagentic order decreases in intensity and transition temperature with increasing x until x < 0.45 while the modulated spin component is measurable at x > 0.35. High resolution measurements revealed that the modulated spin component has two appearances: one with a shorter wavenumber and one with a lower wavenumber. The first one has a stable wavenumber and shows long range order while the latter one is only detectable when a A-type component is present. The wavenumber, intensity and width of the reflection of the latter one is strongly dependent on the A-type component leading to the conclusion that this is a buffer between the two magnetic components existing at low x (A-type order) and low x (incommensurate spin order).