Neutron scattering studies of helicoidal magnet and 2D magnetic materials

Abstract

This thesis focuses on the use of complementary neutron scattering techniques to study both statics and dynamics of multiferroic MnSb₂O₆ and crystal and magnetic structures of 2D van der Waals magnetic materials.

The first part of this thesis is dedicated to the study of the helicoidal magnet MnSb₂O₆ which crystallizes in the trigonal space group P321. Its structural chirality is coupled to its magnetic ground state under Tₙ ≈ 12 K through symmetric Heisenberg exchanges. One chapter is dedicated to neutron diffraction studies on MnSb₂O₆ for a detailed study of both crystal and magnetic structures. While the magnetic ground state could not be unambiguously determined, between a pure cycloidal or a mixture of cycloidal and helical order, the spin rotation plane can be tilted by the application of low external magnetic field. Subsequently, a mechanism based on the coupled structural and magnetic chiralities is proposed to explain the previously reported ferroelectric switching. The following chapter presents the study of the spin dynamics in MnSb₂O₆. By performing sum rules analysis on inelastic neutron scattering data, the seven exchange constants based on a Heisenberg model are extracted. Testing the stability of the spin excitations validates the cycloidal ground state.

The last part of the thesis focuses on the structures of 2D van der Waals magnets. In these materials, the magnetic properties are coupled to the stacking of the twodimensional layers, in particular in Fe₃₋ₓGeTe₂ and Fe₁₊ₓTe, where the magnetism can be tuned by the iron concentration. Single crystal neutron diffraction study on Fe₃₋ₓGeTe₂ is reported as well as spherical neutron polarimetry on Fe₁₊ₓTe. Finally, X-ray powder diffraction results on the crystal structure of honeycomb layered VI₃ are presented.