Thesis presented November 29, 2018

Abstract:
The thesis will focus on the study of quasi-one-dimensional compounds of the family AM₂V₂O₈ (A = Ba, Sr, Pb, ..., M = Co, Mn, Zn, Ni, ...), in the context of the study of incommensurable longitudinal magnetic order induced by magnetic field and in excitations in zero field and under field. Particular emphasis will be placed on research of longitudinal magnetoelectric modes. The first part of the thesis is to continue research on the compound BaCo₂V₂O₈: study by neutron scattering changing magnetic excitations with an applied field (in the phase of Neel and the incommensurate phase) and the effect of applying a uniaxial pressure (in the direction of channels) on the order and magnetic excitations. A second part of the thesis will concern the study of non centrosymmetric SrCo₂V₂O₈ made in the hope of discovering the ferroelectricity and thus measure the longitudinal modes which are also electromagnons. Electrical polarization measures will take place at Institut Néel and THz spectroscopy, in SOLEIL, will be performed for this purpose. Along with this, the same kind of neutron studies as those conducted in BaCo₂V₂O₈ will be done. Finally, a third part of the thesis is to find, in the AM₂V₂O₈ family, a compound with radically different magnetic properties than BaCo₂V₂O₈ or SrCo₂V₂O₈, to continuously pass this new compound XCo₂V₂O₈ (X = Ba, Sr), to establish the complete phase diagram as a function of doping level to identify and study a quantum critical point. It will initially carry out macroscopic measurements (specific heat, magnetization, the CEA and the Institut Néel) and microscopic measurements (neutron scattering) at ILL.

Keywords:
Quantum magnetism, Order and excitations, Quantum phase transitions, Neutron scattering, Spin chains

On-line thesis.