Investigation On Multiferroicity Of Polar Ferrimagnet Mn₂Mo₃O₈

The terminology‘multiferroics'refers to materials where multiple ferroic orders,i.e.ferroelectricity,(anti)ferromagnetism,ferroelasticity,and ferrotoroidicity,coexist and are intimately coupled,which promise potential applications in conceptually new devices,and provide novel physics.The field of multiferroicity has attracted considerable attention during the past two decades,and tens of new materials have been synthesized and a series of unconventional phenomena have been observed.In the present thesis,multiferroicity and magnetoelectric coupling of a polar ferrimagnet Mn₂Mo₃O₈ and its derivates are investigated.The thesis has the following aspects:1.We briefly introduce the research background of multiferroic materials,the microscopic mechanisms of multiferroiticity,and the basic physical properties and research progress of multiferroic A₂Mo₃O₈(A=Fe,Mn,Co,Ni).2.Several methods such as floating zone technique,flux method,and chemical vapor transport,for preparing single crystals are illustrated.Subsequently,the details of preparation of(Mn_1-xA_x)₂Mo₃O₈(A=Fe,Co)single crystals using chemical vapor transport method are presented.3.Magnetism and multiferroicity of Mn₂Mo₃O₈are studied.The ferrimagnetism of Mn₂Mo₃O₈ is revealed by magnetic measurements,arising from interacted magnetic sublattices of Mn²⁺.Electric polarization as a function of temperature and magnetic field are measured using a pyroelectric method.Direct correspondence between the magnetism and ferroelectricity is evidenced,and remarkable magnetoelectric coupling is observed.4.Multiferroicity and magnetoelectric effects of(Mn_1-xFe_x)₂Mo₃O₈are characterized.Upon Fe-doping,the lattice of crystal show anisotropic shrink,i.e.the a-axis is compressed by?0.4%,while the c-axis is shortened by?2%.At x=75%,an antiferromagnetic to ferrimagnetic phase transition is induced.By performing detailed magnetic and ferroelectric measurements,a phase diagram of(Mn_1-xFe_x)₂Mo₃O₈ single crystals is built up,in which various phases of matter are presented.The results show that the physical properties of Mn₂Mo₃O₈can be modulated efficiently by Fe-doping.5.The effect of Co doping on the magnetism of Mn₂Mo₃O₈ is systematically studied.It is found that the lattice of Mn₂Mo₃O₈ is compressed significantly by Co-doping,which is a lot more obvious than Fe-doping.Phase diagram of(Mn_1-xCo_x)₂Mo₃O₈is built up through detailed magnetic characterizations,where magnetic phase transitions can be seen by varying temperature and magnetic field.In comparison with Fe-doping,Co-doping is more efficient to induce a ferrimagnetic to antiferromagnetic transition,and the critical doping content is found to?35%.