Anomalous Magnetic Behaviors And Mechanisms In Titanium-based And Vanadium-based Spinel Oxides

Transition metal spinel oxides are one of the important research fields of condensed matter physics due to their rich physical properties,such as multiferroic,negative thermal expansion,magnetic shape memory effect,and application values in many fields,such as microwave devices,magnetic storage.Geometrical frustrations and the interplays among the orbital,spin,and lattice degrees of freedom determine the magnetic,orbital,and structural transitions in spinel systems.However,there are still a lot of controversies and ambiguities in the study of magnetic,orbital,and structural transitions of titanium-based and vanadium-based spinel oxides.For example,the correlation between geometrical frustration and magnetic structure in the inverse spinel is still unclear.Moreover,the magnetic,orbital,and crystal structure of the spinel systems control the morphology of magnetic domain and crystallographic domain,but the existing researches have paid less attention to the correlation of the arrangement of magnetic domain and crystallographic domain on the physical properties.In order to understand these problems,in this paper,we synthesized a variety of titanium-based and vanadium-based spinel samples,and then investigated their lattice,magnetic,orbital structures and their complex phase transition mechanisms.As a typical Jahn-Teller crystal,MnV₂O₄ shows complex phase transition processes,but their mechanism is still not clear.The correlation among orbital states,crystallographic domains arrangement and physical properties is also uncertain.In this thesis,the Mn_1-xMg_xV₂O₄(x=0,0.05,0.1,0.15)single crystals were grown using the optical floating-zone technique,and their variable temperature XRD,magnetic,specific heat,thermal expansion,and thermal conductivity were measured.We found that a certain degree of orbital fluctuations after structural transition still exists,and was brought about by the anomalous interchain interaction.The orbital fluctuations completely disappear at lower temperature,accompanying by the rotation of the easy magnetization axis,which further leads to the rearrangement of the crystallographic domains,and then causes the large deformation of the sample.In addition,we also found that Mg doping can enhance the orbital fluctuation,and then enhance the magnetic anisotropy of the system,which significantly reduces the remanent strain and improves the magnetostriction performance.Inverse spinel Co₂TiO₄ exhibits a series of unusual physical properties,such as the minimum value of magnetization,the specific heat peak at the compensation temperature,and the sign change of exchange-bias field under H>20 kOe,but their origins are still unclear.We investigated systematically the anomalous properties by using the DC and AC magnetic susceptibilities,specific heat,thermal expansion,magnetostriction,and neutron diffraction measurements,and found that the Co₂TiO₄ exhibits a semi-spin-glass magnetic structure,in which the ferrimagnetic ordered longitudinal component coexists with the disordered transverse component freezing into spin glass state.The oblique spin at B sites can be easily deflected by external magnetic field,bringing about the variation of the longitudinal components.The ferrimagnetic component can be rotated 180°at T_comp under H?15 kOe.These two points play a key role on leading to these anomalous phenomena.The magnetic ordering and structural transition of inverse spinel Fe₂TiO₄ have been controversial.Stoichiometric ratio Fe₂TiO₄ crystal were grown by the optical floating-zone technique,which exhibits the ferrimagnetic ordering at?115 K,and a structural transition from cubic to tetragonal phase at?82 K,accompanied by the rotation of easy magnetization axis.We found that Fe₂TiO₄ single crystal exhibits a magnetic compensation phenomenon at 66 K for the first time.In addition,due to the limitation of easy magnetization axis and tetragonal domains,the hysteresis loops and magnetostriction curves of Fe₂TiO₄ in ZFC and FC processes are significantly different.Moreover,Fe₂TiO₄ single crystal exhibits significant negative thermal expansion effect in the FC process.The reverse spinel Mn₂TiO₄ has a cubic structure when Mn²⁺ions and Ti⁴⁺ions occupy randomly on B sites,but a tetragonal structure when Mn²⁺ions and Ti⁴⁺ions are alternate orderly occupy on B sites.We found that the ordering arrangement of Mn²⁺ions and Ti⁴⁺ions at B sites have no effect on the magnetic structure,but can significantly lower the line width of ferromagnetic resonance and the Kilbert damping.Fe₂VO₄ single crystal exhibits a collinear ferrimagnetic ordering structure below?460 K,and a rotation of easy magnetization axis at?46 K.Co₂VO₄polycrystal shows a ferrimagnetic ordering transition at?162 K and a magnetic compensation behavior at?81.5 K.Mn₂VO₄ polycrystal displays a ferrimagnetic ordering transition at?45 K.Furthermore,through the analysis of the correlation between the magnetic structure and geometrical frustration in titanium-based and vanadium-based inverse spinel,we found that the inverse spinel with low frustration factor(such as Fe₂VO₄ and Co₂VO₄)show the collinear ferromagnetic structure(Neel ordering),and show the non-collinear ferromagnetic structure(YK ordering)with high frustration factor(such as Mn₂TiO₄ and Mn₂VO₄).However,the magnetic structure of inverse spinel(such as Co₂TiO₄ and Fe₂TiO₄)is easily disturbed by the magnetic field,bringing about the deflection of the oblique spin moments at B sites under magnetic field.