Research On Magnetic And Electrical Tunability In Lu Fe₂O₄ And Topological Magnetism In Confined Structures

Multiferroics are defined as materials that exhibit more than one of the primary ferroic properties such as?anti-?ferromagnetism,ferroelectricity and ferroelasticity.Nowadays,the scope of multiferroics has been extended to include any case in which spontaneous magnetic and dielectric orders coexist.A tranditional classification for multiferroics mainly includes so-called type-I and type-II.For type-I,the ferroelectricity and magnetism take place at various temperatures,arising from different mechanisms.For type-II,the magnetic ordering breaks the inversion symmetry and directly causes the ferroelectricity.Lu Fe₂O₄ with rhombohedral structure is one type of charge ordered multiferroics belonging to type-I.LuFe₂O₄ has a layered structure with Fe-containing double layers of triangular connectivity.For stoichiometric sample,the ideal Fe²⁺/Fe³⁺ratio is 1:1,where the triangular lattice produces the frustration.The main results can be described as follows:?a?Ca doped Lu_1-xCa_xFe₂O₄ were prepared by conventional solid state reactions in vacuum environment.Firstly,the dielectric permittivity of Lu_1-xCa_xFe₂O₄ were investigated.It exhibits a linear decrease with increasing applied electric fields.The dielectric tunability of Lu_1-xCa_xFe₂O₄ can be effectively enhanced through Ca doping in a broad temperature range.For example,the vlue of n_r for x=0.1 reaches as high as95.4%at frequency of 2 MHz at RT.For Lu_1-x-x Ca_xFe₂O₄ system,the charge ordering and polarization fluctuation are greatly influenced by the ratio of Fe³⁺/Fe²⁺based on the dielectric,magnetic and transport properties.The high dielectric tunability is critical for the industrial application due to its high signal-to-noise ratio,and the stable status in a wider temperature range can enhance its working performace in real devices.Then,the influence on activation energy by Ca doping was investigate by various models.The frequency dependence of the characteristic temperature in dielectric loss tangent obeys Arrhenius relationship.The fitted activation energy Q is 0.30 eV,0.24eV,0.22 eV for x=0,0.05 and 0.1 respectively.In general,?-T curve can be well explained by the model of thermal activation in paramagnetic temperature range.The fitted activation energy E is 0.34 eV,0.26 eV,0.24 eV,respectively.The values of Q and E are very close,impling that the activation energy can be tuned by Ca doping due to a tuned Fe³⁺/Fe²⁺ratio.?b?Another type of multiferroic RCrO₃ with perovskite structure were prepared because of its abundant physical phenoma.LaCrO₃ and SmCrO₃ were prepared by conventional solid state reactions and their magnetic,dielectric,and transport properties were inverstigated in detail.LaCrO₃ presents a paramagnetic-antiferromagnetic transition at T_N=290 K.However,two transitions were detected for SmCrO₃ at 43 K and 197 K,respectively,corresponding to Neel temperature and spin reorientation transition.The fitted activation energy through Arrhenius relation is 0.37 eV and 0.35eV for LaCrO₃ and SmCrO₃,respectively.?c?Geometrically-confined biskyrmions were inverstigated.Here,our focus is given on a new magnetic structure of skyrmions discovered in recent years.The room temperature biskyrmions material of MnNiGa was chosen as the research sample.MnNiGa nanostripe were fabricated by focused ion beam?FIB?technique.In order to investigate the effect of nanostripe width on biskyrmions,a wedge-shaped nanostripe was also fabricated.In order to overcome the difficulty in TEM sample fabrication with certain crystal orientation by FIB,a new two-step fabrication method was developed.High quality of nonostripe and wedge-shaped nanostripe were successfully fabricated by this method.A single biskyrmion chain was unambiguously observed in the nanostripe.The biskyrmion structrure take place at different magnetic fields in the wedge-shaped nanostripe beacause of the varied confined width.Compared with the nanostripe,a different biskyrmion angle was observed in the wedge-shaped sample because different confined widths can produce various helical configurations.