Magnetization And Electric Polarization Behavior Of Fe/Co Antiferromagnetic Oxides

Generally,the transition metal oxides have abundant physical properties because of the complex electron spin and orbital interactions of the d electrons,such as spin frustration,magnetion platform,magnetoelectric coupling,magnetostriction,magntic phase transition.In this essay,CuFe_1-xM_xO₂?M=Mn,Co?,Na₂Co₂TeO₆ and Na₅Co_15.5Te₆O₃₆ are selected to explore the magnetization and electrical polarization properties,and to investigate the coupling effect between them.In these studies,we hope to deepen our understanding on the complex magnetic interaction of the transition metal antiferromagnetic oxides,to explore and develope new high performance magnetoelectric coupling materials.1.The research background and progress of the three kinds of transition metal oxides are briefly introduced.Firstly,the crystal structure,magnetic structure,magnetization platform induced by magnetic field and magnetoelectric coupling of spin frustration CuFeO₂ are demonstrated,followed by the influence of ion-doping on magnetoelectric coupling behavior.Secondly,the research status of Na₂Co₂TeO₆ polycrystal with honeycomb crystal structure is introduced,including crystal structure,magnetic structure at low temperature and basic magnetic and dielectric properties.Finally,the crystal structure,basic magnetic and electric properties of hexagonal system Na₅Co_15.5Te₆O₃₆ are introduced.2.The magnetoelectric coupling mechanism of CuFeO₂ mainly through regulation of temperature,magnetic field and doping concentration of magnetic Mn³⁺and Co³⁺ions are explored.The influence of Mn³⁺and Co³⁺doping concentrations on CuFeO₂ magnetic structure,electrical polarization and magnetoelectric coupling behaviors are studied by both the static magnetic field and pulsed high magnetic field.Experimental results indicate that the ion-doping affect the magnetic interaction between Fe³⁺ion and release spin frustration of CuFeO₂ and bring in disorder,which make the magnetic transition critical temperature and magnetic transition critical field move to lower magnetic field and lower temperature.A small amount of ion doping will change the 4SL ground state into the coexistence of 4SL state and the antiferromagnetic ferroelectric spiral phase,also expand magnetic field region of the spontaneous electric polarization in CuFeO₂.When the concentration of doped ions become x=0.05,the antiferromagnetic ground state becomes a complete ferroelectric spiral phase.Finally,micro mechanism models for electric polarization and dielectric generation of CuFeO₂ and Mn³⁺(Co³⁺)doped systems are established based on the magnetoelastic models of double exchange and superexchange mechanisms.According to this model,the polarization and dielectric behavior observed in this paper are well explained.3.The Na₂Co₂TeO₆ single crystal with honeycomb layer structure was synthesized by cosolvent method.The crystal structure of Na₂Co₂TeO₆ was analyzed,which shows that Na₂Co₂TeO₆ has a quasi-two-dimensional layered structure which are separated by Na⁺layers.CoO₆ octahedron formes honeycomb structure within the layers.Magnetization behavior shows obvious magnetic anisotropy in the single crystal.At low temperature,with temperature increasing,continuous magnetic phase transitions can be observed.Two magnetic phase transitions were observed at the temperatures T_N1^?14??14?26 K and T_N2^?14??14?4 K with magentic field along c axis,three magnetic phase transitions were observed at the temperatures T^?_N1²7 K,T^?_N2²0 K and T^?_N3⁹.5 K when magnetic field perpendicular to c axis respectively,which results from spin reorientation of magnetic ions.Dielectric constant change and the dielectric loss peak near room temperatur indicate that the system has dielectric polarization behavior near room temperature,which can be used as room temperature dielectric material.4.Complex magnetic and dielectric behaviors of hexagonal symmetry Na₅Co_15.5Te₆O₃₆single crystal were investigated by static and pulsed high magnetic field.The results show that,in the 4.2 K isothermal magnetization measurement,with the magnetic field increase along c axis,the magnetization of Na₅Co_15.5Te₆O₃₆ jumps and occurs spin flip at B⁶.2 T,followed by a magnetization platform,also the magnetocaloric is accompanied by the magnetic phase transition.The time dependence of magnetizaon and mangtic field under the static magnetic field are measured with SQUID.It is found that the system has very long magnetic relaxation time at 2 K,Which may attrucate to the magnetic moment of the system is pinned at low temperatures.PPMS and impedance analyzer were used to study the dielectric behavior at low temperature,which found that the system exhibits a certain magnetic dielectric effect at low temperature.