Ferroelectric/Antiferroelectric Materials Phase Transition Behaviors And Magnetoelectric Coupling Effect

The spontaneous polarization exists in ferroelectrics in a certain range of temperature.When the polarization state changes under an application of external field,various functional effects will be produced,which have been widely used in the applications of medical equipment,defense weapons,and livelihood and environmental protection fields.When the polarization changes rapidly with temperature,especially in the vicinity of phase transitions,a large pyroelectric effect and electrocolaric effect can be generated in ferroelectric and antiferroelectric ceramics,which can be used to develop infrared detectors and solid-state refrigeration technology.Therefore,studying the phase transition behavior of ferroelectric and antiferroelectric ceramics is a basis to enhance the related functional effects.Furthermore,as a member of ferroic materials,ferroelectric and antiferroelectric materials can couple with ferromagnetic films to compose magnetoelectric composites,which show great potential in the application of information storage,microwave tuning,and multi-function electronic devices.The ferroelectric and antiferroelectric ceramics undergo deformation under the application of electric field,which leads them to be important components in the development of strain-mediated multiferroic composites.The study of ferroelectrics/antiferroelectrics-based magnetoelectric composites is a hot topic to achieve large magnetoelectric coupling effect.1.Based on the rich physical effects of ferroelectric/antiferroelectric ceramics in the vicinity of phase transition,this work designed and prepared?Pb,Ca,Sr?TiO₃ferroelectric/antiferroelectric ceramics with multiple phases,and revealed its phase transition behaviors and related functional effects.?1?The Pb_0.3Ca_xSr_0.7-x.7-x TiO₃?x?0.15?ceramics with a single phase transition near room temperature were designed,prepared and studied.It is domenstrated by Devonshire phenomenological theory that the ferroelectric-paraelectric phase transition orders of ceramics changes from secondary to the first with the increase of Ca content.When x?0.10,a secondary-order phase transition occurs;when x=0.15,the phase transition happens drastically and it is a first-order phase transition.At a low DC bias field of 200 V/mm,the Pb_0.3Ca_0.15Sr_0.55TiO₃ ceramic with first-order phase transition exhibits the largest pyroelectric coefficient,where p_max=81.8×10^-8 C/cm²K.Simutaneously,the diffused phase transition happen in these ceramics at room temperature.The electrocolaric effect calculated by indirect method reached the maximum??T=1.71 K?at room temperature in Pb_0.3Ca_0.05Sr_0.65TiO₃ ceramic,and the adiabatic temperature change is higher than 1 K in the temperature range from 5°C to70°C.?2?The Pb_0.4Ca_0.3Sr_0.3TiO₃ ceramic with two-phase-transitions near the room temperature were explored after a series of experiments.It was found that the antiferrodistortive state?i.e.TiO₆ octahedral tilts?existed in Pb_0.4Ca_0.3Sr_0.3TiO₃ ceramic by temperature-dependent characterizations of X-ray diffraction and transmission electron microscope.The antiferrodistortive state favors short-range antiferroelectric order and suppresses the long-range ferroelectric state.The incommensurate modulations and antiferroelectric double hysteresis loops appear in the same temperature range,demonstrating the strong antiferroelectricity in Pb_0.4Ca_0.3Sr_0.3TiO₃ceramic.The electrical properties of Pb_0.4Ca_0.3Sr_0.3TiO₃ ceramic with temperature,isostatic pressure and electric field were comprehensively investigated.The phase transition behaviors were deeply discussed,and the existence of antiferroelectric phase in?Pb,Ca,Sr?TiO₃?PCST?system was demonstrated for the first time.Furthermore,the pyroelectric effect and electrocaloric effect at ferroelectric-antiferroelectric phase transition and the antiferroelectric-paraelectric phase transition are also discussed.?3?The relationship between the composition and the phases was investigated by the study of series of PCST ceramics.The connection between the composition and properties of PCST ceramics was built by the tolerance factor.The average electronegativity-tolerance factor phase diagram of the PCST system was obtained,and critical conditions of the appearences of antiferroelectric phase was given.When t?0.9989,the PCST sample tends to develop ferroelectric orders;while t?0.9981,the antiferroelectric order plays a dominated role in the sample.2.Ferroelectric/antiferroelectric ceramics will deform with electric field.In this work,the ferroelectric ceramic and antiferroelectric ceramic,Pb?Mg,Nb?O₃-Pb?Zr,Ti?O₃?PMN-PZT?and?Pb,La??Zr,Sn,Ti?O₃?PLZST?,are selected as substrates to deposit Y₃Fe₅O₁₂?YIG?and NiMnGa?NMG?films.The study of converse magnetoelectric coupling effect was carried out.?1?YIG/Pt/PMN-PZT/YIG:The strain generated by electric-field-induced deformation of PMN-PZT ceramic can effectively control magnetic properties of YIG film,and the maximum of in-plane converse magnetoelectric coefficient(18.15×10^-8s/m)can be reached at a low bias magnetic field of 20 Oe and an electric field of 4kV/cm.The in-plane electric-field-controlled magnetic susceptibility change can reach65.6%at 20 Oe.The magnetoelectric coupling effect under cycling electric field with different amplitudes is also explored.The reversible and stable magnetization of YIG film can be achieved at 0 Oe with the amplitude of±4.6 kV/cm.Thus,an efficient and energy-saving two-state magnetoelectric storage model can be realized in the YIG/Pt/PMN-PZT/Pt heterostructure by applying a low amplitude of cycling electric field.?2?NMG/PLZST/NMG:Antiferroelectric ceramics display large phase-transition strain under electric field.The magnetization of NMG film changes rapidly at the switching fields of PLZST.At 0 Oe,the magnetization change of NMG film reaches the maximum?15%?.?3?YIG/Pt/PLZST/YIG:The magnetoelectric heterostructure was prepared and studied,and the electric-field-tuning of magnetic susceptibility and magnetization were reported for the first time.When the electric field is higher than the switching field of PLZST,the maximum??/?₀ of the heterostructure can reach 32%,?M/M₀ can achive18%,and the converse magnetoelectric coefficient is 11.5×10^-8 s/m.The magnetic anisotropy of YIG film varies with the electric-field-induced strain,which explains the difference in the in-plane and out-of-plane YIG magnetization as a function of electric field.