Preparation And Study On The Physic Properties Of Ruddlesden-Popper Structural Ca₃M₂O₇?M=Mn,Ti?

Multiferroic materials are single-phase functional materials that posse two or more iron properties?ferroelectric,ferromagnetic and ferroelasticity?.Due to its potential application in advanced spintronic,magnetoelectric device and information storage,multiferroics attract people's attention.In the search for new multi-iron materials,a Ruddlesden-Popper?RP?structure Ca₃M₂O₇?M=Mn,Ti?enter people's horizons because of its rich physical properties.In this thesis work,we prepared Li-dope Ca₃Mn₂O₇ particles by sol-gel method and pure Ca₃Ti₂O₇ ceramics via solid state reaction,and detailed studies of structural,magnetic,electrical and optical properties were investigated.Firstly,we selected alkali metal Li¹⁺ions as dopants to substitute Ca²⁺in Ca₃Mn₂O₇,and prepared Li_2xCa_3-2xMn₂O₇?x=0,0.01,0.03,0.05?particles via a sol-gel method.Ca₃Mn₂O₇ is a G-type antiferromagnetic material and its Néel temperature is about 120 K.In the temperature of 5 K,weak ferromagnetism with strong coercive force was observed in the samples.The magnetism is increased with increasing the Li content,which attributes to the enhance of superexchange interaction in Mn⁴⁺ions and double-exchange interaction between Mn⁴⁺and Mn³⁺.Nevertheless,the oxygen vacancy and leakage current density are decreased with the increase of x,and the band gap shows an increased trend.It is well known that oxygen vacancies modify band width by forming impurity level and influence electrical transport via introducing carriers.Meanwhile,we analyzed the band structure and density of states of Ca₃Mn₂O₇ using first-principles calculations.Then,we successfully prepared pure Ca₃Ti₂O₇ ceramics via solid state action.In the room temperature,significant polarization reversal current was observed in Ca₃Ti₂O₇,which confirm the existence of ferroelectricity.It is found that the coercive field is 115 kV/cm and remanent polarization is about 0.58?C/cm².In the leakage current test,sample exhibits a diode-like effect.Under the forward bias voltage,the sample first shows an off state,and then the current rapidly rises to form an on-state while it is being applied.When the voltage is reversed,sample is returned to the insulated state.To explain the abnormal phenomenon,we constructed an electric transport model.Meanwhile,we bring together optical absorption spectroscopy and first principles calculations to reveal the electronic structure of the room temperature ferroelectric Ca₃Ti₂O₇.