Modification And Mechanism Of Magnetic, Electronic And Thermal Properties In Ferro-films

Ferro-films have drawn enormous attentions and become a hot researchingsubject in the materials science because of their unique physical and chemical properties.Due to the wide application in areas of exchange bias,magnetoelectric coupling,photovoltaic conversion and solid-state cooling,ferro-films have become the hot topic in nanotechnology and condensed matter physics,recently.These applications are mainly focused on the multiferroic nano-electro-mechanical system?NEMS?devices,the weak magnetic filed detection technologies,ferroelectric photovoltaic solar cells and high efficient magnetocaloric and electrocaloric refrigeration.The thes is is mainly about the above new properties and behaviors,which also focuses on the nano-structure and devices related fundamental research.The work mainly includes topics as follows:1.The magnetic domain,magnetostriction and exchange bias behaviors in cluster assembled magnetic nano-films.Cluster assembled SmCo alloy films were prepared by the low energy cluster beam deposition method,and the magnetic domain can be modified by the in-situ annealing induced cluster size change.The cluster size increasing induced the magnetic trans ition from superparamagnetic to single domain states.What's more,single domain SmCo films possess strong magnetostrictive effect.The films with low filed linear behavior for the magnetostrictive coefficient?and possess large saturation values.The cluster assembled Co/CoO core-shell structure films possess magnetic exchange bias at low temperature and enhanced further with the declining of temperature,which originated from the pinning effect at the interface of antiferromagnetic/ferromagnetic?AFM/FM?.In addtion,the strong exchange bias effect was obtained in single phase FeMn cluster films.The antiferromagnetic?AFM?state will partially transfer to spin glass state and induced exchange bias,and the AFM/spin-glass interface interaction can further be enhanced at low temperature.Subsequently,the exchange bias effect is enhanced.Also the magnetic training effect is investigeted at 5 K.2.The magnetoelectric coupling effect and their modification in cluster assembled multiferroic pseudo 1-3 heterostructural films.There is giant magnetoelectric coupling effect in cluster assembled Bi₅Ti₃FeO₁₅/FeGa/Bi₅Ti₃FeO₁₅ pseudo 1-3 heterostructural films.One is the multiferroic Bi₅Ti₃FeO₁₅ buffer layers can greatly release the clamping effect of hard substrates.Another is cluster assembled FeGa plates including enormous interface between the piezoelectric matrices of Bi₅Ti₃FeO₁₅ and FeGa nano-clusters,which is contributed to the interface stress transfer.In addition,the magnetoelectric coupling in heterostructural films contains two sources:the magnetoelectric coupling of single phase mutiferroic Bi₅Ti₃FeO₁₅;magnetostriciton/interface-stress/piezoelectric effect dominated interface coupling.At room temperature the cluster assembled Ni-NiO core-shell structure express exchange bias,which induces biased magnetoelectric coupling effect and magnetostrictive effect at the room temperature in Bi₅Ti₃FeO₁₅/Ni-NiO/Bi₅Ti₃FeO₁₅ pseudo 1-3 heterostructural films.Finally,the relationship between magnetic domain evolution and magnetoelectric bias are built.3.The ferroelectric photovoltaic?FPV?effect and regulation in Bi₅Ti₃FeO₁₅ filmsThe photonics induced electron-hole pairs in ferroelectric films can oppositely separate to electrodes driven by the combination of depolarization field and building in field.The charge accumulation leads to the open circuit voltage V_OCC and short circuit current J_SC.Enduring operation leads to the ferroelectric fatigue and degenerate the FPV effect.Rare earth Ho element doping can regulate the bandgap and improve the anti-fatigue property.What's more both the key parameters V_OC,J_SC obviously increase.However,as solar cells the values are still much less than theoretical prediction.The huge internal resistance of dielectric nature for ferroelectric films has depressed the V_OC,J_SC.Thereby,reducing the internal resistance and improving the FPV effect are necessary,by interducing the narrow bandgap Ag₂O nano-particle and being dispersed into Bi₅Ti₃FeO₁₅ films.Under magnetic field,the Jahn-Teller distortion modulated local coupling at Ag₂O interface lead to photoelectrons transfer in various Fe electron configuration and induced sub-band via e_g2-O_2p-e_g0 interaction.Subsequently,at the interface it formed interfacial conduction,which corresponds to the low internal resistance,high open circuit voltage and strong short circuit current.While the magnetic field is removed,the conductive interface is destroyed and internal resistance increases,which means the depressed V_OC,J_SC.4.The solid state cooling technologies based on electrocaloric and magnetocaloric effectThe films with ferroelectric order and spin order can obtain the solid state cooling and are applied in cooling devices,which satisfy the requirement of local cooling in micro-electro-devices.Near the phase transition point,it occurs huge entropy change and corresponds to large adiabatic change.In Bi₅Ti₃FeO₁₅/BiFeO₃ heterostructural films,the ferroelectric polarization increases with temperature below the Curie temperature,which is defined as negative electrocaloric effect that refrigeration is obtained during applying electric field.While in double perovskite structure La₂Co MnO₆ films,the spin orders are strengthened as the temperature decreases.This is positive magnetocaloric effect that refrigeration is obtianed during removing magnetic field.There is an abrupt change for spin orders near the phase transformation point,which means large change on entropy and adiabatic temperature.