Investigations On Electrocaloric,magnetocaloric Effect And Optical Properties Of EuTiO₃ Materials Under Stress（Strain）

In recent years,multiferroic materials have attracted great attention because of their physical properties and potential applications in magnetic-dielectric coupling and giant magnetic capacitance phenomenon.More importantly,the studies of solid state refrigeration have become one of the research focuses in the refrigeration field,due to its high energy-efficiency and environment friendliness compared with traditional gas compression refrigeration.Furthermore,with the problems of traditional energy source becoming increasingly severe,solar energy has been considered as the most ideal alternative energy because of its low cost and environment-friendliness.Meanwhile,perovskite solar cells become a hot research topic in solar cells,due to their high photo-to-electric conversion efficiency.EuTiO₃（ETO）materials own a typical perovskite structure,and perform antiferromagnetic（AFM）and quantum paraelectric order in bulk.Previous studies have discovered that the coexistence of ferroelectric and ferromagnetic（FM）order can occur in its single phase and induce some novel physical properties due to lattice mismatch,surface tension or external stress.Therefore,the thermodynamic and optical properties of ETO as refrigeration materials or photovoltaic materials can be regulated by applying strain or stress,electric field and magnetic field.Above all,ETO multiferroic materials possess a possibility of practical application in solid-state refrigeration and solar cells.According to the influence of stress（strain）on lattice structure and physical properties of ETO materials,a phenomenological thermodynamic model is employed to investigate electrocaloric（EC）effect and magnetocaloric（MC）effect in ETO nanowires under stress.Meanwhile,the first-principles calculations based on the density functional theory are used to investigate optical properties of ETO thin films with different strains in this paper.The main investigations are as follows:（1）By investigating the EC effect in ETO nanowires under external stress,the giant EC effects（15²0 K）in EuTiO₃ nanowires are revealed for the first time,as well as the maximum EC effect（19.6 K）can be achieved near room temperature by changing radius,external tensile stress and applied electric fields.The results suggest the potential application of ETO nanowires for micro-nano device in refrigeration.（2）The magnetic phase transitions and MC effect of ETO nanowires by considering the radius of the nanowires and the surface tensions have been investigated.A transition mode of paramagnetic/FM skipping AFM phase can be obtained,and the MC effect of ETO nanowires can exceed those of the other low-temperature magnetic materials by increasing the change of applied magnetic fields in this transition,suggesting the potential application of ETO nanowires in low-temperature magnetic refrigeration.（3）The magnetic properties,optical properties,and electron effective mass of ETO thin films with biaxial strains have been investigated.We are surprised that the strains can not only induce ferroelectric polarization and magnetic phase transition,but also regulate the band gap of ETO thin films.Moreover,ideal optical band gap can be obtained by applying 1 4tensile strain to ETO ferroelectric films and photo-generated carriers with spin-polarization can be induced from Eu 4f to Ti 3d states.These results suggest the potential applications of ETO thin films in multiferroic photovoltaic cells.