Study On The Coexistence Of Multiferroicity And Metallicity,and Freestanding Properties In Perovskite Oxide Films

Generally,the ferroelectricity and metallicity is regarded incompatible in a same material because of the screening effect from itinerant electrons to the polarization.Materials with the coexistence of ferromagnetism,ferroelectricity,and metallicity have rarely been reported.However,such multiferroic metallic materials,because of their rich couplings among the degree of freedom of charge,lattice,orbital,and spin,are highly desirable in information storage devices and multi-filed modulation devices.On the other hand,the freestanding perovskite oxide films,which are free from the strains from the substrates,show great potentials in twistronics,flexible and wearable devices because of their super-elasticity and adjustability through mechanical modulations.Therefore,it’s of great significance to investigate their properties and mechanisms.This dissertation is divided into two parts,and the first part is to achieve multiferroic metallic material Pb Nb_0.12Ti_0.88O_3-δthrough controlling the oxygen pressures during the deposition,and the second part is to synthesize freestanding La Mn O₃ thin films and study their properties.（1）.The Pb Nb_0.12Ti_0.88O_3-δfilms were deposited on the Nb:SrTiO₃ substrates under an oxygen partial pressure of 6 and 4 Pa,respectively.The magnetic and ferroelectric hysteresis loops at room temperature of those films confirmed the multiferroicity in those materials,and the measurements of their transport properties further unveiled the metallicity in those materials.The results from scanning transmission electron microscopy revealed that the ferroelectricity originated from the displacements of O ions deviated from their equilibrium positions.The first-principles calculations further revealed that the Pb Nb_0.125Ti_0.875O_3-δwas a half metal,and both the ferromagnetism and the metallicity in those materials were attributed to the contribution of the electrons on the d_xy orbitals of Ti and Nb ions.（2）.The heterostructure was fabricated by successive coherent growth of a water-sacrificial strontium aluminate layer and a ferromagnetic La Mn O₃ layer on the SrTiO₃substrates.The as-grown heterostructure was immersed into de-ion water,and the La Mn O₃ layer was released as freestanding film and would float on water after the intermediate strontium aluminate layer was dissolved.We bended those freestanding La Mn O₃ membranes onto quartz rods with curvatures of 0,0.5,and 1mm^-1,respectively.It was found that the saturation magnetization of La Mn O₃ had been increased by an amplitude of 56%after releasing,and had been further enhanced by 36%at most after bending.Simultaneously,the Curie temperature of La Mn O₃ had been increased by 13K during above processes.The results from scanning transmission electron microscopy revealed that the enhancement of the ferromagnetism originated from the evolution of the structure of La Mn O₃.Two inequivalent Mn O₆ octahedra（denoted as OC1 and OC2,containing Mn1 and Mn2 ions,respectively）alternated three-dimensionally in La Mn O₃when the OC1（OC2）was stretched（compressed）in out-of-plane direction.The degree of stretching（compression）on OC1（OC2）was slight in La Mn O₃ before releasing,and this degree became much stronger when the La Mn O₃ were released,and this degree got further enhanced after the La Mn O₃ were bended.Above changes in the degree of stretching（compression）on OC1（OC2）shortened the distance between adjacent Mn1and Mn2 ions,and produced larger overlaps among the Mn1 half-filled d²²_3z-r,the middle O p_z,and the Mn2 empty d²²_3z-r orbitals,and brought significant enhancement in the ferromagnetism of La Mn O₃ due to enhanced super-exchange interactions according to Goodenough-Kanamori rules.