High Temperature Ferromagnetism Induced By Strain In La_0.7Sr_0.3MnO₃ Films

It is a goal in the field of spintronics to develop high-temperature practical spintronics devices based on high temperature ferromagnetism insulating?FMI?materials,which are of great importance that the acquisition of highly spin-polarized currents,the development of polymorphic memory,etc.The perovskite manganites have attracted a lot of novel physical properties due to its mutual coupling between lattice-obital-charge-spin,which have given us great application prospect.Among the perovskite manganites,La_0.7Sr_0.3MnO₃?LSMO?films have been extensively investigated due to their colossal magnetoresistance effect,high TC of 369 K,and half metallicity.However,degraded ferromagnetism and decreased TC with decreasing thicknesses below a certain critical thickness,limits their practical applications.In view of this problem,we effectively control a preferential orbital occupancy in the material by the stress-induced way and achieve high temperature ferromagnetic insulation in ultrathin LSMO films.The main research contents are as follows:?1?In order to grow LSMO films epitaxially with atomic precision,we prepared an atomically flat?001??110?SrTi O₃?STO?surface by etching with a NH4 F buffered HF solution?BHF?and subsequently annealing in an oxygen atmosphere.The TiO2 terminated STO substrates with regular steps separating the terraces and a fixed step height were obtained by optimizing the parameters such as corrosion time,annealing temperature and holding time,which laid the foundation for the growth of high quality LSMO film.?2?LSMO thin films with varying thicknesses?n = 4-18 u.c.?were grown on?110?Sr Ti O₃ substrates using pulsed laser deposition?PLD?.Here,we report unexpected room temperature insulating ferromagnetism in ultrathin?110?LSMO films?n ? 7 u.c.?,part of the samples Curie temperature of more than 390 K.The relationships between room temperature ferromagnetism,charge transfer,and orbital occupancy are investigated,with X-ray absorption spectroscopy?XAS?.Our results suggest that the room temperature insulating ferromagnetism is originated from super-exchange interaction between Mn2+ and Mn3+.The formation of Mn2+ ions is related to the charge transfer induced by oxygen vacancies.Moreover,a preferential orbital occupancy of eg?3z2-r2?in Mn3+ ions induced by tensile strain is crucial to the in-plane super-exchange coupling in ultrathin?110?LSMO films,resulting in insulating ferromagnetic behavior.?3?In order to further explore the effect of strain on the film TC,we designed and prepared a series of experiments such as LSMO/Ba TiO₃ superlattice and bilayer structure on the?001?STO substrate.The electron-orbit energy of the LSMO is controlled by the tensile strain of the LSMO by BaTiO₃,and the electrons are chosen to occupy the eg?x2-y2?preferentially,so that Mn ions can produce effective ferromagnetic coupling in the plane.Significantly improves the ferromagnetic Curie temperature of LSMO ultrathin films.In the LSMO/BaTiO₃ superlattice,the Curie temperature of the film is close to room temperature by increasing the number of superlattice growth cycles,while a significant room temperature ferromagnetism is shown in BaTiO₃?20 u.c.?bilayer structure,Curie temperature over 375 K.In conclusion,we adjust the orbital energy by strain-induced method,which makes the electrons in the ultrathin LSMO films realize the in-plane orbit coupling,and effectively improve the Curie temperature of the LSMO films,which lays the foundation for its practical application in the spintronics devices.