Weak Ferromagnetism Detection Of Lanthanum Manganate Based On Transition Metal Dichalcogenides

Perovskite oxides have been studied for more than 70 years with rich physical properties including ferroelectricity,ferromagnetism,and piezoelectricity.With these remarkable properties,they have been widely used in the field of energy conversion,optoelectronic devices,and sensors.Magnetic lanthanum manganate(LaMnO3)is a typical perovskite oxide that has been widely studied.Due to the complex coupling of multiple degree of freedom,many exotic physical phenomena have been observed in this system.Furthermore,its magnetic properties can be tuned via various stimulus,making LaMnO3 an excellent platform for the spintronics.In recent years,with the emergence of fabricating ultra-thin freestanding perovskite-type oxides,it is possible to study atomic-scale ferroelectricity,ferromagnetism,and other physical properties at the two-dimensional(2D)limit,and thereby to develop functional devices.However,this technique in-turn brings challenges in characterizing the related physical properties in freestanding samples.For example,the film thickness of freestanding LaMnO3 films fabricated by this technique is only a few unit cells and the sample size is around several microns.Therefore,it is difficult to detect the magnetic moments of the system by conventional macroscopic characterization methods.In view of this,in this thesis,we utilize the arbitrarily transferrable feature of ultra-thin freestanding perovskite oxides to fabricate heterostructure with 2D group-VI transition metal dichalcogenides(TMDs)for the detection of weak magnetism in LaMnO3.Monolayer WS2 is used to combined with ultrathin freestanding LaMnO3 films to construct a ultrathin WS2/LaMnO3 heterostructure.The valley polarization in monolayer WS2 is measured via the photoluminescence(PL).Based on the measured valley polarization,the weak magnetic properties of ultra-thin freestanding LaMnO3 films are investigated indirectly.We observe that the degeneracy of valley excitons in monolayer WS2 is lifted in the fabricated WS2/LaMnO3(8 nm)heterostructure,which proves the existence of magnetic proximity effect induced by freestanding LaMnO3 films at the interface of the heterostructure.Through temperature-dependent PL spectra study,the broken valley degeneracy in monolayer WS2 gradually restores.This behavior is consistent with the ferromagnetic phase transition in pristine LaMnO3 films.These results provide a new method for tuning the magnetic properties and the valley degree of freedom in ultra-thin 2D heterostructure,and demonstrate a new method for detecting weak magnetism in ultra-thin perovskite oxides.The contents of this thesis mainly include:In Chapter 1,we review the background of the study in 2D layered group-VI TMDs and heterostructures,and the valley physics in these systems.In Chapter 2,we review pervious study in conventional perovskite oxides.We introduce the fundamental properties,growth and fabrications of LaMnO3 and the heterostructures with group-VI TMDs.In Chapter 3,we study the sample preparation of atomically thin group-VI TMDs and their heterostructures.Based on the home-made confocal microscopy system,we study the spectroscopic properties of monolayer and few-layer WS2,and explore its macroscopical sample quality and valley polarizations.In Chapter 4,we fabricate ultrathin WS2/LaMnO3 heterostructures.Through polarization-resolved PL measurement,we find that the weak out-of-plane ferromagnetism in freestanding LaMnO3 films induces significant differences in the PL from K valley and-K valley excitons in monolayer WS2.The characterization of weak magnetism in ultra-thin freestanding LaMnO3 is realized by monitoring the difference in valley polarizations.By performing temperature-dependent measurements,the magnetic phase transition is observed in the heterostructure.In Chapter 5,we summarize and prospect the work of this thesis.