Control Of The Dead Layer Effect In La_0.7Sr_0.3MnO₃

The rich interface and bulk effects of correlated oxides make them considered as new materials for the next generation of semiconductor devices to replace silicon.Due to the interaction and correlation of charge,lattice,spin and orbit,many novel phenomena appear in these oxide films or heterojunction interfaces,such as the topological Hall effect of Sr Ru O3 bulk,the room temperature ferromagnetism of La_0.7Sr_0.3MnO₃,and the high mobility two-dimensional electron gas at La Al O3 / Sr Ti O3 interface.However,there is a problem in related oxides,that is,when the number of layers of the film is less than a certain thickness,the material loses its original characteristics.Such a thin layer is often called dead layer.The existence of dead layer has a negative effect on the miniaturization of devices.Therefore,it is of great significance to understand the underlying mechanism of the dead layer for the development of micro electronic devices.The study of dead layer has been started since its discovery.However,despite extensive studies,the mechanism behind the thickness driven dead layer effect is still very complex.For the most representative La_0.7Sr_0.3MnO₃ oxide thin films,the mainstream view is that the dead layer effect is caused by four mechanisms: orbital reconstruction caused by lattice mismatch,oxygen octahedral rotation,polarity discontinuity and structural defects.Among the four reasons,orbital reconstruction has the greatest influence on the film dead layer effect.This is because the lattice mismatch leads to the increase of epitaxial stress,which inhibits the overall exchange effect and makes the film system more inclined to form antiferromagnetic insulator to ensure the lowest energy.For example,the lattice mismatch between La Al O3 and La_0.7Sr_0.3MnO₃ is as high as 2.5%,while the lattice mismatch between Sr Ti O3 and La_0.7Sr_0.3MnO₃ is only 0.6%,so the dead layer thickness of La_0.7Sr_0.3MnO₃ grown on La Al O3 substrate is 200% larger than that on Sr Ti O3 substrate.It is of great significance to understand the effect of epitaxial stress on the dead layer for control the stress and reduce the thickness of the dead layer.In this paper,we prepared high-quality manganese oxide thin films by pulsed laser deposition,and studied the effect of strain on the properties of the thin films by inserting the buffer layer,the phase transition of the thin films with thickness and the magnetic control of the thin films（1）By inserting a layer of Sr₃Al₂O₆ buffer between La_0.7Sr_0.3MnO₃ and La Al O3 substrate,the strain of La_0.7Sr_0.3MnO₃ can be controlled from compressive strain to tensile strain.As the strain decreases from compressive strain to no strain,we observe that the magnetic and metallic properties of the films increase gradually.However,as the stress increases from no strain to tensile strain,the magnetic properties of the films begin to decrease and finally disappear.When Sr₃Al₂O₆ is 6 uc thickness,the films show the best magnetic and transport properties.The RSM shows that the buffer layer releases the lattice strain.Through the buffer layer technology,we make the electrical dead layer thickness of La_0.7Sr_0.3MnO₃ decrease from 27 uc to 12 uc,and the magnetic dead layer thickness decrease from 10 uc to 3uc.At the same time,we have successfully transferred the freestanding La_0.7Sr_0.3MnO₃ to the silicon substrate by using the hydrolysable property of Sr₃Al₂O₆.The no-strain film shows good properties even at a very low thickness of 4uc.We have verified that epitaxial strain can deteriorate the properties of thin films,and also our results provide a promising route to minimizing/eliminating the dead layers of correlated oxides by introduction of a straintunable buffer layer.（2）La_0.7Sr_0.3MnO₃ is an antiferromagnetic insulating phase below the magnetic dead layer,a ferromagnetic metal phase above the electrical dead layer,and a ferromagnetic antiferromagnetic mixture in the intermediate state.In order to study the behavior of the magnetic phase in the process of film thickness change,we studied the thickness dependent R-T curve of La_0.7Sr_0.3MnO₃,the Curie temperature and other physical parameters.For different magnetic phases of La_0.7Sr_0.3MnO₃ thin films,we have carried out the magnetic field control.we found that the external magnetic field can improve the Curie temperature of the film,and increase the metallicity of the film.For La_0.7Sr_0.3MnO₃ mixed with ferromagnetic and antiferromagnetic phases,the R-T curve of La_0.7Sr_0.3MnO₃ shows obvious non-metallic behavior due to Kondo effect at low temperature.We think that this is due to the scattering effect of antiferromagnetic phase on ferromagnetic phase electrons.By suppressing the scattering effect of antiferromagnetic phase on electrons,we successfully suppressed Kondo effect.The phase diagram of La_0.7Sr_0.3MnO₃ is summarized and the general rule of magnetic phase changing with thickness is given.