The Study Of Exchange Bias Effect And Superconductivity Property In LaNiO₃-based Heterostructures

Due to its arduousness and importance,the synthesis of perovskite rare-earth nickelates is a subject undergoing intense study within materials science.With the rapid development of atomic-scale synthesis techniques,the elements from La to Sm?with large atomic radii?can be epitaxially grown on nickelates as high-quality films.Specifically,on LaNiO₃?LNO?,which is the only member of the nickelates family that does not change from paramagnetic metal to antiferromagnetic insulator state with the decrease of temperature.However,metal to insulator transition and novel exchange bias coupling is present in LaNiO₃-based heterostructures.LaNiO₃-based heterostructures have become a major focus of research within the field of nickelates study.Nevertheless,there are still certain issues that require further clarification.?1?In the LNO/manganites heterostructures,the orientation dependence and the generation mechanism of the exchange bias are unclear;?2?There is no direct evidence for the antiferromagnetic order in LNO films,except the theoretical predictions.?3?Superconductivity in LNO-based superlattices has not been experimentally realized.In order to address these issues,we have designed comprehensive experiments presented in this work.Its contents are listed as follows:?1?We have epitaxially grown three different orientations of?001?,?110?,and?111?LNO/LaMnO₃?LMO?superlattices by pulsed laser deposition technique.Through the magnetic measurements,we found that the exchange bias effect along different orientations is an intrinsic characteristic of these superlattices.At the same time,we found that the exchange bias of the samples has a strong dependence on the thickness of the LNO layer,and that a clear critical thickness exists.When the thickness of the LNO layer exceeds the critical value,the exchange bias in the conductive sample vanishes.Conversely,when the thickness is below the critical value,the exchange bias is still present in the sample.Therefore,we have found the cause of the lack of exchange bias in thicker?001?-oriented LNO/LMO superlattice.Further,this study presents a useful method for controlling the exchange bias in LNO/LMO superlattices.?2?In order to prove the antiferromagnetic order in LNO films,we have prepared LNO-based superlattices of various thicknesses.The analysis of exchange bias data,shows that the coupling mechanism of the interfacial exchange bias is caused by an interfacial localized magnetic moment and an antiferromagnetic state in the ultrathin LNO layer,pinning the ferromagnetic LSMO layers together.Further,in order to exclude the interfacial localized magnetic moment induced by charge transfer,we have grown a prototypical superlattice composed of non-magnetic spacer layers as LNO/STO/LSMO systems.The long-range exchange bias effect can still be observed in these LNO/STO/LSMO superlattices.The effective separation of the two factors can support the AFM state in ultrathin LNO layers that exists to pin the adjacent ferromagnetic layer.?3?In the pursuit of high-temperature superconductivity,we have synthesized a prototypical superlattice that consists of an ultrathin LNO?2 u.c.?and an LSMO?3 u.c.?.First,the magnetic susceptibility as a function of the temperature under zero-field cooled and field-cooled processes has unambiguously confirmed the presence of a superconducting phase,which is known as the Meissner effect.Further,a magnetic hysteresis loop showing type II superconductor behavior has also been measured.Using Ar ion beam etching and magnetron sputtering,the resistivity dependence on the temperature of the superlattice is found to show a semiconductor-like behavior with an abrupt superconductivity transition.Therefore,both Meissner effect and zero resistance are observed using resistive and magnetic measurements of the superlattice,representing direct experimental evidence for superconductivity in new superconductors.In summary,this thesis has:shown that interfacial exchange bias is dependent on the orientation and thickness of LNO/Manganites heterostructures,summarized the deep mechanism for exchange bias effect,discovered the existence of antiferromagnetic order in the LNO layer,and confirmed the superconductivity in an LNO-based heterostructure for the first time.These results can improve the understanding of novel physical phenomena in LNO-based heterostructures and further encourage researchers to explore the rich physical phenomena in rare-earth nickelates.