Preparation And Characterization Of Perovskite Nickelate Heterostructures

In recent decades,correlated perovskite oxides represented by nickelates have attracted much attention due to their complex physical properties.In perovskite nickelates heterostructures,these properties are thought to be a consequence of interactions between lattice,charge,spin and orbital degrees of freedom.By manipulating these degrees of freedom via epitaxial strain,dimension and artificially designed interfaces,the electrical and magnetic characteristics of heterostructures can be modulated.In this thesis,we focus on the deposition and characterization of LaNiO₃?LNO?ultrathin films and LaNiO₃/LaMnO₃?LNO/LMO?superlattices.epitaxial strain,dimension,under layers and artificially designed interfaces were used to modulate transport and magnetic properties of these heterostructures.Major achievements are summarized below.?1?A series of LNO ultrathin films and underlayers were epitaxially deposited on SrTiO₃?STO??001?single-crystal substrates by pulsed laser deposition?PLD?monitored in situ by reflective high energy electron diffraction?RHEED?.X-ray diffraction?XRD?measurements indicate that films deposited on STO substrates are under tensile strain.Atomic force microscope?AFM?measurements reveal atomically smooth surface in the films.Sheet resistance of LNO ultrathin films were studied as a function of temperature.The sheet resistance increases continuously as the film thickness decreases.A metal-insulator transition is observed as the film thickness is less than a critical value,5 unit cells?u.c.?on STO and 4 u.c.on LAO.Fitting the data with localization models indicates that the thickness-driven metal-insulator transition may stem from carrier localization.When a 2 u.c.-thick STO underlayer,deposited under 10-3 mbar oxygen pressure,is inserted between the 5 u.c.LNO film and STO substrate,the LNO film shows a metallic behavior above 100 K.X-ray Photoelectron Spectroscopy?XPS?measurements indicates that oxygen vacancy exists in the underlayer.These oxygen vacancies are donors that supplies electrons in the otherwise empty Ti 3d orbitals.The electrons transfer from the STO underlayer to the LNO layer and suppress the carrier localization in the LNO film.Oxygen vacancy density in the underlayer can be greatly decreased by post-annealing the underlayer or improving the oxygen pressure during the deposition of underlayer,the critical thickness in LNO becomes 5 u.c.again.?2?LNO/LMO superlattices with periodicity[?LNO?m/?LMO?n]x where m and n indicate the number of unit cells of LNO and LMO,respectively,and x,the period repetitions,were deposited on STO?001?single-crystal substrates by PLD.The magnetic properties were measured using a Superconducting Quantum Interference Device?SQUID?.Exchange bias is observed as a shift of M-H hysteresis loops at low temperature.The decrease of period repetitions x or the number of unit cells of LNO m leads to a decrease of both exchange bias field HE and coercive field Hc.