Tunable Magnetic And Electrical Properties Of LaMnO₃ And Sr-doped （Pr,Nd） NiO₃ Films Prepared By Pulsed Laser Deposition

By reason of the reduction of dimensionality,the lattice mismatch and symmetry breaking,transition metal oxide films prepared by pulsed laser deposition have emerged some new physical properties different from bulk single crystals,such as interface two-dimensional electron gas,interface superconductivity,electron recombination and so on.Meanwhile,due to the strong coupling among in such freedom degrees include charge,spin,orbit,crystal lattice in the transition metal oxides films,it also provides a unique perspective for the study of the microscopic mechanism of novel physical phenomena.At present,epitaxial films prepared by pulsed laser deposition have been received extensive attention from researchers owing to playing an irreplaceable role in realizing material functional design and inducing novel physical phenomena.As the important components of transition metal oxides,manganates and nickelates systems prepared by pulsed laser deposition have very important practical value and physical significance because of their unique magnetic properties and transport characteristics.This paper studies the magnetic properties of LaMnO₃ epitaxial films and the transport properties of Sr-doped（Pr,Nd）Ni O₃ epitaxial films prepared by pulsed laser deposition.Combined with the work content of the research,this paper is divided into the following two parts:1.The magnetic properties of LaMnO₃ epitaxial films with different thicknesses prepared by pulsed laser deposition were studied.Stoichiometric LMO bulk is known to be a layer-type（A-type）antiferromagnet,in which Mn³⁺is a Jahn–Teller ion with a0)occupancy and the in-plane interaction between adjacent Mn ions is ferromagnetic（FM）while the out-of-plane one is antiferromagnetic（AFM）.In contrast to bulk LMO,there has been lots of controversy about magnetic phase in thin films.While some studies describe the appearance of FM behavior in stoichiometric thin films,other reports have shown that AFM order staying in the films thinner than 5 unit cells（uc）,and FM phase in films thicker than 5 uc.Meanwhile,almost all experiments have unveiled an insulating feature for LMO films in both FM and AFM phases,although many theoretical reports have predicted that LMO thin films possess a Pbnm structure and with a ferromagnetic metallic phase.In our study,we found the 2 to 5 unit cells thickness films are ferromagnetic;another hard ferromagnetic layer with larger coercive field arises as the films deposited to 8-10 uc,and the coupling of these two ferromagnetic layers leads to the distortion of the hysteresis loop.With the further increase of the film thickness,the hysteresis loop with forward exchange bias accompanied by distortion appears in the 15 and 20 uc samples,which indicates the coexistence of the antiferromagnetic layer and two ferromagnetic layers.The step-drop hysteresis loop in the 200 uc sample also demonstrates the existence of the antiferromagnetic layer.This evolution shows that as the thickness of the deposited film increases,the ferromagnetic layer,the hard ferromagnetic layer,and the antiferromagnetic layer occur in order,and the three kinds of magnetism coexist when the film thickness is greater than 15 uc.It is the first time that such magnetic coexistence phenomenon has been observed in LaMnO₃ epitaxial films.Combined with aberration-corrected transmission electron microscopy analysis,the P2_1/n structure exists in the thin LaMnO₃ epitaxial film,and the magnetic delamination can be one-to-one correspondence.We attribute the source of ferromagnetism to the appearance of the P2_1/n structure under stress control,and the antiferromagnetism to the recovery of the Pbnm structure.2.Explore the growth method of depositing 113 perovskite phase Nd_1-xSr_xNi O₃prepared by pulsed laser deposition and obtain superconducting infinite layer structure by topological reduction.Usually,Ni²⁺in the nickel oxides is stable state,instead the Ni³⁺is metastable,so it is very difficult to grow the pure 113 perovskite phase nickel oxides film owing to easy formation of the Ruddlesden-Popper compounds(such as trilayer Ruddlesden-Popper phase(Nd_0.8Sr_0.2)₄Ni₃O₁₀).Though high temperature annealing,we can easily obtain the stable 113 perovskite phase Nd_1-xSr_xNi O₃ film,and the electrical transport properties of Nd_0.8Sr_0.2Ni O₃ with different thicknesses were studied.It was found that since the stress modifies the splitting energy of the e_g band,the film is insulating at 4 uc,restoring the bulk metallic behavior as the film thickness above 6 uc,and the appearance of metal-insulator behavior in the 24 uc film indicates the formation of Ruddlesden-Popper type compounds,which is consistent with the previous result that perovskite phase Nd_1-xSr_xNi O₃ exists stably in the range of about10 nm.In addition,we learned from previous experience of growing the perovskite phase Nd_1-xSr_xNi O₃,successfully grew the Pr_0.82Sr_0.12Ni O₃ film,and used the Ca H₂topotactic reduction to obtain the infinite-layer Pr_0.82Sr_0.18Ni O₂superconducting film.Through collaborative high-pressure studies,we first found that the superconducting temperatureof Pr_0.82Sr_0.18Ni O₂ increases monotonically from 18 K at ambient pressure to over 30 K at 12 GPa.