| Perovskite oxides have numerous physical properties,such as(anti)ferroelectricity,(anti)ferromagnetism,piezoelectricity,pyroelectricity,and superconductivity,while different properties could couple with each other to generate new functionalities,for example,multiferroic property.The structural tolerance and flexibility of the perovskite oxides allows researchers to design and manipulate its properties by substituting,doping,etc.,on atomic scales.Therefore,on one hand,peorovskite oxides are the core materials which are intensively utilized in applications such as storage,catalysis,sensor,and driver;on the other hand,they are research objectives for investigating fascinating physics like the order parameters' competiting and coupling among charge,spin,and orbital.Based on these facts,perovskite oxides are attracting broad attention from both academies of physics and engineering of appllication materials.Room temperature ferromagnetic materials have important applications in spin electronics.Extrinsic room temperature ferromagnetic materials mainly includes defect-induced magnetic materials,like ZnO-based diluted oxides,while a typical example of intrinsic room temperature ferromagnetic materials is double perovskite oxides.Double perovskite ferromagnetic oxides have been verified to possess interesting physical performances,such as Curie temperature above room temperature,extremely high spin polarizability,half-metallic property,therefore,such materials have drawn attention again and much work have been carried out on the structure,magnetism,and transportation property in the past few decades.This dissertation introduces the basic structure,magnetism,and transportation mechanism of double perovskite oxides,on which foundation,ceramics and films of Fe-and Cr-based double perovskite oxides are systematically fabricated and the structures and properties further studied.By studying and analyzing the structure and property of double POs,we obtain a deeper understanding on this type of materials,and may provide some experimental foundations for further basic research on physics and even practical applications.To be more specific,this dissertation includes following aspects:(1)With traditional reducing atmosphere(H2-containing)replaced by N2,we fabricated single-phase iron-rich stoichiometric Sr2Fe1+xMo1-xO6(x=0,1/9,1/6 and 1/3)ceramics.This ceramic system actually mixes Sr2FeMoO6 with Sr3Fe2MoO9 according to different proportions,theoretically averting the oxygen vacancies induced by off-stoichiometry.X-ray diffraction(XRD)results show that all the samples are well-crystallized,and have typical tetragonal structure.X-ray photoelectron spectroscopy(XPS)results demonstrate that,Fe and Mo ions have mixed valences inside the ceramics.Particularly,the ceramic samples with x=1/9 exhibit obvious room-temperature ferromagnetism,and relatively large saturation magnetization,as well as local Fe/Mo ordering indicated by transmission electron microscope(TEM)results.With x increased,the ferromagnetic-paramagnetic transition temperature and magnetoresistance value decrease.Our experiments provide double perovskites with room temperature ferromagnetism and a much safer,low-cost fabrication method.(2)On the foundations of fabricating single-phase bulk Sr2Fe1+xMo1-xO6(x=0,1/9 and 1/6)ceramics in N2 atmosphere,we utilize pulsed laser deposition(PLD)technique to fabricate the corresponding thin films on(001)SrTiO3 and(111)SrTiO3 substrates in N2 atmosphere.TEM results have verified the film growing mechanism to be three-dimensional island growth mode.In N2 atmosphere,epitaxial growth of single-phase Sr2FeMoO6(x=0)films needs a relatively high substrate temperature(>800?),and much higher substrate temeprature is desired in preparing high-quality films with Fe/Mo order.However,at 750? we have obtained high-quality stoichiometric S r2Fe10/9Mo8/9O6(x= 1/9)films with good crystallization and room-temperature magnetism.Nevertheless,under the condition of x=1/6,the films show relatively low saturation magnetization,due to increment of intrinsic B-site disorder.Our experiments have systematically studied Fe-based stoichiometric double perovskite films,the results can help us understand the evolution of the structure and physical properties of double perovskites.Based on these experimental results,with film fabrication conditions considered,we further fabricate Sr2Fe10/9MO8/9O6 films with different thickness on(111)SrTiO3 substrate to systematically study of the influence of stress on structure and property.XPS results indicate that,Fe and Mo ions' mixed valences exist inside all the film samples,similar to the ceramic case.TEM results show that with increment of sample thickness,more defects appear in the films,due to stress relaxation.At low temperature(10K)and room temperature,well-saturated ferromagnetic hysteresis loops have been observed in Sr2Fe10/9Mo8/9O6 with different sample thickness,and particularly the ultra-thin films with thickness of 10nm has a saturation magnetization of 2.30 ??/f.u.,much higher than that of polycrystalline ceramic sample.For the transportation properties,the films with thickness smaller than 30nm exhibit extremely high resistance that could be measured,while the other films show good semiconductor characteristics,with negative magnetoresistance reaching-13%at 130K.Based on these experimental results,we have analyzed the influence of stress on the films' microstructure and macroscopic properties.(3)Compared with widely studied Sr2FeMoO6,little attention has been paid to Cr-based Sr2CrWO6 double perovskites,However,Sr2CrWO6 has some unique advantages,for example,high quality Sr2CrWO6thin film can be easily prepared with a relatively low deposition temperature.In this work,with different atmosphere(O2,N2),PLD technique has been utilized to fabricate Sr2CrWO6 films in purpose to study the atmosphere's influence on properties.We find that,films growing in 0.1 Pa N2 have better property.So,we further systematically fabricate Sr2CrWO6 films on different substrates of(001)SrTiO3,(111)SrTiO3,and(111)LaAlO3.Atomic force microscope results show that the films' surface fluctuation with SrTiO3 substrate is much smaller than that with LaAlO3 substrate,due to smaller lattice mismatch.TEM results indicate that the growing mechanism is two-dimensional layered growth mode on SrTiO3 substrate,but three-dimensional inland growth mode on LaAlO3 substrate.XRD and selected area electron diffraction(SAED)of TEM show no signs of B-site ordering,but Sr2CrWO6 films still exhibit room temperature ferromagetism.Besides,all the samples growing in high vacuum show very small resistance with metallic characteristics and giant positive magnetoresistance with values of 221%at 10K. |
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