Magneto-optical Imaging Of High-throughput Characterization Methods And Dy Yb Bismuth Ferrite Magneto-optical Characteristics Of Combinatorial Screening

The demand for magneto-optical(MO)storage materials with higher Kerr rotation and excellent magnetic properties increased greatly because of the development of information technology and MO storage device.Combinatorial approach is a high efficient method for novel materials screening.The MO storage material research will be accelerated greatly by the application of combinatorial approach.Parallel synthesis and high through-put characterization are the two critical factors which determine the efficiency of combinatorial approach.But the development of high through-put characterization technique is fall behind the development of parallel synthesis.To meet the demand of high-throughput characterization for MO storage materials,we designed a high-throughput magneto-optical Kerr rotation angle characterization system.Comparing with commercial alloy MO storage,Bi-substituted rare earth iron garnet has better stability and higher Kerr rotation.In this thesis,we applied our high-throughput magneto-optical Kerr rotation angle characterization system in combinatorial screening the(Bi_xDy_yYb_3-x-y)Fe₅O₁₂system,and Dy_0.6Yb_0.5Bi_1.9Fe₅O₁₂is found to have the highest Kerr rotation.This thesis is organized according to the logic below.Chapter one is the introduction of the related background knowledge.First,the basic concept,content and history of the combinatorial materials science were introduced,and the latest progresses for combinatorial library design,synthesis and characterization especially the combinatorial synthesis technique and high through-put characterization were reviewed.Secondly,the principle of MO storage and the main MO storage materials are summarized.The MO effect was phenomenologically analysed and the development of MO storage materials(Mn-Bi alloy,RE-TM amorphous alloy,Iron Garnet and Pt/Co multilayer)was summarized, especially the research of rare earth iron garnet.Chapter two is the design and setup of the combinatorial MOKE high-throughput characterization system.The system consists of a He-Ne laser,a micro objective,a pinhole,a double-convex lens,two pieces of Glan-Thompson prism polarizer,a CCD camera,a Helmholtz coil,and many optic stages.The laser beam with diameter of 0.51 mm passes through a beam expander/filter(consisting of a micro objective,a pinhole and a double-convex lens),and a Glan-Thompson prism polarizer to increase the linear polarizability above 10⁵.The parallel laser beam incidence on the library is reflected. The reflected beam passes through a polarization analyzer(ibid)and projects on the CCD camera and is recorded by the camera.Next,the data is transferred in a computer.Finally,the Kerr rotations of all the samples are obtained.To visualize the result,we write a program to show the Kerr rotation angle as the function of position in a graphic mode.In the graph,Kerr rotation is represented by the brightness.The specific features of the system are the use of laser and parallel beam projection imaging.Considering the sample number in one materials library is usually up to 1000 and the individual sample is often as small as 0.5×0.5 mm²,the combinatorial MOKE high-throughput characterization system must use the light source with high brightness.The use of laser can increase the sensitivity of the characterization system while maintain a reasonable spatial resolution for combinatorial materials studies.The use of parallel beam projection imaging has the characterization system the ability of parallel characterization.Because the light intensity signal is determined by the product of reflectivity and Kerr rotation angle, we set the polarization angle of the analyzer a small angle from the perpendicular direction of the polarizer and take two pictures with the samples magnetized and demagnetized.Through these two pictures,we can eliminate the influence of reflectivity and get the Kerr rotation of all the samples.To estimate the spatial resolution of the system,a fully coherent illuminated Fresnel diffraction model was constructed and the diffraction pattern was calculated. Through the calculation,the spatial resolution increases with the decreasing of the sample-screen distance,and the spatial resolution was also estimated at the sample-screen distance of 300 mm.In order to test the above estimation,we deposited an 800 nm thick Gd_1.6Bi_1.4Fe₅O₁₂thin film test library using a RF magnetron sputtering machine.The characterization to this library validates the estimation to the spatial resolution.We measured the hysteresis loop of the Gd_1.6Bi_1.4Fe₅O₁₂film by regular MOKE and our combinatorial MOKE high-throughput characterization system.Two hysteresis loops are consistent.We also fabricated Gd_3-xBi_xFe₅O₁₂(03+to form a ternary (Bi_xDy_yYb_3-x-y)Fe₅O₁₂material system can give rise to more freedom in manipulating material's property,including the potential discovery of new materials with a much higher remanent magnetization while their high coercive field can still be maintained.As the the Bi:RIG film is transparent,the influence of film interference to Kerr rotation was analysed theoretically.To eliminate the influence of interference,we increased the film thickness of the library,reduced the nonuniformity of the equivalent film thickness and used opaque silicon as the substrate.By characterization a test Dy₂BiFe₅O₁₂library,we make sure that the influence of interference was reduced to an acceptable level.So the combinatorial approach can be applied in searching for transparent MO film materials.(Bi_xDy_yYb_3-x-y)Fe₅O₁₂combinatorial libraries were fabricated by co-sputtering of Bi₃Fe₅O₁₂,Dy₃Fe₅O₁₂and Yb₃Fe₅O₁₂targests,and the natural sputtering rate gradient was utilized to generate the composition gradient.The the sputtering rate distribution was measured.The research strategy was to get the potential high MO composition region in the primary spread and identified the highest MO composition in the secondary spread.The combinatorial MOKE high-throughput characterization system which was introduced in chapter two was used to characterize the(Bi_xDy_yYb_3-x-y)Fe₅O₁₂ combinatorial libraries.Dy_0.6Yb_0.5Bi_1.9Fe₅O₁₂was screened out,with the highest MO effect in this ternary system.Scale-up study of Dy_0.6Yb_0.5Bi_1.9Fe₅O₁₂film shows that its hysteresis strongly depends on the annealing temperature.When the annealing temperature was increased from 670℃to 690℃,the remnant magnetism of film enhances significantly.This enhancement is favorable for MO storage.We also studied the effect of film thickness on the interference and found that it could be utilized to enhance the extrinsic Kerr rotation.We deposited a thickness gradient Dy_0.6Yb_0.5Bi_1.9Fe₅O₁₂film and imaged the rotation angle as the function of the film thickness.When the film thickness is 510 nm,the maximum Kerr rotation of 3.27°can be achieved.