Structure Optimization And Magnetism Of Cr Doped Mn-based Chalcogenides

Mn-based chalcogenides exhibit antiferromagnetism,in which the spin of adjacent valence electrons aligns in the opposite direction.Such an alignment results in a zero net magnetic moment,which cannot produce magnetic fields.These properties severely limit their applications.In this thesis,Mn-based chalcogenides were synthesized through the high temperature liquid organic solvent method,and the 3d transition metal Cr was doped into them to control their structure and magnetic properties.The main conclusions are as follows:（1）MnTe nanowires were prepared by high temperature liquid phase organic solvent,and the synthesis conditions were optimized.The structure,magnetic properties,composition and morphology of nanowires were studied by different measurements.By using MnO,MnCl₂,Mn（acac）₃ and Mn（acac）₂ as different Mn sources,the effects of different Mn sourses and addition ratios on the synthesis of MnTe nanowires was studied.It was found that the chemical order degree,and magnetic property and morphology of MnTe nanowires were significantly affected.The antiferromagnetic MnTe nanowires with the best morphology were obtained by using Mn（acac）₂ and Te as the precursors of 1.2:1.The diameter of MnTe nanowires was about 100 nm and the length was more than 10μm.（2）In order to control the magnetic properties of MnTe nanorods,3d transition metal Cr element was successfully doped into MnTe nanowires to form MnTe/Cr₂Te₃ composite nanostructure.by choosing Cr（acac）₃ as Cr source.The effects of different Mn,Cr and Te ratios on MnTe/Cr₂Te₃ heterostructures were studied.By adjusting the molar ratio of Cr and Mn to Cr:Mn=14:86,the best morphology and magnetic property were prepared,where the reaction time was 3 hours,the reaction temperature was 400℃,the ratio of（Cr+Mn）:Te is 1.2:1.The maximum coercivity can be achieve to 6.5 kOe.This is contributed to the synergistic effects of shape anisotropy,pinning effect and ferromagnetic/antiferromagnetic coupling.（3）The high-quality Mn_1-xCr_xSe composite nano-flowers were successfully synthesized using the same method.With MnCl₂ as Mn source and Cr（acac）₃ as Cr source,the influence of different precursors ratios on the synthesis of Mn_1-xCr_x Se composite nanocrystals were studied.By adjusting the molar ratio to Cr:Mn=1:4,the composite nanoflowers with optimum magnetic property and morphology was obtained.In conclusion,MnTe and MnSe chalcogenides were synthesized by high temperature liquid phase organic solvent.MnTe/Cr₂Te₃ and Mn_1-xCr_xSe nanostructureswith the excellent magnetic properties and morphologies were obtained by doping 3d transition metal Cr.This work provides an approach to modify the morphology and magnetic property for Mn-based chalcogenides.