Study On Magnetic Transition Of Metal-organic Frameworks [（CH₃）₂NH₂]M（HCOO）₃（M=Fe,Co） And [（CH₃）₂NH₂]Fe^ⅢFe^Ⅱ（HCOO）₆

Metal-organic frameworks(MOFs)materials are a kind of hybrid materials that periodically combine metal ions/clusters and organic groups by means of hydrogen bond or other ways.Such materials have attracted much attention because of interesting physical and chemical properties,and wide applications in the fields of catalysis,drug transport,gas storage and separation,nonlinear optics,etc.In this thesis,in order to study the magnetic transition of some perovskite-type metal-organic frameworks under external magnetic field,MOFs materials of perovskite-type[(CH3)2NH2]M(HCOO)3(M=FeⅡ,CoⅡ)and niccolite-type[(CH3)2NH2]FeⅢFeⅡ(HCOO)6 have been synthesized by a solvothermal method.The structure and properties of the MOFs have been characterized by using X-ray diffraction,scanning electron microscope,X-ray energy spectrum and physical property measurement system.The[(CH3)2NH2]Fe(HCOO)3 and[(CH3)2NH2]Co(HCOO)3 present weak canted ferromagnetism under the Curie temperature of 19 K and 14.9 K,respectively.Magnetization curves of the[(CH3)2NH2]Fe(HCOO)3 and[(CH3)2NH2]Co(HCOO)3 have been recorded in a magnetic field range 0-140 kOe at different temperatures.From the M-H curves,the spin-flop behavior in[(CH3)2NH2]Fe(HCOO)3 and[(CH3)2NH2]Co(HCOO)3 was observed with the spin-flop field(HSF)of 104 kOe and 90 kOe,respectively.The ZFC/FC and M-H curves of MOF[(CH3)2NH2]FeⅢFeⅡ(HCOO)6 were also measured,indicating the N-type ferrimagnetism with the Curie temperature of about 37 K.When the temperature was 15 K,20 K and 25 K,the magnetization curves of[(CH3)2NH2]FeⅢFeⅡ(HCOO)6 present multi-step transformation in the high magnetic field range.Below about 10 K,the transition field are higher than 140 kOe,while the multi-step transition disappears at the measurement temperature close to the Curie temperature.In the ZFC/FC and ac susceptibility curves of[(CH3)2NH2]FeⅡ(HCOO)3 and[CH3)2NH2]FeⅢFeⅡ(HCOO)6,an abnormal magnetic transition is presented at about 160 K,possibly due to a second phase in them.The second phase in the[(CH3)2NH2]FeⅢFeⅡ(HCOO)6 crystals is characterized by the XRD peaks at the 20 angles of 11.80°,13.30°,16.90°,24.000 and 32.20°,which is a new MOF with a trigonal structure in the space group P3C(no.159).However,given the current available data,understanding the magnetic properties of this second phase is difficult,and it merits further investigation in the future.