Synthesis,Structure And Magnetism Of Mn_3+xA_1-xC（A=Al,Ga） And Mn₄C Composites

As early as the 20th and 60s,the basic physical properties of Mn-based anti-perovskite structural compounds（Mn₃AX）have been systematically studied.In recent years,especially after He et al.discovered that Ni₃MgC has the superconductivity at low temperatures,Mn-based anti-perovskite structural compounds have once again attracted the attention of many scientists.Researchers have discovered new properties of Mn-based anti-perovskite structural compounds with potential applications such as giant magnetoresistance effect,magnetocaloric effect,heat shrinkage effect and constant resistivity and so on.Among them,the application of magnetocaloric effect in the field of magnetic refrigeration has attracted the attention of researchers.As an effective physical property improvement method,element substitution has been fully applied by researchers to the physical properties of Mn-based anti-perovskite structures.However,in order to avoid the elemental substitution resulting in a change in the structure of the Mn-based anti-perovskite structure,the substitution of the elemental element does not exceed0.2,regardless of whether the Mn element at the position of the face or the A element at the position of the corner.Recently,Professor Si Pingzhan of our research group successfully prepared high-purity Mn₄C（typical anti-perovskite structure）compounds by plasma discharge method.Therefore,we tried to replace the A element with a large amount of Mn element,systematically studied the effect of a large number of Mn elements replacing A element on the structure and magnetic properties of Mn_3+xA_1-xC（A=Al,Ga）.At the same time,the magnetic properties of Mn₄C composites were systematically studied.The detail researches are shown as follows:（1）Mn_3+xAl_1-xC（x=0,1/4,1/2,3/4,1）compounds with different chemical compositions were prepared by induction melting followed by heat treatment at 1000^oC for 24 hours.The phase of the samples before and after the heat treatment was characterized by XRD.The magnetic properties of the samples before and after the heat treatment were analyzed by PPMS.The results show that the coercivity and remanence of the Mn_3+xAl_1-xC compound are zero,which means that Mn_3+xAl_1-xC is a soft magnetic material.When x≤0.5,the crystal structure of Mn_3+xA1_1-xC is stable.When≥0.75,the stability of Mn_3+xA1_1-xC deteriorates,and when the temperature rises,the decomposition will be intensified.The Curie temperature of Mn₃AlC after high temperature heat treatment is 309 K.Under the condition of external magnetic field 3 T,the maximum magnetic entropy change of Mn_2.75Al_1.25C at 285 K is about 2.26 J/kg K.（2）Mn_3+xGa_1-xC（x=-1/4,0,1/4,1/2,3/4,1）compounds with different chemical compositions were prepared by induction melting and solid reaction methods.The samples prepared by different methods were characterized by XRD.It was found that the XRD of Mn₃GaC prepared by induction melting contained a large amount of impurities.However,no impurity appeared in Mn_3.25Ga0.₇₅C,Mn_3.5Ga_0.5C,Mn_3.75Ga_0.25C,which may be caused by insufficient reaction or loss of some raw materials in the induction melting process.Analyses the sample prepared by the solid reaction method.when x≤0.5,the crystal structure of Mn_3+xGa_1-xC is stable.When≥0.75,the impurities start to appear in the XRD of Mn_3+xGa_1-xC compounds.The Curie temperature of Mn₃GaC is about 270 K,which is slightly higher than the 246K reported by the predecessors.The saturation magnetizations of Mn_3+xGa_1-xC decrease with increasing the x.（3）The Mn-C nanoparticles composed ofα-Mn（C）solid solution,Mn₄C,manganosite,and trace amount of C were prepared by plasma arc discharge method.The Mn-C nanoparticles exhibit room temperature ferromagnetic behaviours and unusual positive temperature coefficients of magnetization,owing to the presence of Mn₄C,which is Néel’s P-type ferrimagnet.The thermomagnetic behaviours of a system composing materials with positive and negative coefficients of magnetizations may be tunable by tuning the fraction of the component materials.The exchange bias field and the coercivity of the Mn-C nanoparticles decrease with increasing temperature.