The Structure And Magnetic Properties Of Tb₂AlFe_16-xMn_x And Sm₂Fe_17-xCr_x Compounds

The structure and magnetic properties of Tb₂AlFe_16-xMn_x, Sm₂Fe_17-xCr_x and Sm₂Fe₁₇N_x compounds have been studied by means of XRD, magnetization measurement and positron lifetime measurement, etc. The effects of Mn substitution, Cr substitution and interstitial nitrogen on the structure parameter, the Curie temperature and the spontaneous magnetization of Tb₂AlFe_16-xMn_x, Sm₂Fe_17-xCr_x and Sm₂Fe_17-xN_x compounds are analyzed.The results show that at room temperature the Tb₂AlFe_16-xMn_x compounds (x=1, 2, 3, 4, 5, 6, 7, 8) all crystallize in the Th₂Ni₁₇-type structure. The unit-cell volumes have a slight increase initially, followed by a greater linear increase with increasing manganese concentration x. The unit-cell parameter c increases monotonously with increasing x, while a shows a complicated behavior. The investigations of positron lifetime spectra show that Mn substitution for Fe can weaken the ferromagnetic interaction and enhance the antiferromagnetic interaction. The comprehensive factors such as the changing of magnetic ordering, crystal lattice distortion and expanded volume make the positron lifetime show a complicated behavior. Magnetization measurement shows that both the Curie temperature T_C and the spontaneous magnetization of Tb₂AlFe_16-xMn_x compounds decrease rapidly with increasing x. This implies that the magnetic moment of Mn atom is anti-parallel to the magnetic moment of Fe atom in 3d sub-lattice of Tb₂AlFe_16-xMn_x compounds.The research results on the DTA-TG, nitrogen absorption etc of the Sm₂Fe₁₇ show that the unit-cell volume and the Curie temperature of Sm₂Fe₁₇ sample are improved after nitrogen absorption .When the temperature rises up to 700 K, the reaction of nitrogen absorption is violent. In higher temperature than 700K, the compounds are very unstable. The alloy was intruded, meanwhile, the decomposition of the nitride occurred by products SmN andα-Fe. The results show the Sm₂Fe₁₇ or compounds crystallize in only the Th₂Zn₁₇-type structure below 1473K, without Th₂Ni₁₇ phase.The research results of Sm₂Fe_17-xCr_x(x=0, 0.5, 1.0, 1.5, 2.0, 3.0) indicate that the annealing time has great effects on the structure of these compounds. The Sm₂Fe_17-xCr_x samples annealed at 1050℃for 6 days all crystallize in the Th₂Zn₁₇-type structure, while the samples(x>2) annealed at 1050℃for 7 days have a single-phase of Nd₃(Fe,Ti)₂₉-type structure. The unit-cell volumes and parameters a and c show a complicated behavior with increasing x, which implies a strong magneto-elastic effect exists in the compounds. The results on the Curie temperature show that Cr atoms substituting for Fe atoms can enhance the Curie temperature T_C of Sm₂Fe_17-xCr_x compounds generally, which can reach the maximum value 467K for x=1.0. The investigation on the magnetically aligned samples indicates that the Sm₂Fe_17-xCr_x compounds all show the plane anisotropy from 77K to room temperature 300K.