Structural and magnetic properties of Sm2Fe17-xTxM (T = Co, Ti; M = Al, Si) compounds

The structural and magnetic properties of off-stoichiometric 2: 17-type compounds Sm2Fe17-xTxM (T=Co, Ti; M=Al, Si; x=0-4) were studied by x-ray powder diffraction, thermomagnetic analysis, and magnetic measurements. All these compounds were of the Th2Zn17 type. The substitution of Co or Ti for Fe, and the addition of Si or Al into the Sm2Fe17 did not alter the rhombohedral structure. The Co replacement increased the Curie temperature T-c due to the strong coupling between Fe and Co. A small substitution of Ti also led to an increase of T-c. However, when x>3, the exchange coupling became less effective as the separation between the Fe atoms became large, and T-c decreased. The maximum T-c = 713 K was found in Sm2Fe17-xCoxSi when x=4. As for the saturation magnetization M-s, Sm2Fe17-xTixAl had its M-s decreased monotonically with increasing Ti, while M-s of other compounds increased first and then decreased with the content of Co or Ti. The maximum M-s = 159 emu/g was recorded in Sm2Fe15Co2Al. To make use of these compounds as permanent magnets, Sm2Fe15Co2Si is preferred because it has T-c = 641 K and M-s = 142 emu/g. The dual substitutions of Fe by using Co or Ti, and by Al or Si to improve T-c and M-s of Sm2Fe17, are more effective than using single substitution. (C) 1999 American Institute of Physics. [S0021-8979(99)56508-0].