부산대학교 도서관

Tb/sub 2/Fe/sub 3/Si/sub 5/ has been studied by neutron-diffraction techniques at low temperatures. At 20 K the diffraction pattern is characteristic of the tetragonal crystal structure (space group P4/mnc, a = 10.43 A, c = 5.48 A). The atomic positional parameters have been determined by Rietveld refinement of the data. Magnetic-scattering peaks were observed just below 10.3 K, corresponding to the antiferromagnetic ordering temperature determined by susceptibility measurements. Two sets of magnetic-scattering peaks coexist from just below the ordering temperature to the lowest temperature studied, 4.55 K. These data can be accounted for with the following magnetic model. In both cases moments appear only on Tb atoms and are directed along (001). Above 7 K the predominant set is incommensurate, consistent with a sinusoidal modulation along (001) of an antiferromagnetic structure consisting of ferromagnetic (001) sheets coupled antiparallel. Below 7 K the predominant set is commensurate with a and c parameters double those of the chemical cell. In this structure the four Tb moments within (001) squares are coupled parallel. Adjacent squares in the a and c directions are coupled antiparallel. Interpretation of the two sets of peaks may invoke either two physically separate phases or a linear superposition of the two proposed magnetic structures.