MAGNETISM AND STRUCTURAL CHEMISTRY OF RECO13-XGAX ALLOYS (RE = LA, CE, PR, ND, AND MISCHMETAL MM)

Alloys with the composition RECo13-xGax (x≤4.4) have been prepared by arc melting and subsequent annealing for 120 h at 800 °C in evacuated, Mo-lined silica capsules. The Ce,Pr compounds were found to decompose at 900 °C, and no compound formation was observed for the Gd,Sm members in the investigated temperature range from 600 to 900 °C. For the LaCo13-xGax system an extended homogeneity region was observed crystallizing with the cubic NaZn13 type for concentrations x<2.5. For concentrations x>2.5 x-ray powder diffraction analysis for all the rare earth members proved isotypism with a tetragonal NaZn13 -derivative structure, whose crystal symmetry and atom sites were consistent with the Ce2Ni17Si9 type of structure. From magnetization experiments in the range from 5 to 1300 K a generally hard-magnetic behavior was observed for all the RE alloys with tetragonal symmetry and ferromagnetic ordering occurs below Tc<650 K. Curie temperatures in the LaCo13-xGax system monotonically drop from Tc=1290 K for cubic LaCo13 to Tc=145 K for tetragonal LaCo8.7Ga4.3. For the Ce compound ferrimagnetic behavior is suggested. Assuming trivalent rare earth ions, effective paramagnetic moments of the Co atoms decrease as a function of x from 5.9 μB at x=3.1 to ∼3 μB at x=4.3.