MAGNETISM AND CRYSTAL-CHEMISTRY IN REFE12-XGAX (RE = Y, CE, PR, ND, SM, GD, TB, DY, HO, ER, TM, YB, LU AND MM = MISCHMETAL) AND (ZR,PR)(FE1-XCOX)12-YGAY

The existence of ternary compounds according to the formula REFe 12-x Gax which for x ∼6 represents the iron-rich end of a homogeneous range has been confirmed. X-ray powder analysis of alloys annealed at 800 °C or above generally reveal isotypism with the body- centered tetragonal ThMn12 -type structure. For the alloys with heavier rare-earth elements from Gd to Lu a phase transition to a body-centered orthorhombic structure type (ScFe6 Ga6 type) is observed, which has not been reported before. The transition corresponds to a crystallographic group-subgroup relation (I4/mmm→t2 →Immm), and the transition temperature increases with the ordinal number of the rare earth, indicating the higher the stability of the ScFe6 Ga 6 -type structure, the smaller the radius of the rare-earth element. Accordingly, the ThMn12 -type structure is stable for the early rare-earth members and no transition was observed as low as 400 °C. From magnetization curves it is shown that for REFe12-x Gax (RE=rare earth, Y) all magnetic sublattices order simultaneously at temperatures above Tc ∼400 K. For Y, Lu, and light rare-earth-containing alloys collinear or canted ferromagnetism is observed. The vector of magnetization was found to be close to the a,b plane. Strong hysteresis effects are revealed in all alloys. Energy products are highest for (Pr,Sm)Fe ∼6Ga∼6. For the compounds with the heavy rare-earth elements a ferrimagnetic behavior is encountered. Both magnetic sublattices, i.e., Fe and RE, couple antiparallel, exhibiting easy plane anisotropy. The crystallographic transformation, tetragonal-orthorhombic, has little effect on the magnetic behavior of these alloys. (Zr,Hf)(Fe1-x Cox) 12-y Gay remain temperature-independent paramagnets.