A NONMAGNETIC FE PROBE OF MULTIPLE MAGNETIC PHASE-TRANSITIONS IN RMN(2)SI(2-X)GE(X), R EQUALS RARE-EARTH

Dilute Fe-57 Mossbauer-spectroscopy studies of RMn(2)X(2) (X = Si and/ or Ge, R = La, Ce, Pr, Nd, Sm, Eu and Gd) at 4.2 to 650 K yield the fo llowing results: Fe in RMn(2)X(2) does not carry a magnetic moment. It reveals the magnetic order in the Mn and R sublattices through transf erred hyperfine fields (similar to 100 kOe). The compounds LaMn2Si2, L aMn2Ge2, CeMn2Ge2, PrMn2Ge2, NdMn2Ge2 and SmMn2Ge2, known to be ferrom agnets with T-c = 300-350 K, are antiferromagnetically ordered above t heir corresponding T-c. Ther T-N values extend from 385 K (SmMn2Ge2) t o 470 K (LaMn2Si2). At the ferromagnetic-antiferromagnetic phase trans ition, a sharp reorientation of the Mn magnetic moments relative to th e crystalline axes occurs. In RMn(2)Si(2-x)Ge(x) with intermediate x v alues, the Ge is much more dominant in determining the magnetic proper ties of the Mn sublattice. While PrMn2Si2 is an antiferromagnet (T-N = 365 K) and PrMn2Ge2 is a ferromagnet-antiferromagnet (T-c = 328 K, T- N = 415 K), we find that in PrMn2SiGe, the magnetic behaviour is simil ar to that in pure PrMn2Ge2, T-c = 305 K and T-N = 395 K.