NEW MULTIPLE MAGNETIC PHASE-TRANSITIONS AND STRUCTURES IN RMN(2)X(2), X=SI OR GE, R=RARE EARTH

Magnetometry and dilute Fe-57 Mossbauer spectroscopy studies of RMn(2)X(2) (X = Si or Ge, R = La, Ce, Pr, Nd, Sm and Gd) at temperatures 4.2-650 K yield the following results; Fe in RMn(2)X(2) is nonmagnetic. It reveals the magnetic order in the Mn and R sublattices through transferred hyperfine fields. The compounds LaMn2Si2, LaMn2Ge2, CeMn2Ge2, PrMn2Ge2, NdMn2Ge2 and SmMn2Ge2 known to be ferromagnets with T-C = 300-350 K, are antiferromagnetically ordered above their corresponding T-C. Their T-N values extend from 385 K (SmMn2Ge2) to 470 K (LaMn2Si2), similar to the T-N values of the antiferromagnetic heavy rare earth compounds. At the ferromagnetic-antiferromagnetic phase transition, a sharp reorientation of the Mn magnetic moments relative to the crystalline axes occurs. In SmMn2Ge2 we find five magnetic phase transitions, T-C(Sm) = 30 K and T-C(Mn) at 105 and 345 K and T-N(Mn) at 155 and 385 K. In this compound, a superposition of two six-line Fe-57 Mossbauer patterns is seen between 90 and 155 K with changing relative intensities, indicating a competition of two easy magnetization axes, with an anisotropic transferred hyperfine field at the Fe nucleus. In NdMn2Ge2 we find four phase transitions, T-C(Nd) = 21 K, T-C(Mn) = 335 K, T-C(Mn) = 415 K, and one more very sharp transition at 210 K, associated with a discontinuity in Fe-57 hyperfine interaction parameters and a sharp drop in bulk magnetization; this seems to be a transition from pure ferromagnetism to canted antiferromagnetism. The results for antiferromagnetic CeMn2Si2, PrMn2Si2 and GdMn2Ge2 revealed no new phenomena and are in full agreement with previous magnetization studies. In GdMn2Ge2 the transferred hyperfine field at the Fe-57 nucleus is smaller at 4.2 K (below the ordering temperature of Gd) than at 90 K, proving that the transferred hyperfine field from Gd is opposite to that produced by Mn.