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. I t reveals the magnetic order in the Mn and R sublattices through trans ferred hyperfine fields. The compounds LaMn2Si2, LaMn2Ge2, CeMn2Ge2, P rMn2Ge2, 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 co mpounds. At the ferromagnetic-antiferromagnetic phase transition, a sh arp reorientation of the Mn magnetic moments relative to the crystalli ne 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 p atterns is seen between 90 and 155 K with changing relative intensitie s, indicating a competition of two easy magnetization axes, with an an isotropic transferred hyperfine field at the Fe nucleus. In NdMn2Ge2 w e 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 wit h a discontinuity in Fe-57 hyperfine interaction parameters and a shar p drop in bulk magnetization; this seems to be a transition from pure ferromagnetism to canted antiferromagnetism. The results for antiferro magnetic CeMn2Si2, PrMn2Si2 and GdMn2Ge2 revealed no new phenomena and are in full agreement with previous magnetization studies. In GdMn2Ge 2 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.