New multiple magnetic phase transitions and structures in RMn₂X₂, X = Si or Ge, R = rare earth

Magnetometry and dilute ⁵⁷Fe Mössbauer spectroscopy studies of RMn₂X₂ (X = Si or Ge, R = La, Ce, Pr, Nd, Sm and Gd) at temperatures 4.2-650 K yield the following results; Fe in RMn₂X₂ 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 ⁵⁷Fe Mössbauer 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_N(Mn) = 415 K, and one more very sharp transition at 210 K, associated with a discontinuity in ⁵⁷Fe 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 ⁵⁷Fe 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.