Abstract:The temperature dependence of lattice parameters a and c of intermetallic compounds RMn₂Ge₂(R=La, Sm and Gd)were measured in the temperature range of 10-800K by using the X-ray diffractometer. It is found that the high temperature magnetic transitions of Mn-subsystem in light rare earth compounds from paramagnetic to antiferromagnetic state accompany the negative magnetoelastic anomalies of lattice parameters c, where a does not change. This indicates that the antiferromagnetic component of intralayer Mn-Mn exchange coupling is correlated with lattice constant c. The low temperature first order ferromagnetism→antiferromagnetism transitions(or antiferromagnetism→ferromagnetism transition) of Mn-subsystem in SmMn₂Ge₂ and GdMn₂Ge₂ accompany the abruptly decrease(or increase) of lattice parameter a, and Δa/a≈0.15%. This demonstrates that the interlayer Mn-Mn exchange interaction is very sensitive to the intralayer Mn-Mn distance. The critical value of lattice constant a_k, at which the interlayer Mn-Mn coupling changes its sign, is 4.0445×10^-10 m. Based on the molecular field model of exchange interaction the magnetic curves of GdMn₂Ge₂ single crystal at different temperatures were calculated and a good agreement with experimental data had gotten. The Gd-Gd, Gd-Mn, intralayer Mn-Mn and interlayer Mn-Mn exchange coupling parameters were estimated.

Key words:  intermetallic compound; magnetic phase transition; exchange interaction