SPONTANEOUS MAGNETIC PHASE TRANSITION AND MAGNETOELASTIC ANOMALIES AT TRANSITION S IN INTERMETALLIC COMPOUNDS RMn2Ge2 (R=La,Pr,Nd,Sm,Gd,Tb, Y)

The temperature dependence of lattice constants a and c of intermetallic compounds RMn2Ge2 (R=La,Pr,Nd,Sm,Gd,Tb and Y)is measured in the t emperature range 10—800K using the X-ray method. The magnetoelastic anomalies o f a(T) and c(T) are found at different spontaneous magnetic phase transitions. I t is shown that the magnetoelastic anomalies are mainly determined by the Mn-sub system. Mn-Mn exchange interactions depend not only on the lattice constant a bu t also on constant c. Low-temperature ferromagnetic-antiferromagnetic transition s are discussed in the framework of Kittle's exchange inversion model.