MAGNETIC-PROPERTIES OF RARE-EARTH MANGANESE COMPOUNDS OF THE RMN6GE6 TYPE

We determined the magnetic properties of several compounds of the type RMn6Ge6 (R is a heavy rare-earth element). Several of these compounds give rise to two ordering temperatures associated with antiferromagnetic ordering of the R and Mn sublattices. For GdMn6Sn6 and to some extent also for TbMn6Sn6 we observed magnetic behaviour which can be described as ''bootstrap ferrimagnetism'': the antiferromagnetic configurations within both the R and Mn sublattices are broken simultaneously by the R-Mn intersublattice interaction, leading to ferrimagnetic alignment of the R and Mn sublattices. This bootstrap mechanism is sufficiently strong only if the intrasublattice Mn-Mn interaction is sufficiently weak. This condition is met only when the unit cell volume is sufficiently large. The interplay between intersublattice coupling and unit cell volume leads to interesting temperature dependences of the magnetization. In the magnetic isotherms the bootstrap mechanism leads to first-order phase transitions.