Magnetic ordering in RB$_{66}$ boron-rich borides.*

Magnetic ordering in TbB$_{66}$ and GdB$_{66}$ borides which belong to extremely boron-rich borides was investigated at very low temperatures. Measurements of ac susceptibility and of heat capacity of these compounds with a very low concentration of magnetic ions have shown rather clear features of magnetic ordering. They were observed at 0.34 K for TbB$_{66}$ and at 0.2 K for GdB$_{66}$. However, no direct evidence of long range magnetic order was found by neutron scattering experiments at these temperatures. Reasons leading to these observations / results will be discussed.