Antiferromagnetic order in [([Lu.sub.0.8][Ce.sub.0.2]).sub.2][Fe.sub.17] and [Lu.sub.2][Fe.sub.16.5][Ru.sub.0.5]: high pressure study

The antiferromagnetic state seen in [Lu.sub.2][Fe.sub.17] at elevated temperatures is stabilized down to the ground state in [([Lu.sub.0.8][Ce.sub.0.2]).sub.2][Fe.sub.17] and [Lu.sub.2][Fe.sub.16.5][Ru.sub.0.5]. The pressure and substitution effects are understood in terms of exchange interaction varying with changes in the lattice parameter c for [([Lu.sub.0.8][Ce.sub.0.2]).sub.2][Fe.sub.17], while for [Lu.sub.2][Fe.sub.16.5][Ru.sub.0.5], the change in magnetic structure has occurred due to the weakening of the positive exchange interaction as a result of the reduction in the Fe magnetic moments.