MAGNETIC-ORDERING OF CUFE1-XALXO2

The magnetic orderings of the rhombohedrally stacked triangular-lattice antiferromagnet CuFe1-xAlxO2 has been investigated by powder neutron diffraction measurements on samples with x = 0.0-0.2, and its magnetic phase diagram has been obtained. The four-sublattice collinear magnetic ordering seen in the sample with x = 0.0 is easily destroyed with only 1% substitution, and non-collinear helical magnetic ordering with propagation wave vector similar to (0.2 0.2 3/2) is observed down to 1.5 K instead of collinear five-sublattice magnetic ordering. This is reasonable for a Heisenberg-spin triangular-lattice antiferromagnet of orbital singlet Fe3+ magnetic ions. These results suggest that four-sublattice collinear magnetic ordering is stabilized by a delicate balance of exchange interactions.