Reinvestigation of magnetic structures for the thermally induced states of CuFe_1-xAl_xO₂ (x = 0.00, 0.02 and 0.05) using a four-circle neutron diffractometer

We have reinvestigated the effect of non-magnetic impurity on thermally-induced magnetic orderings in geometrically frustrated triangular lattice antiferromagnets CuFe_1-xAl_xO₂ with x = 0.00, 0.02 and 0.05, using a four-circle neutron diffractometer. Although we inferred in our previous work with a three-axis diffractometer that a proper screw type magnetic structure is realized in the intermediate-temperature (IM) phase of the samples with x = 0.02 and x = 0.05, in the present work we found that a sinusoidally amplitude-modulated magnetic structure with the magnetic moments canted by about 50° from the c axis toward the hexagonal (110) direction is realized. These experimental findings suggest that the quasi-Ising magnetic orderings in CuFeO₂ are stabilized not with the strong Ising anisotropy expected from the Ising-like successive magnetic phase transitions in CuFeO₂ but with the small Ising anisotropy added to strongly competing exchange interactions.