Structural and magnetic properties of CuCr1-xMgxO2 by neutron powder diffraction

Delafossite antiferromagnetic oxide CuCrO2 was investigated by means of neutron powder diffraction versus temperature. Evolution of the structural parameters (in the R (3) over barm hexagonal setting) shows a steady decrease in the a lattice parameter but negative thermal expansion along c as the temperature decreases from 300 to 1.5 K. Below T-N=24 K, both a and c become constant and a relaxation of the rhombohedral distortion of the CrO6 octahedra is observed. The magnetic structure of CuCrO2 is noncommensurate and shows a short magnetic correlation length of about 200 angstrom along c. Symmetry analysis considerations allow us to propose two possible magnetic structures, helicoidal or cycloidal, compatible with ferroelectric properties. Despite having a strong impact on transport and magnetic properties, Mg doping affects only slightly the crystal and magnetic structures, as shown in CuCr0.98Mg0.02O2.