Correlation between crystal structure and magnetism in the frustrated antiferromagnet CuFe O2 under high magnetic fields

The results of synchrotron x-ray-diffraction and magnetization measurements on a triangular lattice antiferromagnet CuFe O2 under pulsed high magnetic fields are reported. This material exhibits a distorted triangular lattice structure below ∼11 K to relieve partially the geometric frustration. We find stepwise changes in the lattice constants, associated with the magnetization changes parallel (H) and perpendicular (H) to the trigonal c axis. The relative changes in the lattice constant b with H are reproduced by a calculation based on a model in which the number of bonds connecting two parallel spins along the b axis increases with increasing field and the lattice contracts to gain the ferromagnetic direct and antiferromagnetic superexchange energies. For H, we find a discontinuous change in b and c at ∼24 T, and a plateau in b and c at 24< H <30 T. The change in b with increasing H agrees also with the same calculation. We discuss the anisotropic behavior in CuFe O2 observed at the low fields and find that the anisotropy is closely correlated with the lattice distortion.