The results of synchrotron x-ray-diffraction and magnetization measurements on a triangular lattice antiferromagnet $\mathrm{Cu}\mathrm{Fe}{\mathrm{O}}_2$ under pulsed high magnetic fields are reported. This material exhibits a distorted triangular lattice structure below $\sim 11\mathrm{K}$ to relieve partially the geometric frustration. We find stepwise changes in the lattice constants, associated with the magnetization changes parallel $\left(H^{\Vert }\right)$ and perpendicular $\left(H^{\perp }\right)$ to the trigonal $c$ axis. The relative changes in the lattice constant $b$ with $H^{\Vert }$ 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 $H^{\perp }$, we find a discontinuous change in $b$ and $c$ at $\sim 24\mathrm{T}$, and a plateau in $b$ and $c$ at $24<H^{\perp }<30\mathrm{T}$. The change in $b$ with increasing $H^{\perp }$ agrees also with the same calculation. We discuss the anisotropic behavior in $\mathrm{Cu}\mathrm{Fe}{\mathrm{O}}_2$ observed at the low fields and find that the anisotropy is closely correlated with the lattice distortion.

• Received 7 March 2007

DOI:https://doi.org/10.1103/PhysRevB.75.224411