| Due to antiferromagnetic interactions between nearest-neighbor Fe3+ spins in each hexagonal plane, CuFeO2 is a geometrically-frustrated antiferromagnet. We construct a spin Hamiltonian that includes the nearest-neighbor (NN), second-NN, third-NN exchange interactions, J1, J2, J3, respectively, the single-ion anisotropy D and the antiferromagnetic interaction Jz between adjacent hexagonal planes. The spin-wave (SW) frequencies are obtained from a 1/S Holstein-Primakoff expansion for the four inequivalent spins at sites Fitting the measured SW frequencies of pure CuFeO2 gives J1S = ~ 1.14 meV, J2S = ~ 0.50 meV, J3S = ~ 0.65 meV, JzS = ~ 0.33 meV, and DS = ~ 0.17 meV. The predicted frequencies are given by the black (upper) solid and dashed curves on the right. Replacing Fe3+ with Al3+ in CuFe1-xAlxO2 lowers the SW frequencies. Within our model, the same effect is produced by decreasing DS. In the blue (lower) curves, we compare the predicted and experimental frequencies for DS = 0.12 meV and x = 0.02. This work was done by Randy S. Fishman in collaboration with Neutron Scattering Science Division at ORNL, Argonne National Laboratory, NIST, University of Maryland, and Osaka University (Japan). Phys. Rev. Lett. 99, 157201 (2007). |
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