Inelastic neutron scattering study of frustrated Heisenberg triangular magnet CuFeO2

We have studied the magnetic excitation spectrum of the geometrically frustrated triangular lattice antiferromagnets of CuFeO2 with collinear ground state and CuFe1-xAlxO2( 0.014 <= x <= 0.030) with noncollinear ground state, using inelastic neutron scattering measurements on the single crystals. We have reported almost complete spin- wave dispersion relations and the temperature dependence of the magnetic excitation spectrum in both pure and slightly diluted samples. While the higher-energy spin- wave branches remain almost unchanged between the pure and diluted samples, the energy gap of the lowest-energy branch in CuFeO2 becomes zero as a result of the dilution, suggesting the retrieval of the original Heisenberg spin character of the orbital singlet Fe3+. Furthermore, by comparing with recent elastic neutron diffraction and x- ray measurements, we discuss the interrelation among the magnetic ordering, crystal lattice distortion and magnetic excitation qualitatively.