Magnetic correlations and the influence of atomic disorder in frustrated isosceles triangular lattice antiferromagnet CuMnO 2

We have studied the magnetic correlations of the isosceles triangular lattice antiferromagnet Cu1+xMn1-xO2 with x = 0.00 and 0.04, using the magnetic susceptibility, specific heat, elastic, and inelastic neutron-scattering experiments. Above T-N, the magnetic correlations in both samples are characterized by a two-dimensional (2D) short-range correlation, as indicated by the broad peak in the susceptibility and the asymmetric diffuse scattering observed in neutron diffraction experiments. Below T-N, the magnetic structures, with the propagation vectors Q = ((1/2) over bar 1/2 1/2) in x = 0.00 and Q = ((1/2) over bar 1/2 0) in x = 0.04, have been identified to be collinear magnetic structures with the magnetic moments almost parallel to the d(z)2 orbitals. Although the atomic disorder does not affect the direction and length of the magnetic moments in CuMnO2, the stacking sequence along the c direction is changed from antiferromagnetic to ferromagnetic. Considering the interlayer exchange interactions between isosceles triangular lattice layers, we find that the second-nearest-neighbor interlayer interaction plays an important role for the magnetic stacking along the c axis. The magnetic excitation below T-N is characterized by collective spin-wave excitation with an energy gap of 6 meV from the three-dimensional long-range magnetic order and by spin-liquid-like 2D excitation, as indicated by C(T) proportional to T-2 in CuMnO2.