Magnetic correlations and the influence of atomic disorder in frustrated isosceles triangular lattice antiferromagnet CuMnO₂

We have studied the magnetic correlations of the isosceles triangular lattice antiferromagnet Cu_1+xMn_1-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=(12̄1212) in x=0.00 and Q=(12̄120) in x=0.04, have been identified to be collinear magnetic structures with the magnetic moments almost parallel to the d_z2 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) T2 in CuMnO₂.