Non-Spin-Wave type of Magnetic Excitations in a Well-characterized Nearest-Neighbor Triangular Antiferromagnet

The S = 3/2 Mn⁴⁺ ions of La₂Ca₂MnO₇ forms perfect triangular lattice in the ABC stacking. Therefore when the 120^o antiferromagnetic order forms on the triangular lattice, interlayer magnetic interaction cancel out exactly. The two-dimensional long-range antiferromagnetic order develops below 2.8K, evidenced by the perfect Warren magnetic peak profile as well as in heat capacity and magnetic susceptibility data, substantially lower than the Weiss temperature of 25K. High temperature susceptibility data fitting to the Rushbrooke-Wood formulus yields a nearest-neighbor exchange constant which is identical to that derived from the low-energy stiffness of magnetic excitations, validating the nearest-neighbor magnetic interaction in plane. Different from magnetic excitation spectra previously reported in neutron scattering studies using samples with 3-dimensional long-rang 120^o antiferromagnetic order, in our sample of 2-dimensional 120^o antiferromagnetic order, the Spin-Wave spectrum definitely fails to describe our observed spectra. There are conflicting theories as to what kind of magnetic excitation beyond the Spin-Wave theory should be at the moment, ranging from the fractionalization of spin at the low dimensions to the mode-mode coupling decay of the spin-wave modes.