Triangular lattice antiferromagnet RbFe (Mo O4) 2 in high magnetic fields

The saturation of magnetization was studied in pulsed magnetic fields (H) for the triangular lattice antiferromagnet RbFe (Mo O4) 2 at temperatures (T) below the magnetic ordering point TN. The field-driven transition from the antiferromagnetic to the paramagnetic phase was studied by specific-heat measurements in a steady magnetic field. The specific heat shows a sharp peak at a value of the magnetic field below which a singularity in the magnetization curve occurs, indicating a splitting of the field-induced transition to the paramagnetic phase. This splitting vanishes at low temperatures. A complete T-H phase diagram is derived experimentally both for the magnetic field lying in the spin plane and perpendicular to the plane. A spin-flip magnetic-resonance mode with the softening of the spin-wave spectrum at the saturation field has been observed. The gap of the spin-wave spectrum was found to be reduced by zero-point fluctuations by a factor of 0.82±0.08 with respect to the prediction of the molecular-field theory.