Magnetic Resonance in Quasi-Twodimensional Antiferromagnets

In this work two low-dimensional antiferromagnets Ba2MnGe2O7 and Cu(en)(H2O)2SO4 were investigated in ordered and disordered states by magnetic resonance in the frequency range from 4.5 to 120 GHz at temperatures down to 450 mK. In Ba2MnGe2O7 we confirmed the formation of a collinear antiferromagnetic order with easy-plane anisotropy, the gap in the spin waves spectrum equal to 24 GHz was determined. We have observed effect of hyperfine coupling between nuclear and electronic moments of manganese, the interaction constant of the nuclear and electronic subsystems was determined. We also proved the presence of weak anisotropy in a plane perpendicular to the main axis of anisotropy. We showed that the temperature dependence of the EPR line width above the Neel point can be described in the Berezinsky-Kosterlitz-Thouless transition model. In Cu(en)(H2O)2SO4 the formation of a collinear antiferromagnetic order parameter with “easy”, “second easy” and “hard” anisotropy axes was confirmed, the anisotropy axes and anisotropy parameters in the ordered phase were determined. We also observed the splitting of the antiferromagnetic resonance line, which can be associated with the formation of inequivalent weakly coupled antiferromagnetic subsystems below the Neel temperature.