Noise pumping of nuclear spin waves in an antiferromagnet

-Parametric excitation and the above-threshold behavior of a nonequilibrium system of nuclear magnons are investigated both under microwave noise pumping and under monochromatic pumping with noise modulation of the magnon spectrum. It is established that there exist two critical pumping amplitudes: the first corresponds to the onset of nonlinear absorption of the microwave field, and the second corresponds to the development of strong phase correlations-a nonequilibrium Bose condensate (NBC)-in a system of excited magnon pairs. The formation of the NBC was recorded by two methods: by intense electromagnetic radiation from a sample and by a coherent response of parametric magnons to a harmonic modulation of their spectrum (by a modulation response). The existing theories provide a satisfactory description for the functional dependence of pumping thresholds on the parameters of experiment, except for the region of minimal temperatures. The observed discrepancy between theoretical and experimental results can be explained under the assumption that quasi-elastic two-magnon relaxation processes make a nonadditive contribution to the threshold amplitudes of noise pumping. (C) 2005 Pleiades Publishing, Inc.