Parallel Pumping of the Brillouin-Zone-Boundary Magnons in a Two-Dimensional Ferromagnet K₂CuF₄

Abstract

There is a very flat branch in the spin-wave spectrum of two-dimensional ferromagnets. Since the energy of the lowest Brillouin-zone-boundary magnon is very small in this case, those magnons can be excited by a conventional parallel pumping experiment. Instability of the zone-boundary magnons in a two-dimensional ferromagnet K 2 CuF 4 is observed and a magnon relaxation time is measured at a pumping frequency of 8.9 GHz. Another feature of this case is the existence of measurable steady state absorption below the instability threshold (subthreshold absorption), because the magnon density of states of the lowest branch is very large. It is usually difficult to observe this absorption in three-dimensional ferromagnets. From the shape of the subthreshold absorption observed at pumping frequencies of 8.9, 35 and 73 GHz, the interlayer exchange field in K 2 CuF 4 is determined to be 126 Oe (or equivalent to 0.017 K). The ratio of the interlayer to the intralayer exchange fields is estimated to be 0.00042.