NMR study of two-dimensional ferromagnet K2CuF4 below 1 K

Two-dimensional ferromagnet K2CuF4 is an ideal compound to study two-dimensional magnetic systems because the interplane exchange coupling constant is much smaller than the intraplane exchange coupling constant. We measured the temperature dependence of the NMR frequencies of the 1/2 <----> 3/2 transition for Cu-63 and Cu-65 nuclei in K2CuF4 single crystal far below T-c = 6.25 K. The NMR measurements were made by spin-echo method at zero magnetic field. The experimental results are discussed in the framework of the two-dimensional spin wave theory at millikelvin temperatures. (C) 2003 Published by Elsevier Science B.V.