Temperature and Frequency Dependence of the ESR Line Width in Two-Dimensional Ferromagnet K2CuF4

The temperature dependence of the ESR line widths of newly found two-dimensional ferromagnet K 2 CuF 4 (which has the K 2 NiF 4 type crystal structure and T c =6.25 K) has been observed at various frequencies. When the temperature is lowered from room temperature, the gradual narrowing of the line width occurs at first, reaching a minimum value at the temperature of several times as large as T c , and then it grows up and makes a peak near T c . Below the temperature where the peak appears, steep narrowing of the line width occurs with further decreasing temperature. At high temperatures, peculiar angular dependence of the line widths is observed in the a c -plane. This angular dependence has been explained by taking account of the effect of the anisotropic exchange interaction besides the magnetic dipolar broadening effect.