Half-field EPR line in the two-dimensional ferromagnet K2CuF4

Room temperature measurements of the half-field EPR line in the two- dimensional ferromagnet K₂CuF₄ are reported. Changes of the intensity and line-width vs the angle between the applied field and the c-axis indicate the necessity of an additional term in the perturbing Hamiltonian besides the dipolar interaction. Its appearance at half the main Zeeman field at each angle means that there is no observable dynamic effect on the shift of the resonance position in this salt.