Magnetic Properties of K2CuF4 –A Transparent Two-Dimensional Ferromagnet–

It has been found that K 2 CuF 4 is ferromagnetic below 6.25 K. The two-dimensionality of this compound as expected from its K 2 NiF 4 type structure is strongly supported by magnetic and specific heat measurements. The torque as well as zero field NMR measurements suggest that the c -plane is an easy plane, in which the spins are very weakly bounded to the direction of a -axis. On the assumption that this magnetic system to be purely two-dimensional, the exchange interaction J between the spins within the c -plane is found to be the Heisenberg type with about one percent XY-like anisotropy and j / k =10.0 K is obtained from the series expansion analysis of the high temperature susceptibility as well as linear dependence of the low temperature specific heat. The transition at 6.25 K may be caused by inter-layer exchange interaction. The exchange field between the adjacent layers is expected at least larger than 400 Oe.