Critical Temperature of Randomly Diluted Two-Dimensional Heisenberg Ferromagnet, K2CuxZn(1-x)F4

The susceptibility of randomly diluted two-dimensional Heisenberg-like ferromagnet K 2 Cu x Zn (1- x ) F 4 was measured down to 50 mK, using the 3 He– 4 He dilution refrigerator and a SQUID magnetometer. The ferromagnetic critical temperature T c ( x ) was obtained for x =0.98, 0.94, 0.85, 0.82, 0.68, 0.60, 0.54, 0.50 and 0.42. The value of [1/ T c (1)][(d/d x ) T c ( x )] x =1 was approximately 3.0. The critical temperature versus x curve exhibits a noticeable tail near the critical concentration, which may stem from the second nearest-neighbor interaction. The critical concentration x c , below which concentration there is no long range order down to T =0 K, was estimated to be 0.45∼0.50. The susceptibility of sample with x =0.42 behaves as if it obeys the Curie law down to 50 mK.