EPR study of the randomly diluted two-dimensional heisenberg ferromagnet K 2Cu xZn 1−xF 4

Abstract

We report a room temperature EPR study of K 2Cu x Zn 1− x F 4, a randomly site-siluted system of the two-dimensional Heisenberg ferromagnet K 2CuF 4. Since the ferromagnetism of K 2CuF 4 comes from the orbital ordering of d z 2− x 2 and d z 2− y 2 due to the cooperative Jahn-Teller distortion of F −-octahedra, particular attention has been paid to the x-dependences of g-values in order to see how the orbital ordering is disturbed by Zn 2+ ions which have no Jahn-Teller effect. From the almost unchanged g-values obtained for x≳0.4, we infer that the orbital ordering remains for x≳0.4, which assures the in-plane exchange interactions to be ferromagnetic over this range of x. Both the linewidth of the main-line and the intensity of the satellite (2ω 0)-line show different aspects above and below x≊0.4. This value of x coincides with the percolation limit 0.41 for the next-nearest-neighbor interactions on a square lattice. Taking account of the appearance of T c for x<0.59, the percolation limit for the nearest-neighbor interactions, observed in the susceptibility-measurement by Okuda et al., we conclude that x≊0.4 should be the critical concentration and thus the next-nearest-neighbor exchange plays an important role in the present diluted system.