Unusual Critical Behavior of the Diluted Uniaxial Dipolar Ferromagnet LiTb0.3Y0.7F4

The experimental critical behavior of the susceptibility of LiTb0.3Y0.7F4 is described by the power law χ=Γ[(T-Tc) / Tc]-γ with Tc=0.520±0.003 K and γ=1.80±0.04. This behavior is dramatically different from that previously observed in LiTbF4 and is evidence of a departure from marginal dimensionality when magnetic ions have been randomly replaced by nonmagnetic ions. Series expansion of (χT)-1 in powers of T-1 for a diluted Ising dipolar ferromagnet gives a good description of experimental results described in this Letter.