Unusual Critical Behavior of the Diluted Uniaxial Dipolar Ferromagnet Li${\mathrm{Tb}}_{0.3}$${\mathrm{Y}}_{0.7}$${\mathrm{F}}_4$

The experimental critical behavior of the susceptibility of Li${\mathrm{Tb}}_{0.3}$ ${\mathrm{Y}}_{0.7}$ ${\mathrm{F}}_4$ is described by the power law $\chi =\Gamma {\left[\frac{(T-T_c)}{T_c}\right]}^{-\gamma }$ with $T_c=0.520\mathrm{}\pm 0.003$ K and $\gamma =1.80\mathrm{}\pm 0.04$. This behavior is dramatically different from that previously observed in LiTb${\mathrm{F}}_4$ and is evidence of a departure from marginal dimensionality when magnetic ions have been randomly replaced by nonmagnetic ions. Series expansion of ${\left(\chi T\right)}^{-1\mathrm{}}$ in powers of $T^{-1\mathrm{}}$ for a diluted Ising dipolar ferromagnet gives a good description of experimental results described in this Letter.