Investigation of the Competition between Structural and Ferromagnetic Transitions in GdRu$_2$ using the Mossbauer Effect*

We use the $^{99}$Ru Mossbauer Effect(ME) to investigate magnetism in the Laves phase Ce$_{1-x}$Gd$_x$Ru$_2$ series. This series of compounds is superconducting for small $x$ and has been thought to be ferromagnetic for large $x$ on the basis of extrapolations of Curie-Weiss fits to the $\chi$(T) data. We have studied the temperature dependence of the ME spectra for a number of these compounds and found no evidence of a hyperfine magnetic field due to ferromagnetic order. In GdRu$_2$, a Curie-Weiss fit to the $\chi$(T) data above 120K yields a Curie-Weiss temperature of 93K. However, at $\sim 120K$, $\chi^{-1}$ deviates from a straight line fit. The ME spectra show that there is a qualitative change in the environment of the Ru nucleus in this temperature re gion pointing to a structural phase transition. Below this transition the width of the ME spectrum grows with decreasing temperature by 50$\%$ between 100K and 4.2K. We analyze this temperature dependence in terms of a slowing down of the fluctuations of the local field due to Gd moments.