Collapse of the Hyperfine Magnetic Field at the Ru site in GdRu$_2$ and HoRu$_2$

The M\"{o}ssbauer Effect(ME) is frequently used to investigate magnetically ordered systems. One usually assumes that the magnetic order induces a hyperfine magnetic field, $H_{hyper}$, at the ME active site. This is the case in the ruthenates where the temperature dependences of $H_{hyper}$ at $^{99}$Ru sites track the magnetic order. This is not the case in GdRu$_2$ and HoRu$_2$. Specific heat, magnetization, and susceptibility show that there is ferromagnetic order below 93K in GdRu$_2$. Neutron diffraction data reveal that HoRu$_2$ orders ferromagnetically at 15.30(4) K with an ordered moment of 7.98(8) $\mu_B$. However there is no evidence of a correspondingly large $H_{hyper}$ in the $^{99}$Ru ME in either material. {\it Ab initio} calculations shows that spin polarization occurs only on the rare earth sites with $ H_{hyper} < 5T$ on the Ru sites. The results are compared with the corresponding calculations for ferromagnetic SrRuO$_3$.