We have used specific heat and neutron diffraction measurements on single crystals of URu$_{2-x}$Fe$_x$Si$_2$ for Fe concentrations $x$ $leq$ 0.7 to establish that chemical substitution of Ru with Fe acts as chemical pressure $P_{ch}$ as previously p
roposed by Kanchanavatee et al. [Phys. Rev. B {bf 84}, 245122 (2011)] based on bulk measurements on polycrystalline samples. Notably, neutron diffraction reveals a sharp increase of the uranium magnetic moment at $x=0.1$, reminiscent of the behavior at the hidden order (HO) to large moment antiferromagnetic (LMAFM) phase transition observed at a pressure $P_xapprox$ 0.5-0.7~GPa in URu$_2$Si$_2$. Using the unit cell volume determined from our measurements and an isothermal compressibility $kappa_{T} = 5.2 times 10^{-3}$ GPa$^{-1}$ for URu$_2$Si$_2$, we determine the chemical pressure $P_{ch}$ in URu$_{2-x}$Fe$_x$Si$_2$ as a function of $x$. The resulting temperature $T$-chemical pressure $P_{ch}$ phase diagram for URu$_{2-x}$Fe$_x$Si$_2$ is in agreement with the established temperature $T$-external pressure $P$ phase diagram of URu$_2$Si$_2$.
We have used high-resolution neutron spectroscopy experiments to determine the complete spin wave spectrum of the heavy fermion antiferromagnet CeRhIn$_5$. The spin wave dispersion can be quantitatively reproduced with a simple $J_1$-$J_2$ model that
also naturally explains the magnetic spin-spiral ground state of CeRhIn$_5$ and yields a dominant in-plane nearest-neighbor magnetic exchange constant $J_0$ = 0.74 meV. Our results pave the way to a quantitative understanding of the rich low-temperature phase diagram of the prominent Ce$T$In$_5$ ($T$ = Co, Rh, Ir) class of heavy fermion materials.
