Chemical pressure tuning of URu2Si2 via isoelectronic substitution of Ru with Fe

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 isoelectronic substitution of Ru with Fe acts as ``chemical pressure'' $P_{ch}$. Neutron diffraction reveals a sharp increase of the uranium magnetic moment at $x=0.1$, reminiscent of the ``hidden order'' (HO) to large moment antiferromagnetic (LMAFM) phase transition in URu$_2$Si$_2$. Using the unit cell volume, and the isothermal compressibility $\kappa_{T}$ for URu$_2$Si$_2$, we determine $P_{ch}$ as function of $x$. The resulting temperature $T$-chemical pressure $P_{ch}$ phase diagram for URu$_{2-x}$Fe$_x$Si$_2$ is in good agreement with the established temperature $T$-external pressure $P$ phase diagram of URu$_2$Si$_2$. Thus, URu$_{2-x}$Fe$_x$Si$_2$ provides a new opportunity to study the close relationship between the HO and LMAFM phases with methods that cannot be used under pressure, and may shed some new light on the on the elusive order parameter of the HO.