URu$_{2-x}$Fe$_x$Si$_2$: magnetic excitations in hidden order and antiferromagnetic phases

The substitution of Fe for Ru in the heavy fermion compound URu$_2$Si$_2$ drives a transition from Hidden Order (HO) to long-range antiferromagnetic order (AFM) at roughly 5$\%$ Fe. The resulting phase diagram is analogous to that under applied pressure. Using inelastic neutron scattering, we have studied the magnetic excitations in both the HO and AFM phases as a function of Fe concentration. The strong interband scattering that sits at the Brillouin zone edges persists across the HO/AFM boundary, although the energy gap grows. However, the excitations centered on the AFM zone center, which are prominent in the HO phase, disappear in the AFM phase. Implications of these results will be discussed. PRB 94, 201102(R) (2016)