The easy $c$-axis collinear antiferromagnetic order of the bilayer square-lattice iridate ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ has motivated a diverse range of microscopic models. A previous high-pressure Raman study suggested a magnetic phase transition near 15 GPa at low temperatures, but the fundamental nature of this transition has remained unclear. Here, we utilize high-pressure resonant inelastic x-ray scattering, x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD) to directly probe the evolution of magnetic excitations in ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ under compression. We identify three distinct pressure regimes. At low pressures, the system hosts a well-defined magnon characteristic of long-range antiferromagnetic order. Beyond a characteristic pressure, the magnon broadens substantially while its energy continues to harden, marking a crossover into a heavily damped paramagnon regime. Upon further compression, the paramagnon spectral weight is fully suppressed at a distinct second boundary. Both the crossover and the phase boundary exhibit a pronounced temperature dependence, shifting to higher pressures at lower temperatures. The crossover pressure lies near the previously identified Raman transition, whereas the second boundary extends beyond 22.5 GPa at 33 K. Throughout the explored pressure-temperature phase space, XAS and XMCD spectra reveal no discernible changes, ruling out any substantial departure from the $J_{\mathrm{eff}}=1/2$ state or a transition to an alternative magnetic configuration such as a spin-flip phase. We discuss these findings in the context of the competing microscopic models for magnetism in ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$.

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• Antiferromagnetism
• Exchange interaction
• High-pressure studies
• Magnons
• Spin dynamics
• Spin state transition
• Antiferromagnets
• Layered crystals
• Resonant elastic x-ray scattering