Approaching the quantum critical point in a highly-correlated all-in-all-out antiferromagnet

All-in-all-out (AIAO) antiferromagnetic order in strongly spin-orbit-coupled 5d compounds can demonstrate exotic electronic phases and strongly-coupled quantum critical phenomena. Here we experimentally explore this scenario by directly tracing the AIAO spin order in Sm₂Ir₂O₇ using resonant x-ray magnetic diffraction techniques under high pressure. We find that the magnetic order is suppressed at a critical pressure P_c=6.30 GPa, while the lattice symmetry remains in the cubic Fd-3m space group across the quantum critical point. Comparing pressure tuning and the chemical series R₂Ir₂O₇ reveals that the suppression of the AIAO order and the approach to the spin-disordered state is characterized by contrasting evolutions of both the pyrochlore lattice parameter a and the trigonal distortion x. The former affects the 5d bandwidth, the latter the Ising anisotropy, and as such we posit that the opposite effects of pressure and chemical tuning lead to spin fluctuations with different Ising and Heisenberg character in the quantum critical region. Finally, we compare the iridates to AIAO spin order in osmate analogues, where spin fluctuations, lattice breathing modes, and quasiparticle excitations all interact in the quantum critical region.