  Continuous quantum phase transitions involving all-in-all-out (AIAO)
antiferromagnetic order in strongly spin-orbit-coupled 5d compounds could give
rise to various exotic electronic phases and strongly-coupled quantum critical
phenomena. Here we experimentally trace the AIAO spin order in Sm2Ir2O7 using
direct resonant x-ray magnetic diffraction techniques under high pressure. The
magnetic order is suppressed at a critical pressure Pc=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 constant 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, the observed low-pressure scale of the AIAO quantum phase transition
in Sm2Ir2O7 identifies a circumscribed region of P-T space for investigating
the putative magnetic Weyl-semimetal state.
