  Rare earth pyrochlore Iridates (RE2Ir2O7) consist of two interpenetrating
cation sublattices, the RE with highly-frustrated magnetic moments, and the
Iridium with extended conduction orbitals significantly mixed by spin-orbit
interactions. The coexistence and coupling of these two sublattices create a
landscape for discovery and manipulation of quantum phenomena such as the
topological Hall effect, massless conduction bands, and quantum criticality.
Thin films allow extended control of the material system via symmetry-lowering
effects such as strain. While bulk Pr2Ir2O7 shows a spontaneous hysteretic Hall
effect below 1.5K, we observe the effect at elevated temperatures up to 15K in
epitaxial thin films on (111) YSZ substrates synthesized via solid phase
epitaxy. Similar to the bulk, the lack of observable long-range magnetic order
in the thin films points to a topological origin. We use synchrotron-based
element-specific x-ray diffraction (XRD) and x-ray magnetic circular dichroism
(XMCD) to compare powders and thin films to attribute the spontaneous Hall
effect in the films to localization of the Ir moments. We link the thin film Ir
local moments to lattice distortions absent in the bulk-like powders. We
conclude that the elevated-temperature spontaneous Hall effect is caused by the
topological effect originating either from the Ir or Pr sublattice, with
interaction strength enhanced by the Ir local moments. This spontaneous Hall
effect with weak net moment highlights the effect of vanishingly small lattice
distortions as a means to discover topological phenomena in metallic frustrated
magnetic materials.
