Emergent magnetic and topological phenomena in (111) pyrochlore iridate thin films

During the past decade, quantum materials with nontrivial band topology have become a focal theme in condensed matter physics. In this thriving field, great efforts have been recently made for the experimental discovery of correlated topological semimetals. Pyrochlore iridates R₂Ir₂O₇ (R is a rare-earth ion) being the very first magnetic Weyl semimetal (WSM) candidate, direct demonstrations of its characters with the peculiar "all-in-all-out" antiferromagnetic (AFM) ordering has remained elusive thus far, due to limitations in both available materials and probing techniques.

In this talk, we first report on a new synthesis protocol "in-situ solid phase epitaxy", where (111)-oriented high-quality single crystalline R₂Ir₂O₇ thin films of millimeter-scale can be achieved. Next, we have done comprehensive exploration on the magnetic and topological properties of Eu₂Ir₂O₇. Using synchrotron resonant X-ray magnetic scattering, we directly demonstrate the formation of the Ir "all-in-all-out" AFM order in R₂Ir₂O₇ thin film, as required by the theoretical proposal. Concurrent with the magnetic transition, we reveal the emergence of an intrinsic anomalous Hall conductivity, which exhibits a colossal coercive field of ~100 T at low temperatures connected to the large spin-orbit interaction of Ir. These findings collectively single out the pyrochlore Eu₂Ir₂O₇ as a rare example of AFM WSMs that was proposed a decade ago. In the end, we discuss the observations on distinctive behaviors between Eu₂Ir₂O₇ and Y₂Ir₂O₇, which indicate the emergence of an exotic chiral quantum disordered state in (111) Y₂Ir₂O₇ thin films.

These results may have several broad implications for (i) exploiting the interplay of chiral edge excitations, spin-charge collective modes, topological Weyl magnons; (ii) investigating a new type of quantum criticality of the Luttinger liquid in R₂Ir₂O₇; (iii) conceivably exploring an idea of quantum hydrodynamics in the ultra-clean limit. From the applied perspective, high-quality, well-characterized magnetic WSM films open a prospect to push the nascent field of topological AFM spintronics.