Resonant scattering and diffraction-contrast imaging studies of order and fluctuations in quantum materials under magnetic field


Understanding the role of mesoscale effects (such as magnetic and twin domains, lattice distortions, or magneto-striction) in phase transitions of quantum materials (QM) as well as hysteresis and related phenomena (e.g. memory effects, domain dynamics) tuned in by magnetic fields at cryogenic temperature would require a holistic approach merging both diffraction and imaging methodologies. Contemporary condensed-matter physics research shows how complex magnetostructural transition into novel forms of magnetism might arise in 5d-electron systems such as iridates and double perovskites with a strong interplay of exchange and spin-orbit interactions. In particular, Ba2NaOsO6 (BNOO), a nominally cubic system, behaves as a ferromagnet (FM, Tc~6.8 K, ordered moment ~0.2 μB per Os) with an unusual easy axis. Recent studies claim this ordered phase to be a novel canted FM (cFM) preceded by a local-symmetry breaking [4].

Indeed, x-ray scattering reveal correlation of lattice symmetry and magnetism in BNOO. Even at 300K BNOO is tetragonal (T), undergoing a bulk global symmetrybreaking transition into an orthorhombic (O) phase at low temperature. While there is some evidence of structural fluctuations in paramagnetic phase, calorimetric data, concurrently measured with diffraction, indicated T-to-O transition to occur just above Tc. Below Tc, x-ray resonant scattering revealed a commensurate superlattice peak at q=(1,0,0) along orthorhombic a* (i.e. shortest axis). In a magnetic field, this staggered order saturates, indicating a coherent rotation of domains as the FM component aligns with field, revealing qualitative differences with bulk M(H) data. Due to twins, bulk measurements are susceptible to effects of field applied along all three directions simultaneously, while scattering probes staggered component of individual twins. Such a magneto-structurally inter-twined transition gives rise to an inhomogeneous phase. While detailed resonant and diffraction studies indicate a long-range orbital-ordered phase with quantization (or principal) axis confined to the orthorhombic bc plane, field-temperature evolution of mesoscopic 3D network of orbital and twin domains remains hidden, which is quintessential in determining functional properties of QM. A full-field diffractioncontrast imaging method to reconstruct aperiodic mesoscopic topology of QM will be discussed.

Use of the Advanced Photon Source (6-ID-B,C and 6-ID-D) was supported by the DOE, under Contract No. DE-AC02-06CH11357.