This is a past event. Its details are archived for historical purposes.
The contact information may no longer be valid.
Please visit our current events listings to look for similar events by title, location, or venue.
Friday, December 7, 2018 at 10:00am to 11:00am
Physical Sciences Building, 401
245 East Avenue
A comprehensive understanding of the role of mesoscale features (such as magnetic and twin domains, lattice distortions, or magneto-striction) in phase transitions of quantum materials (QM) as well as hysteresis, memory effects, domain dynamics,etc., tuned in by magnetic fields would require a holistic approach merging both diffraction and imaging. Some exemplary cases include 5d-electron systems such as iridates and double perovskites with a strong interplay of exchange and spin-orbit interactions1,2 giving rise to complex magneto-structural transition into novel forms of magnetism. This talk focuses on, Ba2NaOsO6 (BNOO), a presumed cubic system, which behaves as a ferromagnet (FM, Tc~6.8 K, ordered moment ~0.2 µB per Os) with an unusual easy axis3. Recent studies claim this ordered phase to be a novel canted FM (cFM) preceded by a local-symmetry breaking.4
A series of x-ray scattering studies aided by novel instrumentation at the Advanced Photon Source (APS) reveals correlation of lattice symmetry and magnetism in BNOO. Surprisingly, even at room temperature BNOO is tetragonal (T), undergoing a bulk global symmetry breaking transition into an orthorhombic (O) phase at low temperature. While there is some evidence of structural fluctuations in paramagnetic phase, calorimetric data5, concurrently measured with diffraction, indicated T-to-O transition to occur just above Tc. Below Tc, a commensurate superlattice peak at q=(1,0,0) along orthorhombic a* (i.e. shortest axis) was observed. While a coherent rotation of domains aligns FM component with external field this primary staggered order survives revealing qualitative differences with bulk M(H) data. Due to structural twins, bulk measurements below Tc are susceptible to effects of field applied simultaneously along all three directions, while scattering probes staggered component of individual twins. Such a magneto-structurally inter-twined transition gives rise to an inhomogeneous phase. While detailed resonant-scattering 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, being developed to visualize aperiodic mesoscopic topology of QM, will be presented.
Use of the APS 6-ID-B,C and 6-ID-D beamlines was supported by the DOE, under Contract No. DE-AC02-06CH11357.
1 H. Ishizuka, L. Balents, PRB 90, 184422 (2014).
2 S. Gangopadhyay, W. E. Pickett, PRB 91, 045133 (2015).
3 S. Erickson, et al., PRL 99, 016404 (2007).
4 L. Lu, et al., Nat. Comm. 8, 14407 (2017).
5 K. Willa, et al., RSI 88, 125108 (2017).