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CLASSE Seminar - Zahir Islam

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).

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Nahla Minges

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Zahir Islam

Speaker Affiliation

Advanced Photon Source, Argonne National Lab


9:30 am PSB 401

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