Characterizing Coupled Order Parameters Across Successive Phase Transitions at Atomic Resolution in SmBaMn₂O₆

Rich phase diagrams and highly tunable ground states arising from strong coupling between electronic, magnetic, and lattice degrees of freedom make complex oxides an ideal playground for exploring connections between material structure and exotic strongly correlated properties. The A-site ordered double-perovskite manganite SmBaMn₂O₆ is a paradigmatic example: a complex set of structural distortions host a charge and orbital ordered (COO) polar antiferromagnetic ground state [1,2]. With increasing temperature, the material undergoes a series of structural, magnetic, and COO transitions governed by competing order parameters [3]. Coupling of these order parameters to the lattice generates intricate structural distortions and offers a tuning knob to control and stabilize new phases such as an anticipated ferromagnetic metallic phase under biaxial compressive strain [4].<br/><br/>To characterize these distortions, we measure SmBaMn₂O₆in bulk and epitaxial thin films at atomic resolution with <i>in situ</i> scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Studying the ground states and successive phase transitions in both the strain-free single crystal and epitaxial thin films under a series of strain states allows the role of the lattice in stabilizing electronic and magnetic order to be systematically investigated. Cryogenic STEM reveals the polar COO distortion defining the ground state structure as well as the low temperature phase transitions which reshape the COO and extinguish the polar and antiferromagnetic order. Characterization at higher temperatures can in turn clarify the structural mechanisms at play as the system undergoes an insulator-metal transition into a charge disordered phase.<br/><br/>[1] Morikawa, et al. <i>J. Phys. Soc. Jpn.</i>, <b>81</b>, 093602 (2012).<br/>[2] Sagayama, et al. <i>Phys. Rev. B</i>, <b>90</b>, 241113. (2014).<br/>[3] Yamada, et al. <i>J. Phys. Soc. Jpn.</i>, <b>81</b>, 113711. (2012).<br/>[4] Nowadnick, et al. <i>Phys. Rev. B</i>, <b>100</b>, 195129. (2019).<br/><br/>* This work made use of the electron microscopy and synthesis facilities of the Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM), which are supported by the National Science Foundation under Cooperative Agreement No. DMR-2039380. The authors acknowledge the use of facilities and instrumentation supported by NSF through the Cornell University Materials Research Science and Engineering Center DMR-1719875, a Helios FIB supported by NSF (DMR-1539918), and FEI Titan Themis 300 acquired through NSF-MRI-1429155, with additional support from Cornell University, the Weill Institute and the Kavli Institute at Cornell.