Browsing by Author "Sheptyakov, D"

Now showing 1 - 2 of 2
  • No Thumbnail Available
    Item
    Chemical pressure effects on crystal and magnetic structures of bilayer manganites PrA2Mn2O7 (A = Sr or Ca)
    (AIP Publishing, 2016-06-03) Deng, G; Sheptyakov, D; Pomjakushin, V; Medarde, M; Pomjakushina, E; Conder, K; Kenzelmann, M; Studer, AJ; Gardner, JS; McIntyre, GJ
    The crystal and magnetic structures of the bilayer manganites PrSr2Mn2O7 (PSMO) and PrCa2Mn2O7 (PCMO) have been studied by neutron powder diffraction. It was found that PSMO crystallizes in space group I4/mmm, while PCMO adopts space group Cmc21 at room temperature. The difference in the structure arises from chemical pressure induced by the Ca substitution for Sr on the A sites, which causes different Jahn-Teller distortions. In PSMO, the MnO6 octahedra suffer a small elongated distortion, while those in PCMO adopt strong compressed distortion along the axial direction. In addition, the octahedra in PCMO show a+b0c0 rotation and a0b+c+ tilting in the Glazer notation in comparison to PSMO. As a result, these two compounds adopt very different magnetic structures: The magnetic structure of PSMO is an A-type magnetic structure (Im'm'm) with propagation vector k = (0, 0, 1) and magnetic moments in the ab plane. In contrast, a C-type antiferromagnetic magnetic structure (Cm'c2′1) with the multiple propagation vectors (k = (0, 12, 12) and (0, 12, 0)) and magnetic moments mainly along the b axis is found in PCMO. The critical exponent of the magnetic phase transition is around 0.345 for PSMO and 0.235 for PCMO, indicating 3D and 2D XY transitions, respectively. The strong Jahn-Teller distortion induced by the chemical pressure is believed to suppress the double exchange and favour super-exchange in PCMO, leading to the dramatic difference in the magnetic structure. © 2016 Author(s). Published by AIP Publishing.
  • No Thumbnail Available
    Item
    Jahn-Teller versus quantum effects in the spin-orbital material LuVO 3
    (American Physical Society, 2015-04-13) Skoulatos, M; Toth, S; Roessli, B; Enderle, M; Habicht, K; Sheptyakov, D; Cervellino, A; Freeman, PG; Reehuis, M; Stunault, A; McIntyre, GJ; Tung, LD; Marjerrison, C; Pomjakushina, E; Brown, PJ; Khomskii, DI; Rüegg, A; Kreyssig, A; Goldman, AI; Goff, JP
    We report on combined neutron and resonant x-ray scattering results, identifying the nature of the spin-orbital ground state and magnetic excitations in LuVO3 as driven by the orbital parameter. In particular, we distinguish between models based on orbital-Peierls dimerization, taken as a signature of quantum effects in orbitals, and Jahn-Teller distortions, in favor of the latter. In order to solve this long-standing puzzle, polarized neutron beams were employed as a prerequisite in order to solve details of the magnetic structure, which allowed quantitative intensity analysis of extended magnetic-excitation data sets. The results of this detailed study enabled us to draw definite conclusions about the classical versus quantum behavior of orbitals in this system and to discard the previous claims about quantum effects dominating the orbital physics of LuVO3 and similar systems. © 2015 American Physical Society