Hole-doping-induced melting of spin-state ordering in PrBaCo2O5.5+x

Abstract

The layered perovskite cobaltite RBaCo2O5.5 (R: rare-earth elements or Yttrium) exhibits an abrupt temperature-induced metal−insulator transition (MIT) and has been attributed to spinstate ordering (SSO) of Co 3+ ions. Here we investigated the hole doping member of PrBaCo2O5.5+x (0 ≤ x ≤ 0.24) with multiple techniques. The analysis on crystal and magnetic structures by electron and neutron diffraction confirm the SSO in the insulating phase of undoped PrBaCo2O5.5, which is melted by increasing the temperature across the MIT. In addition, we discovered that hole doping to PrBaCo2O5.5 also melts the SSO in conjunction with an insulator-metal transition. The experimental results from electron/neutron diffraction and soft x-ray absorption spectroscopy (XAS) all lead to the conclusion that hole-doping induced MIT occurs is in the same manner as the temperatureinduced MIT. Therefore, we propose a unified mechanism that dominates the temperature-and hole-doping-induced MITs in the PrBaCo2O5.5+x system. Specifically, this mechanism involves symmetry breaking coupled with a SSO in the paramagnetic phase. arXiv:1602.05687v2 [cond-mat.str-el]

First-Principles Study of Anisotropic Oxygen Diffusion in PrBaCo2O5.5

“…Co Oct and Co Py possess, respectively, 2.60 and 2.21 μ B in the paramagnetic-metal phase, which correspond to the IS states with slightly larger values than the ideal value (2 μ B ) for the IS state. This is consistent with the experimental suggestion by Miao et al 12 but differs from other suggestions that explain the metal–insulator transition. 10,11 In the ferromagnetic phase, Co Oct has 2.63 μ B and Co Py does 3.10 μ B , and they have the same values in the ferrimagnetic phase.…”

Large Magnetovolume Effect Induced by Embedding Ferromagnetic Clusters into Antiferromagnetic Matrix of Cobaltite Perovskite

“…The temperature dependence of the magnetic reflection 11 and in Figure a indicates two successive magnetic ordered phases for x = 0. In our previous study, we assigned these magnetic structures to two types of SSO‐AF structures (see Figure b), where the magnitude of magnetic moment varies between different nonequivalent Co site . The SSO melts when hole doping level x exceeds ≈0.06 and the magnetic structure transforms to a G‐type AF structure with the uniform magnitude of magnetic moment at all the Co sites (see Figure c) .…”

“…Therefore, the two nonequivalent Co sites of the Pmmm structure are splitting into four sites because of the SSO of Co 3+ ions, so that the crystal structure transforms to Pmma (2 a p × 2 a p × 2 a p ) within the orthorhombic crystal system (see Figure S1a in the Supporting Information) . Our previous study reveals that hole doping ( x ) to PrBaCo 2 O 5.5 melts the SSO at x = 0.06, accompanied by crystal structure transition into the Pmmm structure . Studying the newly synthesized x = 0.41 sample using high‐resolution neutron powder diffraction (NPD) and electron diffraction, (see results in Figure S2b–d, Supporting Information) leads to the conclusion that the Pmmm structure survives until x = 0.41.…”

“…In our previous study, we assigned these magnetic structures to two types of SSO‐AF structures (see Figure b), where the magnitude of magnetic moment varies between different nonequivalent Co site . The SSO melts when hole doping level x exceeds ≈0.06 and the magnetic structure transforms to a G‐type AF structure with the uniform magnitude of magnetic moment at all the Co sites (see Figure c) . Taking into account the coexistence of F and AF correlations as shown by the magnetic susceptibility (see Figure S5 in the Supporting Information), the ground state of x = 0.24 may consists of the short‐range ordered F‐clusters embedded in the AF matrix (see Figure a).…”

Photo-induced Antiferromagnetic-ferromagnetic and Spin-state Transition in GdBaCo2O5.5 Thin Film

“…When x ≃ 5.5, the oxygen vacancies can order into columns along a-axis, resulting in non-equivalent Co sites coordinated by either oxygen octahedra or oxygen pyramids [15-17, 19, 21]. The nominal valence state of Co is 3+ and the competition between crystal field splitting and Hund coupling results in various spin-states such as low-spin (LS, t 6 2g e 0 g , S=0), high-spin (HS, t 4 2g e 2 g , S=2) and intermediate-spin (IS, t 5 2g e 1 g , S=1) states [15,16,[19][20][21][22][23][24].…”