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Cation Ordering by Atomic Layer Engineering of Strongly Correlated Oxides

Subject Area Experimental Condensed Matter Physics
Synthesis and Properties of Functional Materials
Term from 2017 to 2025
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 387838520
 
Final Report Year 2022

Final Report Abstract

We have carried out detailed structural, magnetic/electric and optical (Raman) experiments on well-selected film samples of strongly correlated double perovskites of Co/Mn and Ni/Mn series with the general aim to study the short- and long-range B-site cation ordering as well as their influences on electron transport, magnetism and lattice structure. The experiments were performed also on artificially layered/ordered oxide superlattices (SL) of different chemical composition aiming to answer the following fundamental scientific questions and proposal objectives. • How does the coupling between charge, spin and lattice degrees of freedom depend on the thickness of individual layers in SL? What is the characteristic length scale of charge-, spin- and lattice-spreading within a superlattice (SL)? By using different ex situ (magnetic, Raman spectroscopy, TEM) and in situ (optical ellipsometry) techniques we have established the characteristic length scale of charge/spin and spin/lattice couplings to be 2-3 u.c. Particularly, the spin-phonon coupling constant was found to be strongly influenced by lattice epitaxy strain modifying the orbital overlap. The best values were obtained for the relaxed and/or slightly compressed films. • How does the charge reconstruction manifests itself in artificially ordered SLs? What is the mechanism of charge/orbital reconstructions at the interfaces? It was demonstrated that to obtain the ordered double perovskite emergent phase the charge transfer or charge reconstruction should occur together the with lattice (orbital) reconstruction, both resulting in the formation of a new crystalline structure at the interfaces which is not present in the volume of individual layers. • How do the long- and short-range cation orderings interplay in a realistic artificial SL? By direct imaging of the local structure (TEM, SAED) and by measuring the superstructure peaks in XRD patterns a direct correlation between the long- and short-range B-site ordering was elucidated. Moreover, the lattice superstructures usually correlate well with the optimal magnetic properties. An important exclusion was found in DP films with small A-site cations: here the relaxation of chemical pressure accompanied by tilting of bond angles and shifting of the A-site cations or oxygen anions leads to an artificial doubling of the lattice constant along the [111] crystallographic direction, imitating the high degree of B-site ordering. At the same time the short-range of Co/Mn was also observed in such DP films (Nd2CoMnO6). • What are the factors limiting the long-range cation ordering in SLs? Important limiting factors of the long-range B-site ordering seem to be the defects of real substrates, like atomic steps at the terrace edges, point defects and dislocations. The former, being almost unavoidable, result in antiphase boundaries and prevent the full long-range B-site ordering. To minimize their influence, the crystals of very high crystalline quality should be chosen and the substrates with a very small miscut angle (<0.1°) should be used. Finally, we have demonstrated that by fine tuning of chemical composition of layers in artificially ordered SLs one can considerably improve both short- and long-range of ordering. • What is the driving force for the B-site ordering? Basic driving forces of the B-site ordering were found to be charge and size mismatch of the corresponding B-cations. Probably, these two factors are interrelated with each other and, thus, enable to obtain a high degree of B-site ordering in the Co/Mn and Ni/Mn double perovskites. Indeed, for an LMCO with Co2+ and Mn4+ cations in the fully ordered state, the valence and site mismatches are coupled with each other, yielding size mismatch ∆R=|RCo2+ - RMn4+|=21 pm, which is considerably larger than the size mismatch between the undesirable Co3+ and Mn3+ cations in a disordered variant ∆R=|RCo3+-RMn3+|=3 pm. A similar issue is valid for the Ni/Mn ordering as ∆R=|RNi2+-RMn4+|=16 pm>>∆R=|RNi3+-RMn3+|=4 pm. From the energetic point of view, the gain in the potential electrostatic (Coulomb) energy will be obtained when both Co2+ and Mn4+ are separated from each other. But this has to be considered with the increased elastic energy of the lattice due to local strains as the size mismatch between cations in the ordered phase (Co2+ and Mn4+) is much larger than the size mismatch in a fully disordered state (Co3+ and Mn3+). It looks probable that the NaCl type of ordering with alternating B/B’ cations in all three spatial directions is favorable for the compensation of both charge and size mismatches. • How does the microstructure of the substrate influences the ordering? We used commensurate perovskite (STO, LaAlO3, LSAT) and incommensurate (Al2O3, MgO) substrates with different orientations ((100), (110) and (111)). The (100) ordering was found to be less favored due to the increased elastic energy as the (111) ordering. In addition, the large miscut angles and small width of terraces resulted in a reduced degree of short-range ordering visualized by TEM.

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