Browsing by Author "Ehrenberg, H"

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    Magnetic structure and spin correlations in magnetoelectric honeycomb Mn4Ta2O9
    (American Physical Society, 2018-10-22) Narayanan, N; Senyshyn, A; Mikhailova, D; Faske, T; Lu, T; Liu, Z; Weise, B; Ehrenberg, H; Mole, RA; Hutchison, WD; Fuess, H; McIntyre, GJ; Liu, Y; Yu, DH
    We elucidate the magnetic interactions and the role of spin (electron) correlation in determining the ground state of the honeycomb compound Mn4Ta2O9, by neutron powder diffraction, inelastic neutron scattering (INS), specific-heat (CP) measurements, and electronic-structure calculations. The antiferromagnetic long-range order with moments along c occurs at 102 K with strong exchange striction and small anisotropy. It is escribed by the three-dimensional Ising model. Diffuse magnetic scattering has been observed above TN, which is attributed to the two-dimensional spin correlations within the Mn2+ honeycombs. This is confirmed by the calculated exchange constants. INS experiments and spin-wave simulations together with CP measurements reveal two gapped modes on the ab plane, originating from the rotation of the spins away from the easy axis c. The magnetic anisotropy is mainly determined by an electron-correlation-assisted dipole-dipole interaction. This work provides insight into the competing origins of the magnetic anisotropy, which leads to different magnetic ground states in the family of honeycomb compounds. ©2018 American Physical Society
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    The structural behavior of electrochemically delithiated LixNi0.8Co0.15Al0.05O2 (x<1) battery cathodes
    (Elsevier, 2023-04-30) Hölderle, T; Monchak, M; Baran, V; Dolotko, O; Bette, S; Mikhailova, D; Voss, A; Avdeev, M; Ehrenberg, H; Müller-Buschbaum, P; Senyshyn, A
    A full series of variously delithiated LixNi0.8Co0.15Al0.05O2 (NCA) (x < 1) battery cathodes is extracted from cylinder Li-ion batteries of 18650-type, and structural studies of electrode materials are performed ex situ applying high-resolution neutron powder diffraction. A set of structural and microstructural parameters at different state-of-charges (SOC) is gathered from full-profile Rietveld refinements. Accurate cell metrics of the NCA lattice are obtained and compared for both, ex situ and operando modes of data collection. A detailed analysis of lithium occupancies and interatomic distances in the NCA cathode is carried out, revealing a shrinking material response of the NCA structure during delithiation, while the experimental quantification of Li/Ni cation mixing showed an SOC independent behavior. The observed deintercalation driven anisotropic broadening of the Bragg reflections in real-life NCA cathodes were analyzed and assigned to an increasing internal anisotropic microstrain, which is linked to the non-uniform distribution of Li in the structure of NCA, resulting in an unstable and unpredictable performance of LIBs. © 2023 Elsevier B.V.
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    Symmetry analysis of the ferroic transitions in the coupled honeycomb system (Fe, Co, Mn)4Ta2O9
    (Australian Institute of Physics, 2020-02-04) Narayanan, N; Faske, T; Lu, T; Liu, Z; Brennan, M; Hester, JR; Avdeev, M; Senyshyn, A; Mikhailova, D; Ehrenberg, H; Hutchison, WD; Mole, RA; Fuess, H; McIntyre, GJ; Liu, Y; Yu, DH
    Exotic phenomena such as spin liquid, spin-orbital entities, magnetic order induced multiferroicity (type ii) or quantum criticality have recently triggered extensive research on the ground state properties of frustrated magnetic systems. The ground states of these compounds are determined by the coupling of the spin to the orbital, charge and lattice degrees of freedom. One of the extensively investigated lattices is the honeycomb lattice due to the development of the Kitaev model for quantum spin liquids [1-2]. In this work, we are interested in the coupled honeycomb system M4A2O9 (M=Fe, Co and Mn and A=Nb, Ta). All members have two crystallographically distinct M sites, which are in the distorted octahedral oxygen cages. These cages form edge-shared coplanar and corner-shared buckled honeycombs respectively which are interconnected in the perpendicular direction leading to competing exchange paths. The M=Co and Mn members were magnetoelectrics, whereas Fe2Ta2O9 was reported to exhibit both magnetoelectric and (type ii) multiferroic phases depending on the temperature [3-4]. Magnetoelectrics and multiferroics are technically highly relevant with a variety of applications such as MRAMs and field sensors. However, the coupling mechanism is very complicated [5]. Furthermore, due to the group properties of the symmetry analysis methods such as representation analysis and magnetic space groups, the magnetic structure of the Nb counterpart Co4Nb2O9 is controversially discussed. It is therefore apparent that the above discussed diversities of the properties are determined by the magnetic structure and the closely related electronic structure. These can be elucidated by investigating the structure and dynamics of these compounds, which will help to understand the emergence of different ground states and the diverse phase transitions in this family of materials In this work, we systematically investigate the magnetic and electronic structure of the (Fe, Co, Mn)4Ta2O9 system. We combined several different techniques of neutron powder diffraction, inelastic neutron scattering, heat capacity, electronic band structure calculations and spin wave modeling based on linear spin wave theory.