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arXiv · 1605.06067

Hydration induced spin glass state in a frustrated Na-Mn-O triangular lattice

Ioanna Bakaimi·Rosaria Brescia·Craig M. Brown·Alexander A. Tsirlin·Mark A. Green·Alexandros Lappas
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Abstract

Birnessite compounds are stable across a wide range of compositions that produces a remarkable diversity in their physical, electrochemical and functional properties. These are hydrated analogues of the magnetically frustrated, mixed-valent manganese oxide structures, with general formula, NaxMnO2. Here we demonstrate that the direct hydration of layered rock-salt type a-NaMnO2, with the geometrically frustrated triangular lattice topology, yields the birnessite type oxide, Na0.36MnO2 0.2H2O, transforming its magnetic properties. This compound has a much-expanded interlayer spacing compared to its parent a-NaMnO2 compound. We show that while the parent a-NaMnO2 possesses a Neel temperature of 45 K as a result of broken symmetry in the Mn3+ sub-lattice, the hydrated derivative undergoes collective spin-freezing at 29 K within the Mn3+/Mn4+ sub-lattice. Scaling-law analysis of the frequency dispersion of the AC susceptibility, as well as the temperature-dependent, low-field DC magnetization confirm a cooperative spin-glass state of strongly interacting spins. This is supported by complementary spectroscopic analysis (HAADF-STEM, EDS, EELS) as well as by a structural investigation (high-resolution TEM, X-ray and neutron powder diffraction) that yield insights into the chemical and atomic structure modifications. We conclude that the spin-glass state in birnessite is driven by the spin-frustration imposed by the underlying triangular lattice topology that is further enhanced by the in-plane bond-disorder generated by the mixed-valent character of manganese in the layers.

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Ioanna Bakaimi, Rosaria Brescia, Craig M. Brown, Alexander A. Tsirlin, Mark A. Green, Alexandros Lappas. 2016-05-19. Hydration induced spin glass state in a frustrated Na-Mn-O triangular lattice. https://doi.org/10.1103/physrevb.93.184422

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