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

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, Na x MnO 2 . Here we demonstrate that the direct hydration of layered rock-salt type α-NaMnO 2 , with the geometrically frustrated triangular lattice topology, yields the birnessite type oxide, Na 0.36 MnO 2 ·0.2H 2 O, transforming its magnetic properties. This compound has a muchexpanded interlayer spacing compared to its parent α-NaMnO 2 compound. We show that while the parent α-NaMnO 2 possesses a Néel temperature of 45 K as a result of broken symmetry in the Mn 3+ sub-lattice, the hydrated derivative undergoes collective spin-freezing at 29 K within the Mn 3+ /Mn 4+ 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 2 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.

Cluster glass behavior of the frustrated birnessites AxMnO2·yH2O(AKulish1, Scholtens2, Blake3 2019Phys. Rev. B70152View full textAdd to dashboardCite

“…The 001 peaks at 2θ ≈ 2.5° are consistent with the ~7 Å inter-layer spacing expected for the birnessite compounds. 14,23 The space group is monoclinic C2/m for Na-bir and triclinic C1 ̅ for K-bir. The refined lattice parameters are listed in Table I, and details of the XRD analysis are given in Appendix B.…”

Spin-glass behavior in KxRu4−yNiyOStimpson1, Powell2, Stenning3 et al. 2018Phys. Rev. B9010View full textAdd to dashboardCite

“…This value falls within the expected range of between 0.005 and 0.06 typical of spin glass systems as discussed in previous works. 36,37,39,40 Typically in spin glass materials the relationship between the relaxation time and the transition temperature can be described by the power law given in…”

“…Figure 5(a) shows the linear plot of lnf vs 1/Tf; typically a linear Arrhenius relationship is indicative of weakly or non-interacting spins. 37 For example Anand et al saw a deviation from linearity at low frequencies which they attributed to the formation of strongly interacting clusters. 39 We note that despite the large values of τ0 observed for our materials from the fit to the Power Law we see no evidence of a deviation from linearity to support the formation of strongly correlated clusters.…”

“…for NaxMnO2.yH2O and PrRhSn3 materials. 37,39 Fitting of the Volger-Fulcher to determine f0, Ea and T0 proved difficult due to the limited frequency range that the data has been collected over meaning it is impossible to fit the expected curve to the data. We have seen similar problems in dielectric relaxor data whereby unrealistic values are obtained as a result of the sensitivity of fitting to the curvature of Tm(f) data and subsequently extrapolating over several orders of magnitude.…”

“…Additionally, the divergence between the ZFC and FC data diminishes disappearing almost completely by 7 T suggesting that the spin-glass state is destroyed under high applied magnetic fields as expected for these types of systems. 36,37 This perhaps suggests that there may be more than one contribution to the spin glass behaviour at low applied fields. The temperature dependence of both the divergence between ZFC and FC data (feature 1 in figure 6) and Tf (feature 2 in figure 6) allows us to probe the temperature-field phase diagram and the field dependent paramagneticspin-glass phase transition as shown in figure 6(c).…”