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arXiv · 2208.12197

Incommensurate magnetic modulation in K-rich cryptomelane, K$_x$Mn$_8$O$_{16}$ ($x\approx1.45$)

Liam A. V. Nagle-Cocco·Joshua D. Bocarsly·Krishnakanth Sada·Nicola D. Kelly·Mathias A. Kiefer·Emannuelle Suard·Sarah J. Day·Cheng Liu·Clare P. Grey·Prabeer Barpanda·Clemens Ritter·Siân E. Dutton
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Abstract

Cryptomelane is a hollandite-like material consisting of K$^+$ cations in an $\alpha$-MnO$_2$ tunnel-like crystallographic motif. A sample with stoichiometry K$_{1.448(3)}$Mn$_8$O$_{16}$ has been synthesised and its magnetic properties investigated using variable-temperature magnetic susceptibility, heat capacity, and neutron powder diffraction. Three distinct transitions at $T_1=184$\,K, $T_2=54.5$\,K, and $T_3=24$\,K are observed. At $T_1$ there is a subtle tetragonal$\rightarrow$monoclinic transition associated with Mn$^{3+}$/Mn$^{4+}$ ordering, and a set of non-magnetic superstructure peaks emerge; these could not be indexed definitively and are indicative of an ordering that is incommensurate with the unit cell. Magnetic Bragg peaks emerge below $T_2=54.5$\,K, and their positions indicate an incommensurate modulated magnetic structure. The model consistent with the data is a dual-$\vec{k}_\mathrm{mag}$ structure with a ferromagnetic $|\vec{k}_\mathrm{mag}|=0$ component and an incommensurate $\vec{k}_\mathrm{mag}$ parallel to the $\alpha$-MnO$_2$ tunnels [$|\vec{k}_\mathrm{mag}|=0.36902(15)$], with the latter most likely to be helical. The period of oscillation of the helical component is in line with predictions based on a Heisenberg spin Hamiltonian [Mandal \textit{et al}. Phys. Rev. B 90, 104420 (2014)]. Below $T_3=24$\,K, there is a magnetic transition, which gives rise to a different set of magnetic Bragg peaks indicative of a highly complex magnetic structure.

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Liam A. V. Nagle-Cocco, Joshua D. Bocarsly, Krishnakanth Sada, Nicola D. Kelly, Mathias A. Kiefer, Emannuelle Suard, Sarah J. Day, Cheng Liu, Clare P. Grey, Prabeer Barpanda, Clemens Ritter, Siân E. Dutton. 2022-08-25. Incommensurate magnetic modulation in K-rich cryptomelane, K$_x$Mn$_8$O$_{16}$ ($x\approx1.45$). https://arxiv.org/abs/2208.12197

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