Abstract
The high-pressure synthesized quasi-one-dimensional compounds NaMn$_2$O$_4$ and Li$_{0.92}$Mn$_2$O$_4$ are both antiferromagnetic insulators, and here their atomic and magnetic structures were investigated using neutron powder diffraction. The present crystal structural analyses of NaMn2O4 reveal that Mn3+/Mn4+ charge-ordering state exist even at low temperature (down to 1.5 K). It is evident from one of the Mn sites shows a strongly distorted Mn3+ octahedra due to the Jahn-Teller effect. Above TN = 39 K, a two-dimensional short-range correlation is observed, as indicated by an asymmetric diffuse scattering. Below TN, two antiferromagnetic transitions are observed (i) a commensurate long-range Mn3+ spin ordering below 39 K, and (ii) an incommensurate Mn4+ spin ordering below 10 K. The commensurate magnetic structure (kC = 0.5, -0.5, 0.5) follows the magnetic anisotropy of the local easy axes of Mn3+, while the incommensurate one shows a spin-density-wave order with kIC = (0,0,0.216). For Li$_{0.92}$Mn$_2$O$_4$, on the other hand, absence of a long-range spin ordered state down to 1.5 K is confirmed.
Highlights
Low-dimensional and frustrated spin systems have attracted attention in experimental and theoretical research fields [1,2]
This confirms that NaMn2O4 is in a CaFe2O4 structure (CFO)-type structure with a charge ordering of Mn3+ (t23ge1g) and Mn4+ (t23ge0g)
In the CFO structure, two different Mn2O4 zigzag chains are formed by a network of edge-sharing MnO6 octahedra (Mn-Mn bond labeled as J2) aligned along the c axis [Figs. 3(a) and 3(c)]
Summary
Low-dimensional and frustrated spin systems have attracted attention in experimental and theoretical research fields [1,2]. The crystal structure of CFO is based on edge-sharing Fe octahedron chains running along the c axis and are connected with each other by corner sharing in the ab plane These six chains build up a distorted hexagonal void (tunnels), where Ca ions are situated (see Fig. 3). They pose a geometrically frustrated lattice, based on a honeycomblike mesh of triangular or zigzag ladders with magnetic ions.
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