Short-range and long-range incommensurate magnetic ordering in the frustrated antiferromagnets Ca3Co2−xFexO6: A neutron diffraction study

Abstract

We present the results of a detailed neutron powder diffraction study on the spin-chain compounds Ca${}_{3}$Co${}_{2\ensuremath{-}x}$Fe${}_{x}$O${}_{6}$ ($x$ $=$ 0.2 and 0.4), crystallizing in the rhombohedral structure (space group $R\overline{3}c$), in the temperature range of 1.5--20 K. At low temperature, these compounds show a long-range magnetic ordering (LRO) with an incommensurate structure having an amplitude modulation of the moments along the crystallographic $c$ axis. Our analysis also shows that a short-ranged spin-spin correlation along the $c$ axis (incommensurate type such as that of the LRO), however, with a long-range spin-spin correlation in the $\mathit{ab}$ plane, coexists with the LRO below the LRO temperature of 20 and 17 K for $x$ $=$ 0.2 and 0.4 samples, respectively. The observed long-range as well as the short-range magnetic ordering can be described by a model of platelet-like-shaped magnetic crystallites, disorderly stacked along the crystallographic $c$ direction. At 1.5 K, the LRO and short-range ordering (SRO) contribute in the magnetic phase in volume ratios of 13:7 and 3:2 for $x$ $=$ 0.2 and $x$ $=$ 0.4, respectively. The magnetic correlation lengths at 1.5 K for the LRO (${\ensuremath{\xi}}_{\mathrm{I}}^{c}$) and SRO (${\ensuremath{\xi}}_{\mathrm{II}}^{c}$) along the $c$ axis are \ensuremath{\sim}1027 and 60 \AA{} for $x$ $=$ 0.2, and 1021 and 47 \AA{} for $x$ $=$ 0.4, respectively. The relative contribution of the LRO and SRO in the magnetic phase remains constant down to 1.5 K (below transition temperature).