Short-range and long-range incommensurate magnetic ordering in the frustrated antiferromagnets Ca_3Co_(2-x)Fe_xO_6: A neutron diffraction study

摘要

We present the results of a detailed neutron powder diffraction study on the spin-chain compounds Ca_3Co_(2-x)Fe_xO_6 (x = 0.2 and 0.4), crystallizing in the rhombohedral structure (space group R3c), 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 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 a = 0.2 and x = 0.4, respectively. The magnetic correlation lengths at 1.5 K for the LRO (ζ_I~c) and SRO (ζ_II~c,) along the c axis are ～1027 and 60 A for x = 0.2, and 1021 and 47 A 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).