Crystal and magnetic structures of AFe(II)Fe(III)(C2O4)(3) (A = organic cation): two-dimensional honeycomb ferrimagnets

Ferrimagnetic tris-oxalato-ferrate(II,III) salts with general formula AFe(I I)Fe(III)(C2O4)(3) (A = quaternary ammonium, phosphonium) all crystallise i n two-dimensional hexagonal honeycomb lattices but while some show conventi onal Neel type Q magnetisation at low temperature others show negative Mel type N magnetisation below a compensation temperature T-comp. The compounds having negative magnetisation show a discontinuity in the magnetisation at a temperature between T-comp and T-c suggesting a magneto-strictive transi tion. For the type Q compounds, hysteresis and frequency dependent AC susce ptibility show that the low temperature magnetic state is glassy, either be cause of frustration of random anisotropy domains below a blocking temperat ure, or imbalance in the fractions of Fe-II, and Fe-III. (C) 2001 Elsevier Science B.V. All rights reserved.