Christopher J. Nuttall

Crystal and magnetic structures of afeIIFeIII(C2O4)3 (A = organic cation): two-dimensional honeycomb ferrimagnets.

Ferrimagnetic tris-oxalato-ferrate(II,III) salts with general formula AFeIIFeIII(C2O4)3 (A = quaternary ammonium, phosphonium) all crystallise in two-dimensional hexagonal honeycomb lattices but while some show conventional Néel type Q magnetisation at low temperature others show negative Néet type N magnetisation below a compensation temperature Tcomp. The compounds having negative magnetisation show a discontinuity in the magnetisation at a temperature between Tcomp and Tc suggesting a magneto-strictive transition. For the type Q compounds, hysteresis and frequency dependent AC susceptibility show that the low temperature magnetic state is glassy, either because of frustration of random anisotropy domains below a blocking temperature, or imbalance in the fractions of FeII and FeIII.

Effect of Organic Cation A on the Crystal Structure and Magnetisation of the Layer Molecular Ferrimagnets AFeIIFeIII(C2O4)3

Abstract We survey the crystal and magnetic structures and bulk magnetisation for representative examples of the series of ferrimagnetic tris-oxalato-ferrate(II,III) salts with general formula AFeII FeII(C2O4)3 (A = quaternary ammonium, phosphonium). The compounds all crystallise in two-dimensional hexagonal honeycomb lattices but while some show positive magnetisation at low temperature (Neel type Q) others show a large negative magnetisation below a compensation temperature Tcomp (Neel type N). In compounds that exhibit negative magnetisation a discontinuity in the magnetisation is observed at Tt (Tcomp < Tt < Tc) indicative of a magneto-strictive transition. In compounds that exhibit positive magnetisation, hysteresis and frequency dependent AC susceptibility indicate a glassy magnetic order at low temperatures, possibly arising from the frustration of random anisotropy domains below a blocking temperature, or imbalance and disorder of Fe(II) and Fe(III).

Synthesis and magnetism of mixed valency [N(n-C4H9)4 or P(C6H5)4]CrIICrIII(C2O4)3

Mixed valency compounds ACrIICrIII(C2O4)3[A = N(n-C4H9)4, P(C6H5)4] have been synthesised and characterised structurally and magnetically, the structures consisting of hexagonal layers of Cr bridged by C2O42–; although short range antiferromagnetic correlations are observed below 100 K no transition to a long range ordered state is observed above 2 K.