Magnetization of the Layer Compounds AFe^IIFe^III(C₂O₄)₃ (A = Organic Cation), in Low and High Magnetic Fields:  Manifestation of Néel N and Q Type Ferrimagnetism in a Molecular Lattice

Abstract

Detailed bulk magnetization and magnetic susceptibility measurements are reported for representative examples of the series of ferrimagnetic tris-oxalato-ferrate(II,III) salts with general formula AFe II Fe III (C 2 O 4 ) 3 (A = quaternary ammonium, phosphonium, or arsonium). The compounds all crystallize in two-dimensional hexagonal honeycomb lattices, but while some show conventional low field positive magnetization at low temperature (Néel type Q), others show a large negative magnetization below a compensation temperature T comp (Néel type N) under the same measurement protocol. Isothermal and temperature-dependent magnetizations are measured after zero field and field cooling. Temperature-dependent hysteresis measurements are also presented. In compounds that exhibit negative magnetization a discontinuity in the magnetization is observed at T t ( T comp < T t < T c ) which correlates with the onset of a giant anisotropy and is indicative of a magnetostrictive transition. In compounds that exhibit positive magnetization, hysteresis behavior and frequency-dependent ac susceptibility data indicate a glassy magnetic order at low temperatures, possibly arising from the frustration of random anisotropy domains below a characteristic blocking temperature or imbalance and disorder of Fe(II) and Fe(III).