Magnetic and spectroscopic studies of ferricenium tetrabromoferrate(III), [Fe(C⁵H⁵)²]^.+[FeBr⁴]^.-, a soft ferromagnet and its iodo analogue

Abstract

Detailed studies of the properties of ferricenium tetrabromoferrate (III) using electron paramagnetic resonance (EPR) and Mossbauer spectroscopy reveal that this material undergoes a magnetic transition at 13.2 K. The origin of this transition has been traced to the near neighbor ferrimagnetic coupling as evidenced by a broadening and the subsequent disappearance of an EPR line due to the ₆A¹ ground state of FeBr⁴- and an appearance of the Zeeman split Mossbauer spectrum, the latter in concurrence with earlier reports. Infrared spectrum shows the emergence of new bands around the magnetic ordering temperature. Detailed magnetization measurements as a function of temperature and field clearly indicate long range ferromagnetic ordering at a temperature below 4 K. This is corroborated by Mossbauer results with the appearance of another transition and a six line pattern due to the ferricenium cation at 2.9 K. The mechanism for ferromagnetic coupling is discussed. The change in EPR line-width and the appearance of two new EPR lines below 20 K give additional evidence to the other experimental observations. The EPR, Mossbauer and IR studies suggest that no such ordering exists in the analogous C¹⁰H¹⁰Fe²I⁴, probably due to a structural transition at low temperature hindering magnetic ordering.