Evidence for the breakdown of simple classical pictures of organic molecule-based ferrimagnetics: Low-temperature crystal structure and single-crystal ESR studies of an organic heterospin system

A model compound for purely organic ferrimagnets has been studied by single -crystal cw-ESR spectroscopy. A heterospin system under study is composed o f two kinds of nitronylnitroxide molecules with the ground states of S = (1 )/(2) and S = 1. These molecules are stacked in an alternating chain in the crystal. The cw-ESR signal of the compound was found to split into two sig nals below 10 K, which were reproduced by the superposition of two Lorentzi an signals. The X-ray measurements at 9.6 K disclosed that the crystal stru cture remained unchanged at low temperatures, indicating that the origin of the ESR line splitting is not attributable to symmetry reduction associate d with structural change but to some change in the spin state: Two distingu ishable paramagnetic species are found from the ESR spectra, The appearance of two kinds of paramagnetic species in the alternating chain at finite te mperature is expected as a primitive model of thermal excitation in the fer rimagnetic chain which is based on a theoretical calculation. The present e xperimental results demonstrate that the classical picture of ferrimagnetic spin state (antiparallel alignment of adjacent spins with different spin q uantum numbers) is invalidated to describe organic molecule-based exchange coupled spin systems and overlooks an essential part of the nature of ferri magnetics.