Magnetic structure of the 4,4,4′,4′,5,5,5′,5′-octamethyl-2,2′-m-phenylenebis(4,5-dihydroimidazol-1-oxyl 3-oxide) biradical: quantum spin effect of S= 1 species associated with structural change

Abstract

A peculiar magnetic structure has been found in the crystal of a stable organic biradical molecule with S = 1,4,4,4′,4′,5,5,5′,5′-octamethyl-2,2′-m-phenylenebis(4,5-dihydroimidazol-1-oxyl 3-oxide)(m-BNN). A high-field magnetization process up to 40 T at 1.8 K exhibits a two-fold saturation process. Powder X-ray diffraction reveals the breakdown of the translational equivalence between the biradical dimers at low temperature. From these results, it is concluded that there is a four-fold quasi-degeneracy associated with a singlet pair formation between two S= 1/2 spins in only half the biradical dimers in the crystal.