Magnetic transitions in the positional isomers (4-HOPNN and 2-HOPNN) of an organic nitronyl nitroxide radical using muon–spin rotation

The zero-field muon–spin rotation/relaxation technique has been used in the direct observation of spontaneous magnetic order below the Curie temperature of TC∼500 mK in 2-hydroxyphenyl-α-nitronyl nitroxide (2-HOPNN) and two successive magnetic transitions in 4-hydroxyphenyl-α-nitronyl nitroxide (4-HOPNN) at TC1≈700 and TC2∼100 mK. The different position of the hydroxyl group in these positional isomeric compounds causes different crystal packings and magnetic dimensionalities. The results reveal different magnetic transitions in these two α-nitronyl nitroxides radicals suggesting a 2D–3D magnetic crossover in 4-HOPNN.