Magnetic studies reveal near-perfect paramagnetism in the molecular semiconductor vanadyl phthalocyanine (C32H16N8VO)

Abstract

Temperature (0.5–300K) and magnetic field (H up to 90kOe) dependences of the magnetization (M) of a powder sample of vanadyl phthalocyanine (VOPc) having the Phase II-triclinic structure are measured and analyzed. The data of χ = M/H vs. T measured in H = 1kOe fit the modified Curie-Weiss (CW) law, χ = χo+C/(T−θ), with C = 6.266×10−4 emuK/gOe, θ = −0.1K and χo = −9.3×10−7emu/gOe. The Curie constant C yields magnetic moment μ = 1.704μB, S = 1/2, and g = 1.967 characteristic of VO2+. The magnitude of θ = −0.1K signifying very weak inter-ion antiferromagnetic exchange coupling is supported by the analysis of the variable frequency (9.8–336GHz) electron paramagnetic resonance data. The isothermal data of M vs. H at ten temperatures between 0.5K and 300K when plotted as M vs. H/(T+0.1) collapses on to a single curve given by M = Motanh {gμBH/[2kB(T+0.1)]} with Mo = NgμBS = 9.48emu/g expected for S = 1/2 system, thus signifying near perfect paramagnetism in VOPc.