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

Temperature (0.5–300 K) and magnetic field (H up to 90 kOe) 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 = 1 kOe fit the modified Curie-Weiss (CW) law, χ = χ_o+C/(T−θ), with C = 6.266×10⁻⁴ emuK/gOe, θ = −0.1 K and χ_o = −9.3×10⁻⁷ emu/gOe. The Curie constant C yields magnetic moment μ = 1.704 μ_B, S = 1/2, and g = 1.967 characteristic of VO²⁺. The magnitude of θ = −0.1 K signifying very weak inter-ion antiferromagnetic exchange coupling is supported by the analysis of the variable frequency (9.8–336 GHz) electron paramagnetic resonance data. The isothermal data of M vs. H at ten temperatures between 0.5 K and 300 K when plotted as M vs. H/(T+0.1) collapses on to a single curve given by M = M_otanh {gμ_BH/[2k_B(T+0.1)]} with M_o = Ngμ_BS = 9.48 emu/g expected for S = 1/2 system, thus signifying near perfect paramagnetism in VOPc. - Highlights: • Magnetization M vs. temperature T from 0.5 K to 300 K in H = 1 kOe is reported. • M vs. T data fit Curie-Weiss law with θ = −0.1 K, S = 1/2, &g = 1.967 for VO²⁺. • Isothermal M vs. H data (H up to 90 kOe) for ten T from 0.5 K to 300 K is reported. • M vs. H/(T−θ) plot for various T fit a single Brillouin function curve for S = 1/2. • Analysis of magnetic and EPR data in VOPc shows near perfect S = 1/2 paramagnetism.