Magnetic Determination of the Electronic State of Cu and Exchange Interactions in the - and

Using the analysis of the magnetization data in the α- and β-phases of copper phthalocyanine (CuPc) samples, the electronic state of Cu and exchange interactions are reported. After verifying the crystal structure of the powder samples using X-ray diffraction, the temperature dependence (2-250 K) of the magnetization M of both samples was measured in magnetic field H = 1 kOe and isothermally at 2 and 5 K in H up to 90 kOe. The data were analyzed first using the modified Curie-Weiss law, χ = χ₀ + C/(T + θ), showing good fit for T > 4 K and yielding θ = 2.3 K (0.2 K) for α-CuPc (β-CuPc) and spin S = 1/2 characteristic of Cu²⁺. The data were next fitted to the Bonner-Fisher model for S = 1/2 antiferromagnetic Heisenberg chain showing an excellent fit to all the M versus T data and yielding the Cu²⁺-Cu²⁺ exchange constant J/k_B = 3.4 K (0.4 K) for the α-CuPc (β-CuPc). The isothermal data of M versus H are analyzed taking exchange coupling into account. The large difference in the magnitudes of J/k_B for the two phases is discussed in terms of the differences in their crystal structures.