\textbf{Magnetic Determination of the Electronic State of Cu and Exchange Interactions in the }$\alpha $\textbf{- and }$\beta $\textbf{-Phases of Molecular Semiconductor Copper Phthalocyanine (C}$_{\mathbf{16}}$\textbf{H}$_{\mathbf{32}}$\textbf{CuN}$_{\mathbf{8}}$\textbf{)}

Among the transition metal phthalocyanines (TMPc, TM $=$ Mn, Fe, Co, Ni, and Cu) in which the TM atoms form linear chains along the b-axis, the electronic structure of CuPc has become of particular recent interest for applications in spintronics [1]. Here, using analysis of magnetization data in the $\alpha $- and $\beta $-phase of powder CuPc samples, the electronic state of Cu and exchange constant are reported. After verifying the crystal structure using x-ray diffraction, the temperature dependence (2 K to 250 K) of the magnetization M of both samples was measured in magnetic field H $=$ 1 kOe and isothermally at 2 K and 5 K in H up to 90 kOe. The data were analyzed first using the modified Curie-Weiss law, $\chi \quad = \quad \chi_{o}$ $+$ C / (T$+\theta )$, showing good fit for T \textgreater 4 K and yielding $\theta \quad =$ 2.3 K (0.2 K) for $\alpha $-CuPc ($\beta $-CuPc) and spin S $=$ 1/2 characteristic of Cu$^{2+}$. The data were next fitted to the Bonner-Fisher model for S $=$1/2 antiferromagnetic Heisenberg chain showing excellent fit to all the M vs. T data and yielding the Cu$^{2+}$-Cu$^{2+}$ exchange constant J/k$_{B}=$3.4 K ( 0.4 K) the for $\alpha $-CuPc ($\beta $-CuPc). The isothermal data of M vs. H is analyzed taking exchange into account. The large difference in the magnitudes of J/k$_{B}$ for the two phases is discussed in terms of the differences in their crystal structures [2]. [1] M. Warner et al, Nature, 503, 504-508 (2013). [2] Z. Wang et al, IEEE. Trans. Magn. ( in press, Nov. 2015 issue).