\textbf{Nature of magnetism in the molecular semiconductor Cobalt Phthalocyanine (C}$_{\mathrm{\mathbf{16}}}$\textbf{H}$_{\mathrm{\mathbf{32}}}$\textbf{CoN}$_{\mathrm{\mathbf{8}}}$\textbf{): Low temperature, high magnetic field investigations}

Transition metal doped phthalocyanines (MPc, M $=$ Mn, Fe, Co, Ni and Cu) are molecular semiconductors with many potential applications in which the M atoms form linear chains along the b-axis [1]. A recent report [2] on $\beta $-CoPc based on the temperature dependence (8 K to 310 K) of magnetization (M) in magnetic field H $=$ 70 kOe suggested it to be a linear chain magnet. Here we report results from detailed investigations of the magnetic properties of two powder samples of $\beta $-CoPc covering wider temperature range of 0.4 K to 300 K and in H up to 90 kOe. X-ray diffraction confirmed the $\beta $-phase and SEM showed needle-like (plate-like) morphology for the samples from Sigma-Aldrich (Alfa-Aesar). Magnetically, both samples are quite similar, the M vs. T data in H $=$ 10 kOe fitting the Curie-Weiss (CW) law above T \textgreater 3 K yielding $\theta \quad =$ 2.5 K, \textmu $=$ 2.16 \textmu $_{\mathrm{B}}$ per Co$^{\mathrm{2+}}$ and g $=$ 2.49 for S $=$ 1/2. Below 3 K, the data deviates from the CW law yielding a peak in M near 2 K, but the data from 0.4 K to 300 K fits well with the prediction of the Bonner-Fisher model for S $=$ 1/2 AFM Heisenberg chain [3] yielding the Co$^{\mathrm{2+}}$-Co$^{\mathrm{2+}}$ exchange constant J/k$_{\mathrm{B}}$ $=$ 3 K (\textbf{H} $=$ J $\Sigma $ \textbf{S}$_{\mathrm{\mathbf{i}}}$\textbullet \textbf{S}$_{\mathrm{\mathbf{i+1}}})$. [1] A. Mugarza et al, Phys. Rev. B. 85, 155437, (2012); [2] M. Serri et al, Nature. Commun. \textbf{5, }3079 (2014); [3] J. Bonner and M. Fisher, Phys. Rev. 135, A640 (1964).