Low temperature, high magnetic field investigations of the nature of magnetism in the molecular semiconductor β- cobalt phthalocyanine (C32H16CoN8)

Abstract

Results from detailed investigations of the magnetic properties of a powder sample of β-CoPc for the temperatures T=0.4K to 300K and in magnetic fields H up to 90kOe are reported. X-ray diffraction confirmed the β-phase and scanning electron microscopy showed plate-like morphology of the sample. For T>3K, the data of magnetic susceptibility χ vs. T fit the Curie–Weiss (CW) law yielding θ=−2.5K, µ=2.16µB per Co2+ and g=2.49 for spin S=1/2 of the low spin-state of Co2+. However for T<3K, the χ vs. T data deviates from the CW law yielding a peak in χ at Tmax=1.9K. It is shown that the χ vs. T data from 0.4K to 300K fits well with the predictions of the Bonner–Fisher (BF) model for S=1/2 Heisenberg linear chain antiferromagnet with the Co2+–Co2+ exchange J/kB = −1.5K (Ĥ=−2J Σ Si•Si+1). The data of magnetization M vs. H at T=1K agrees with the predictions of the BF model with J/kB=−1.5K, yielding saturation magnetization MS=12.16emu/g above 60kOe corresponding to complete alignment of the spins.