Ising-like chain magnetism, Arrhenius magnetic relaxation, and case against 3D magnetic ordering in β -manganese phthalocyanine (C ₃₂ H ₁₆ MnN ₈ )

Previous magnetic studies in the organic semiconductor β-manganese phthalocyanine (β-MnPc) have reported it to be a canted ferromagnet below T(C) ≈ 8.6 K. However, the recent result of the lack of a λ-type anomaly in the specific heat versus temperature data near the quoted T(C) has questioned the presence of long-range 3-dimensional (3D) magnetic ordering in this system. In this paper, detailed measurements and analysis of the temperature (2 K-300 K) and magnetic field (up to 90 kOe) dependence of the dc and ac magnetic susceptibilities in a powder sample of β-MnPc leads us to conclude that 3D long-range magnetic ordering is absent in this material. This is supported by the Arrott plots and the lack of a peak in the ac susceptibilities, χ' and χ″, near the quoted T(C). Instead, the system can be best described as an Ising-like chain magnet with Arrhenius relaxation of the magnetization governed by an intra-layer ferromagnetic exchange constant J/k(B) = 2.6 K and the single ion anisotropy energy parameter |D|/k(B) = 8.3 K. The absence of 3D long range order is consistent with the measured |D|/ > J.