Ferromagnetism of Ni(SCN)2(C2H5OH)2

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Abstract
The magnetic properties of Ni(SCN)2(C2H5OH)2 have been studied, the first Ni(II) system in the general family of compounds M(SCN)2(ROH)2 (where M=divalent Mn, Fe, Co, or Ni and R=CH3, C2H5, i‐ or n‐C3H7) to be examined at low temperatures. In contrast to previously studied Mn(II) and Co(II) members of this family, which exhibit predominant antiferromagnetism, the present compound is ferromagnetic. The susceptibility of a polycrystalline sample is of Curie–Weiss form only above 75 K, with ḡ = 2.175 ± 0.01 and S=1 and with θ=24.1±1.0 K. The initial susceptibility is well accounted for by an asymptotic critical law, χ0 = Γ[T/Tc − 1]−γ, in the reduced temperature range 0.147–0.013, with Tc = 13.081 ± 0.01 K, γ=1.354±0.02, and Γ=0.0925±0.003 emu/mol. The γ value is between 3D‐XY and 3D‐Heisenberg model predictions. The susceptibility in the paramagnetic regime well above Tc is analyzed including the effects of axial and rhombic crystal field distortions, represented by D[Ŝ2z − S(S + 1)/3] and E[Ŝ2x − Ŝ2y] terms in the spin Hamiltonian, and incorporating exchange interactions in a mean‐field approximation. An excellent fit is obtained with g=2.175, D/k=−64.3±5 K, E/k=23.1±4 K, and zJ/k=19.4±1 K. The magnitude of D is the largest we know of in a Ni(II) compound. The three spin states of the 3A crystal field ground term are strongly split, probably contributing to the relatively low value of Tc compared to zJ/k. Some two‐dimensional character in the exchange, as has been found for other members of this family of compounds, may also contribute.