Ferromagnetism of Ni(SCN)2(C2H5OH)2

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.