Abstract:

Two cobalt hydroxysulfates Na₂Co₃(OH)₂(SO₄)₃(H₂O)₄ (1) and Co₃(OH)₂(HSO₄)₂(SO₄)(H₂O)₄ (2) have been prepared and characterized by single-crystal X-ray diffraction, thermal analysis, and magnetic measurements. The structure of 1 consists of helical Co₃(OH)₂ ferrimagnetic chains that are connected by μ₃- and μ₄-sulfates into a two-dimensional anionic [Co₃(OH)₂(SO₄)₃(H₂O)₂]²⁻ layer. The adjacent [Co₃(OH)₂(SO₄)₂(H₂O)₂]²⁻layers in 1 are further linked by Na⁺ ions into a three-dimensional framework. The related compound 2 also consists of helical Co₃(OH)₂ ferrimagnetic chains but has a two-dimensional layered structure due to the absence of Na⁺ ions and partial replacement of sulfates by hydrosulfates. Preliminary magnetic measurements show that 1 and 2 have similar behavior. A detailed magnetic study of 1 reveals a stepped hysteresis loop at 2 K with a coercive field of 1800 Oe and remnant magnetization of 2.8 Nβ attributed to the different anisotropy axes of Co(II) spins in the helical Co₃(OH)₂ chain. Field-cooled (FC) and zero-field-cooled (ZFC) magnetization measurements show that 1 has a blocking temperature ca 28 K. The study of frequency dependence of real χ_m′ component of ac susceptibility by the equation φ = ΔT_f/[T_fΔ(log ω] exhibits that 1 is close to a spin glass. The fitting of Arrhenius law reveals that τ₀ seems a little longer but the U/k_B is normal for a typical single-chain magnet following Glauber dynamics. The −zν value of 3.3 obtained by the conventional critical scaling law of the spin dynamics τ = τ₀ (T_p − T_f/T_f)^−zν is out of the range for various spin glasses. The plot of dlog(T)/dlog(χT)_ferro versus T eliminates the phase transition to long-range ordering and suggests that 1 is a real one-dimensional system. The unusual slow magnetic relaxation behavior of 1 is tentatively ascribed to be a ferrimagnetic chain with spin-glass-like dynamic relaxation, possibly arising from the movement of the domain walls.