Field-induced spin-structural transition and giant magnetostriction in Ising chain α-CoV₂O₆

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We have investigated the temperature and magnetic field dependence of magnetization, specific heat (Cₚ), and relative sample length change (ΔL/L₀) for understanding the field-induced spin-structural change in quasi-one-dimensional spin chain α-CoV₂O₆ which undergoes antiferromagnetic (AFM) transition below Tₙ=15 K. Analysis of Cₚ(T) shows that an effective S=1/2 Ising state is realized below 20 K, though the magnetic fluctuations persist well above Tₙ. Cₚ and the coefficient of linear thermal expansion (α) exhibit strong H dependence in the AFM state. We also observe a huge positive magnetostriction [ΔL(H)/L₀] below 20 K which does not show any tendency of saturation up to 9 T. With increasing field, a sharp and symmetric peak emerges below Tₙ in both Cₚ(T) and α(T) due to field-induced first order ferrimagnetic/ferromagnetic-paramagnetic transitions. The large value of magnetostriction below Tₙ suggests strong spin-lattice coupling in α-CoV₂O₆.