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Field-induced spin-structural transition and giant magnetostriction in Ising chain $α$-CoV$_2$O$_6$

We have investigated the temperature and magnetic field dependence of magnetization, specific heat ($C_p$), and relative sample length change ($ΔL/L_0$) for understanding the field-induced spin-structural change in quasi-one-dimensional spin chain $α$-CoV$_2$O$_6$ which undergoes antiferromagnetic (AFM) transition below $T_N$$=$15 K. Analysis of $C_p$($T$) shows that an effective $S$$=$1/2 Ising state is realized below 20 K, though the magnetic fluctuations persist well above $T_N$. $C_p$ and the coefficient of linear thermal expansion ($α$) exhibit strong $H$ dependence in the AFM state. We also observe a huge positive magnetostriction [$ΔL$($H$)/$L_0$] 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 $T_N$ in both $C_p$($T$) and $α$($T$) due to field-induced first order ferrimagnetic/ferromagnetic-paramagnetic transitions. The large value of magnetostriction below $T_N$ suggests strong spin-lattice coupling in $α$-CoV$_2$O$_6$.