Spontaneous magnetization $m$ in an Ising ferromagnet appears below the critical temperature $T_c$. The derivative of $m$ diverges in the limit $T \to T_c^-$.

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% Spontaneous magnetization m in an Ising ferromagnet appears below the critical temperature T_c. % The derivative of m diverges in the limit $T \to T_c^-$. \documentclass{standalone} \usepackage{pgfplots} \pgfplotsset{compat=newest} \begin{document} \begin{tikzpicture}[ declare function={ % arcsinh arcsinh(\x) = ln(\x + sqrt(\x^2+1)); % magnetization m(\x) = and(\x>=0,\x<1) * (1 - (sinh(arcsinh(1)/x))^(-4))^(1/8) + or(\x<0,\x>=1) * (0); }] \begin{axis}[ xlabel = {$T/T_c$}, ylabel = {$m(0,T)$}, smooth,thick, domain=0:1.8, ymax=1.6, axis lines = center, every tick/.style = {thick}] \addplot[color=blue,very thick,samples=300]{m(x)}; \addplot[color=black,dashed,domain=0:1]{x}; \end{axis} \end{tikzpicture} \end{document}

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