Spontaneous magnetization in an Ising ferromagnet appears below the critical temperature . The derivative of diverges in the limit .
\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}