m[T_]:=(1-1/Sinh[1/(2*T)]^4)^(1/8);
Tc=1/(2*ArcSinh[1]);

Plot[m[T],{T,0.001,Tc},PlotPoints->2000,PlotRange->{{0,1},{0,1}},AxesLabel->{"T","M"}]
Tc=N[4/(2*ArcSinh[1])]
