data=Import["/Users/mohamedelhanoty/Desktop/Master_stuttgart/Advanced_lab/Hall_effect/A06/Task3/coolingdata2.txt","Table"];

p=3.5346*10^21;

k=8.6173*10^-5;
g=0.6361;
i=0.03;
b=0.2;
e=1.6*10^-19;
d=0.001;
curve=NonlinearModelFit[data,p-0.5*p*Sqrt[0.5*p^2+n^2*(Exp[-(g/k*x)])]*(1-m^2)/(p-0.5*p*Sqrt[0.5*p^2+n^2*(E^-(g/k*x))]*(1+m^2))^2,{n,m},x]