Here D and k are constans to be evaluated.

Equation:
M(t)=Mo - (Mo - Ms)*Pi*8*R*Ms*D [Sum((k*R^2 + Pi^2*D n^2)*k*R^2*t -Pi^2*R^2*D n^2 - (Pi^2*R^2*D n^2*
Exp[-t*(k*R^2 + D*Pi^2 n^2)/R^2])/(k*R^2 + n^2*D*Pi^2)^2]

[Sum] goas from 1 to 1000

Ms=140
Mo=11.58
R=0.004
Variables and Data
t M(t)
0 11.58
30 72.61
60 97.89
90 106.05
120 109.63
150 117.02
180 119.57
210 125.99
240 129.56
270 132.85