As in my previous question, I am confused about something from the 2011 condensed matter paper, question 8 this time.
In the last paragraph of the question, you are asked to estimate the value of the mean field B_e, which is given earlier as:
B_e = mu_0 * lambda * M
Where M is the magnetization. A value for lambda has already been estimated in this question but I'm not so sure about M.
In the solutions from the physics website [http://www.physics.ox.ac.uk/expapers/ExamSolutions_for%20Students/2012-2013/B3%20V.pdf.pdf], the value for M used is n*g*mu_B*J.
I'm presuming that this is taken from the original magnetization formula in the limit for B_J(x) when x>>1? If so, why is this approximation valid?
Any help is appreciated.