as far as my knowledge was concerned arrot plots are used to find out for first or second order magnetic transitions. BFO is very weak ferromagnetic character almost linear (anti ferro magnetic part) at RT. but use expression of M(H)= susceptability(AF component)* H + saturation magnetisation( weak ferro). in order to get saturation magnetisation make first part equal to zero i.e extrapolate the linear part of M-H curve to zero.