can anyone tell me how to measure anisotropy field from the VSM measurement along hard axis direction?
The anisotropy field is defined as H_aniso = 2*K/J_s, where K is the anisotropy constant and J_s the Saturation magnetization. This Definition is unambiguous only in the case of a uniaxial magnetic anisotropy described with the free Energy f = K*cos^2 phi. In this case, assuming an external field applied along the hard axis, the minimization of the free enthalpy g = E_Zeeman + f, results in an initial linear increase of the magnetization with external field until it saturates at the Saturation magnetization (hard axis loop). The anisotropy field is identical with the field that is necessary to reach this Saturation Point.
Very often the magnetic anisotropy comprises higher order Terms. One can still define an anisotropy field from the second derivative of the free Energy evaluated at the Minimum. In that case one has to extrapolate the initial linear increase of the magnetization versus field function to the Saturation Point.
Details can be found in Ref. PHYSICAL REVIEW B 70, 184423 (2004), if you Neglect the extra discussion of the Details stemming from the magnetooptical Kerr effect in that paper.
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