To get directly the magnetic moment you should do neutron diffraction measurements. The Moessbauer measurements will give you moment indirectly.

Even though the magnetic hyperfine field can be measured quite precisely with Mossbauer spectroscopy, the relationship between the magnetic hyperfine field at the position of the resonant nucleus and the magnetic moment of the electron shell of same atom or of a unit cell is not obvious. However it is generally agreed on that the magnetic hyperfine field is proportional to the magnetic moment with a proportionality factor specific to the material composition and structure.

Given the mentioned proportionality the measurement of trends of the magnetic hyperfine field in lieu of the magnetic moment with MB spectroscopy are useful and mostly accurate. The absolute value of the magnetic moment can either be obtained by calculation or using another experimental method such as neutron diffraction. The latter is particularly suitable because of its ability to provide magnetic structure.

The application of MB spectroscopy to magnetism is often useful for particular studies: materials must contain a Mossbauer isotope (57Fe is very popular); very thin film and layer systems can be studied; the experimental setup using a MB source is quite inexpensive and accessible. In addition, a version of time-resolved MB spectroscopy available at synchrotron facilities can be applied to micron-sized samples and sub-nanometer particles.

H=A.mu (A is a constant)

Can you please suggest any refrance for NiFe alloy

Please see the appended text. It's not dealing with FeNi unfortunately but a list of references at the end may be useful.

Thanks Wolfgang Sturhahn Sir .  I try to read it and try to grap maximum i can.

Thanks

 In my case，for Nd2Fe14B, the A is about 150!