Magnetisation definition

The magnetic field seen by the probe neutron is solely due to the magnetic dipole moment density of the unpaired electrons. In other words, the magnetisation density is simply related to the electron spin density by a multiplicative factor, and there is no ambiguity in its definition. 

Fig. 38. In-plane magnetisation (a) and magnetostriction (b) of a Terfecohan TtHFe0.55Co0.45) 1.5 single layer and a Terfecohan/Fe multilayer film. Definitions of the magnetostriction remanence (A.r) and the coercive field UHc) are illustrated for the Terfecohan film. After Due et al. (2001a).

The magnetic susceptibility is introduced as a thermodynamic quantity which requires a partial differentiation of the magnetisation according to the applied field. This differential (isothermal) magnetic susceptibility differs from the frequent definition of mean magnetic susceptibility if the behaviour of the magnetic material is nonlinear. 

However, in a rigid solid, the definition of a transverse relaxation time is more difficult since the shape of the decay is more complex. T2 is therefore often taken as a characteristic time of the FID, usually the time taken to decay to 1/e of its initial value, and as such can be used as an effective lifetime for the transverse magnetisation. If we consider the effects of increasing the temperature of a solid sample, we find that as various motions come into play, the second moment decreases and the FID lengthens. Eventually there comes a point where there is no residual static interaction on the timescale of the... 

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