Magnetocrystalline anisotropy, single-domain

In a single-domain particle of a-Fe203 the magnetization vector is held in the c-plane perpendicular to the c-axis by the magnetocrystalline field. Mossbauer studies use the 57Fe nucleus as the "observer to record when the relaxation time t becomes shorter than the period for precession of the nuclear spin about the direction of the effective field. Substitution into the equation for the Larmor frequency, or observer relaxation time, with an expression for the frequency factor proportional to the specific volume and anisotropy constant of the oxide gave (26, 27) the relationship ... 

In order to illustrate how precession-aided relaxation effects may manifest themselves in a ferrofluid, it will be useful to briefly summarize the differences in the relaxation behavior for axially symmetric and nonaxially symmetric potentials of the magnetocrystalline anisotropy and apphed held, when the Brownian relaxation mode is frozen. Thus only the solid-state (Neel) mechanism is operative that is, the magnetic moment of the single-domain particle may reorientate only with respect to the crystalline axes. 

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