of the ferromagnetic anisotropy and demagnetizing fields, would be expected from powdered material (and was seen in a test of powdered pure Co). The g values lie near the g (2.22) of pure Co, and the H a lie between the anisotropy field values for hexagonal (5850 Oe) and cubic (1100 Oe) cobalt. We conclude that the resonances in these materials are ferromag-netic resonances and not paramagnetic resonance of non-S-state ions.
We consider the irreversible dynamics of the magnetization vector M in a singledomain particle. The dynamics is given by a stochastic phenomenological equation due to Gilbert. It contains a damping field proportional to 19I and a corresponding white noise field component. The probability distribution function satisfies a Fokker-Planck equation derived by Brown. We give the overbarrier decay rate ~ out of a metastable minimum. First we rederive the well-known expression for ~c for an axially symmetric model. We argue that this result is unphysical. For systems of general point symmetry of the magnetic anisotropy energy we give ~: in both the low-damping and intermediate-to high-damping limits.
%'e consider a ferromagnet whose magnetization is opposite to an applied magnetic field. The rate of quantum nucleation is calculated for a film and is estimated for a bulk solid. The temperature corresponding to the crossover from thermal to quantum nucleation is estimated. The effect may be large enough to be observed in materials with high anisotropy constants.
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