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We calculate the quantum tunneling rate of a ferromagnetic particle of $\sim100 \AA$ diameter in a magnetic field of arbitrary angle. We consider themagnetocrystalline anisotropy with the biaxial symmetry and that with thetetragonal symmetry. Using the spin-coherent-state path integral, we obtainapproximate analytic formulas of the tunneling rates in the small $\epsilon(=1- H/H_c)$-limit for the magnetic field normal to the easy axis ($\theta_H =\pi/2$), for the field opposite to the initial easy axis ($\theta_H = \pi$),and for the field at an angle between these two orientations ($\pi/2 <<\theta_H << \pi$). In addition, we obtain numerically the tunneling rates forthe biaxial symmetry in the full range of the angle $\theta_H$ of the magneticfield ($\pi/2 < \theta_H \leq \pi$), for the values of \epsilon =0.01 and0.001.Comment: 25 pages of text (RevTex) and 4 figures (PostScript files), to be  published in Phys. Rev.

Magnetic-field dependence of macroscopic quantum tunnelingand coherence of ferromagnetic particles