Thermally Activated Relaxation Time of a Single Domain Ferromagnetic Particle Subjected to a Uniform Field at an Oblique Angle to the Easy Axis: Comparison with Experimental Observations

New asymptotes of the relaxation time of the magnetic moment of a single domain particle with a uniform magnetic field applied at an oblique angle to the easy axis (in excellent agreement with exact numerical results from the Fokker-Planck equation for the Néel-Brown model) are used to model the experimental angular variation of the switching field for individual Co and BaFeCoTiO particles. Good agreement is obtained, justifying the Néel-Brown (in effect, the Kramers) conception of the superparamagnetic relaxation process and allowing one to deduce the value of the damping constant (hitherto almost unknown).