THE EFFECT OF AN OBLIQUE MAGNETIC-FIELD ON THE SUPERPARAMAGNETIC RELAXATION-TIME

The effect of a constant magnetic field, applied at an angle psi to the easy axis of magnetization, on the Neel relaxation time tau of a single domain ferromagnetic particle with uniaxial anisotropy is investigated by calculating the smallest non-vanishing eigenvalue of the appropriate Fokker-Planck equation as a function of psi. The curve of lambda(1) versus psi is symmetric about psi = 90 degrees. Thus the maximum decrease in tau occurs at psi = 45 degrees with maximum increase to a value exceeding that at psi = 0, at psi = 90 degrees, the psi = 0 value being again attained at psi = 180 degrees. The results are shown to be consistent with the behaviour predicted by the Kramers theory of the rate of escape of particles over potential barriers. This theory when applied to the potential barriers for the psi = 90 degrees orientation for rotation in space yields a simple approximate formula for the escape rate which is in reasonable agreement with the exact lambda(1) calculated from the Fokker-Planck equation.