Quantum tunneling nucleation for superparamagnetic spin cluster reversals

Usual theories for time-dependent effects in superparamagnetism (over-turning of large spin clusters) assume a flip rate ^∝ exp (-U/kT). Instead, following Lifshitz and Kagan, we calculate the flip rate due to quantum-mechanical zero-point motion. The resulting “zero temperature” flip rate is R₀ exp (-cn) for a cluster with n spins. R₀ and c depend on domain wall energy and mass; typically, $\text{R}{}_0\text{= 10}{}^{13}\text{n}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{sec}{}^{-1}$ and c = 1.