Escape rate of the nanospin system in a magnetic field: the first-order phase transition within quantum regime

Abstract

We have investigated the escape rate of the nanospin particle with a magnetic field applied along the easy axis. The model studied here is described by the Hamiltonian $\hat{\mathcal{H}}=K_1{\hat{S}}_z^2+K_2{\hat{S}}_y^2+g{\mu }_{b}H{\hat{S}}_x$, $(K_1>K_2>0)$ from which the escape rate is calculated within the semiclassical approximation. We have obtained a diagram for the orders of the phase transitions depending on the anisotropy constant and the external field. For $K_2/K_1>0.85$ the present model reveals, for the first time, the existence of the first-order transition within the quantum regime.

14 pages, 5 postscript figures included