MACROSCOPIC QUANTUM TUNNELING IN ANTIFERROMAGNETS

Starting from the Néel state of a uniaxial antiferromagnetic particle, we show that, due to the tunneling of the Néel vector between easy directions, the ground state of a sufficiently small particle is a quantum superposition of two equivalent Néel states. A certain orientation of the Néel vector becomes frozen as the volume of the particle grows, or the dissipation due to the interaction of the Néel vector with microscopic degrees of freedom increases. For the weak dissipation, which is mostly the case, the crossover from classical to quantum regime occurs at temperature T*∼(ε{lunate}aε{lunate}e) 1 2TN, where ε{lunate}a and ε{lunate} e are anisotropy and exchange constants, TN is the Néel temperature. © 1990.