Quantum Coherence in Small Antiferromagnets

An antiferromagnetic particle with an excess spin $\overrightarrow{j}$ has 2 degrees of freedom. The one associated with $\overrightarrow{j}$ is ferromagnetic, while the antiferromagnetic behavior involves Néel vector $\overrightarrow{n}$. It is shown that the spin-parity effect described by Loss et al. has to do with the ferromagnetic degree of freedom and does not imply the localization of $\overrightarrow{n}$ for small enough particles for half-integer spin. It is also shown that the tunnel splitting associated with $\overrightarrow{n}$ is insensitive to applied fields.