Rotational states of a nanomagnet

We study a nanomagnet that exhibits spin tunneling and is free to rotate about its anisotropy axis. Hamiltonian of a rotated two-state spin system is derived and exact low-energy eigenstates of the nanomagnet that are superpositions of spin and rotational states are obtained. We show that parameter α=2(ℏS)²/(IΔ) determines the ground state of the magnet. Here ℏS is the spin, I is the moment of inertia, and Δ is the tunnel splitting. The ground-state magnetic moment is zero at α<α_c=[1-1/(2S)²]^-1 and nonzero at α>α_c . At α→∞ the spin of the nanomagnet localizes in one of the directions along the anisotropy axis.