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 alpha=2(hS)(2) /( I Delta) determines the ground state of the magnet. Here hS is the spin, I is the moment of inertia, and Delta is the tunnel splitting. The ground-state magnetic moment is zero at alpha <alpha(c)=[1-1/( 2S)(2)]-1 and nonzero at alpha <alpha(c). At alpha ->infinity the spin of the nanomagnet localizes in one of the directions along the anisotropy axis.