Spin Tunneling in a Rotating Nanomagnet

We study spin tunneling in a magnetic nanoparticle with biaxial anisotropy that is free to rotate about its anisotropy axis. Exact instanton of the coupled equations of motion is found that connects degenerate classical energy minima. We show that mechanical freedom of the particle renormalizes magnetic anisotropy and increases the tunnel splitting. \\[4pt] M. F. O'Keeffe and E. M. Chudnovsky, cond-mat, arXiv:1011.3134.