We study tunneling of the magnetic moment in a particle that has full rotational freedom. Exact energy levels are obtained and the ground-state magnetic moment is computed for a symmetric rotor. The effect of the mechanical freedom on spin tunneling manifests itself in a strong dependence of the magnetic moment on the moments of inertia of the rotor. Energy of the particle exhibits quantum phase transitions between states with different values of the magnetic moment. Particles of various shapes are investigated and quantum phase diagram is obtained.
O'Keeffe, M. F., Chudnovsky, E. M., & Garanin, D. A. (2012). Quantum tunneling of the magnetic moment in a free nanoparticle. Journal of Magnetism and Magnetic Materials, 324(18), 2871-2878. DOI: 10.1016/j.jmmm.2012.04.032