The uniaxial Tb ions have elongated electronic orbitals (shaped like ellipsoids) and the Tb moment points along the ellipsoid due to spin-orbit coupling. The isotropic Tb ions have spherically shaped orbitals, so spin-orbit coupling gives no preference for the moment direction. The image shows that Tb atoms are either ellipsoidal (purple) or spherical (green) at intermediate temperatures (on some time scale). The number of spherical Tb ions determines when the transition occurs.