We consider a class of general spin Hamiltonians of the form
Hs(t)=H0(t)+H′(t) where
H0(t) and
H′(t) describe the dipole interaction of the spins with an arbitrary time-dependent magnetic field and the internal interaction of the spins, respectively. We show that if
H′(t) is rotationally invariant, then
Hs(t) admits the same dynamical invariant as
H0(t). A direct application of this observation is a straightforward rederivation of the results of Yan et al [Phys. Lett. A, Vol: 251 (1999) 289 and Vol: 259 (1999) 207] on the Heisenberg spin system in a changing magnetic field.