  Hamiltonians describing the relativistic quantum dynamics of a particle with
an arbitrary spin are shown to exhibit a supersymmetric structure when the even
and odd elements of the Hamiltonian commute. For such supersymmetric
Hamiltonians an exact Foldy-Wouthuysen transformation exits which brings it
into a block-diagonal form separating the positive and negative energy
subspaces. Here the supercharges transform between energy eigenstates of
positive and negative energy. The relativistic dynamics of a charged particle
in a magnetic field is considered for the case of a scalar (spin-zero) boson
obeying the Klein-Gordan equation, a Dirac (spin one-half) fermion and a vector
(spin-one) boson characterised by the Proca equation.
