  In this study, we investigate the decoherence of a spatially superposed
electrically neutral spin-$\frac12$ particle in the presence of a relativistic
quantum electromagnetic field in Minkowski spacetime. We demonstrate that
decoherence due to the spin-magnetic field coupling can be categorized into two
distinct factors: local decoherence, originating from the two-point correlation
functions along each branch of the superposed trajectories, and nonlocal
decoherence, which arises from the correlation functions between the two
superposed trajectories. These effects are linked to phase damping and
amplitude damping. We also show that if the quantum field is prepared in a
thermal state, decoherence monotonically increases with the field temperature.
