We study the response of an array of quantum-mechanical spin-1/2 particles in a classical background magnetic field to quantum fluctuations of the electromagnetic field. We show that there is a depolarization of the spins due to these fluctuations in that the magnetic moment of the system is reduced by the vacuum fluctuations. We further show that this depolarization may be partially canceled by interaction with a negative-energy flux. Thus quantum states such as squeezed states in which the energy density may be negative can cause an increase in the magnetic moment relative to the vacuum state. Consequently, spin systems can serve as model detectors of negative energy. We estimate the size of these effects and discuss the possibility of their observation.