  The entropy stabilized oxide
Mg$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$O exhibits antiferromagnetic
order and magnetic excitations, as revealed by recent neutron scattering
experiments. This observation raises the question of the nature of spin wave
excitations in such disordered systems. Here, we investigate theoretically the
magnetic ground state and the spin-wave excitations using linear spin-wave
theory in combination with the supercell approximation to take into account the
extreme disorder in this magnetic system. We find that the experimentally
observed antiferromagnetic structure can be stabilized by a rhombohedral
distortion together with large second nearest neighbor interactions. Our
calculations show that the spin-wave spectrum consists of a well-defined
low-energy coherent spectrum in the background of an incoherent continuum that
extends to higher energies.
