Individual atoms can have their spin reversed by interaction with energy from elsewhere. A spin reversal itself can influence the spin of the electrons in neighbouring atoms. A wave of change in electron spins can then move through the electrons in the crystal lattice as it dissipates like waves on a pool of water. Since the electrons are very small and governed by quantum mechanics, this wave, can be treated as though it is a number of particle like entities. These are magnetic quasi particles or magnons. 