The microscopic Hamiltonian for a spin-glass is diagonalized using the Holstein-Primakoff and Bogoliubov transformations. Both an external, Zeeman, and internal anisotropy field are included. The Zeeman energy enters the resonance condition with an extra factor of one-half which is shown to be of geometrical origin. For single ion anisotropy the resonance condition is angular dependent. It is implied that single crystal ESR measurements will yield information about the nature of the anisotropy energy in a spin-glass.
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