The required bias field of a sperimagnetic film is governed by a finite exchange coupling coefficient (&lgr;) mainly determined by the subnetwork coupling. From the study of a series of (Dy,Tb)FeCo films at compensation composition, it is found that &lgr; is mainly determined by the concentration of the rare earth components. Moreover, the exchange integral between rare earth and transition metal subnetworks can be derived from &lgr; so that the number of uncertain parameters for mean field modeling is reduced. Accordingly, the bias field required for magneto-optical recording is mainly proportional to &lgr; that can be obtained quantitatively from either measurement and/or mean field modeling. ©1997 American Institute of Physics.