Measurements of coercive force H c and remanent magnetization 4πMr have been made as a function of temperature (−195?T?150°C) on dense polycrystalline Li ferrite substituted with Mn (0?y?⋅.20) and Co (0?z?.02). We report an abrupt degradation in loop squareness (decreased 4πMr, increased H c ) at a characteristic temperature T ϑ, which increases with z but is independent of y. Single‐crystal data on anisotropy K 1 show a correlation between T ϑ and the temperature at which sign reversal in K 1 is found with Co substitution. The absence of a dependence on Mn content is in accord with the fact that K 1 is relatively insensitive to Mn substitutions, which have been shown to control magnetostriction effects. The deterioration in the hysteresis loop is discussed in terms of the nucleation of reverse domains as K 1 passes through zero.