  ac susceptibility has been studied for polycrystalline Zn$_{1-x}$Mn$_x$O.
Stoichiometric samples demonstrate Curie-Weiss behavior, which indicates mostly
antiferromagnetic interactions. Magnetic susceptibility can be described by a
diluted Heisenberg magnet model developed for semimagnetic semiconductors.
High-pressure oxygen annealing induces spin-glass like behavior in
Zn$_{1-x}$Mn$_x$O by precipitation of ZnMnO$_3$ in the paramagnetic matrix.
