Absence of ferromagnetism in Co and Mn substituted polycrystalline ZnO

We discuss the properties of semiconducting bulk ZnO when substituted with the magnetic transition metal ions Mn and Co, with substituent fraction ranging from x=0.02 to x=0.15. The magnetic properties were measured as a function of magnetic field and temperature and we find no evidence for magnetic ordering in these systems down to T=2 K. The magnetization can be fit by the sum of a Curie-Weiss term with a Weiss temperature of Theta>100 K and a Curie term. We attribute this behavior to contributions from both tM ions with tM nearest neighbors and from isolated spins. This particular functional form for the susceptibility is used to explain why no ordering is observed in tM substituted ZnO samples despite the large values of the Weiss temperature. We also discuss in detail the methods we used to minimize any impurity contributions to the magnetic signal.