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 = 0.01 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 those 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. We also discuss in detail the methods we used to characterize any impurity contributions in these samples.