Extrinsic origin of ferromagnetism in doped ZnO

Different synthetic routes have been tested to prepare bulk doped ZnO. The doping elements were Mn, Fe and Co. We were able to prepare single-phase compounds at high temperature by using ceramic procedures and at very low temperature using a sol-gel method. No ferromagnetism has been observed for these samples. In contrast, spontaneous magnetization is observed in samples obtained from non-optimal conditions and exhibiting secondary phases. Magnetic impurities have been easily identified for Fe- and Co-doped samples. However, the nature of the room temperature magnetic impurity for the Mn-based compound is not so clear and different possibilities are discussed. We have also checked the effect of the grain size on the magnetic properties by synthesizing nano-materials using a sol-gel route. The single-phase nano-compounds are also not ferromagnets showing similar properties to the well-sintered polycrystalline materials.