T. Shripathi

Study of ferromagnetism in Mn doped ZnO dilute semiconductor system

The wide band gap semiconductor ZnO when doped with a very low percent of some transition metal ions can exhibit above room temperature ferromagnetism, transforming it into a unique compound for spin-electronic applications. In the present work we have compared the electronic structure of two polycrystalline Zn1-xMnxO pellets (for x=0.02 and 0.04), prepared by low temperature processing, and carefully characterized. The Rietveld refinement of the XRD patters established that the samples have the ZnO lattice with ZnS type Wurtzite hexagonal symmetry and no detectable impurities. The samples exhibit distinctly different magnetic properties. The pure ZnO pellet shows a diamagnetic behaviour, the 2% sample displayed a clear FM ordering at 300 K while the 4% sample did not show any ordering even upon cooling. Their electronic structure has been investigated using x-ray absorption and x-ray photoemission spectroscopy with an aim to find out how the changes in the electronic structure can correlate to the magnetic properties in such diluted magnetic semiconductor materials. The results show that most of the Mn ions of the ferromagnetic sample are in the divalent state. For the higher Mn percent nonmagnetic sample, a larger contribution of higher oxidation Mn states are dominant and the oxygen content also increases. The two factors can be correlated to the suppressed ferromagnetism, though it is hard to exactly predict that which of these two factors weighs more.

Study of room temperature ferromagnetism for cobalt and manganese doped ZnO diluted magnetic semiconductor

We report the results of electronic and magnetic properties of poly-crystalline Zn1-xMxO (M = Co and Mn) pellets studied by XRD, VSM and XPS. The specimens were synthesized by solid state reaction method using high purity oxides. Samples exhibit Wurtzite hexagonal symmetry and show interesting magnetic properties. The Co (5%) doped sample prepared by heating in air shows a paramagnetic state in agreement with several claims. But the sample prepared by heating in Ar atmosphere depicts ferromagnetic ordering. Addition of 2% Mn in it further enhances its magnetic moment. The specimens ZnCoO (Ar heated) and ZnCoMnO were hydrogenated at 550°C that caused a significant change in their magnetization. The Co doped ZnO shows a robust increment in the magnetic moment but the Mn doping does not seem to respond to hydrogenation. O 1s XPS results show clear evidence of oxygen depletion for the samples prepared by heating in Ar atmosphere and a further depletion upon hydrogenation. Results suggest oxygen vacancies as the basic origin for room temperature ferromagnetism in doped ZnO.