Exchange bias and the origin of magnetism in Mn-doped ZnO tetrapods

Wurtzite-type ZnO tetrapod nanostructures were prepared by evaporating Zn metal under humid argon flow. After the fabrication, Mn was doped into ZnO nanostructures by diffusion at 600degreesC. The average concentration of Mn was determined to be 8.4 mol % by x-ray fluorescence. X-ray diffraction patterns obtained from the doped and undoped samples are almost the same. High-resolution transmission electron microscopy observations reveal the existence of surface layers. Magnetic measurements show that the sample has a very large coercivity H-C=5500 Oe at 5.5 K and a Curie temperature T-C=43 K, which may suggest that ferrimagnetic (Zn,Mn)Mn2O4 exists at the surface. Exchange bias is clearly observed below 22 K. Exchange bias is attributed to the exchange interaction between ferrimagnetic (Zn,Mn)Mn2O4 and spin-glass-like (or antiferromagnetic) phase in manganese oxides. (C) American Institute of Physics.