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 600 ° C . 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 ) Mn 2 O 4 exists at the surface. Exchange bias is clearly observed below 22 K . Exchange bias is attributed to the exchange interaction between ferrimagnetic ( Zn , Mn ) Mn 2 O 4 and spin-glass-like (or antiferromagnetic) phase in manganese oxides.