Role of point defects on the enhancement of room temperature ferromagnetism in ZnO nanorods

Abstract

Vertically aligned undoped ZnO nanorod arrays (NRAs) have been fabricated on silicon (111) substrates by radio frequency magnetron sputtering. The structural studies illustrate a hexagonal wurtzite structure of the ZnO NRAs with compressive stress. Raman analysis of the E₂^high phonon mode corroborates the partial relaxation of stress in NRAs by the post growth treatment under oxygen and vacuum atmospheres. The anomalous Raman modes have been attributed to the local vibrations and it corresponds to the silent modes of wurtzite ZnO. The appearance of forbidden modes illustrates the breakdown of the Raman selection rules. The role of point defects on the ferromagnetic behaviour of NRAs was analyzed by optical transitions and correlated with the magnetic properties. Post growth treatment of NRAs under oxygen and vacuum atmospheres significantly suppresses the point defects owing to the enhancement of the crystalline quality. The temperature dependent zero-field cooled and field cooled magnetizations reveal the coexistence of antiferromagnetism and ferromagnetism below 7 K. However, the ferromagnetism is dominant and stable between 7 K and room temperature. The decrease of ferromagnetism in NRAs is directly associated with the compensation of point defects such as zinc and oxygen vacancies as substantiated by the radiative transition between shallow donor and acceptor energy levels. These results confirm that point defects play an important role in enhancing the room temperature ferromagnetism in ZnO NRAs.