Magnetic and optical properties of single crystals of transition metal doped ZnO

Considerable effort has been devoted to the study of transition metal doped zinc oxide following various theoretical predictions of room temperature ferromagnetism in these materials. Near equilibrium growth techniques may be suitable for the production of dilute transition metal doped ZnO because the transition metal solubility in ZnO can be relatively high. This work reports on the optical, structural, and magnetic properties of single crystals of Co- and Mn-doped ZnO grown by a modified melt-growth technique. X-ray diffraction reveals that the as grown crystals are pure single crystals with no second phases. Mn doping up to 5% results in an increase in c-axis lattice parameter (5.207 angstrom to 5.211 angstrom), and X-ray linewidths (78 arcsec to 252 arcsec) at doping levels of 5% Mn. Optical transmission shows distinct absorption spectra related to the color of the Zn1-xTMxO sample resulting from interatomic transitions within the divalent transition metal ion. Electron paramagnetic resonance confirms the divalent nature of the substitutional transition metal ions. Raman spectroscopy shows the predominantly the E, (high) peak at 407 cm(-1), but with increasing doping shows additional disorder-related modes. The magnetic behavior reveals a paramagnetic behavior at all temperatures for both Mn- and Co-doped ZnO up to transition metal doping levels of 5% and 3% respectively. The dominant exchange mechanism in the Mn- and Co-doped ZnO crystals is found to be antiferromagnetic superexchange. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.