Optical constants correlated electrons-spin of micro doughnuts of Mn-doped ZnO films

Abstract

Diluted magnetic semiconductor (DMS) Mn:ZnO thin films with “ring-like or doughnut-like” structures were grown using aerosol spray pyrolysis for 20 and 30 min. Electron paramagnetic resonance revealed the ferromagnetic ordering which varies with Mn concentration. These ferromagnetic films obtained coexist with dielectric state. Spectroscopic ellipsometry results showed that the dielectric constants ɛ1 and ɛ2 also vary in definite pattern as Mn concentration. We used and fitted Bruggeman theory to explain these trends and to find ɛ1 and ɛ2 for both ZnO and Mn which compare well with literature. The results showed that the structural, optical and magnetic properties of these DMSs are strongly sensitive to the preparation parameters. It is unambiguously demonstrated that the room-temperature ferromagnetism is strongly correlated with dielectric constants. The effective medium approach Bruggeman model used to fit our experimental data demonstrated a decrease in the dielectric constant with the addition of Mn concentration. These findings revealed that the refractive index of the films increases with ferromagnetic ordering while the extinction coefficient reduces. This indicates that when the Mn:ZnO film becomes more opaque (high reflecting) to UV (∼370 nm) and absorbs less of that light, the ferromagnetic ordering is enhanced. A relatively new phenomenon of d-band resonance at 3.35 eV (∼370 nm polarized light) from Zn O:O Mn molecule at 0.25 at.% Mn was observed for both 20 (thinner film) and 30 min (thicker film) deposited Mn:ZnO films. A good correlation between ferromagnetic intensity signal with its width and angular dependence for the films was also observed.