Speakers
Description
Structural, optical and magnetic properties of combustion synthesized $Zn_{0.96}Ni_{0.01}Cr_{0.03}O$ and $Zn_{0.90}Ni_{0.05}Cr_{0.05}O$ have been investigated. X-ray diffraction (XRD) analyses confirm that samples are in the hexagonal wurtzite structure. No impurity peaks where detected in $Zn_{0.96}Ni_{0.01}Cr_{0.03}O$, while a weak secondary spike $ZnCr_2O_4$ phase $(2\Theta=43^o)$ was identified in $Zn_{0.90}Ni_{0.05}Cr_{0.05}O$. Lattice parameters, obtained from Rietveld refinement, were found to be a=b=3.2535±0.0002 Å for both the samples, while c=5.2132±0.0003 Å for $Zn_{0.96}Ni_{0.01}Cr_{0.03}O$ decreasing to 5.2129±0.0002 Å for $Zn_{0.90}Ni_{0.05}Cr_{0.05}O$. These values are comparable with the standard data (PDF#36-1451). Diffuse reflectance spectra show weak absorption bands at 422, 610 and 660 nm, characteristic of tetrahedral $Ni^{2+}$ ions in the ZnO lattice [1]. Band-gap values, calculated using the Kubelka−Munk function [2], was found to be 3.287±0.003 and 3.272±0.003 eV for $Zn_{0.96}Ni_{0.01}Cr_{0.03}O$ and $Zn_{0.90}Ni_{0.05}Cr_{0.05}O$, respectively. Magnetization as a function of field measurements, $M(\mu_0H)$, was performed at room temperature using a vibrating sample magnetometer. The $Zn_{0.96}Ni_{0.01}Cr_{0.03}O$ and $Zn_{0.90}Ni_{0.05}Cr_{0.05}O$ samples show ferromagnetic (FM) and antiferromagnetic (AFM) behaviour, respectively. Point defects are the source for the obtained FM in $Zn_{0.96}Ni_{0.01}Cr_{0.03}O$. The exchange interaction between $Ni^{2+}$ and/or $Cr^{3+}$ dopants and formation of a $ZnCr_2O_4$ phase is responsible for AFM behaviour in $Zn_{0.90}Ni_{0.05}Cr_{0.05}O$. This study reveals that the $Ni^{2+}$ and $Cr^{3+}$ ions successfully substituted into $Zn^{2+}$ sites at lower concentrations of $Cr^{3+}$ ions, while at higher concentrations some of the Cr ions are in an octahedral rather than tetrahedral coordination causing the formation of secondary phase. Interestingly, in Ni-Cr doped ZnO, the weak FM behaviour is transformed to AFM behaviour depending on the Cr content.
Keywords: ZnO, Combustion synthesis, Rietveld refinement, Magnetic properties
References
- J.J. Beltrán, C.A. Barrero and A. Punnoose, The Journal of Physical Chemistry C., Vol. 120, p.8969–8978(2016).
- N. Srinatha, K.G.M. Nair and Basavaraj Angadi, Physica B., Vol. 474, p.97–104 (2015).
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