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Description
This study focuses on the magnetic properties of $Zn_{1-x}Co_xO$, with x = 0.01, 0.03, and $Zn_{0.96}Co_{0.01}Cr_{0.03}O$, synthesized by solution combustion method. X-ray diffraction (XRD) revealed samples are in a hexagonal wurtzite structure. Rietveld refinement gives lattice parameters, a = b = 3.246 Å, and c = 5.201 Å, for $Zn_{0.99}Co_{0.01}O$; matching standard data (PDF#36-1451), and marginally increased in $Zn_{0.96}Co_{0.01}Cr_{0.03}O$, attributed to defects near dopants sites [1]. The particle size determined using transmission electron microscope images was found to be 48±2, and 39±3 nm for $Zn_{1-x}Co_xO$ (x = 0.01, 0.03), respectively, and 15±2 nm for $Zn_{0.96}Co_{0.01}Cr_{0.03}O$. Diffuse reflectance spectra show the absorption bands in all samples at 569 nm $(^4A_2 (F)→^4A_1 (G))$, 610 nm $(^4A_2 (F)→^4T_1 (P))$ and 660nm $(^4A_2 (F)→^2E(G))$ are transitions of $Co^{2+}$ ions replacing $Zn^{2+}$ sites [3]. In Co-Cr doped ZnO, an absorption band at 541 nm $(^4A_2 (F)→^4T_{2g} (F))$ reflects the transition of $Cr^{3+}$ ions [2] in the lattice. Band-gap values found are 3.306±0.003, and 3.289±0.004 eV for $Zn_{1-x}Co_xO$ (x = 0.01, 0.03, respectively) and 3.285±0.003 eV for $Zn_{0.96}Co_{0.01}Cr_{0.03}O$. Magnetization as a function of field curves, $M(\mu_0H)$, measured at room temperature (RT) using a vibrating sample magnetometer, of $Zn_{0.96}Co_{0.01}Cr_{0.03}O$ and $Zn_{0.97}Co_{0.03}O$ samples are hysteretic, signifying RT ferromagnetism (FM). Cobalt-doped ZnO shows diamagnetism for x = 0.01, while RTFM is seen for the x = 0.03 sample. The observed RTFM are explained based on bound magnetic polaron (BMP) mechanism. The number of BMPs created in $Zn_{0.97}Co_{0.03}O$ was found to be $2.5 \times 10^{14} cm^{-3}$. It is suggested that the exchange interaction of $Co^{2+}$ and/or $Cr^{3+}$ dopants mediated BMPs is ordering RTFM.
Keywords: ZnO, Combustion synthesis, Ferromagnetism, TEM.
References
- M. Ashokkumar, S. Muthukumaran, Enhanced room temperature ferromagnetism and photoluminescence behavior of Cu-doped ZnO co-doped with Mn, Phy. E 69 (2015) 354–359. https://doi.org/10.1016/j.physe.2015.02.010.
- J.J. Beltrán, C.A. Barrero, A. Punnoose, Combination of Defects Plus Mixed Valence of Transition Metals: A Strong Strategy for Ferromagnetic Enhancement in ZnO Nanoparticles, J. Phys. Chem. C 120 (2016) 8969–8978. https://doi.org/10.1021/acs.jpcc.6b00743.
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