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In this work, the role of so-called magnetic ions such as transitional metal (TM)/rare-earth (RE) ions and structural defects in stabilizing room-temperature ferromagnetism (RTFM) in wide-band oxide semiconductors are investigated in details, considering their three different length scales such as bulk, thin films and nanostructures. We have found that although the TM or RE ions substitute at host cation site, no evidence of intrinsic FM is detected in bulk phase, instead an evidence of paramagnetism (in dilute doping range) and anti-ferromagnetism (when heavily doped) is observed. However, with the reduction of particle size to nanoscale regime (˂ 20 nm), ferromagnetism is found to be stabilize in case of pure low-dimensional (1D/2D) SnO2 nanorods, nanowires, thin films in which the singly ionized oxygen vacancies (VO+) defects are responsible for inducing FM in undoped SnO2, not the magnetic ions like Co2+ or Gd3+ ions.
On the other hand, the evidence of cation vacancy-induced d0 ferromagnetism is observed in pure and nonmagnetic-doped ZnO nanowires and thin films. In this context, 1D nanowires and thin films of various alkali-metal (Li, Na, K) doped ZnO prepared by template-assisted sol-gel and pulsed laser deposition methods. Both the value of saturation magnetization moment (MS) and Cuire temperature (TC) found to enhance significantly after cationic substitution of such non-magnetic alkali-metals in ZnO. Various experimental evidence have shown that Zn vacancy (VZn) defects are the dominant magnetic source in the alkali-doped ZnO thin films and nanostructures and are responsible for stabilization of high-TC d0 ferromagnetism. The strength of ferromagnetic signal in alkali-doped ZnO can also be tuned using N or F co-doping, film thickness, annealing, and experimental pressure. The evidence of Zn vacancy defects due to Li-doping is also confirmed by Positron annihilation spectroscopy. Hence our studies have shown that the stabilization of RTFM in wide band oxides is possible through defect engineering that can be an effective alternative way to prepare high-TC oxide-based magnetic semiconductor for spintronic and opto-spintronic application.
References: [1] S. Ghosh et. al, J. Magn. Magn. Mater. 322, 1979, (2010).
[2] S. Ghosh et. al, J. Appl. Phys. 107, 123919, (2010).
[3] S. Ghosh et. al, J. Magn. Magn. Mater. 323, 1083, (2011).
[4] S. Ghosh et. al, ACS Appl. Mater. Interfaces 4, 2048, (2012).
[5] S. Ghosh et. al, J. Solid State Chemistry, 186, 278, (2012).
[6] S. Ghosh et. al, J. Appl. Phys. 109, 23927, (2011).
[7] S. Ghosh et. al, J. Appl. Phys. 112, 043910 (2012).
[8] S. Ghosh et. al, ACS Appl. Mater. Interfaces, 5, 2455 (2013).
[9] S. Ghosh et. al, J. Physical Chemistry C 117, 8458 (2013).
[10] S. Ghosh et al, Crystal Eng. Comm. 15, 7748 (2013).
[11] S. Ghosh et. al, J. Alloys and Compounds, 590 396 (2014). |