Robust Room Temperature Superparamagnetic Properties of ZnO Nanostructures: Li-Based Fe Dual Dopants.

Strong room temperature superparamagnetic properties have been achieved in Zn0.94Li0.03Fe0.03O nanoparticles within a magnetic field of ± 20,000 Oe. High-purity ZnO, Zn0.97Li0.03O, Zn0.94Li0.03Mn0.03O, and Zn0.94Li0.03Fe0.03O nanostructures were synthesized by means of the coprecipitation method. The XRD crystallographic planes and HRTEM of the synthesized compositions showed that a single phase of a ZnO hexagonal wurtzite structure was obtained with the absence of any secondary phases or magnetic cluster. Pure ZnO powders show asymmetrical nanoparticles with a certain degree of agglomeration and approximately have an average particle size of 37 nm. The TEM image of Li-monodoped ZnO powders displays uniform spherical nanoparticles with less agglomeration, and the average particle size was reduced to 35 nm. The image of the (Li, Mn) codoped ZnO sample illustrates that the particles of ZnO are transformed to elongated shapes without agglomeration with an average particle size of 32 nm. In case of (Li, Fe) codoped ZnO powders, the image clearly shows a mixture of uniform nanospherical and elongated particles with a small average particle size of 27 nm. The absorption edge of ZnO is red shifted to more wavelength absorption due to (Li, Mn) and (Li, Fe) codoping, and it becomes sharper after Li monodoping which is a dynamic factor in the optoelectronic applications. Interestingly, the Zn0.94Li0.03Fe0.03O composition exhibits a superparamagnetic behavior at room temperature; it obviously shows a semi-saturation magnetization of 0.02 emu/g but has a nearly very small coercivity of 14 Oe. The instantaneous presence of both ferromagnetism and antiferromagnetism in Zn0.94Li0.03Fe0.03O gives rise to the disordered state of superparamagnetism. [ABSTRACT FROM AUTHOR]