The influence of the grain boundary (GB) specific area sGB on the appearance of ferromagnetism in Fe-doped ZnO has been analysed. A review of numerous research contributions from the literature on the origin of the ferromagnetic behaviour of Fe-doped ZnO is given. An empirical correlation has been found that the value of the specific grain boundary area sGB is the main factor controlling such behaviour. The Fe-doped ZnO becomes ferromagnetic only if it contains enough GBs, i.e., if sGB is higher than a certain threshold value sth = 5 × 104 m2/m3. It corresponds to the effective grain size of about 40 μm assuming a full, dense material and equiaxial grains. Magnetic properties of ZnO dense nanograined thin films doped with iron (0 to 40 atom %) have been investigated. The films were deposited by using the wet chemistry “liquid ceramics” method. The samples demonstrate ferromagnetic behaviour with Js up to 0.10 emu/g (0.025 μB/f.u.ZnO) and coercivity Hc ≈ 0.03 T. Saturation magnetisation depends nonmonotonically on the Fe concentration. The dependence on Fe content can be explained by the changes in the structure and contiguity of a ferromagnetic “grain boundary foam” responsible for the magnetic properties of pure and doped ZnO.

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Ferromagnetic behaviour of Fe-doped ZnO nanograined films.

Nanograined (grain size 6–15 nm) ZnO films with various Fe content (between 0 and 40 at.%) were synthesized by the novel liquid ceramics method. The films with 0, 0.1, 5 and 10 at.% Fe contain only ZnO-based solid solution with wurtzite structure. The films with 20 at.% Fe contain mainly amorphous phase. The peaks of the second phase (ZnFe2O4 with cubic lattice) become visible in the X-ray diffraction spectra at 30 at.% Fe. Therefore, the overall solubility of Fe in nanograined ZnO films at 550 °C is about 20 at.% Fe. The solubility limit in the bulk is about 1.5 at.% Fe. The recently published papers on the structure and magnetic behaviour of Fe-doped ZnO allowed us to obtain the dependence of Fe solubility in ZnO on the grain size. The overall Fe solubility drastically increases with the decreasing grain size. The quantitative estimation shows that, close to the bulk solubility limit, the thickness of a Fe-enriched layer in grain boundaries is that of several monolayers.

Increase of Fe solubility in ZnO induced by the grain boundary adsorption

Structural, optical and magnetic properties of Zn0.97−xAlxCr0.03S nanoparticles

Iron (Fe)-doped zinc oxide (ZnO) thin films were deposited using two techniques: (i) radio-frequency (RF) sputtering of Fe-doped ZnO targets, and (ii) co-sputtering, where ZnO was RF-sputtered and iron was direct-current (DC)-sputtered. The as-deposited films were polycrystalline, with predominant growth along the (002) direction of hexagonal ZnO, and possessed a considerable concentration of oxygen vacancies. From an optoelectronic point of view, the films were highly transparent, with a band gap of 3.25 eV, and had electrical resistivity values in the range of 100–103 Ω cm. To improve the electrical conductivity of the films, they were annealed in a vacuum and in a hydrogen atmosphere. The annealing process did not affect the optical properties of the films. However, there were substantial structural and chemical changes in the films. Moreover, the electrical conductivity of the films was enhanced drastically upon annealing in hydrogen, where the electrical resistivity was reduced to 3.2 × 10−3 Ω cm.

Influence of iron doping on the structural, chemical, and optoelectronic properties of sputtered zinc oxide thin films

Zn1−xMnxO (x = 0, 0.02, 0.04, 0.06, 0.08 and 0.1) nanocrystalline diluted magnetic semiconductors (DMSs) were synthesized using sol–gel-based auto-combustion technique. X-ray diffraction (XRD) analysis of all the synthesized samples indicated pure wurtzite hexagonal structure without any secondary phase with perfect solubility of Mn contents in ZnO matrix. The increase in Mn concentration reduced the average crystallite size as evaluated by the Scherrer’s formula. Morphological analysis performed using a field emission scanning electron microscope (FESEM) indicated nearly an ordered surface morphology with some porosity and few cracks. Energy dispersive X-ray (EDX) spectroscopy confirmed the stoichiometric contents of Zn, Mn and O in each sample. Micrographs of all the samples exhibited a decrease in grain size with increase of Mn contents. Magnetic measurements performed using vibrating sample magnetometer (VSM) confirmed room temperature ferromagnetism exhibiting an increase in saturation magnetization and coercivity with increasing Mn contents in all the prepared samples.

Enhanced ferromagnetic characteristics in Mn-doped ZnO: A promising diluted magnetic semiconductor

Zn1-xFexO (x = 0.04, 0.06, 0.08, 0.10, 0.12) thin films were grown on Si substrates using reactive magnetron sputtering. X-ray diffraction analyses show that the samples have wurtzite structures with the c-axis orientation. X-ray photoelectron spectroscopy results indicate that the Fe ions are in a +2 charge state in the films. Magnetization measurements indicate that room temperature ferromagnetism is present in films annealed in vacuum while films annealed in air were non-magnetic. The presence of oxygen vacancies in these films may mediate exchange coupling of the dopant ions, resulting in room temperature ferromagnetism.

FERROMAGNETISM IN Fe-DOPED ZnO THIN FILMS

A series of Zn0.95-xCo0.05AlxO (x=0, 0.01, 0.02, 0.05, 0.08, 0.10) powders with different percentages of aluminum was fabricated using the sol-gel method. X-ray diffraction (XRD) revealed that the Co ions and Al ions substitute for Zn2+ ions without changing the wurtzite structure. No impurity phases were found. No clusters or precipitates of cobalt or aluminum were found using scanning electron microscope analysis. Fourier transform-infrared reflection (FT-IR) spectrometry was used to examine the infrared transmission properties and revealed that Co ions were incorporated into the lattice as Co2+ substituting for Zn2+. Ferromagnetic behavior in the samples was obtained at room temperature. As the Al content x increased, the room temperature ferromagnetism of the samples was reduced, and when x increased to 0.08, the room temperature ferromagnetism disappeared.

Structural and Magnetic Properties of Cobalt and Aluminum Co-Doped ZnO Powders

“Green” multiferroic BaTiO3/FeBSi composite films were grown by pulsed laser deposition and ion beam sputtering on general Pt/Ti/SiO2/Si substrates. Room temperature X-ray diffraction and Raman scattering show that the crystal structures of BaTiO3 and FeBSi are tetragonal and amorphous, respectively, and no additional or intermediate phase peaks appears in the composite films. A cross-sectional scanning electron microscopy image clearly demonstrates a 2-2 type structure with sharp interface between the top FeBSi layer and bottom BaTiO3 layer. The magnetic properties of the top FeBSi are obviously modified by the bottom BaTiO3. The composite films show obvious ferroelectric feature.

Zhen Zhen Zhou

Papers

FERROMAGNETISM IN Fe-DOPED ZnO THIN FILMS

Structural and Magnetic Properties of Cobalt and Aluminum Co-Doped ZnO Powders

Novel Multiferroic Lead-Free BaTiO3/FeBSi Composite Films