A.A. Mazilkin

Ferromagnetism of Zinc Oxide Nanograined Films

The reasons for the appearance of ferromagnetic properties of zinc oxide have been reviewed. It has been shown that ferromagnetism appears only in polycrystals at a quite high density of grain boundaries. The critical size of grains is about 20 nm for pure ZnO and more than 40 μm for iron-doped zinc oxide. The solubility of manganese and cobalt in zinc oxide increases significantly with a decrease in the size of grains. The dependences of the saturation magnetization on the concentrations of cobalt, manganese, and ion are nonmonotonic. Even if the size of grains is below the critical value, the ferromagnetic properties of zinc oxide depend significantly on the texture of films and the structure of amorphous intercrystallite layers.

Interfacial dominated ferromagnetism in nanograined ZnO: a μSR and DFT study

Diamagnetic oxides can, under certain conditions, become ferromagnetic at room temperature and therefore are promising candidates for future material in spintronic devices. Contrary to early predictions, doping ZnO with uniformly distributed magnetic ions is not essential to obtain ferromagnetic samples. Instead, the nanostructure seems to play the key role, as room temperature ferromagnetism was also found in nanograined, undoped ZnO. However, the origin of room temperature ferromagnetism in primarily non–magnetic oxides like ZnO is still unexplained and a controversial subject within the scientific community. Using low energy muon spin relaxation in combination with SQUID and TEM techniques, we demonstrate that the magnetic volume fraction is strongly related to the sample volume fraction occupied by grain boundaries. With molecular dynamics and density functional theory we find ferromagnetic coupled electron states in ZnO grain boundaries. Our results provide evidence and a microscopic model for room temperature ferromagnetism in oxides.

Effective Temperature of High Pressure Torsion in Zr-Nb Alloys

Abstract Severe plastic deformation by the high pressure torsion (HPT) leads to the phase transitions and strong grain refinement. The starting α Zr-phase in Zr alloyed by 2.5 and 8 mass% Nb transforms into β + ω mixture. This β + ω phase mixture can be found in the equilibrium phase diagram at higher (effective) temperature (Teff = 620°C for Zr-2.5 mass% Nb and (Teff = 550°C for Zr-8 mass% Nb). The published papers on phase transitions during HPT are analysed and the values of effective temperature are estimated. Contrary to the increasing temperature, the increasing pressure slows down the diffusion and grain boundary migration. Therefore, the forced atomic movement during HPT produces the states equivalent to higher temperature, but not to the higher pressure.

Heat effect of grain boundary wetting in Al-Mg alloys

Grain boundary wetting transitions were previously observed in the Al–Mg system. The melting of as-cast Al–5 wt% Mg and Al–10 wt% Mg alloys was studied by the differential scanning calorimetry. The asymmetric shape of the melting curve permitted the observation of the thermal effect of grain boundary wetting. The difference in the shape of the melting curve for the two studied alloys is explained by the different temperature dependence of the fraction of completely wetted grain boundaries.

Structural changes in aluminum alloys upon severe plastic deformation

The structure and phase composition of Al-Zn, Al-Mg, and Al-Mg-Zn alloys were studied before and after severe plastic deformation of these alloys. The deformation was performed by high pressure torsion with true strain of ∼6. It was established that, as a result of severe plastic deformation, the grains of Al and Zn and also of the β and τ phases revealed in the structure decrease significantly in size and reach nanometer scales. A supersaturated solid solution of Zn in Al decomposes completely in this case and achieves the equilibrium state corresponding to room temperature. The decomposition is less pronounced for the magnesium-containing alloys. Based on the obtained experimental data, a conclusion is drawn concerning the possible mechanisms of this process. Microhardness measurements revealed softening of the alloys as a result of the deformation, which is due to the decomposition of the supersaturated solid solution.

Pseudopartial Wetting of Grain Boundaries in Severely Deformed Al-Zn Alloys

After severe plastic deformation by the high-pressure torsion, Al-Zn alloys have three various classes of Al/Al grain boundaries (GBs) wetted with a second zinc-rich phase. Completely wetted Al/Al GBs are coated with the layer of a zinc-rich phase more than 30 nm thick. Partially (incompletely) wetted Al/Al GBs contact particles of the zinc-rich phase with a contact angle >60°, but contain no measurable zinc concentration. Pseudopartially wetted Al/Al GBs also contact Zn particles with a contact angle >60°. However, they have a thin interlayer of the zinc-rich phase with a uniform thickness of 2–4 nm, the presence of which explains the unusually high ductility of Al-Zn alloys after high-pressure torsion.

Hardness of nanostructured Al-Zn, Al-Mg and Al-Zn-Mg alloys obtained by high-pressure torsion

Microstructure and hardness of ternary Al–Zn–Mg alloys were studied both in as cast state and after high pressure torsion (HPT) with 5 torsions (shear strain about 6). The size of (Al) grains and of reinforcing second phase precipitates decreases drastically after HPT reaching nanometer range. During HPT, the Zn- and Mg-rich supersaturated (Al) solid solution decomposes and reaches the equilibrium state corresponding to the room temperature. In the as cast state the hardness of the supersaturated solid solutions increases with increasing Zn and Mg content due to the solid-solution hardening. However, after HPT the work hardening and Hall-Petch hardening due to the decreasing grain size competes with softening due to the decomposition of a supersaturated solid solution. In the net effect, the severe plastic deformation results in softening of ternary Al–Zn– Mg alloys.

Corrigendum: Interfacial dominated ferromagnetism in nanograined ZnO: a μSR and DFT study

Diamagnetic oxides can, under certain conditions, become ferromagnetic at room temperature and therefore are promising candidates for future material in spintronic devices. Contrary to early predictions, doping ZnO with uniformly distributed magnetic ions is not essential to obtain ferromagnetic samples. Instead, the nanostructure seems to play the key role, as room temperature ferromagnetism was also found in nanograined, undoped ZnO. However, the origin of room temperature ferromagnetism in primarily non–magnetic oxides like ZnO is still unexplained and a controversial subject within the scientific community. Using low energy muon spin relaxation in combination with SQUID and TEM techniques, we demonstrate that the magnetic volume fraction is strongly related to the sample volume fraction occupied by grain boundaries. With molecular dynamics and density functional theory we find ferromagnetic coupled electron states in ZnO grain boundaries. Our results provide evidence and a microscopic model for room temperature ferromagnetism in oxides.

Effect of copper on dislocation luminescence centers in silicon

The effect of copper on dislocation luminescence centers in silicon has been investigated using photoluminescence and transmission electron microscopy. It has been demonstrated that there exist two main mechanisms responsible for quenching of dislocation luminescence by the copper impurity. The first mechanism is dominant at high copper concentrations and associated with the decrease in the time of nonradiative recombination of nonequilibrium charge carriers due to the formation of copper precipitates in silicon. This leads to the quenching of the entire dislocation luminescence and the edge exciton luminescence. The second mechanism is associated with the interaction of individual copper atoms with deep dislocation centers D1/D2, which results in the passivation of the recombination activity of these centers. This mechanism takes place even at room temperature and is highly effective at low copper concentrations.

Comparative study of the spectral and structural properties of EuAl3(BO3)4 single crystals with different morphologies

Europium alumoborate EuAl3(BO3)4 microcrystals have been synthesized by the flux method at a temperature of 1050°C. The obtained crystals have different morphologies: both plane-faced and skeletal microcrystals have been observed. Infrared spectroscopy, cathodeluminescence, and transmission electron microscopy investigations of individual microcrystals showed that the spectral and structural characteristics of these morphological forms coincide. The obtained crystals are characterized by the rhombohedral symmetry (sp. gr. R32) with the inclusions of C2/c monoclinic phase domains.