Study of the oxygen vacancy influence on magnetic properties of Fe- and Co-doped SnO2 diluted alloys

Transition-metal (TM)-doped diluted magnetic oxides (DMOs) have attracted attention from both experimental and theoretical points of view due to their potential use in spintronics towards new nanostructured devices and new technologies. In the present work, we study the magnetic properties of Sn_0.96TM_0.04O₂ and Sn_0.96TM_0.04O_1.98(V_O)_0.02, where TM = Fe and Co, focusing in particular in the role played by the presence of O vacancies nearby the TM. The calculated total energy as a function of the total magnetic moment per cell shows a magnetic metastability, corresponding to a ground state, respectively, with 2 and 1 μ_B/cell, for Fe and Co. Two metastable states, with 0 and 4 μ_B/cell were found for Fe, and a single value, 3 μ_B/cell, for Co. The spin-crossover energies (E_S) were calculated. The values are E_S^0/2 = 107 meV and E_S^4/2 = 25 meV for Fe. For Co, E_S^3/1 = 36 meV. By creating O vacancies close to the TM site, we show that the metastablity and E_S change. For iron, a new state appears, and the state with zero magnetic moment disappears. The ground state is 4 μ_B/cell instead of 2 μ_B/cell, and the energy E_S^2/4 is 30 meV. For cobalt, the ground state is then found with 3 μ_B/cell and the metastable state with 1 μ_B/cell. The spin-crossover energy E_S^1/3 is 21 meV. Our results suggest that these materials may be used in devices for spintronic applications that require different magnetization states.     

Tunable structural transition and multiferroic properties of the composite thin films through the structural transition of magnetic layer

The Bi_0.9Er_0.1Fe_0.96Mn_0.02Co_0.02O₃/Co_1-xMn_xFe₂O₄ (BEFMCO/CM_xFO) thin films have been deposited by sol-gel method. Structural distortion is observed in the BEFMCO with the appearance of trigonal-R-3m: H in the CM_xFO. The enhanced multiferroic properties, well electrically writable and ferroelectric switching properties are obtained in BEFMCO/CM_xFO thin films. The investigation indicates that the structural transformation of the CM_xFO influences the structure and multiferroic properties of BEFMCO and the interfacial effects between BEFMCO and CM_xFO layers. This transformation and Mn-doping cause the change of carriers, which solves the problem that the magnetic layer exacerbates the ferroelectric properties. It promotes to form the weak local electric field, which causes the weak interface effect, and brings out the weak resistive switching in the BEFMCO/CM_xFO thin films. Therefore, it is believed that the BEFMCO/CM_xFO films can offer a potentially tunable structural transformation of composite films for practical applications.     

Electrocrystallisation of CoFe alloys under the influence of external homogeneous magnetic fields—Properties of deposited thin films

The influence of homogeneous magnetic fields with flux density up to 1 T superimposed during the deposition of CoFe thin films on their properties has been studied. It has been clearly demonstrated that the superimposition of magnetic fields influences the resulting layer properties significantly. A pronounced impact on the layer morphology has been observed. The layers deposited under the influence of the parallel-to-electrode magnetic field appear denser and more homogenous than those obtained without a magnetic field. On the contrary, the layers deposited in the perpendicular-to-electrode magnetic field appeared more diverse. A scaling analysis revealed a smoothing effect of a parallel- and a roughening effect of a perpendicular-to-electrode magnetic field. No influence of magnetic fields neither on the deposited layers chemical composition nor the structure and texture has been found, whereas the internal stress state of the layer is affected by the superimposition. The effects are discussed with respect to the Lorentz force driven convection, which increases the electrochemical reaction's rates and improves desorption of hydrogen from the electrode surface. The alterations of magnetic properties of the CoFe thin films correlate well with the observed microstructural changes. Moreover, an in-plane magnetic anisotropy is induced by a parallel magnetic field superimposition. This phenomenon origins from a preferential next neighbour atomic pair-ordering in the direction of the magnetic field, e.g. magnetization, during deposition of the ferromagnetic alloy.     

Magnetic anisotropy of (100)- and (110)-oriented epitaxial CrO2 films grown on SnO2- the effects of strain and interface

The crystal structures and magnetic properties of CrO₂ films of different orientations and thicknesses grown on SnO₂ were investigated and compared with those on TiO₂. Results reveal that large anisotropic strains were found in (100) films, but only small strains existed in (110) films. The magnetic anisotropies of CrO₂ (110) films on SnO₂ and TiO₂ were found to be similar. As for CrO₂ (100) films, magnetic anisotropies were quite different-at the same thickness, the magnetic anisotropy of the film on SnO₂ substrate was higher than that on TiO₂. However, the easy axes of the films were found to be always along c axis even at very small thickness, and no easy axis switching was observed. First-principle calculations reveal that magnetic anisotropy energy is not only concerned with anisotropic strain but also with the substrate. The competition between the anisotropic preference of interface and the effect of strains may be one important factor for determining the actual easy axis direction. The combined effects of strain and interface may also be one reason for the magnetic anisotropic difference between CrO₂ (100) films on SnO₂ and TiO₂.     

Controlled synthesis of Mg(OH)2 thin films by chemical solution deposition and their thermal transformation to MgO thin films

The chemical solution deposition of Mg(OH)₂ thin films on glass substrates and their transformation to MgO by annealing in air is presented. The chemical solution deposition consists of a chemical reaction employing an aqueous solution composed of magnesium sulfate, triethanolamine, ammonium hydroxide, and ammonium chloride. The as-deposited films were annealed at different temperatures ranging from 325 to 500?°C to identify the Mg(OH)₂-to-MgO transition temperature, which resulted to be around 375?°C. Annealing the as-deposited Mg(OH)₂ films at 500?°C results in homogeneous MgO thin films. The properties of the Mg(OH)₂ and MgO thin films were analyzed by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, UV–Vis spectroscopy, and by circular transmission line model. Results by X-ray diffraction show that the as-deposited thin films have a brucite structure (Mg(OH)₂), that transforms into the periclase phase (MgO) after annealing at 500?°C. For the as-deposited Mg(OH)₂ thin film, a nanowall surface morphology is found; this morphology is maintained after the annealing to obtain MgO, which occurred with the evident formation of pores on the nanowall surface. The assessed chemical composition from X-ray photoelectron spectroscopy yields Mg_0.36O_0.64 (O/Mg ratio of 1.8) for the as-deposited Mg(OH)₂ film, where the expected stoichiometric composition is Mg_0.33O_0.67 (O/Mg ratio of 2.0); the same assessment yields Mg_0.60O_0.40 (O/Mg ratio of 0.7) for the annealed thin film, which indicates the obtainment of a MgO material with oxygen vacancies, given the deviation from the stoichiometric composition of Mg_0.50O_0.50 (O/Mg ratio of 1.0). These results confirm the deposition of Mg(OH)₂ films and the obtainment of MgO after the heat-treatment. The energy band gap of the films is found to be 4.64 and 5.10?eV for the as-deposited and the film annealed at 500?°C, respectively. The resistivity of both Mg(OH)₂ and MgO thin films lies around 10⁸?Ω·cm.