Experimental evidence for ferromagnetism at room temperature in MgO thin films

Superior removal of dyes by mesoporous MgO/g-C3N4 fabricated through ultrasound method: Adsorption mechanism and process modeling

The present research concerns the synthesis of a mesoporous composite characterized with high surface area and superior adsorption capacity in order to investigate its efficacity in removing hazardous and harmful dyes molecules from water. The synthesized mesoporous composite, MgO/g-C₃N₄ (MGCN), was successfully prepared through the sonication method in a methanolic solution followed by an evaporation and a calcination process. The configuration, crystalline phase, surface properties, chemical bonding, and morphological study of the fabricated nanomaterials were investigated via XRD, BET, FESEM, HRTEM, XPS, and FTIR instrumentation. The obtained nanomaterials were used as sorbents of Congo Red (CR) and Basic Fuchsin (BF) dyes from aqueous solutions. Batch elimination experimental studies reveal that the elimination of CR and BF dyes from an aqueous solution onto the MGCN surface was pH-dependent. The highest removal of CR and BF pollutants occurs, respectively, at pH 5 and 7. The absorptive elimination of CR and BF dyes into the MGCN surface was well-fitted with a pseudo-second-order kinetics and Langmuir model. In this concern, the maximum nanocomposite elimination capacity for CR and BF was observed to be 1250 and 1791 mg g^-1, respectively. This investigation confirms that MGCN composite is an obvious and efficient adsorbent of CR, BF, and other organic dyes from wastewater.

Origin of Ferroelectricity and Multiferroicity in Binary Oxide Thin Films

The observation of ferroelectric, ferromagnetic, and ferroelastic phases in thin films of binary oxides attracts the broad interest of scientists and engineers. However, the theoretical consideration of the physical nature of the observed behavior was performed mainly for HfO₂ thin films from the first principles, and in the framework of Landau-Ginzburg-Devonshire (LGD) phenomenological approach with special attention to the role of oxygen vacancies in both cases. Allowing for the generality of the LGD theory, we applied it to the group of binary oxides in this work. The calculations have been performed based on the assumption that oxygen vacancies, as elastic dipoles, can be partially transformed into electric dipoles due to the defect site-induced and/or surface-induced inversion symmetry breaking (via, e.g., piezoelectric effect), and can "migrate" throughout the depth of an ultrathin film. Since many films of binary oxide are ferroelectric and ferromagnetic due to the oxygen vacancies, they can be multiferroics. Performed calculations have shown that thin films of binary oxides can be considered as new multiferroics with physical properties useful for broad spectra of applications in nanoelectronics and nanotechnology. The properties can be controlled by the choice of oxygen vacancy concentration, film thickness, and special technological treatment, such as annealing.

d° Ferromagnetism of Magnesium Oxide

Magnetism without d-orbital electrons seems to be unrealistic; however, recent observations of magnetism in non-magnetic oxides, such as ZnO, HfO2, and MgO, have opened new avenues in the field of magnetism. Magnetism exhibited by these oxides is known as d° ferromagnetism, as these oxides either have completely filled or unfilled d-/f-orbitals. This magnetism is believed to occur due to polarization induced by p-orbitals. Magnetic polarization in these oxides arises due to vacancies, the excitation of trapped spin in the triplet state. The presence of vacancies at the surface and subsurface also affects the magnetic behavior of these oxides. In the present review, origins of magnetism in magnesium oxide are discussed to obtain understanding of d° ferromagnetism.

Magnetism of Amorphous and Nano-Crystallized Dc-Sputter-Deposited MgO Thin Films

We report a systematic study of room-temperature ferromagnetism (RTFM) in pristine MgO thin films in their amorphous and nano-crystalline states. The as deposited dc-sputtered films of pristine MgO on Si substrates using a metallic Mg target in an O₂ containing working gas atmosphere of (N₂ + O₂) are found to be X-ray amorphous. All these films obtained with oxygen partial pressure (P_O2) ~10% to 80% while maintaining the same total pressure of the working gas are found to be ferromagnetic at room temperature. The room temperature saturation magnetization (M_S) value of 2.68 emu/cm³ obtained for the MgO film deposited in P_O2 of 10% increases to 9.62 emu/cm³ for film deposited at P_O2 of 40%. However, the M_S values decrease steadily for further increase of oxygen partial pressure during deposition. On thermal annealing at temperatures in the range 600 to 800 °C, the films become nanocrystalline and as the crystallite size grows with longer annealing times and higher temperature, M_S decreases. Our study clearly points out that it is possible to tailor the magnetic properties of thin films of MgO. The room temperature ferromagnetism in MgO films is attributed to the presence of Mg cation vacancies.

Ferromagnetism in sphalerite and wurtzite CdS nanostructures

Room-temperature ferromagnetism is observed in undoped sphalerite and wurtzite CdS nanostructures which are synthesized by hydrothermal methods. Scanning electron microscopy and transmission electron microscopy results indicate that the sphalerite CdS samples show a spherical-like shape and the wurtzite CdS ones show a flower-like shape, both of which are aggregated by lots of smaller particles. The impurity of the samples has been ruled out by the results of X-ray diffraction, selected-area electron diffraction, and X-ray photoelectron spectroscopy. Magnetization measurements indicate that all the samples exhibit room-temperature ferromagnetism and the saturation magnetization decreases with the increased crystal sizes, revealing that the observed ferromagnetism is defect-related, which is also confirmed by the post-annealing processes. This finding in CdS should be the focus of future electronic and spintronic devices.

The magnetic proximity effect in a ferrimagnetic Fe3O4 core/ferrimagnetic γ-Mn2O3 shell nanoparticle system

We report the magnetic proximity effect in a ferrimagnetic Fe(3)O(4) core/ferrimagnetic γ-Mn(2)O(3) shell nanoparticle system, in terms of an enhancement of the Curie temperature (T(c)) of the γ-Mn(2)O(3) shell (~66 K) compared to its bulk value (~40 K), and the presence of magnetic ordering in its so-called paramagnetic region (i.e. above 66 K). The ferrimagnetic nature of both core and shell has been found from a neutron diffraction study. The origin of these two features of the magnetic proximity effect has been ascribed to the proximity of the γ-Mn(2)O(3) shell with a high-T(c) Fe(3)O(4) core (~858 K in bulk form) and an interface exchange coupling between core and shell. Interestingly, we did not observe any exchange bias effect, which has been interpreted as a signature of a weak interface exchange coupling between core and shell. The present study brings out the importance of the relative strength of the interface coupling in governing the simultaneous occurrence of the magnetic proximity effect and the exchange bias phenomenon in a single system.