We performed temperature dependent x-ray linear dichroism (XLD) experiments on an iron pnictide system, Ba(Fe(1-x)Co(x))2As2 with x=0.00, 0.05, 0.08, and 0.10 to experimentally verify the existence of orbital ordering (OO). Substantial XLD was observed in polarization dependent x-ray absorption spectra of Fe L edges. By exploiting the difference in the temperature dependent behaviors, OO, and structure contributions to XLD could be clearly separated. The observed OO signal indicates different occupation numbers for d(yz) and d(zx) orbitals and supports the existence of ferro-OO. The results are also consistent with the theoretical prediction. Moreover, we find substantial OO signal well above the structural and magnetic transition temperatures, which suggests the existence of strong OO fluctuations up to high temperatures.

We report the room temperature ferromagnetism and a metal–semiconductor transition at 227 K in 200 MeV Ag15+-ion irradiated thin films of Fe-implanted ZnO. The single phase nature of Fe-doped ZnO after ion irradiation is confirmed by x-ray diffraction. Magneto-resistance measurements show spin polarization below 150 K. X-ray absorption spectroscopy and x-ray magnetic circular dichroism studies at room temperature reveal that Fe is oxidized in a mixed valence (Fe2+ and Fe3+) state and the magnetic signal is due to the Fe2+ state. The observations are explained on the basis of the combined effect of carrier doping by Fe3+ and oxygen vacancies.

A study of the electronic structure and magnetic properties of Co doped ZnO thin films synthesized by ion implantation followed by swift heavy ion irradiation is presented using near-edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) measurements. The spectral features of NEXAFS at the Co L(3,2)-edge show entirely different features than that of metallic Co clusters and other Co oxide phases. The atomic multiplet calculations are performed to determine the valence state, symmetry and the crystal field splitting, which show that in the present system Co is in the 2+ state and substituted at the Zn site in tetrahedral symmetry with 10Dq = -0.6 eV. The ferromagnetic character of these materials is confirmed through XMCD spectra. To rule out the possibilities of defect induced magnetism, the results are compared with Ar annealed and Ar-ion implanted pure ZnO thin films. The presented results confirm the substitution of Co at the Zn site in the ZnO matrix, which is responsible for room temperature ferromagnetism.

We report on the structural and electronic properties of swift heavy ion (SHI) irradiated pristine TiO2 thin films, deposited by radio frequency magnetron sputtering on sapphire substrates. The high resolution x-ray diffraction and Raman measurements show a structural phase transition from anatase to admixture of brookite and rutile phases of TiO2 with increasing SHI fluence followed by a significant distortion in the TiO6 octahedra. The modification in the electronic structure stimulated by SHI irradiation has been investigated using x-ray absorption (XAS) experiments at the O K and Ti L3,2 absorption edges. The O K edge spectra clearly indicate the splitting of the pre-edge spectral features having t2g and eg symmetry bands due to structural disorder/distortion induced by irradiation. The intensity of the SHI generated components at the O K edge increases monotonically, which can be correlated to the modification in unoccupancies associated with O 2 p orbitals hybridized with Ti 3 d states. The XAS spectra at the Ti L3,2 edge further authenticate that SHI creates a controlled structural disorder/distortion in the TiO6 octahedra.

Room temperature ferromagnetic behavior in cluster free, Co doped Y2O3 dilute magnetic oxide films

We present a detailed study on the structural, electronic, and magnetic properties of chemically synthesized Sn1−xCoxO2 (x=0.00 to 0.05) nanoparticles. X-ray diffraction and transmission electron microscope measurements were performed to analyze the crystal structure and morphology of Sn1−xCoxO2 nanoparticles. The energy dispersive x-ray analysis measurements were performed to check the possible presence of any impurity elements in the nanocrystals. The near edge x-ray absorption fine structure (NEXAFS) experiments at Sn M5,4-edge and Co L3,2-edge were performed to probe the local environment of Sn and Co ions in the SnO2 matrix. The NEXAFS at Co L3,2-edge, along with multiplet calculations, indicate that the Co is substituted at the Sn site in SnO2 matrix with +2 charge state and do not form metallic clusters and other oxide phases. The ferromagnetic nature of these materials was confirmed by x-ray magnetic circular dichroism and room temperature magnetization hysteresis loop measurements.

Phase-change materials are attracting much attention in the scientific and engineering communities owing to their applications and underlying basic phenomena. Ge 2 Sb 2 Te 5 is reversible-phase-change material (amorphous to crystalline and vice versa) that is used for optical data storage and phase-change random-access memory and has recently been explored for use as a reversible near-infrared (NIR) window [Singh et al., Appl. Phys. Lett. 111, 261102 (2017)]. For a reversible NIR window, large transmission contrast between two phases and low phase-transition temperature are required to reduce the power consumption. In the present work, phase transition in thermally deposited (Ge 2 Sb 2 Te 5) 100−x Ag x (x = 0, 1, 3, 5, and 10) thin films is achieved by vacuum thermal annealing. Transmission sharply decreases with phase transition in the NIR region. Ge 2 Sb 2 Te 5 shows large transmission contrast (more than 50%) in the wavelength range from 1600 to 3200 nm with phase transition from an amorphous to a hexagonal-closepacked structure at 260 • C. In (Ge 2 Sb 2 Te 5) 90 Ag 10 thin films, a similar transmission contrast is achieved at a comparatively lower temperature (160 • C) due to reduction of the rocksalt phase. Distortion of the host lattice with addition of 10% Ag is confirmed from the drastic change in the density of states in the valence band and the shift in core-level (3d) spectra of Ag, Sb, and Te. This distortion enables a hexagonal-closepacked phase in (Ge 2 Sb 2 Te 5) 90 Ag 10 thin films to be obtained at 160 • C. (Ge 2 Sb 2 Te 5) 90 Ag 10 could be a potential candidate for a reversible NIR window as it requires less power to achieve phase transition and high transmission contrast.

Ge2Sb2Te5 (GST) is one of the best phase change materials because of its splendid set of properties, viz., high thermal stability, fast crystallization speed, good endurance, scalability, and reliability. Phase transition [amorphous → face centered cubic (fcc) → hexagonal close packed (hcp)] of GST thin films with annealing was studied using X-ray diffraction. Thin films in amorphous, fcc, and hcp phases are highly, medium, and negligible transparent in the near infra-red region, respectively. The optical transmission in amorphous, fcc, and hcp phases is ∼92%, ∼46%, and ∼2%, respectively, at the wavelength of 2740 nm. At 2740 nm, a high transmission contrast (∼90%) is observed with phase transition from the amorphous to hcp phase. By utilizing large transmission contrast, it is demonstrated that GST can be availed as a potential candidate for reversible near infra-red-window. The sharp change in optical transmission with phase transition can be understood from the change in density of states in the valence band.