P. Thakur

Modifications in structural and electronic properties of TiO2 thin films using swift heavy ion irradiation

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 2u2009p orbitals hybridized with Ti 3u2009d states. The XAS spec...

NEXAFS studies of La0.8Bi0.2Fe1−xMnxO3(0.0 ⩽ x ⩽ 0.4) multiferroic system using x-ray absorption spectroscopy

We report the electronic structure studies of well-characterized La0.8Bi0.2Fe1−xMnxO3 (LBFM, 0.0 ≤ x ≤ 0.4) multiferroic samples investigated by soft x-ray absorption spectroscopy at O K-, Mn L3,2-, Mn K-, Fe L3,2- and La M4,5-edges along with the spectra of reference compounds. The investigations are performed to find out the chemical states and crystal field symmetry of ions present in the LBFM multiferroic samples after the substitution of Mn ions at Fe sites. Studies reveal a slight shift in the peak positions of O K-edge spectra along with growth in intensities of low-energy features. An analysis of the Mn L3,2- and K-edge, in addition to the comparison with the spectra of reference compounds, indicates that the Mn ions are present in mixed valence states. It is found that the Mn ions are mainly in +3 states along with a small contribution of +2 and +4 ones. The study of Fe L3,2-edge confirms the trivalent state of Fe ion. M4,5-edges of La corroborate the +3 state of La in the system. Atomic multiplet calculations are also performed on Fe L3,2-, Mn L3,2- and La M4,5-edges to further verify the valency, symmetry and the crystal field splitting.

Irradiation induced modification in transport properties of LaNiO3 thin films: An x-ray absorption study

Highly c-axis oriented LaNiO3 thin films are grown on LaAlO3 (001) single crystal substrates using rf-magnetron sputtering. Swift heavy ion irradiation induced variations in structural and electrical transport properties of deposited films are studied. Pristine film shows unusual insulating character while the irradiated film exhibits metallic behaviour. X-ray diffraction study indicates that irradiation improves the crystalline character of films while retaining the oriented single phase growth. Electronic structure measurements performed using x-ray absorption spectroscopy at O K, La M5,4, and Ni L3,2-edges reveal that Ni-O hybridization-controlled localization of charge carriers is responsible for the observed transport behaviour.

Swift heavy ion irradiation induced magnetism in magnetically frustrated BiMn2O5 thin films

The swift heavy ion (SHI) irradiation induces weak ferrimagnetism (FM) in magnetically frustrated polycrystalline BiMn{sub 2}O{sub 5} thin films. This is manifested from irradiation induced higher energetic configuration that accounts for evolution of the Mn{sup 2+} state in the Mn{sup 3+}/Mn{sup 4+} network. Basically, this is the root of large magnetic moment in the irradiated samples. X-ray diffraction and Raman-scattering data of the samples indicate considerable modifications in the crystal structure after the SHI irradiation. FM in the irradiated samples and magnetically frustrated behavior of the pristine sample is apparent from dc magnetization measurements. Element specific characterizations such as near-edge x-ray absorption fine structure spectroscopy at O K and Mn L{sub 3,2} edges along with x-ray magnetic circular dichroism at Mn L{sub 3,2} edge show the evolution of the Mn{sup 2+} at disbursement of the Mn{sup 4+}. The microscopic origin behind the induced weak FM is found to be the increased orbital moment in the irradiated thin films.

Magnetic properties of planar arrays of Fe-nanowires grown on oxidized vicinal silicon (111) templates

Planar arrays of Fe nanowires (NW) grown on oxidized self assembled Si templates are shown to be ferromagnetic at room temperature with wire width down to 30 nm as revealed by magnetometery and x-ray magnetic circular dichorism studies. The atomic terrace low angle shadowing (ATLAS) method used to produce these NW arrays allows one to grow planar arrays that are several nanometers thick as opposed to monolayer thickness attained with step flow and step decoration methods. These NW arrays possess much smaller width fluctuations along the wire length owing to the highly periodic nature of the step-bunched templates. Magnetic anisotropy of the NW array is dominated by the shape anisotropy which keeps the magnetization in-plane with easy axis along the length of the wires.

Ion beam synthesis of Ni nanoparticles embedded in quartz

Fabrication and fluence dependent growth of Ni nanoparticles in quartz matrix using a low energy ion implantation followed by thermal annealing are reported. 100keV Ni ions are implanted (at room temperature) in quartz at different ion fluences using a low energy ion beam facility. The samples are postannealed at 600°C and then characterized using atomic force microscopy, magnetic force microscopy, field cooled and zero field cooled magnetization measurements, x-ray absorption spectroscopy, and UV-visible spectroscopy. Nanoclusters of Ni of nearly uniform size are synthesized at an ion fluence of 5×1016ions∕cm2. The size of these nanoclusters could be varied by varying ion fluence. Formation of NiO nanoclusters is observed for ion fluence of 2×1017ions∕cm2.

Structural and Magnetic Characterizations of 200 MeV Ag15+ Irradiated BixCo2-xMnO4 Thin Films

The effect of swift heavy ion (SHI) irradiation on the structural and magnetic properties of thin films of Bi-substituted Co2MnO4 prepared by Pulsed Laser Deposition (PLD) is presented. XRD analysis reveals that the BixCo2-x MnO4 (x = 0.0, 0.1 & 0.3) films grown on amorphous quartz, and crystalline LaAlO3 (LAO) exhibited single phase cubic spinel structure with low strain before and after the irradiation at 200 MeV Ag15+ ions at three fluence values 1 × 1011, 5 × 1011 and 1 × 1012 ions/cm2. DC magnetization hysteresis loop study of the films revealed a ferrimagnetic ordering below the transition temperature ~ 185 K and the saturation magnetization was increased by the irradiation at optimal fluence value 5uf0b41011 ions/cm2. X-ray Magnetic Circular Dichroism (XMCD) studies showed the antiparallel alignments of Co and Mn magnetic moments.