Yogesh Kumar

Ellipsometric investigation of ZnFe2O4 thin films in relation to magnetic properties

We report an influence of disorder on structural and magnetic properties of ZnFe2O4 thin films grown at temperatures ranging from 400u2009°C to 600u2009°C by pulsed laser deposition in O2 atmosphere on SrTiO3 (100) substrates evidenced by properties of electronic transitions observed in the dielectric function. Inversion of the normal spinel structure was found to be one of the main mechanisms responsible for the increase in the magnetic response for the lowest growth temperature. The enhanced feature in the dielectric function located at ∼3.5u2009eV, related to the transition involving tetrahedrally coordinated Fe3+ cations, corresponds to the dominating magnetic coupling between the octahedral and tetrahedral lattice sites, responsible for the overall ferrimagnetic behaviour of the film grown at the lowest temperature.

Detection of defect-induced magnetism in low-dimensional ZnO structures by Magnetophotocurrent

The detection of defect-induced magnetic order in single low-dimensional oxide structures is in general difficult because of the relatively small yield of magnetically ordered regions. In this work, the effect of an external magnetic field on the transient photocurrent measured after light irradiation on different ZnO samples at room temperature is studied. It has been found that a magnetic field produces a change in the relaxation rate of the transient photocurrent only in magnetically ordered ZnO samples. This rate can decrease or increase with field, depending on whether the magnetically ordered region is in the bulk or only at the surface of the ZnO sample. The phenomenon reported here is of importance for the development of magneto-optical low-dimensional oxides devices and provides a new guideline for the detection of magnetic order in low-dimensional magnetic semiconductors.

Conductivity fluctuations in proton-implanted ZnO microwires

Electric noise can be an important limitation for applications of conducting elements in the nanometer size range. The intrinsic electrical noise of prospective materials for opto-spintronics applications like ZnO has not yet been characterized. In this study, we have investigated the conductivity fluctuations in 10 nm thick current paths produced by proton implantation of ZnO microwires at room temperature. The voltage noise under a constant dc current bias in undoped, as well as in Li-doped microwires, is characterized by [Formula: see text] power spectra with [Formula: see text]. The noise intensity scales with the square of the bias current pointing to bias-independent resistivity fluctuations as a source of the observed noise. The normalized power spectral density appears inversely proportional to the number of carriers in the probed sample volume, in agreement with the phenomenological Hooge law. For the proton-implanted ZnO microwire and at 1 Hz we obtain a normalized power spectral density as low as [Formula: see text] Hz(-1).

Disappearance of metal-insulator transition in NdNiO3/LaAlO3 films by ion irradiation controlled stress

The effect of strain on the metal-insulator transition (MIT) of the epitaxial NdNiO3 film on the c-axis oriented LaAlO3 single crystal, grown by pulsed laser deposition, has been investigated. Swift heavy ion irradiation was used to vary the strain state of the deposited film. X-ray diffraction confirmed a systematic fluence dependent rise in the in-plane compressive strain, while maintaining the epitaxy of the film. This in-plane compressive strain has been found to reduce the MIT temperature, which finally disappeared for the highest fluence of the irradiation. This is also corroborated with the Raman spectroscopy measurements which suggest that the ion irradiation induced stress is responsible for the suppression of the metal-insulator transition.

Photo-enhanced magnetization in Fe-doped ZnO nanowires

An emerging branch of electronics, the optospintronics, would be highly boosted if the control of magnetic order by light is implemented in magnetic semiconductors nanostructures being compatible with the actual technology. Here, we show that the ferromagnetic magnetization of low Fe-doped ZnO nanowires prepared by carbothermal process is enhanced under illumination up to temperatures slightly below room temperature. This enhancement is related to the existence of an oxygen vacancy VO in the neighborhood of an antiferromagnetic superexchange Fe3+-Fe3+ pair. Under illumination, the VO is ionized to VO+ giving an electron to a close Fe3+ ion from the antiferromagnetic pair. This light excited electron transition allows the transition of Fe3+ to Fe2+ forming stable ferromagnetic double exchange pairs, increasing the total magnetization. The results presented here indicate an efficient way to influence the magnetic properties of ZnO based nanostructures by light illumination at high temperatures.

Modification in transport properties of NdNiO3 thin films by substrate induced strain

NdNiO3 thin films have been grown on c- axis oriented LaAlO3 single substrates using pulsed laser deposition technique. Raman spectroscopy data confirms the formation of NdNiO3 phase. Along with φ-scans, XRD θ-2θ measurement reveals the highly c-axis oriented epitaxial growth of NdNiO3 films on the LaAlO3 substrates. Electrical resistivity measurement reveals stabilization of metallic phase at lower temperatures. Increased bandwidth due to straitening of Ni-O-Ni bond angle which is induced by the in-plane compressive strain is responsible for this metallic behavior.

Structural and electrical properties of NdCo1−xNixO3

Effect of Ni doping on the structural and electrical properties of NdCoO3, a compound of great interest for its foreseen applications as a catalyst, sensor, and thermoelectric material, has been investigated. Single phase NdCo1-xNixO3 (0 ≤ x ≤ 0.2) were synthesized using solid state reaction method. To determine space group, lattice constants and atomic positions, Rietveld refinement of powder diffraction data has been performed. Four-probe direct current conductivity measurements were also carried out to investigate transport behavior of compounds. The unit cell parameters found to increase with Ni doping. The drastic improvement of the room-temperature conductivity of NdCo1-xNixO3 (x = 0.1, 0.2) with respect to the pure compound has been observed and explained in terms of decrease in band gap. Behavior in different regions of temperature investigated by fitting Arrhenius and variable range hopping (VRH) models.