M. Wasi Khan

Structural, electrical transport and x-ray absorption spectroscopy studies of LaFe1−xNixO3 (x⩽0.6)

Electronic structures of LaFe1−xNixO3 (x⩽0.6) have been studied by x-ray absorption near edge structure spectra of OK, FeL2,3 and LaM4,5 edges. Upon substitution of Ni at Fe site in LaFeO3, the OK-edge spectra show a feature about 2.0eV lower than that of LaFeO3. This feature is growing as the concentration of Ni is increasing. This is consistent with our resistivity data which show that the resistivity decreases very fast with Ni substitution from GΩcm for LaFeO3 to a few mΩcm for the sample with 60% Ni substitution. The resistivity data have been fitted with a variable-range hopping model and it is found that the gap parameter reduces from 2eV to 2.1meV with the Ni substitution. This gap parameter decreases very systematically with the increase in Ni concentration. The structural analysis of these samples shows that they have single-phase orthorhombic structure with space-group Pnma in the studied range (0⩽x⩽0.6). The study of FeL2,3-edge structures confirm the trivalent state of Fe. The observed featur...

Ferromagnetism in 200-MeV Ag+15-ion-irradiated Co-implanted ZnO thin films

Structural, electrical resistivity, and magnetization properties of 200-MeV Ag+15-ion-irradiated Co-implanted ZnO thin films are presented. The structural studies show the presence of Co clusters whose size is found to increase with increase of Co implantation. The implanted films were irradiated with 200-MeV Ag+15 ions to fluence of 1×1012ions∕cm2. The Co clusters on irradiation dissolve in the ZnO matrix. The electrical resistivity of the irradiated samples is lowered to half. The magnetization hysteresis measurements show ferromagnetic behavior at 300K, and the coercive field increases with the Co implantation. The ferromagnetism at room temperature is confirmed by magnetic force microscopy measurements. The results are explained on the basis of the close interplay between the electrical and the magnetic properties.

Swift heavy ion irradiation-induced modifications in structural, magnetic and electrical transport properties of epitaxial magnetite thin films

The effect of swift heavy ion (SHI) irradiation (190MeVAg) on structural, electrical transport and magnetic properties of epitaxial magnetite (Fe3O4) thin films (thickness ∼70nm) grown on MgO⟨100⟩ oriented substrate have been investigated. The x-ray diffraction shows that at low fluence values up to 5×1011ions∕cm2, the strain in the films is relaxed, whereas, at higher fluence range 1×1012–1×1013ions∕cm2, the epitaxial relationship with the substrate is lost along with a phase transformation from magnetite to more oxidized magnetite phase (i.e., maghemite). The Verwey transition temperature measured by electrical transport is found to increase from 109 to 117K with the low fluence SHI irradiation, which is related to the irradiation induced strain relaxation and structural modifications. At higher fluences the system did not show Verwey transition and the resistance is also increased. The similar results were obtained by magnetization studies. The observed magnetization at 1T field is increased at low flu...

Single phase formation of Co-implanted ZnO thin films by swift heavy ion irradiation: Optical studies

Low temperature photoluminescence and optical absorption studies on 200MeV Ag+15 ion irradiated Co-implanted ZnO thin films were studied. The Co clusters present in as implanted samples were observed to be dissolved using 200MeV Ag+15 ion irradiation with a fluence of 1×1012ions∕cm2. The photoluminescence spectrum of pure ZnO thin film was characterized by the I4 peak due to the neutral donor bound excitons and the broad green emission. The Co-doped ZnO films show three sharp levels and two shoulders corresponding to 3t2g and 2eg levels of crystal field splitted Co d orbitals, respectively. The ultraviolet-visible absorption spectroscopy also shows the systematic variation of band gap after 200MeV Ag+15 ion irradiation.

Exposition of semiconducting and ferromagnetic properties of pulsed-laser-deposited thin films of LaFe1-xNixO3 (x=0.3, 0.4, and 0.5)

We have explored the possibility of ferromagnetic semiconducting property in the epitaxial thin films of LaFe1−xNixO3 (x=0.3, 0.4, and 0.5) grown on (001) oriented LaAlO3 substrate. We observe that substitution of Ni in the series leads to the increase in conductivity of the samples with conduction being controlled by the disorder-induced localization of charge carriers. All these samples show ferromagnetic behavior at room temperature while their magnetization decreases with increase in Ni concentration in the composition. The results have been explained on the basis of the close interplay between the electrical and magnetic properties.

Electrical transport and 1/f noise properties of LaFe1−xNixO3 (x = 0.3, 0.4 and 0.5) thin films

The electrical transport and 1/f conduction noise properties of LaFe1−xNixO3 (x = 0.3, 0.4 and 0.5) thin films have been studied. The temperature dependent resistivity data have been fitted with Motts variable-range hopping model. From the resistivity data we have estimated various parameters, namely, the hopping distance (Rh), the hopping energy (Eh) and the density of states at the Fermi level (N(EF)). The decrease in the hopping energy and the increase in the density of states at room temperature have been observed with the increase in Ni doping for all samples. The Ni doping increases the density of charge carriers and enhances delocalization induced phenomena in the system. The magnitude of the normalized noise SV/V2 increases with the decrease in the Ni concentration over the whole temperature range. The conduction noise is also proportional to the square of the bias current, which confirms that the noise arises from the conduction fluctuations. The Hooge parameter (γ) calculated at room temperature is compared with its value in semiconductors and manganite/oxide materials. The observed features have been explained on the basis of charge-carrier doping in LaFeO3. The noise measurements in conjugation with the electrical properties of the film propose the propitious characteristic of the system.

Influence of antiphase boundary density on the conduction noise properties of epitaxial magnetite thin films

Low frequency conduction noise (1∕f noise) properties of epitaxial magnetite (Fe3O4) thin films having a varying density of antiphase boundaries (APBs) were investigated as a function of temperature and frequency. Temperature dependence of noise exhibits a similar behavior to that of resistivity for all the films. The magnitude of normalized noise (Sv∕V2) decreases with the increasing film thickness, which correlates well with the density of APBs. The quantitative feature of noise, i.e., Hooge parameter has a strong thickness dependence at low temperatures which implies that the APBs play an important role in determining the transport mechanism in epitaxial Fe3O4 films.

Influence of 190 MeV Ag+15 ion irradiation on electrical transport and magnetic properties of LaFe1―xNixO3 (x=0.3 and 0.4) thin films

In the present work we have studied the effect of 190 MeV Ag ion irradiation on the structural, electrical, and magnetic properties of LaFe1−xNixO3 (x=0.3 and 0.4) thin films. The films were grown on LaAlO3 ⟨001⟩ oriented substrates using pulsed laser deposition technique. The pristine and irradiated samples were investigated using x-ray diffraction, temperature dependent resistivity, and magnetic measurements. The x-ray diffraction patterns of the irradiated films show that the lattice of the composition strain relaxed with enhanced c-axis orientation. The temperature dependence of the resistivity indicates that all the films (pristine and irradiated) exhibit semiconducting behavior for entire studied temperature range (80–300 K). The pristine and irradiated samples show the activated variable range hopping behavior throughout the studied temperature range. The magnetization hysteresis measurements show ferromagnetic behavior at 300 K for all samples. The results are explained on the basis of the close i...

Small polaron hopping conduction mechanism in Ni-doped LaFeO3

The electrical transport properties of LaFe1− x Ni x O3 (0.1 ≤ x ≤ 0.6) bulk samples were investigated over a wide temperature range, i.e. 9–300 K. Powder x-ray diffraction patterns at room temperature showed that all samples were formed in a single phase. However, a structural transformation was observed from orthorhombic (Pnma) to rhombohedral crystal symmetry at x > 0.5 in Ni-doped samples, which is supported by the electrical transport analysis. Temperature-dependent resistivity data were fitted using Motts variable-range hopping model for a limited range of temperatures to calculate the hopping distance and the density of states at Fermi level. It was found that all parameters vary systematically with an increase in Ni concentration. Moreover, the resistivity data were also fitted using the small polaron hopping (SPH) model. The non-adiabatic SPH conduction mechanism is followed up to 50% Ni concentration, whereas an adiabatic hopping conduction mechanism is active above it. Such a change in the conduction mechanism is accompanied by subtle electronically induced structural changes involving Fe3+–O–Fe3+ and Fe3+–O–Ni3+ bond angles and bond lengths. Thus, we suggest that the transport properties can be explained according to the additional delocalization of charge carriers induced by Ni doping.

Solubility of Co clusters in Co-implanted ZnO thin films by 200 MeV Ag15+ ions irradiation

We have investigated the structural, electrical resistivity, pink noise (1/f noise) and magnetic properties of 200 MeV Ag15+ ions (fluence ~1 × 1012 ions cm−2) irradiated Co-implanted ZnO thin films. The ZnO films were grown on Al2O3 substrate by the PA-MBE technique and 80 keV Co ion implantation with 1 × 1016 ions cm−2 dose value. The structural studies of an unirradiated film show the presence of Co clusters, which dissolve in the ZnO matrix on swift heavy ion (SHI) irradiation. The temperature-dependent electrical resistivity plots of pristine (unirradiated) and irradiated films demonstrated semiconducting nature. The resistivity data were fitted in the Motts variable range hopping (VRH) model and the activation energies were estimated. The magnitude of normalized noise SV/V2 increases with decrease in temperature and estimated Hooges parameters have higher values as compared to other semiconductors. We observe a clear magnetic hysteresis loop with coercivity ~65 Oe for both the films at room temperature, establishing the ferromagnetic nature. The correlation between the electrical transport and magnetic properties in the present system formulates it to be a potential aspirant for the spintronics-oriented devices.