arXiv subjects

P. Samarasekara

Publications and source records attributed to P. Samarasekara.

At least 19 recordsLinked to original sources

arXiv2019

Second order perturbed Heisenberg Hamiltonian of Fe3O4 ultra-thin films

Due to the wide range of applications, theoretical models of Fe3O4 films are found to be important. Ultra thin Fe3O4 films with ferrite structure have been theoretically investigated using second order perturbed modified Heisenberg Hamiltonian. Matrices for ultra thin films with two and three spin layers are presented in this manuscript. Total magnetic energy was expressed in terms of spin exchange interaction, magnetic dipole interaction, second order magnetic anisotropy and stress induced magnetic anisotropy. Magnetic properties were observed for films with two spin layers and variant second order magnetic anisotropy. For the film with three spin layers, second order anisotropy constant was fixed to avoid tedious derivations. Magnetic easy axis rotates toward the in plane direction as the number of spin layers is increased from two to three because the stress induced anisotropy energy dominates at higher number of spin layers. According to some other experimental data, the magnetic easy axis of thin films rotates toward the in plane direction as the thickness is increased. For ferrite film with two spin layers, magnetic easy and hard directions can be observed at 0.75 and 1.2 radians, respectively, when the ratio of stress induced anisotropy to the long range dipole interaction strength is 3.9. For ferrite film with three spin layers, magnetic easy and hard directions can be observed at 2.4 and 2.3 radians, respectively, when the ratio of stress induced anisotropy to the long range dipole interaction strength is 4.2.

cond-mat.mtrl-sci↗

arXiv2018

Effect of PEG binder on gas sensitivity of ferric oxide films synthesized by spin coating and doctor blade methods

Ferric oxide (Fe2O3) thin films were fabricated on amorphous nonconductive and FTO conductive glass substrates starting from iron acetate nanoparticles by the doctor blade and the spin coating techniques. Thin films of Fe2O3 with and without the PEG binder were investigated. Films were subsequently annealed at 500 0C in air for 1 hour to crystallize the phase Fe2O3. The structural and chemical properties of Fe2O3 thin films were determined using XRD and FTIR. According to XRD, the single phase of Fe2O3 crystallized in the thin film after annealing. The optical band gap of synthesized Fe2O3 thin films were determined by UV-Visible spectrums. Gas sensitivity, respond time and recovery time of Fe2O3 thin films were measured in 1000ppm of CO2 gas using AUTO LAB. Thin films synthesized by the doctor blade method were thicker and more uniform than the thin films prepared by the spin coating technique. Gas sensitivity of Fe2O3 films synthesized by the doctor blade method is slightly higher than that of the films fabricated by the spin coating technique. Fe2O3 films with the PEG binder posses a higher gas sensitivity, lower respond time and lower recovery time compared to films without PEG binder. After adding the PEG binder, the optical band gap of Fe2O3 thin films reduces. The reduction of the activation energy is the reason for the improvement of the gas sensitivity after adding the PEG. However, the peak positions of FTIR do not indicate any significant change due to the adding of the PEG. Adding the binder increased the gas sensitivity of iron oxide by 15.44%. Both FTIR and UV-Visible data confirm the formation of Fe2O3 in the thin film.

cond-mat.mtrl-sci↗

arXiv2018

Effect of annealing temperature on photovoltaic properties of spin coated CdS films

CdS thin film samples were fabricated using spin coating technique on normal glass and conductive glass substrates and annealed at different temperatures. According to the XRD patterns of films prepared on normal glass substrates, polycrystalline films are consist of single phase of CdS. Films synthesized on conductive glass substrates were employed for photovoltaic measurements. Variation of photovoltaic properties with annealing temperature was investigated. The liquid junction photocell was measured in the electrolyte of Na2S2O3/I2. Then, the short circuit photo current and open circuit photo voltage were measured at room temperature under illumination and under dark using a solar cell simulation system. Optical band gap of CdS film samples was determined by measuring UV-visible spectroscopy.

cond-mat.mtrl-sci↗

arXiv2018

Optical properties of spin coated and sol-gel dip coated cupric oxide thin films

Spin coating technique was employed to fabricate CuO films at different spin speeds for different time duration, and they were annealed at different temperatures for different time durations in air. These thin films were characterized using UV Visible spectrometer and solar cell simulator. As the spinning rate increases, the corresponding band gap energy of the thin film increases. However, the annealing temperature does not affect the optical band gap. Moreover, the optical band gap does not depend on the annealing time and spin time. It was observed that the optical band gap decreases with the number of CuO layers. The optical band gap values of CuO thin films were between 1.843 and 1.869 eV. Inclusion of the two additives enhances the photocurrent, photovoltage and efficiency. Photocurrent density can be enhanced from 0.1464 to 1.172 mA per cm2 using the additive ethylene glycol. Properties of CuO films synthesized using sol gel dip coating technique were compared with those of CuO films grown using spin coating method.

physics.app-ph↗

arXiv2018

Characterization of spin coated Zn doped cupric oxide thin films

Thin films of Zn2+ doped cupric oxide (CuO) were synthesized using spin coating technique starting from a solution with Cu and Zn. The speed of spin coating and time duration were varied to fabricate a film with required uniform thickness. Samples were subsequently annealed in air at different annealing temperatures to crystallize the phase of CuO. Doping concentration of Zn2+ was varied up to weight percentage of 10%. Structural properties of samples were determined using X-ray diffraction technique. Particle size, dislocation density and strain at different doping concentrations were determined using XRD patterns. Optical properties were measured by means of UV/Vis spectrometer. Optical band gap of CuO could be tailored by doping a trace amount of Zn2+. The optical band gap decreases with increase of particle size. The particle size, dislocation density, strain and optical band gap have a turning point close to the doping concentration of 6%.

cond-mat.mtrl-sci↗

arXiv2017

Third order perturbed modified Heisenberg Hamiltonian of fcc structured ferromagnetic films with seventy spin layers

Magnetic properties of fcc structured ferromagnetic films with the number of spin layers up to seventy was described using third order perturbed Heisenberg Hamiltonian. The variation of magnetic easy direction, magnetic energies in easy and hard directions, magnetic anisotropy energy and the angle between easy and hard directions was investigated by varying the number of spin layers. Spin exchange interaction, magnetic dipole interaction, second and fourth order magnetic anisotropies, in and out of plane applied magnetic fields, demagnetization factor and stress induced anisotropy were considered in the model. Because magnetic dipole interaction and demagnetization factor represent microscopic and macroscopic properties of the sample, respectively, both these terms were incorporated in our theoretical model. Although our model is a semi-classical model, some discrete variations of angle of easy axis were observed. Our theoretical data qualitatively agree with experimental data of Fe and Ni ferromagnetic films.

cond-mat.mtrl-sci↗

arXiv2017

Optical properties of spin coated Cu doped ZnO nanocomposite films

Spin coating technique was used to synthesize pure ZnO and Cu doped ZnO films on amorphous and conducting glass substrates. The doped amount of Cu in ZnO was varied up to 5% in atomic percentage. Speed of spin coating system, coating time, initial chemical solution and annealing conditions were varied to optimize the properties of samples. Transmittance of samples was measured for ZnO doped with 1, 2, 3, 4 and 5% of Cu. Absorbance, reflectance and refractive index were derived from the measured transmittance. Film thickness of each film was calculated using the graphs of refractive index versus wavelength. Film thickness varies in a random manner depending on the amount of ZnO or Cu doped ZnO solutions spread on the substrate. The energy gap of each film was calculated using the graph of square of absorption coefficient time photon energy versus photon energy. The calculated energy gap values of Cu doped ZnO film samples decrease with the Cu concentration in ZnO. This means that the conductivity of ZnO can be increased by adding a trace amount of conducting material such as Cu.

physics.app-ph↗

arXiv2017

Third order perturbed Heisenberg Hamiltonian of sc ferromagnetic films with fifty spin layers

Magnetic energy of simple cubic structured ferromagnetic films with 10 to 50 spin layers was determined using third order perturbed Heisenberg Hamiltonian. By plotting 3-D plot of energy versus angle and stress induced anisotropy, the values of stress induced anisotropy corresponding to energy minimums and maximums were determined. By plotting the graphs of energy versus angle at these different stress induced anisotropy values, the easy and hard directions were determined. Magnetic easy and hard directions are related to energy minimums and maximums of the curve. Similar graphs were plotted for spin exchange interaction to determine the easy and hard directions. Graphs of energy versus angle were plotted by keeping all the magnetic energy parameters at constant values for each number of spin layers to determine the variation of magnetic easy directions, hard direction and corresponding energies with the number of spin layers. The magnetic easy axis gradually rotates from out of plane to in plane direction as the number of spin layers is increased. In addition, the magnetic anisotropy energy increases with the number of spin layers.

cond-mat.mtrl-sci↗

arXiv2017

Photocurrent enhancement of spin coated CdS thin films by adding Cu

Cu added CdS films were synthesized using spin coating technique at different spin speeds for different time durations. Films were subsequently annealed at different temperatures for different time periods in air to crystallize the phase of CdS in thin film form. Films were characterized using XRD, UV- visible spectrometer and solar simulator. According to XRD patterns, addition of trace amount of Cu did not change the structure of CdS. However, the optical band gap gradually decreases with percentage of Cu as expected. As a result, the photocurrent, fill factor and efficiency measured in KI/I2 electrolyte gradually increase with the amount of Cu. Photovoltaic properties could be improved without altering the structure of CdS. Efficiency enhanced CdS films find potential applications in solar cell industry.

cond-mat.mtrl-sci↗

arXiv2017

Magnetic Dipole Interaction and Total Magnetic Energy of Lithium Ferrire Thin Films

The total magnetic energy of Lithium ferrite thin films was determined using the classical Heisenberg Hamiltonian. The short range magnetic dipole interactions between spins within one unit cell and the interactions between spins in two adjacent unit cells have been determined in order to find the total magnetic energy of lithium ferrite films. Only the spin pairs with separation less than cell constant have been taken into account to calculate dipole interaction and spin exchange interaction. Theoretically several easy and hard directions of lithium ferrite film were found for one set of energy parameters included in our modified Heisenberg Hamiltonian. The dependence of total magnetic energy of a lithium ferrite film on number of unit cells, spin exchange interaction, dipole interaction, second order magnetic anisotropy, fourth order magnetic anisotropy, internal applied magnetic field and stress induced magnetic anisotropy has been explained in this manuscript.

cond-mat.other↗

arXiv2017

Impedance and electrical properties of Cu doped ZnO thin films

Cu doped transparent ZnO thin films were spin coated on conductive glass substrates. The samples were subsequently annealed in air for 1 hour at 500 0C in order to form the phase of ZnO. ZnO samples were doped with different Cu molar percentages up to 5%. The impedance and photovoltaic properties of sample were measured. Photocurrent and photovoltage of doped and undoped samples were measured in KI/I2 electrolyte. Adding a trace amount of Cu improved the conducting properties of ZnO samples without changing other basic properties of ZnO. The photocurrent gradually increases with the doping concentration due to the high conducting properties of Cu. Investigation was carried out only up to the doping concentration of 5%, because higher doping concentrations may significantly influence the other properties of ZnO such as transparence of the film. Impedance of samples was determined by fitting the data to an equivalent circuit. The impedance reaches the maximum value at Cu concentrations of 3%.

cond-mat.mtrl-sci↗

arXiv2017

Ferromagnetic films with three to twenty spin layers as described using second order perturbed Heisenberg Hamiltonian

Modified second order perturbed Heisenberg Hamiltonian was employed to describe the magnetic properties of ferromagnetic films with three to twenty spin layers for the first time. Previously, the solution of second order perturbed Heisenberg Hamiltonian was found for ferromagnetic films with two to five layers under some assumptions by us. In this report, the exact solution is presented without any assumptions by calculating the pseudo inverse of matrix C. All eight magnetic parameters such as spin exchange interaction, second order magnetic anisotropy, fourth order magnetic anisotropy, stress induced magnetic anisotropy, demagnetization factor, in plane magnetic field, out of plane magnetic field and magnetic dipole interaction were taken into account. The easy and hard directions were found using 3-D and 2-D plots of total magnetic energy versus these magnetic parameters. Angle between easy and hard directions was nearly 107 degrees in many cases. The magnetic easy axis gradually rotates from out of plane to in plane direction by indicating a preferred in plane orientation of magnetic easy axis for films with higher number of spin layers. These theoretical results agree with some experimental research data of ferromagnetic thin films.

cond-mat.str-el↗

arXiv2016

Magnetic anisotropy dependence of the energy of oriented thick ferromagnetic films

Heisenberg Hamiltonian was employed to describe the variation of energy of thick ferromagnetic films with second and fourth order anisotropies. At angle of 1.36 degrees and anisotropies of 1.25, energy is minimum for thick film of sc(001) with 1000 layers. Energy becomes minimum at angle of 1.18 degrees and fourth order anisotropy of 1.15 for thick film of bcc(001) with the same thickness. According to these simulations, these lattices can be easily oriented in some certain directions under the influence of some particular values of anisotropies. Energy varies with second and fourth order anisotropies in similar passion for both types of lattices. The energy gradually decreases with second and fourth order anisotropy for both types of lattices in the range described here.

cond-mat.mtrl-sci↗

arXiv2016

Thick ferromagnetic films and their anisotropies as described by second order perturbed Heisenberg Hamiltonian

Second and fourth order anisotropy dependence of energy of thick simple cubic ferromagnetic films with 10000 layers is explained using Heisenberg Hamiltonian with second order perturbation in this manuscript. The second and fourth order anisotropy constants were assumed to be constants through out the film. When the fourth order anisotropy is given by fourth order anisotropy of 6, the sc(001) ferromagnetic thick films with 10000 layers can be easily oriented in 0.6 radians direction for the energy parameters given this report. Under the influence of the second order anisotropy given by second order anisotropy of 6.3, the easy direction of sc(001) film with 10000 layers is given by 0.66 radians. Although the energy varies periodically in all cases, the maximum energy considerably decreases with fourth order anisotropy constant. According to 3-D plots, energy under influence of second order anisotropy is larger than energy under influence of fourth order anisotropy.

cond-mat.mtrl-sci↗

arXiv2016

Effect of stress induced anisotropy and applied magnetic field on second order perturbed energy of thin ferromagnetic films

All the simulations were carried out for sc(001) film with three (N=3) layers. The effect of applied magnetic field and stress induced anisotropy on energy of ultra-thin ferromagnetic films has been investigated using Heisenberg Hamiltonian with second order perturbation. The energy becomes minimums at certain values of angles, applied magnetic field and stress, by indicating that film can be easily oriented in theses certain directions by applying particular applied magnetic field or stress. For example, when applied magnetic field perpendicular to film plane is 3.6, the film can be easily oriented along 0.97 and 2.65 radians directions. When in plane applied magnetic field is 3, energy minimums can be detected at 1 and 2.6 radians. When applied stress is 2.7, film can be easily oriented in 2.8 direction.

cond-mat.mtrl-sci↗

arXiv2016

Third Order Perturbed Heisenberg Hamiltonian of Thick Spinel Ferrite Films

The third order perturbed Heisenberg Hamiltonian was employed to investigate the spinel thick nickel ferrite films. The variation of energy up to N=10000 was studied. At N=75, the energy required to rotate from easy to hard direction is very small. For film with N=10000, the first major maximum and minimum can be observed at 202 and 317 degrees, respectively. This curve shows some abrupt changes after introducing third order perturbation. The maximum energy of this curve is higher than that of spinel thick ferrite films with second order perturbed Heisenberg Hamiltonian. At some values of stress induced anisotropy, the maximum energy is less than that of spinel thick ferrite films with second order perturbed Heisenberg Hamiltonian derived by us previously.

cond-mat.mtrl-sci↗

arXiv2016

Easy Axis orientation of Ferromagnetic Films as Described by Third Order Perturbed Heisenberg Hamiltonian

The magnetic easy axis orientation was studied by the third order perturbed Heisenberg Hamiltonian. Ferromagnetic CoPt/AlN multilayer thin films with number of layers N=11, 16 and 21 synthesized on fused quartz substrates using dc magnetron sputtering technique have been employed as experimental data. According to experimental research performed by some other researchers, easy axis of these fcc structured ferromagnetic films is oriented in the plane of the film at one particular temperature. Average value of out of plane spin component was plotted against temperature in order to determine the spin reorientation temperature. The spin reorientation temperature was highly sensitive to 2nd order magnetic anisotropy constant.

cond-mat.mtrl-sci↗

arXiv2016

Spin Coated Multilayered Cupric Oxide Thin Films and their structural properties

Sol-gel spin coating technique was employed to synthesize CuO thin films at 1500 rpm, 2200 rpm and 2400 rpm for 30 seconds, and subsequently annealed at 150-550°C for 1 hour. Multi-layered CuO thin films with 3 and 5 layers were prepared. These thin films were characterized using XRD, FTIR and polarizing light microscopy. The XRD and FTIR analysis confirmed the crystallization of CuO phase in thin films annealed above 350 °C, and corresponding crystallite sizes were observed to be increasing with the annealing temperature. The Scherrer formula was applied for (111) and peaks of XRD patterns of CuO thin films. According to the data of peak, average crystallite size increased from 0.801 to 55.2 0A as annealing temperature was increased from 350 to 500 0C.

cond-mat.mtrl-sci↗