Anup Bandyopadhyay

Modeling of Plastic Deformation Effects in Ferromagnetic Thin Films

To explain the magnetic behavior of plastic deformation of thin magnetic films (Fe and permalloy) on an elastic substrate (nitinol), it is noted that unlike in the bulk, the dislocation density does not increase dramatically because of the dimensional constraint. As a result, the resulting residual stress, even though strain hardening is limited, dominates the observed magnetic behavior. Thus, with the field parallel to the stress axis, the compressive residual stress resulting from plastic deformation causes a decrease in remanence and an increase in coercivity; and with the field perpendicular to the stress axis, the resulting compressive residual stress causes an increase in remanence and a decrease in coercivity. These elements have been inserted into the model previously developed for plastic deformation in the bulk, producing the aforementioned behavior, which has been observed experimentally in the films.

Thickness dependence of the magnetic hysteresis of NiFe-31% films as a function of an applied isotropic in-plane stress

The magnetic hysteresis of dc magnetron-sputtered Ni69Fe31 films that were sandwiched between titanium layers was investigated as a function of an externally applied isotropic in-plane strain. The hysteresis curves were measured with a Kerr magnetometer that monitored the longitudinal Kerr ellipticity as a function of the in-plane magnetic field. The strain was created by bending the samples in two dimensions using a pressure cell. Measurements were performed on films with different thicknesses. The magnetoelastic properties appeared to be much smaller for films with a thickness of 100 nm than for films with a thickness of 288 or 500 nm. This might be due to a change of the domain wall pinning, or a change of the domain wall density as a function of the film thickness. Measurements under compressive isotropic in-plane stress were shown to be possible by flipping the sample in the pressure holder and measuring through the glass substrate. The Faraday effect and stress-induced birefringence in the glass sub...

Linear birefringence control and magnetization in sputter-deposited magnetic garnet films

Birefringence control is essential in the fabrication of on-chip magnetooptic Faraday isolators. We report on film thickness and stress studies of birefringence in sputter-deposited single-layer bismuth-substituted iron garnet films and double-layer iron garnet films with gadolinium gallium garnet (GGG) covers. We examine compressively and tensilely strained films and analyze the photoelastic response of the sputter-deposited garnet films. We show that the net birefringence can be nulled out under planar compressive strain conditions. Bilayer GGG on iron garnet film yields a reduced birefringence. Temperature control during the sputter deposition of the GGG cover is critical and strongly influences the magnetic moment and birefringence level in the waveguide. High-temperature deposition lowers the magnetization in the underlying iron garnet film.

Silicon nanoparticle synthesis by short-period thermal anneals at atmospheric pressure in argon

Silicon nanoparticles have been studied for a wide variety of applications including nanoelectronic, photovoltaic, and optoelectronic devices. In this work, silicon nanoparticles were synthesized by short-period annealing of silicon-on-insulator substrates to temperatures ranging between 600 and 900 °C in argon gas at atmospheric pressure. Two different top silicon layers were deposited by ion-beam sputtering onto oxidized substrates. The thinner 6 nm top layer samples were annealed to temperatures for 30 s periods while thicker 15 nm top layer samples were annealed for 60 s periods. For both sets of samples, nanoparticles were observed to form at all the anneal temperatures through imaging by AFM. One long-period UHV anneal study, with 30-min anneal times, observed nanoparticle formation at temperatures similar to the current work while another similar long-period UHV anneal reported nanoparticle formation only above well-defined formation temperatures that depended upon the starting top layer thickness. In the current work, the average nanoparticle radius was found to increase both with the final anneal temperature and anneal period. For the highest anneal temperatures of the 6 nm top layer samples, a changing surface topography indicated that the thinner Si source layer was becoming depleted and the nanoparticle formation process was nearing completion. No such changes were observed for the thicker 15 nm samples at the same temperatures.Silicon nanoparticles have been studied for a wide variety of applications including nanoelectronic, photovoltaic, and optoelectronic devices. In this work, silicon nanoparticles were synthesized by short-period annealing of silicon-on-insulator substrates to temperatures ranging between 600 and 900 °C in argon gas at atmospheric pressure. Two different top silicon layers were deposited by ion-beam sputtering onto oxidized substrates. The thinner 6 nm top layer samples were annealed to temperatures for 30 s periods while thicker 15 nm top layer samples were annealed for 60 s periods. For both sets of samples, nanoparticles were observed to form at all the anneal temperatures through imaging by AFM. One long-period UHV anneal study, with 30-min anneal times, observed nanoparticle formation at temperatures similar to the current work while another similar long-period UHV anneal reported nanoparticle formation only above well-defined formation temperatures that depended upon the starting top layer thickness....

Functionalization dependence of the electron beam sensitivity for a conformationally immobile calix[6]arene resist

Calixarenes form a group of compounds that have been studied as high resolution negative electron beam resists. In this work, resists based on a conformationally immobile calix[6]arene molecule were synthesized and their sensitivities measured while the number of attached functional groups was systematically varied. This provided a direct test of the effect of the degree of functionalization on the electron beam resist sensitivity. The synthesis produced two basic molecular conformations: a cone conformer and a 1,2,3-alternate conformer. The number of attached allyl groups was deliberately varied from 0 to 8 allyls in steps of 2 allyls per synthesis. The resulting nine different resists were exposed and contrast curves were measured by atomic force microscopy. The least sensitive were the unfunctionalized calix[6]arenes with threshold doses of about 2 mC/cm2. The fully functionalized conformers had threshold doses around 0.1 mC/cm2. The sensitivities of both conformers were found to scale directly with th...