Michael F. Ruane

Mean-field analysis of amorphous rare earth-transition metal alloys for thermomagnetic recording

A mean-field model is developed for amorphous ferromagnetic materials with potential applications in thermomagnetic recording/magneto-optical readout systems. The emphasis is on the reduction of the number of adjustable parameters, so that important variables and their effects on magnetic properties can be investigated. The available experimental data on GdCo-, GdFe-, and TbFe-based alloys is compared with the model predictions and good agreement is obtained in all cases. Expressions for the exchange stiffness coefficient and macroscopic anisotropy energy constant are derived and the latter is compared with available experimental data. The results have been used to study domain wall characteristics of the three material systems.

Domain wall structure in Permalloy films with decreasing thickness at the Bloch to Néel transition

The Bloch to Neel wall transition is investigated in Permalloy films between 160 and 10 nm thickness using direct integration of the Landau–Lifshitz–Gilbert equation in a three-dimensional Cartesian lattice. At 80 nm, the wall is a symmetric Bloch wall characterized by two adjoining vortices with the magnetization at the wall center pointing perpendicular to the plane of the material throughout the thickness. The Bloch to Neel transition takes place between 35 and 30 nm, below which the wall becomes a symmetric Neel wall. For the Bloch walls, our wall energy per unit area calculations match reasonably well the results of A. Hubert’s Ritz method calculations [Magnetic Domains (Springer, New York, 1998), p. 251] and A. E. Labonte’s numerical calculations [J. Appl. Phys. 40, 2450 (1969)]. For the Neel walls, however, our results indicate an approximately 70% higher energy for thicknesses of 30 nm and below, since the Neel wall tails are included. For thicknesses below 160 nm, the anisotropy energy component ...

Magneto-optical measurements of hysteresis loop and anisotropy energy constants on amorphous TbxFe1−x alloys

Measurements of the Kerr magneto‐optic effect, coercivity, and anisotropy energy constants are performed in a magneto‐optical system that combines the Kerr rotation angle and ellipticity to enhance the strength of the signal. The temperature dependence of the polar Kerr effect is compared with the magnetization of the iron subnetwork in the mean‐field approximation and good agreement is obtained in all cases. Coercivity is found to be a strong function of the sweeping frequency and the saturating field and the results are found to be in qualitative agreement with the magnetization reversal model based on the existence of initial nuclei in a saturated film and the subsequent growth of these nuclei under a reverse field. Finally, perpendicular magnetic anisotropy energy is studied by magneto‐optical techniques and the first two coefficients in the expansion of anisotropy energy in powers of the angle of deviation from the easy axis are determined.

Measurement of reflectivities for magnetooptical media

Reflectivities for magnetooptical media are measured as functions of angle of incidence in a series of related experiments. Measurements yield sufficient information to determine absolute magnitude and relative phase for the Fresnel amplitude reflection coefficients, r‖(p) and r‖(s), as well as for r⊥, which describes the polar Kerr effect on incident polarized light. Using dielectric tensor values measured at normal incidence, theoretical curves for magnitude and phase of r‖(p),r‖(s), and r⊥ vs angle of incidence are developed. Experimental measurements are compared to theoretical functions of the reflectivities and found to be in good agreement throughout the range of angle of incidence. The theoretical analysis and related experimental procedures explicitly treat the presence of a transparent overcoating layer.

Thickness dependent wall mobility in thin Permalloy films

Domain wall mobility in Permalloy films has been calculated as a function of thickness at 10, 80, and 160 nm which reflects the structure change of Neel, symmetric Bloch and C-shaped (asymmetric Bloch) domain walls. The mobility has been derived from the dynamics of a single nonperiodic domain wall using direct integration of the Landau–Lifshitz–Gilbert equation in a Cartesian lattice. This investigation allows for a detailed examination of spin precession, wall motion and overall magnetization distortion as the wall is moved in the presence of fields ranging from 0.5 to 5 Oe applied in the easy axis direction. At 10-nm-thick films, the mobility of a Neel wall is 30 m/s Oe. Wall motion takes place without noticeable distortion in the magnetization distribution in the vicinity of the Neel wall. For 80 nm-thick films, the mobility of a symmetric Bloch wall is 5 m/s Oe, or 20% less than the theoretical prediction for the mobility of a 180° domain wall model. At dynamic equilibrium, the symmetric Bloch wall h...

Residential end-use load shape estimation from whole-house metered data

A methodology for estimating end-use load shapes using hourly whole-house metered load data, household demographic survey data, and weather data (temperature) is presented. Although the focus on the residential sector, the techniques developed can also be applied to the industrial and commercial sectors. The load shape for each member of a set of customers is first compressed into a small number of representative parameters. These are then related to customer characteristics, such as appliance holdings, demographic and socioeconomic data, and information on the dwelling. The results obtained show that the overall methodology provides an effective means for end-use load shape modeling and estimation. >

Resonant Cavity Imaging: A Means Toward High-Throughput Label-Free Protein Detection

The resonant cavity imaging biosensor (RCIB) is an optical technique for detecting molecular binding interactions label free at many locations in parallel that employs an optical resonant cavity for high sensitivity. Near-infrared light centered at 1512.5 nm couples resonantly through a Fabry-Perot cavity constructed from dielectric reflectors (Si/SiO2), one of which serves as the binding surface. As the wavelength is swept using a tunable laser, a near-infrared digital camera monitors cavity transmittance at each pixel. A wavelength shift in the local resonant response of the optical cavity indicates binding. Positioning the sensing surface with respect to the standing wave pattern of the electric field within the cavity controls the sensitivity with which the presence of bound molecules is detected. Transmitted intensity at thousands of pixel locations is recorded simultaneously in a 10 s, 5 nm scan. An initial proof-of-principle setup has been constructed. A test sample was fabricated with 25,100-mum wide square features, each with a different density of 1-mum square depressions etched 12 nm into the SiO2 surface. The average depth of each etched region was found with 0.05 nm rms precision. In a second test, avidin, bound selectively to biotin conjugated bovine serum albumin, was detected.

Self-starting mode locking in a Cr:forsterite laser by use of non-epitaxially-grown semiconductor-doped silica films

We demonstrate RF sputtered, non-epitaxially-grown semiconductor nanocrystallite-doped silica films for mode locking a Cr:forsterite laser. We controlled the size and the optical properties of the nanocrystallites by varying the ratio of InAs to SiO(2) during fabrication. Femtosecond pump-probe measurements were performed to characterize the nonlinear optical properties of these films, revealing their lower saturation fluences. Using the InAs-doped silica films as saturable absorbers permitted self-starting Kerr-lens mode locking (KLM), generating pulses of 25-fs duration with 91-nm spectral bandwidth at 1.3 microm . We also describe saturable-absorber mode-locked operation without KLM and investigate its dependence on intracavity dispersion.

Simulation of three-dimensional nonperiodic structures of π-vertical Bloch line (pi-VBL) and 2π-VBL (2pi-VBL) in Permalloy films

Three-dimensional magnetic structures of pi-vertical-Bloch line (VBL) and 2pi-VBL are investigated in 80–320-nm-thick Permalloy films using direct integration of the Landau–Lifshitz–Gilbert equation in an 128×128×80 point Cartesian lattice. A pi-VBL reflects the shapes of its adjacent walls, which change with film thickness. The pi-VBL conducts the flux between walls of opposite chirality by letting the magnetization rotate out of the plane of the walls via a vortex structure. The Neel caps switch chirality via a “converging point,” or cross-tie, flux at one surface and a vortex flux, or a swirl, at the other surface. The pi-VBL energy per unit area is 0.44, 0.20, and 0.30 erg/cm2 in 80, 160, and 320 nm films, respectively. The corresponding pi-VBL widths are 88, 60, and 86 nm. A stable winding 2pi-VBL structure was also computed by combining two pi-VBL structures of appropriate chirality. The Neel caps intersect via the pair-(swirl, converging point) like flux at both surfaces. The width of the 2pi-VBL i...

Structure dependent stray fields from domain walls in permalloy films

In this article, we present magnitudes and distributions of stray magnetic fields from a single domain wall in a permalloy film are calculated for film thickness 10nm to 2.5 /spl mu/m, using direct integration of the Landau-Lifshitz-Gilbert equation in a Cartesian lattice.