ABSTRACT The experimental fact that measured elastic and structural properties of superlattices are strongly correlated can be understood on the basis of a simple model based on the packing of hard spheres. The model is consistent with features of many models that have been proposed to explain the supermodulus effect, but contrary to previous explanations, it allows predictions for a given pair of constituents to be made. For an arbitrary pair of elements, it predicts the existence or non-existence of an elastic anomaly, and a rough estimate of its magnitude.

Changes in transverse phonon sound velocity were measured during low temperature irradiation of amorphous Pd80Si20 with 3.5 MeV krypton ions. The sound velocity decreases as a function of the ion fluence and shows a tendency to saturate at large fluences at a relative change of -4.7%. The changes in sound velocity were used to determine the changes in shear elastic constant and in Debye temperature both of which were in reasonable agreement with the value reported in the literature.

The magnetization reversal process of interacting elongated nanoislands is presented here. The magnetization reversal has been investigated by means of magneto-optical Kerr effect magnetometry, analyzing the beams reflected and diffracted by the array, magnetic force microscopy, and micromagnetic simulations. The nanoislands have an aspect ratio of 4.2 and are arranged in chiral square units forming a checkerboard array. Due to this particular arrangement, each island is subjected to a spatially asymmetric dipolar interaction field. We found that for certain directions of the applied field this specific character of the dipolar interaction affects the reversal process profoundly. In these cases the magnetization reversal takes places via the nucleation and displacement of a vortex state in two of the four nanoislands in every square unit, at variance with single domain rotation process generally observed for other directions of the applied field.

In this study, we demonstrate experimentally using longitudinal and diffracted magneto-optical Kerr effect measurements and confirmed by theoretical modeling that localized magnetic field sources can be used to actively induce and finely control the magnetization states and reversal paths of nanomagnets beyond what achievable via lateral confinement and anisotropy engineering, and that such localized field sources can be easily facilitated within the appropriate array structure.

Many metallic superlattices are known to exhibit dramatic anomalous elastic properties as a function of modulation wavelength. All measurements to date measure either a shear modulus or a longitudinal modulus in the superlattice plane. Here we present a method which also probes longitudinal elastic behavior perpendicular to the layers using a nano-hardness tester. In this method, a diamond tip is indented into the material and both the indentation depth and applied force are constantly monitored during loading and unloading. The elastic properties are extracted from the unloading part of the force versus displacement curve. Contrary to the anomaly found in a shear modulus, no anomaly in Young’s modulus was found in the present study on Mo/Ni superlattices.

Doubly resonant Raman electron paramagnetic transitions in ruby (Al(2)O(3):Cr(3+)) have been investigated using dye laser excitation tuned across the range of the Zeeman components of its well-known R(1) emission line. With magnetic field B normal to c, the optic axis of ruby, it is seen that the Raman electron paramagnetic resonance (EPR) lines exhibit significant intensity enhancements due to the simultaneous occurrence of "in resonance" and "out resonance," as visualized in the Kramers-Heisenberg formalism of inelastic light scattering. The specific Raman EPR features observed in the present study, together with their resonances, however, differ markedly from those observed in our previous investigation with B parallel to c. These differences can be traced to level mixing effects within the Zeeman multiplet of the (4)A(2) (ground) state on the one hand, and the negligible value of g perpendicular to for the (E) over bar (excited) state of the R(1) emission line on the other. Furthermore, we note that with B perpendicular to c and under resonant conditions, the experimentally observed Lambda m = +/- 1 pair of Stokes and anti-Stokes Raman lines are not connected by time-reversal symmetry.

The effect of an external magnetic field on the eight antiferromagnetic resonance (AFMR) modes of NiO has been studied experimentally using Brillouin light scattering. The results are reproduced by a model that includes the effects of exchange, dipolar coupling, a small cubic anisotropy, and Zeeman terms. Magnetic fields up to 7 T were applied along several NiO crystalline directions. The agreement between theory and experiment provides additional proof that the model, recently introduced to explain the existence of the AFMR multiplet, is indeed valid. Deviations between simulations and experiments, together with a review of previously published results, indicate that large magnetostrictive effects are present in NiO.

Magnetic vortices in restricted geometries, characterized by a circulating in-plane magnetization and an out-of-plane vortex core, exhibit a rich excitation spectrum, of which the fundamental mode is a non-degenerate translational excitation that corresponds to circular motion of its core at a characteristic frequency. Recently there has been considerable interest in the unique dynamics of these vortices, motivated in part by the potential applications of field- and current-driven core polarization reversals. Here we investigate the dynamics of magnetic vortices confined in lithographically defined, micron-sized Permalloy disks with circular and elliptical symmetry. The resonance frequencies are detected experimentally using a microwave reflection technique where an r.f. current in a coplanar waveguide generates an oscillating magnetic field that is absorbed preferentially at the eigenfrequencies of the magnetic disks patterned on its central strip. The eigenfrequency of a single vortex depends primarily on the magnetostatic energy profile and, consequently, can be tuned by varying the geometry of the disk or through effective confinement by shifting the vortex to an energetically distinct position using a static magnetic field, with frequencies typically in the range of 50 MHz to 1 GHz. In single vortex systems the out-of-plane magnetic core defines the direction that the core circulates around its equilibrium position. For dynamically interacting vortex pairs, the relative polarizations of the two cores leads to four modes with distinct eigenfrequencies and motion patterns, three of which can be excited by a spatially uniform magnetic field. For small amplitude perturbations the energy profile is harmonic in form but for larger perturbations, micromagnetic simulations show that higher order terms are necessary to describe the energy profile, suggesting that nonlinearities in the excitation mode should emerge. Experimentally we find that as the amplitude of the r.f- . driving field is increased, the translational-mode peak first takes on a distorted shape and then splits into two well-defined peaks that differ in frequency by up to 25 % as the field is increased. Although the translational mode frequency increases substantially as a function of an in-plane dc field H applied along the ellipse minor axis, the critical driving field and the magnitude of the splitting change little with H. The thickness and field dependence of this mode-splitting phenomenon are examined via measurements of lithographically patterned micron-sized Permalloy ellipses with thicknesses of 20, 40, and 60 nm. The experimental results compare well with numerical calculations that incorporate a critical velocity parameter, providing new insight into the origin of the observed vortex dynamic mode splitting.

We report the Raman electron paramagnetic resonance (EPR) of ${\text{Cr}}^{3+}$ in ruby $({\text{Al}}_{2}{\text{O}}_{3}:{\text{Cr}}^{3+})$ in the ${^{4}A}_{2}$ (ground) and $\overline{E}$ (excited) states of its well-known ${R}_{1}$ emission line. Using tunable dye laser excitation within the range of the Zeeman components of ${R}_{1}$, we observe highly selective doubly resonant enhancements of the Raman EPR lines. The double resonances confirm the assignments of the Raman EPR lines, and they underscore the simultaneous occurrence of both ``in resonance'' and ``out resonance'' as visualized in the Kramers-Heisenberg quantum-mechanical picture of inelastic light scattering. The $g$ factors of the ${^{4}A}_{2}$ and $\overline{E}$ states are consistent with the observed magnetic field dependence of the Raman EPR shifts. Through the interplay of Raman effect and the sharp Zeeman components of ${R}_{1}$, the results provide clear insights into the underlying microscopic mechanism of these resonant Raman EPR spectra of ruby.

We study the magnetization reversal in elliptical nanodots with the external field applied exactly along the minor (hard) axis. By varying the magnitude of the applied field, several first and second order transitions take place and the system proceeds through magnetic configurations characterized by different symmetry properties. The dynamical matrix method is used to calculate the spin excitations as function of the applied field. This model system allows us to investigate the relationship between the singularities of the magnetization, the presence of soft spin excitations, and the symmetry properties of the static and dynamic magnetization fields. Rules that govern the transitions are formulated.

We demonstrate a magneto-optic technique to measure Brownian relaxation of magnetic nanoparticles suspended in liquids. We used AC susceptibility data as a function of frequency of the applied AC magnetic field to verify that the results agree with those obtained via a conventional inductive detection technique. However, compared with a commercial AC susceptometer using the conventional detection scheme, our magneto-optic setup is able to detect a density of nanoparticles at least three orders smaller. This technique has the potential of being used as a sensor for magnetic nanoparticles such as in local temperature, viscoelasticity, or molecular-binding measurements.

In a combined experimental and numerical study, we investigated the details of the motion and pinning of domain walls in isolated and interacting permalloy triangular rings (side 2 mu m, width 250 nm, and thickness 25 nm). To induce interaction between the rings, they were arranged either in vertical chains with an apex of each triangle in proximity to the edge center of the triangle above it or in horizontal chains where the proximity is between the adjacent corners of the triangles. Using longitudinal and diffraction magneto-optic Kerr effects, magnetic force microscopy, and micromagnetic simulations, we determined the field dependence of the spin structure in the rings. In all cases the remnant state of each ring is an "onion" state characterized by two domain walls-one head to head the other tail to tail-pinned at the apexes. In isolated rings the magnetization reversal occurs between two onion states via the formation of an intermediate vortex state, which arises from the motion and annihilation of the two domain walls. In the case of the horizontal chains the reversal mechanism is unchanged except that the dipolar interaction affects the field range in which the rings are in the vortex state. In the case of vertical chains an additional intermediate state is observed during reversal. The new state involves a domain wall pinned at the center of the edge that is in close proximity to the apex of its neighbor. We show that the domain-wall motion in this last case can be modeled by a triple potential well. Because the new state requires that a domain wall be pinned at the neighboring apex, our observations can be viewed as a very elementary form of magnetic logic.

Using the time evolution of the dynamic magnetization, we study the frequencies of spin excitations in small ferromagnetic bars with particular attention to the uniform mode. The calculation is done using micromagnetics, and the frequency of various modes is found as a function of the cell size used in the calculation. We find that the uniform mode and some standing spin-wave modes are relatively insensitive to changes in the cell size. In contrast, edge modes and corner modes show significant frequency changes as the cell size is changed in the micromagnetics calculation. These results justify the use of micromagnetics with larger cells to estimate the frequency of the uniform mode of micronsized magnetic bars. A comparison of experiment and theory shows that the micromagnetics estimate is significantly better than the usual Kittel formula with effective demagnetizing factors; however appreciable differences remain.

Brillouin scattering was used to study the effect of high-power microwave fields on an array of permalloy particles and the results are compared with simulations. The simulations are of two types: one is based on a model in which each particle is treated as a single spin, the second model relies on generalized micromagnetic codes that include external driving fields and enable magnon–magnon coupling. Experimental results as well as simulations show clear, but sometimes different, evidence of non-linear behavior.

The effect of surface roughness on the properties of the oxide scale formed on Fe–Cr–Al alloys during oxidation in air at high temperatures has been investigated. Large and systematic differences in scale thickness, in the composition of the oxides forming the scale, and in the residual stess levels are found.

We present the results of a study of the magnetization reversal in Permalloy triangular rings using diffracted magneto-optics Kerr effect combined with numerical micromagnetic simulations and in-field magnetic force microscopy. Diffracted loops show a two-step switching process for external fields along or perpendicular to any of the ring edges. The diffracted loops calculated from micromagnetic simulations reproduce the measured ones and show that the switching occurs from one asymmetric onion state to the reversed state, for both directions of the applied field. In both cases a stable intermediate vortex state appears during the switching, accounting for the diffracted loop structures. The stability and the magnetization chirality of the vortex state depend on the direction of the applied field relative to the ring orientation. Magnetization configurations occurring during reversal imaged with magnetic force microscopy operated applying an external field confirm the above reversal process.

A magnetic vortex in a restricted geometry possesses a nondegenerate translational excitation that corresponds to circular motion of its core at a characteristic frequency. For 40-nm thick, micron-sized permalloy elements, we find that the translational-mode microwave absorption peak splits into two peaks that differ in frequency by up to 25% as the driving field is increased. An analysis of micromagnetic equations shows that for large driving fields two stable solutions emerge.