Showing papers by "Sheldon Schultz published in 1991"
TL;DR: It is concluded that studies of this model scattering system allow the quantitative evaluation and testing of ideas regarding wave propagation and localization in strongly scattering media.
Abstract: We have calculated and measured the properties of X-band microwaves propagating in a 2D array of low-loss high-dielectric-constant cylinders. Transmission bands and photonic band gaps are conclusively identified in excellent agreement with the theoretical predictions. Detailed data on the properties of isolated defect states are also presented. We conclude that studies of this model scattering system allow the quantitative evaluation and testing of ideas regarding wave propagation and localization in strongly scattering media.
239 citations
TL;DR: In this article, the authors measured the electric field energy density for microwave radiation localized in essentially two-dimensional space by scattering from a random array of dielectric cylinders placed between a pair of parallel conducting plates.
Abstract: WAVEFUNCTIONS of electrons or photons in a strongly scattering random medium may become localized owing to the underlying wave nature of the particles1,2. Particularly surprising and counterintuitive is the prediction that, under appropriate conditions, scatterers placed randomly in space will always produce fully localized states—that is, an energy distribution of the normal modes whose envelope decays exponentially in all directions. In consequence, energy at the resonant frequency of a localized mode, injected into that mode's region of space, cannot diffuse away, but remains trapped until dissipated. Here we report measurements of the electric-field energy density for microwave radiation localized in essentially two-dimensional space by scattering from a random array of dielectric cylinders placed between a pair of parallel conducting plates. We detect regions of high energy density representing the signature of localized modes. The available range of measured variables, scattering materials and cylinder configurations offer the opportunity to provide quantitative answers to important general questions about strong localization. In particular, a better understanding of two-dimensional localization raises the possibility of using localized-mode resonances as a diagnostic tool for situations in which localization phenomenon may occur naturally3—for example, in investigations of the internal distribution of media and defects in geological strata, under-ocean topology or electronic thin films, all of which may exhibit pseudo-two-dimensional characteristics.
210 citations
TL;DR: In this paper, the effects of particle size and aspect ratio on the hysteresis in controlled arrays of small magnetic particles were investigated using an alternating gradient magnetometer for particles ∼5−0.1 μm.
Abstract: We have investigated the effects of particle size and aspect ratio on the hysteresis in controlled arrays of small magnetic particles. The arrays of permalloy particles were fabricated via electron‐beam lithography. Each array consists of ∼ 106 identical, uniformly spaced particles. Hysteresis loops measured with an alternating‐gradient magnetometer for particles ∼5–0.1 μm are presented. We find an increase in the coercive force as the particle width decreases below 0.3 μm due to a change in the switching mechanism from domain‐wall nucleation and wall motion to vortex nucleation and vortex motion. A novel angular dependence of the loops is described in detail. Results from ab initio micromagnetic calculations on isolated rectangular Permalloy particles are compared, where applicable, with the measurements. We find excellent qualitative and, in selected cases, quantitative agreement between the experiments and calculations.
191 citations
TL;DR: Magnetization distributions in a rectangular permalloy particle with an 8:1 aspect ratio are presented in this article, where a magnetic field is applied along the easy axis of the particle, there is considerable change in the vortex pattern as the applied field varies, but the moment is nearly constant.
Abstract: Magnetization distributions in a rectangular permalloy particle with an 8:1 aspect ratio are presented. When a magnetic field is applied along the easy axis of the particle, there is considerable change in the vortex pattern as the applied field varies, but the moment is nearly constant. Magnetization reversal is sudden. When a field is applied oblique to the easy axis the magnetization is rather uniform in most of the particle. When an in‐plane field is applied exactly along the hard axis, a complex domain structure is formed, so that the remanence is zero, in agreement with experiment. Formation of this state requires that the applied field exceed a critical value. Upon application of an easy axis field to this complex state, the domain structure is shifted so that the magnetization increases linearly with the applied field.
56 citations
TL;DR: In this article, a technique has been developed for measuring the switching fields of individual submicron magnetic particles using a magnetic force microscope (MFM) in which an in situ magnetic field can be applied.
Abstract: A technique has been developed for measuring the switching fields of individual submicron magnetic particles using a magnetic force microscope (MFM) in which an in situ magnetic field can be applied. This allows the study of the evolution of the particles' magnetic states as a function of applied field and the direct observation of cooperative switching. Observations of the switching fields for individual nanolithography Permalloy particles are compared with remanent magnetization data, taken with an alternating gradient magnetometer on both isolated and interactive arrays of these particles. Comparison of MFM images of the particles with numerical simulations has provided insight into the magnetic behavior of the sensing tips. >
51 citations
TL;DR: In this paper, the Foucault mode of Lorentz microscopy has been applied to detect the polarity of the magnetic field produced by ferromagnetic particles in the single-domain size range.
Abstract: The Foucault mode of Lorentz microscopy has been applied to detect the polarity of the magnetic field produced by ferromagnetic particles in the single-domain size range. One can follow the polarity of any given particle through multiple cycles of removal and return of the sample from the transmission electron microscope. Hence, one can determine the particle coercivity by applying a sequence of known fields to the sample. The proposed technique makes it possible to obtain unambiguous data for the dependence of an individual particle's coercivity on the orientation of the applied field, and the particle's dimensions. Both these data are needed to determine the magnetization reversal mechanism in single-domain particles. The authors demonstrate the ability to detect the polarity and measure the coercivity of nearly ellipsoidal gamma -Fe/sub 2/O/sub 3/ particles which have a moment of approximately 10/sup -13/ emu. >
24 citations
TL;DR: A strong dependence of the Gd EPR spectrum and the microwave absorption on the magnetic history of the samples has been observed, suggesting the formation of magnetic domains or a glassy state.
Abstract: We have measured the EPR spectrum of dilute ${\mathrm{Gd}}^{3+}$ ions substituting for ${\mathrm{Eu}}^{3+}$ in ${\mathrm{Eu}}_{2}$${\mathrm{CuO}}_{4}$. Each one of the resonance lines shows a splitting into two or more weaker lines below ${\mathit{T}}_{\mathit{N}}$\ensuremath{\approxeq}215 K. The simultaneous appearance of a low-field microwave-absorption signal is attributed to the onset of long-range antiferromagnetic ordering with a weak ferromagnetic component. The splitting of the EPR spectrum is associated with different Gd sites in the magnetically ordered structure. A reduction of the local symmetry of the rare-earth sites below ${\mathit{T}}_{\mathit{N}}$ is described in terms of an internal magnetic field lying in the ab plane and additional crystal-field terms related to local crystallographic distortions. Dipolar and exchange contributions to the internal field are discussed. Possible random static displacements of the oxygen atoms in the ${\mathrm{CuO}}_{2}$ planes and their role in the development of the weak ferromagnetism and the presence of internal fields at the Gd sites are also analyzed. A strong dependence of the Gd EPR spectrum and the microwave absorption on the magnetic history of the samples has been observed, suggesting the formation of magnetic domains or a glassy state.
14 citations
TL;DR: In this article, results of some numerical simulations are presented that model the behavior of the magnetic force microscope under various probe/sample configurations, assuming uniform magnetization of an axially symmetric probe of various types.
Abstract: Results of some numerical simulations are presented that model the behavior of the magnetic force microscope under various probe/sample configurations. Analysis is carried out under the assumption of uniform magnetization of an axially symmetric probe of various types including: sphere, cylinder, conical tip, and rounded magnetically coated conical tip. In these cases the analysis can be made by studying the effective magnetic surface charge on the probe. We also consider the case of a spherical probe with magnetization that can rotate in response to the local field. The sample configurations studied are also of the axially symmetric type, which can be used to model the fields generated by arrays of cylindrical samples, e.g., alumite.
11 citations
TL;DR: The low-temperature ESR spectra of Gd3+ and Er3+ in Pr2CuO4 show symmetry properties appropriate to the crystal tetragonal symmetry, and the absence of any splittings of the ESR lines below the Neel temperature implies that the magnetostatic dipole field at the rare-earth-ion site due to the antiferromagnetically ordered Cu moments is <45 Oe.
Abstract: The low-temperature (T<300 K) electron-spin-resonance (ESR) spectra of Gd3+ and Er3+ in Pr2CuO4 show symmetry properties appropriate to the crystal tetragonal symmetry. The completely resolved Gd3+ spectra allowed us to measure, at T=2 K, the principal g values g?=1.985(8), g=2.040(8), and the crystal-field parameters [b20=-399(2)×10-4 cm-1, b40=-33.1(7)×10-4 cm-1, and b44=205(3)×10-4 cm-1]. The large broadening of the ESR lines, observed above T40 K, is due to a relaxation via the thermally populated crystal-field excited Pr levels. For Er3+ in Pr2CuO4 we observe a single ESR line corresponding to a ground-state doublet with g?=17.94(5) and g0.2. The absence of any splittings of the ESR lines below the Neel temperature implies that the magnetostatic dipole field at the rare-earth-ion site due to the antiferromagnetically ordered Cu moments is <45 Oe. © 1991 The American Physical Society.
3 citations
TL;DR: In this article, the authors reported the observation of an unusual electron spin resonance (ESR) signal in single crystals of Eu2CuO4, which was associated with a resonance mode of the CuO2 planes, similar to the midfield and low-field absorptions they have reported previously.
Abstract: We report the observation of an unusual electron spin resonance (ESR) signal in single crystals of Eu2CuO4. The signal appears to be associated with a resonance mode of the CuO2 planes, similar to the midfield and low‐field absorptions we have reported previously [Phys. Rev. B 41, 1934 (1990)]. However, it is only observed when the projection of the applied dc magnetic field in the CuO2 plane is within a few degrees of the 〈110〉 crystallographic direction. Additionally the sample must be field cooled in the CuO2 plane, but with a component of the cooling field perpendicular to the 〈110〉 ESR observation direction. Both the field for resonance and the linewidth exhibit a 1/cos θ dependence, where θ is the angle of the applied dc field between the c axis and the 〈110〉 observation direction. Additional constraints for observation of the resonance are that the microwave rf magnetic field must have a component in the CuO2 plane, but perpendicular to the dc field. The signal disappears above ∼215 K, which we ass...
2 citations
TL;DR: In this paper, an apparatus for making in situ measurements of the magnetic moment of submonolayer through multimonolayer samples deposited in a UHV chamber is described, which is designed to be used with a commercial SQUID magnetometer with a sensitivity of ∼10−8 emu.
Abstract: An apparatus for making in situ measurements of the magnetic moment of submonolayer through multimonolayer samples deposited in a UHV chamber is described. The UHV chamber is designed to be used with a commercial SQUID magnetometer with a sensitivity of ∼10−8 emu. The component of magnetic moment parallel to the sample surface may be measured over the temperature range 4–400 K and for magnetic fields (applied parallel to the surface) up to 5 T. A thin re‐entrant metal tape is used both as an initial substrate for the deposition of any desired films, and as the means of transporting the deposited sample from the evaporation and evaluation regions of the vacuum chamber to the SQUID analysis region. Some results for Fe evaporated on polycrystalline Ag films are presented.