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We discuss the apparent paradox that the energy released in a fission bomb explosion is independent of the energy ϵ liberated by each fissioning nucleus. The paradox is resolved by noting that a larger value of ϵ results in a shorter "burn time"; this stops the chain reaction sooner and results in fewer nuclei being fissioned. Editor's Note: Both awe-inspiring and terrifying, a nuclear explosion is a spectacular display of microscopic physics profoundly affecting the human scale. This paper explores the physics of nuclear explosions and addresses the counter-intuitive fact that the total energy released by a fission bomb is independent of the energy released in a single fission. While this was known to the scientists who developed the atomic bomb, much of their work is classified and their knowledge remains inaccessible to the general public. With only a few historical documents to go on, the authors of this article attempt to recreate a calculation by Manhattan Project scientists and clearly show why the total energy yield is independent of the energy released by each fissioning nucleus. This article will be of interest to readers interested in nuclear physics or in the history of the Manhattan Project. [ABSTRACT FROM AUTHOR]

A superconducting Transition-Edge Sensor (TES) with low- T c is essential in high resolution calorimetric detection. With the motivation of developing sensitive calorimeters for applications in cryogenic neutrinoless double beta decay searches, we have been investigating methods to reduce the T c of an Ir film down to 20 mK. Utilizing the proximity effect between a superconductor and a normal metal, we found two room temperature fabrication recipes for making Ir-based low- T c films. In the first approach, an Ir film sandwiched between two Au films, a Au/Ir/Au trilayer, has a tunable T c in the range of 20–100 mK depending on the relative thicknesses. In the second approach, a paramagnetic Pt thin film is used to create the Ir/Pt bilayer with a tunable T c in the same range. We present a detailed study of fabrication and characterization of Ir-based low- T c films and compare the experimental results to the theoretical models. We show that Ir-based films with a predictable and reproducible critical temperature can be consistently fabricated for use in large scale detector applications. [ABSTRACT FROM AUTHOR]

The formation of the vortex-antivortex-vortex (v-av-v) metastable remanent states in elongated magnetic elements have been systematically investigated using micromagnetic modeling. It is demonstrated that the v-av-v magnetization pattern can be effectively stabilized by exciting the single vortex state with an external RF field. Furthermore, we show that a set of different polarity combinations of the vortex cores can be achieved by adjusting the frequency and amplitude of the excitation field. The corresponding dynamic response in time- and frequency-domain has also been presented. Owing to the diversity of the collective modes with different vortex-antivortex combinations, this system may open promising perspectives in the area of spin transfer torque oscillators. [ABSTRACT FROM AUTHOR]

Micropore optics have recently been implemented in a lobster eye geometry as a compact X-ray telescope. Fields generated by rare-earth magnets are used to reduce the flux of energetic electrons incident upon the focal plane detector in such a setup. We present the design and implementation of the electron diverters for X-ray telescopes of two upcoming missions: the microchannel X-ray telescope onboard the space-based multiband astronomical variable objects monitor and the soft X-ray instrument onboard the solar wind magnetosphere ionosphere link explorer. Electron diverters must be configured to conform to stringent limits on their total magnetic dipole moment and be compensated for any net moment arising from manufacturing errors. The two missions have differing designs, which are presented and evaluated in terms of the fractions of electrons reaching the detector, as determined by relativistic calculations of electron trajectories. The differential flux of electrons to the detector is calculated, and the integrated electron background is determined for both designs. [ABSTRACT FROM AUTHOR]

Exchange bias effects have been studied in elliptical dots composed of ferromagnetic Ni80Fe20–antiferromagnetic Ir20Mn80 bilayers. The magnetization reversal mechanisms and magnetic configurations have been investigated by magneto-optic Kerr effect and magnetic force microscopy. Although the obtained bias fields in these dots are relatively small, the magnetization reversal is found to be influenced by the ferromagnetic–antiferromagnetic coupling. Namely, for some off-axis angles of measurement, the magnetization reversal mechanism of the Ni80Fe20–Ir20Mn80 ellipses depends on whether exchange bias is induced along the minor or major axis of the ellipses. Hence, exchange bias is shown to be an effective means for tailoring the magnetization reversal of elliptical dots after sample fabrication. [ABSTRACT FROM AUTHOR]

An in situ environmental transmission electron microscopy study of the nucleation and growth of oxide islands during the early-stage oxidation of (001) Cu1-xAux alloys (x<=38 at. %) was undertaken in order to investigate the effects of alloying on oxide island nucleation behavior and growth kinetics. The kinetic data reveal that Au enhances the nucleation density of oxide islands and suppresses their growth rate. Our results provide insight into reasons for the decreased passivation properties of Cu when alloyed with Au. [ABSTRACT FROM AUTHOR]

We explore the magnetic properties of Co and Fe on a Pt(997) vicinal single crystal by means of reflection high energy electron diffraction and the surface magneto-optical Kerr effect. Both Co and Fe are grown at room temperature as wedges with thickness ranging from 0 to 3.5 ML (monolayer). We find that Co and Fe show layer-by-layer growth followed by three dimensional growth as the coverage increases due to the large lattice mismatch. Co and Fe exhibit ferromagnetism down to 0.2 ML coverage due to the strong anisotropy. The symmetry breaking at the surface results in the magnetic easy axis of the atomic layers along the surface normal direction. The coercivity of both Fe and Co atomic layers fit an exponential decay with temperature that can be understood as being due to thermally activated, nonequilibrium spin dynamics. The magnetic anisotropy energy per atom is extracted by fitting the experimental data. [ABSTRACT FROM AUTHOR]

The original estimate of the fissility of very heavy nuclei made by Meitner and Frisch can be derived without resorting to Rayleigh's theory of vibrating liquid drops and Legendre polynomials. A simple expression, familiar from the theory of the Wilson cloud chamber, suffices. [ABSTRACT FROM AUTHOR]

A very slight change in some nuclear properties would have been enough to make the fission chain reaction impossible. If the alpha-decay energy of 235U, the only naturally occurring fissile nuclide, were just 0.1 MeV higher than it actually is so little would remain in the Earth that extracting a critical mass would be close to impossible. Moreover, even with a sufficient abundance of 235U, if its scission point were shifted inwards by at most 2 fm not enough secondary neutrons would be emitted for a chain reaction to be possible. Speculating within the framework of the multiverse concept, we ask whether there could be a universe similar to ours in which intelligent life can emerge but in which the fission chain reaction is impossible. [ABSTRACT FROM AUTHOR]

A microscopic computer experiment is set up to investigate the statistical properties of far from equilibrium homogeneous chemical systems undergoing instabilities. Sustained periodic behavior of limit cycle type is observed. Both the frequency and the amplitude of the oscillations are found to be in good agreement with the macroscopic description. A comparison with the stochastic theory of chemical systems based on master equation formalism is also carried out. The dynamical and static correlation functions obtained by these two procedures are in very good agreement. [ABSTRACT FROM AUTHOR]

Investigates the effect of uniaxial deformation on the giant magnetoresistance (GMR) effect in melt-spun Co-Cu ribbons. Increase of structural disorder and resistivity; Degradation of the GMR effect; Effect of the deformation of the metastable alloy prior to precipitation of the Co particles on the particle nucleation process.

Deals with a study which examined the antiferromagnetic coupling between two magnetic layers across intervening nonmagnetic layers found in iron/chromium sandwiches. Growth and characterization of the sandwiches; Basic principles of magnetic coupling; Oscillation of the magnetic coupling in the sandwiches.

Investigates axial profiles of thermal wall loading and radiation output in a direct current vortex-stabilized arc. Information on the theory; Description of the experimental apparatus; Results of the experiments.

Microscopic simulation is used to clarify the status of stochastic theories of homogeneous chemical systems operating in the multiple steady state region. The results demonstrate the failure of the Langevin approach, but show excellent agreement with the master equation formulation. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Describes a study which compared the structure and stoichiomestry of the films on aluminum[sub2]oxygen[sub3] with the films on yttria-stabilized zirconium dioxide (YSZ) and magnesium oxide. Importance of aluminum[sub2]oxygen[sub3] substrate; Superconducting onset temperature of the film; Features of film-substrate interactions.

We report the fabrication process, magnetic behavior, as well as the surface modification of ferromagnetic microdisks suspended in aqueous solution. They posses unique properties such as high magnetization of saturation, zero remanence due to spin vortex formation, intrinsic spin resonance at low frequencies, and the capability of delivering various biomolecules at once. Furthermore, because of their anisotropic shape, our magnetic particles rotate under alternating magnetic fields of small amplitude. This can be used to promote the idea of advanced therapies, which include combined drug delivery and magnetomechanical cell destruction when targeting tumor cells. The approach enables us to fabricate suitable magnetic carriers with excellent size tolerances, and then release them from the wafer into solution, ready for surface modification and therapeutic use. The particles have a magnetic core and are covered with few nanometers of gold on each side to provide stability at ambient conditions as well as biocompatibility and selective adhesion functions. A successful attempt to bind thiolates, including SH-modified antibody, to the disk’s surface was demonstrated. [ABSTRACT FROM AUTHOR]

In two-phase nanocomposite magnets, open recoil loops have shown to be sensitive to interphase interfacial conditions and have been often used to characterize the interphase exchange coupling. Typically, the open recoil loops are attributed to the soft phase volume that is decoupled from the hard phase. Our element-specific magnetic measurements on bilayer Sm–Co/Fe exchange-spring magnets reveal that open recoil loops are present not only in the soft Fe layer but also in the hard Sm–Co layer and that the Fe- and Sm-specific remanence curves are similar to each other. The experimental results and micromagnetic modeling reveal that the observed open recoil loops can originate from the anisotropy variations in the hard Sm–Co layer. [ABSTRACT FROM AUTHOR]

It is demonstrated here that an already ideal exchange-spring magnet can be further improved by intermixing the interface. This is counter-intuitive to the general expectation that optimal exchange- spring magnet behavior requires an ideal, atomically coherent soft-hard interface. Epitaxial Sm- Co/Fe thin-film exchange-spring bilayers are thermally processed, by annealing or high-temperature deposition, to induce interdiffusion. With increasing processing temperature, the hysteresis loop becomes more single-phase-like, yet the magnetization remains fully reversible. The interface is characterized via synchrotron x-ray scattering and electron microscopy elemental mapping. The magnetization behavior is modeled by assuming a graded interface where the material parameters vary continuously. The simulations produce demagnetization curves similar to experimental observations. [ABSTRACT FROM AUTHOR]

Co has been epitaxially grown onto flat and modified Ru(0001) substrates via molecular beam epitaxy at elevated temperature and characterized ex situ with atomic force and magnetic force microscopy. On flat Ru(0001), Co grows into three-dimensional Co islands (dots) of ∼20–900 nm or a flat Co film network with deep holes (antidots) in truncated pyramidal shapes, which randomly distribute on the surface. When the Ru surface morphology is modified via either scratches or e-beam lithography, the dots align along the edges of the surface features (∼4–100 nm deep), and form dot chains, wires, arrays, and dots around lithographic patterns. The dots exhibit in-plane single domains with magnetic easy axis along the edges of these surface features. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Remagnetization processes in a SmCo/NdCo exchange spring system are studied using superconducting quantum interference device magnetometry and magneto-optical imaging. The magneto-optical images reveal that the reversal of the soft NdCo layer and its return to the polarized state at cycling around the exchange bias field occur due to the asymmetric nucleation and growth of twisted-phase domains. The observed peculiarities of the domain-wall kinetics are discussed and a way of tailoring exchange biased systems by regulating the motion of the twisted-phase domains is suggested. [ABSTRACT FROM AUTHOR]

Submicron quasihexagonal Co dots and dot chains have been grown epitaxially on Ru(0001) substrate with molecular beam epitaxy and investigated with magnetic force microscopy. All the dots exhibit in-plane single domains. The magnetization of the dots in a chain prefers to align along the chain direction due to inter-dot magnetostatic interactions. Micromagnetic calculations suggest that the observed single domains are metastable energetically and both vortex and single domain can exist in zero field. The inter-dot magnetostatic coupling, modeled with a dot pair, introduces a uniaxial anisotropy with easy axis along the pair/chain direction. The induced anisotropy field decreases with increasing inter-dot distance and approaches the dipolar limit. [ABSTRACT FROM AUTHOR]

A method for the analysis of temperature and field dependent magnetization data near the Curie temperature T[sub C] is devised, which allows the reliable determination of critical exponents even in strongly inhomogeneous ferromagnets. It is shown, both experimentally and theoretically, that the field-induced broadening of the ferromagnetic phase transition follows a power law with a characteristic critical exponent β-δ where β and δ are the conventional temperature and field dependent critical exponents of the magnetization. Experimental results on inhomogeneous La[sub 2/3]Ca[sub 1/3]MnO[sub 3] films yield critical exponents β = 0.3682 ± 0.0033 and δ = 4.76 ± 0.32, even though the intrinsic T[sub C] distribution width is larger than the field-induced effect. [ABSTRACT FROM AUTHOR]

We have studied the magnetic stability in exchange-bias and exchange-spring systems during repeated reversal of the pinned layer. The samples were prepared by epitaxial sputter deposition. The exchange-biased system consists of a combination of ferromagnetic and antiferromagnetic Fe/Cr superlattices. The exchange-spring system is a bilayer structure made of hard Sm-Co and soft Fe ferromagnetic layers. The magnetic properties were investigated using the magneto-optic Kerr effect during repeated reversal of the soft layer magnetization by field cycling up to 10[sup 7] times. The experimentally observed decay behaviors are discussed in terms of microstructure and spin configuration of the pinning layers. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effect of magnetic disorder on the magnetization reversal process in thin Co/CoO films has been investigated. The antiferromagnetic CoO layer allows a reversible tuning of the magnetic disorder by simple temperature variation. For temperatures above a critical temperature T[sub c], we observe a discontinuous magnetization reversal, whereas smooth magnetization loops occur for T

Studies the effects of stress on the magnetic properties of ferromagnetic materials, dealing specifically with the field and stress dependence of the irreversible magnetization changes in pure iron. Identification of three ways in which stress affects magnetization; Description of the pure iron rod in this experiment; Measurement of the magnetization of the pure iron rod; Results of this study.

Presents a study which investigated spin polarization of the conduction bands and secondary electrons of gadolimium (Gd). Explanation for the polarization dip of the secondary electron spectra of Gd surfaces; Temperature dependence of the bulk bands; Correlation between spin polarization and oxygen.

Discusses a study on the magnetic and structural instabilities of ferromagnetic and antiferromagnetic iron/copper(100). Properties of iron films grown onto copper(100); Details of the low-energy electron diffraction; Magnetic properties accompanying the structural phases of regions I-III.

The vortex translational mode frequency is known to be only weakly dependent on the magnitude of an in-plane magnetic field (e.g. the vortex core position) for circular ferromagnetic dots. Here we demonstrated that the frequency-field dependence becomes discrete when a nanoscale vortex barrier is introduced in the dot structure. We found that the frequency is mostly defined by the outer diameter of the dot or the barrier size for the vortex core located outside or inside the barrier, correspondingly. The experimental results are in good agreement with the micromagnetic simulation. [ABSTRACT FROM AUTHOR]

Mapping the distribution of currents inside a superconductor is usually performed indirectly through imaging of the stray magnetic fields above the surface. Here, we show that by direct imaging of the Doppler shift contribution to the quasiparticle excitation spectrum in the superconductor using low temperature scanning tunneling microscopy, we obtain directly the distribution of supercurrents inside the superconductor. We demonstrate the technique at the example of superconductor/ ferromagnet hybrid structure that produces intricate current pattern consisting of combination Meissner shielding currents and Abrikosov vortex currents. [ABSTRACT FROM AUTHOR]