Exchange-spring magnets are nanocomposites that are composed of magnetically hard and soft phases that interact by magnetic exchange coupling. Such systems are promising for advanced permanent magnetic applications, as they have a large energy product--the combination of permanent magnet field and magnetization--compared to traditional, single-phase materials. Conventional techniques, including melt-spinning, mechanical milling and sputtering, have been explored to prepare exchange-spring magnets. However, the requirement that both the hard and soft phases are controlled at the nanometre scale, to ensure efficient exchange coupling, has posed significant preparation challenges. Here we report the fabrication of exchange-coupled nanocomposites using nanoparticle self-assembly. In this approach, both FePt and Fe3O4 particles are incorporated as nanometre-scale building blocks into binary assemblies. Subsequent annealing converts the assembly into FePt-Fe3Pt nanocomposites, where FePt is a magnetically hard phase and Fe3Pt a soft phase. An optimum exchange coupling, and therefore an optimum energy product, can be obtained by independently tuning the size and composition of the individual building blocks. We have produced exchange-coupled isotropic FePt-Fe3Pt nanocomposites with an energy product of 20.1 MG Oe, which exceeds the theoretical limit of 13 MG Oe for non-exchange-coupled isotropic FePt by over 50 per cent.

Exchange-coupled nanocomposite magnets by nanoparticle self-assembly

Now more than ever, Gravitation and Spacetime, Second Edition, by Hans C. Ohanian and new coauthor Remo Ruffini, deserves John Wheeler's praise as "the best book on the market today of 500 pages or less on gravitation and general relativity." Gravitation and Spacetime has been thoroughly updated with the most exciting finds and hottest theoretical topics in general relativity and cosmology. Highlights of the revision include the rise and fall of the fifth force, principles and applications of gravitational lensing, COBE's spectacular confirmation of the blackbody spectrum of the cosmic thermal radiation, theories of dark matter and inflation, and the early universe as a testing ground for particle physicists' unification theories, and much, much more. The ideal choice for a graduate-level introduction to general relativity, Gravitation and Spacetime is also suitable for an advanced undergaduate course.

Gravitation and spacetime.

Carbon nanotubes can act as electron sources1 with very rigid structures2, making them particularly interesting for use as point electron sources in high-resolution electron-beam instruments. Promising results have been reported with respect to some important requirements for such applications: a stable emitted current3,4,5 and a long lifetime6,7. Two parameters of an electron source affect the resolution of these instruments: the energy spread of the emitted electrons and a parameter called the reduced brightness, which depends on the angular current density and the virtual source size. Several authors have measured a low energy spread associated with electron emission3,7,8,9. Here we measure the reduced brightness, and find a value that is more than a factor of ten larger than provided by state-of-the-art electron sources in electron microscopes. In addition, we show that an individual multi-walled carbon nanotube emits most current into a single narrow beam. On the basis of these results, we expect that carbon nanotube electron sources will lead to a significant improvement in the performance of high-resolution electron-beam instruments.

High brightness electron beam from a multi-walled carbon nanotube

If frequent measurements ascertain whether a quantum system is still in its initial state, transitions to other states are hindered and the quantum Zeno effect takes place. However, in its broader formulation, the quantum Zeno effect does not necessarily freeze everything. In contrast, for frequent projections onto a multidimensional subspace, the system can evolve away from its initial state, although it remains in the subspace defined by the measurement. The continuing time evolution within the projected 'quantum Zeno subspace' is called 'quantum Zeno dynamics': for instance, if the measurements ascertain whether a quantum particle is in a given spatial region, the evolution is unitary and the generator of the Zeno dynamics is the Hamiltonian with hard-wall (Dirichlet) boundary conditions. We discuss the physical and mathematical aspects of this evolution, highlighting the open mathematical problems. We then analyze some alternative strategies to obtain a Zeno dynamics and show that they are physically equivalent.

/pdf/quantum-zeno-dynamics-mathematical-and-physical-aspects-a9nz26r56z.pdf

Quantum Zeno dynamics: mathematical and physical aspects

We measured field emission energy distributions of electrons emitted from individual multiwalled carbon nanotubes mounted on tungsten tips. The shape of the energy distribution is strongly sample dependent. Some nanotube emitters exhibit an almost metallic behaviour, while others show sharply peaked energy distributions. The smallest half-width we measured was only 0.11 eV, without correction for the broadening of the energy analyzer. A common feature of both types of carbon nanotube energy spectra is that the position of the peaks in the spectrum depends linearly on the extraction voltage, unlike metallic emitters, where the position stays in the vicinity of the Fermi level. With a small modification to the field emission theory for metals we extract the distance between the highest filled energy level of the nanotube and the vacuum potential, the field on the emitter surface, the emitter radius and the emitting area, from the energy distribution and the Fowler–Nordheim plot. The last two parameters are in good agreement with transmission electron micrographs of such samples. The sharply-peaked energy distributions from other samples indicate that resonant states can exist at the top of the nanotube.

Field emission energy distributions from individual multiwalled carbon nanotubes

The field equations of general relativity are shown to derive from a limit to force or to power in nature. The limits have the value of c4/4G and c5/4G. The proof makes use of a result of Jacobson. All known experimental data are consistent with the limits. Applied to the universe, the limits predict its darkness at night and the observed scale factor. Other experimental tests of the limits are proposed. The main counterarguments and paradoxes are discussed, such as the transformation under boosts, the force felt at a black hole horizon, the mountain problem, and the contrast to scalar–tensor theories of gravitation. The resolution of the paradoxes also clarifies why the maximum force and the maximum power have remained hidden for so long. The derivation of the field equations shows that the maximum force or power plays the same role for general relativity as the maximum speed plays for special relativity.

General Relativity and Cosmology Derived From Principle of Maximum Power or Force

Decapitation of tungsten field emitter tips during sputter sharpening

The principle of maximum power makes it possible to summarize special relativity, quantum theory and general relativity in one fundamental limit principle each. Special relativity contains an upper limit to speed; following Bohr, quantum theory is based on a lower limit to action; recently, a maximum power given by c
 5/4G was shown to be equivalent to the full field equations of general relativity. Taken together, these three fundamental principles imply a limit value for every physical observable, from acceleration to size. The new, precise limit values differ from the usual Planck values by numerical prefactors of order unity. Among others, minimum length and time intervals appear. The limits imply that elementary particles are not point-like and suggest a lower limit on electric dipole values. The minimum intervals also imply that the non-continuity of space–time is an inevitable result of the unification of quantum theory and relativity, independently of the approach used.

Simple Derivation of Minimum Length, Minimum Dipole Moment and Lack of Space–Time Continuity

The invention relates to a device actuated with a transponder (60) for generating a switch signal (S). The device is based on an oscillating circuit (10) with a capacitance (C1), an identification coil (L1) and an oscillator amplifier (12). A frequency observer (20) is connected to the oscillating circuit (10), said frequency observer evaluating the frequency (f1) tuned in the oscillating circuit and emitting a switch signal (S) if a change is detected. The frequency in the oscillating circuit (10) is changed by the approach of a transponder (60). The inventive device enables an almost powerless transponder recognition.

Switching device actuated with a transponder

Motion Mountain is a free and
entertaining physics textbook - 5 volumes in pdf format with more than 2000 pages -
for pupils, students, teachers and anybody interested in nature and motion. The text,
guaranteed to be fascinating, surprising and challenging on every page, is
a guide on the following questions: How do things, images and empty space move?
What is motion? Why does it exist?

Christoph Schiller

Papers

General Relativity and Cosmology Derived From Principle of Maximum Power or Force

Decapitation of tungsten field emitter tips during sputter sharpening

Simple Derivation of Minimum Length, Minimum Dipole Moment and Lack of Space–Time Continuity

Switching device actuated with a transponder

Motion Mountain - The Adventure of Physics