Fundamental Magnetic Properties and Structural Implications for Nanocrystalline Fe-Ti-N Thin Films

The magnetization (M) as a function of temperature (T) from 2 to 300 K and in-plane field (H) up to 1 kOe, room temperature easy and hard direction in-plane field hysteresis loops for fields between -100 and +100 Oe, and 10 GHz ferromagnetic resonance (FMR) profiles have been measured for a series of soft-magnetic nano-crystalline 50 nm thick Fe-Ti-N films made by magnetron sputtering in an in-plane field. The nominal titanium concentration was 3 at. % and the nitrogen concentrations (xN) ranged from zero to 12.7 at. %. The saturation magnetization (Ms) vs. T data and the extracted exchange parameters as a function of xN are consistent with a lattice expansion due to the addition of interstitial nitrogen in the body-centered-cubic (bcc) lattice and a structural transition to body-centered-tetragonal (bct) in the 6-8 at. % nitrogen range. The hysteresis loop and FMR data show a consistent picture of the changes in both the uniaxial and cubic anisotropy as a function of xN. Films with xN > 1.9 at. % show an overall uniaxial anisotropy, with an anisotropy field parameter Hu that increases with xN. The corresponding dispersion averaged uniaxial anisotropy energy density parameter = HuMs/2 is a linear function of xN, with a rate of increase of 950 erg/cm3 per at. % nitrogen. The estimated uniaxial anisotropy energy per nitrogen atom is 30 J/mol, a value consistent with other systems. For xN below 6 at. %, the scaling of coercive force Hc data with the sixth power of the grain size D indicate a grain averaged effective cubic anisotropy energy density parameter that is about an order of magnitude smaller that the nominal K1 values for iron, and give a quantitative vs. D response that matches predictions for exchange coupled random grains with cubic anisotropy.Comment: 13 pages, 7 figure

Magnetization precession due to a spin polarized current in a thin nanoelement: numerical simulation study

In this paper a detailed numerical study (in frames of the Slonczewski formalism) of magnetization oscillations driven by a spin-polarized current through a thin elliptical nanoelement is presented. We show that a sophisticated micromagnetic model, where a polycrystalline structure of a nanoelement is taken into account, can explain qualitatively all most important features of the magnetization oscillation spectra recently observed experimentally (S.I. Kiselev et al., Nature, vol. 425, p. 380 (2003), namely: existence of several equidistant spectral bands, sharp onset and abrupt disappearance of magnetization oscillations with increasing current, absence of the out-of-plane regime predicted by a macrospin model and the relation between frequencies of so called small-angle and quasichaotic oscillations. However, a quantitative agreement with experimental results (especially concerning the frequency of quasichaotic oscillations) could not be achieved in the region of reasonable parameter values, indicating that further model refinement is necessary for a complete understanding of the spin-driven magnetization precession even in this relatively simple experimental situation.Comment: Submitted to Phys. Rev. B; In this revised version figure positions on the page have been changed to ensure correct placements of the figure caption

The Projective Line Over the Finite Quotient Ring GF(2)[xx]/<x3−x>< x^{3} - x> and Quantum Entanglement I. Theoretical Background

The paper deals with the projective line over the finite factor ring $R\_{\clubsuit} \equiv$ GF(2)[$x$]/$$. The line is endowed with 18 points, spanning the neighbourhoods of three pairwise distant points. As $R\_{\clubsuit}$ is not a local ring, the neighbour (or parallel) relation is not an equivalence relation so that the sets of neighbour points to two distant points overlap. There are nine neighbour points to any point of the line, forming three disjoint families under the reduction modulo either of two maximal ideals of the ring. Two of the families contain four points each and they swap their roles when switching from one ideal to the other; the points of the one family merge with (the image of) the point in question, while the points of the other family go in pairs into the remaining two points of the associated ordinary projective line of order two. The single point of the remaining family is sent to the reference point under both the mappings and its existence stems from a non-trivial character of the Jacobson radical, ${\cal J}\_{\clubsuit}$, of the ring. The factor ring $\widetilde{R}\_{\clubsuit} \equiv R\_{\clubsuit}/ {\cal J}\_{\clubsuit}$ is isomorphic to GF(2) $\otimes$ GF(2). The projective line over $\widetilde{R}\_{\clubsuit}$ features nine points, each of them being surrounded by four neighbour and the same number of distant points, and any two distant points share two neighbours. These remarkable ring geometries are surmised to be of relevance for modelling entangled qubit states, to be discussed in detail in Part II of the paper.Comment: 8 pages, 2 figure

Harmonic Generation from Relativistic Plasma Surfaces in Ultra-Steep Plasma Density Gradients

Harmonic generation in the limit of ultra-steep density gradients is studied experimentally. Observations demonstrate that while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale-lengths ($L_p/\lambda < 1$) the absolute efficiency of the harmonics declines for the steepest plasma density scale-length $L_p \to 0$, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the Relativistic Oscillating Mirror (ROM) was estimated to be in the range of 10^{-4} - 10^{-6} of the laser pulse energy for photon energies ranging from 20-40 eV, with the best results being obtained for an intermediate density scale-length

Structure peculiarities of cementite and their influence on the magnetic characteristics

The iron carbide $Fe_3C$ is studied by the first-principle density functional theory. It is shown that the crystal structure with the carbon disposition in a prismatic environment has the lowest total energy and the highest energy of magnetic anisotropy as compared to the structure with carbon in an octahedron environment. This fact explains the behavior of the coercive force upon annealing of the plastically deformed samples. The appearance of carbon atoms in the octahedron environment can be revealed by Mossbauer experiment.Comment: 10 pages, 3 figures, 3 tables. submitted to Phys.Rev.

Projective Ring Line Encompassing Two-Qubits

The projective line over the (non-commutative) ring of two-by-two matrices with coefficients in GF(2) is found to fully accommodate the algebra of 15 operators - generalized Pauli matrices - characterizing two-qubit systems. The relevant sub-configuration consists of 15 points each of which is either simultaneously distant or simultaneously neighbor to (any) two given distant points of the line. The operators can be identified with the points in such a one-to-one manner that their commutation relations are exactly reproduced by the underlying geometry of the points, with the ring geometrical notions of neighbor/distant answering, respectively, to the operational ones of commuting/non-commuting. This remarkable configuration can be viewed in two principally different ways accounting, respectively, for the basic 9+6 and 10+5 factorizations of the algebra of the observables. First, as a disjoint union of the projective line over GF(2) x GF(2) (the "Mermin" part) and two lines over GF(4) passing through the two selected points, the latter omitted. Second, as the generalized quadrangle of order two, with its ovoids and/or spreads standing for (maximum) sets of five mutually non-commuting operators and/or groups of five maximally commuting subsets of three operators each. These findings open up rather unexpected vistas for an algebraic geometrical modelling of finite-dimensional quantum systems and give their numerous applications a wholly new perspective.Comment: 8 pages, three tables; Version 2 - a few typos and one discrepancy corrected; Version 3: substantial extension of the paper - two-qubits are generalized quadrangles of order two; Version 4: self-dual picture completed; Version 5: intriguing triality found -- three kinds of geometric hyperplanes within GQ and three distinguished subsets of Pauli operator

Observation of energetic terahertz pulses from relativistic solid density plasmas

We report the first experimental observation of terahertz (THz) radiation from the rear surface of a solid target while interacting with an intense laser pulse. Experimental and two-dimensional particle-in-cell simulations show that the observed THz radiation is mostly emitted at large angles to the target normal. Numerical results point out that a large part of the emission originates from a micron-scale plasma sheath at the rear surface of the target, which is also responsible for the ion acceleration. This opens a perspective for the application of THz radiation detection for on-site diagnostics of particle acceleration in laser-produced plasmas

Reduction of the Yb valence in YbAl3 nanoparticles

Measurements of specific heat, dc magnetic susceptibility, and Yb LII and LIII x-ray absorption near-edge structure XANES and extended x-ray absorption fine structure EXAFS on YbAl3 milled alloys are reported. X-ray diffraction patterns are consistent with a reduction in particle size down to 10 nm and an increase in the lattice strain up to 0.4% for 120 h of milling time. A decrease in the mean valence from 2.86 for the unmilled alloy to 2.70 for 120 h milled YbAl3 is obtained from the analysis of XANES spectra. From the analysis of spectra in the EXAFS region, an increase in the mean-square disorder of neighbor distance with milling time is detected in good agreement with the results of x-ray diffraction. Size effects strongly influence the magnetic and thermal properties. The value for the maximum of the magnetic susceptibility decreases around 30% for 120 h milled alloy and an excess specific heat, with a peak around 40 K in the milled samples, is derived. These changes in the physical properties along the milled YbAl3 alloys are associated with the reduction in particle size. Such a reduction leads to the existence of a large number of Yb2+ atoms at the surface with respect to the bulk affecting the overall electronic state