Surface plasmons in small metal particles

The dielectric response of a small metal particle to a perturbing potentialv _L =r ^L Y _LM e ^iωt is considered within the random phase approximation (RPA). The static dielectric polarizability is found and the size dependence of the surface plasmon frequencies are then determined from sum rule calculations. When the particle radiusa is large compared to the Thomas-Fermi screening lengthr ₀ the RPA equation is transformed into a form appropriate for an analytical solution. The dynamic electric polarizability, the position and the width of the surface plasma resonance are found in the limita/r ₀?1.     

Investigation of magnetostrictive/piezoelectric multilayer composite with a giant zero-biased magnetoelectric effect

In this paper, we investigate the resonance magnetoelectric (ME) effect in the middle supported multilayer composites consisting of high-permeability Fe-based nanocrystalline soft magnetic alloy Fe_73.5Cu₁Nb₃Si_13.5B₉ (FeCuNbSiB), Nickel (Ni), and piezoelectric Pb(Zr_1?x Ti _x )O₃ (PZT). The coupling effect between positive magnetostrictive FeCuNbSiB and negative magnetostrictive Ni results in the build-in magnetic bias due to their different magnetic permeability and coercivity. As a result, a giant resonance ME voltage coefficient (α _ME,r ) at zero DC magnetic bias field (H _dc) and multi-peaks of α _ME,r for FeCuNbSiB/Ni/PZT/Ni/FeCuNbSiB composite are observed. The experimental results show that the giant zero-biased α _ME,r strongly depends on the thickness of FeCuNbSiB ribbon. The maximum zero-biased α _ME,r is up to 86 V/cm?Oe for FeCuNbSiB/Ni/PZT/Ni/FeCuNbSiB with four-layer FeCuNbSiB ribbons, which is ～500 times higher than that of the previously reported NKNLS-NZF/Ni/NKNLS-NZF trilayer composite. Compared with the peak α _ME,r and the optimum H _dc of Ni/PZT/Ni composite, the largest peak α _ME,r of FeCuNbSiB/Ni/PZT/Ni/FeCuNbSiB composite with four-layer FeCuNbSiB ribbons increases ～185 %, and the optimum H _dc decreases ～300 Oe, respectively. Based on the nonlinear magnetostrictive constitutive relation and the magnetoelectric equivalent circuit, a theoretical model of α _ME,r versus H _dc is built under free boundary conditions. Calculated zero-biased α _ME,r and α _ME,r versus H _dc are in good agreement with the experimental data. This laminate composite shows promising applications for high-sensitivity power-free magnetic field sensors, zero-biased ME transducers and small-size energy harvesters.     

Phase transitions in a ferromagnetic semiconductor. III

We investigate the ferromagnetic phase transition in a heavily doped semiconductor or semimetal containing magnetic ions. The coupling between conduction electrons and magnetic ions is treated in the molecular field approximation. We show that the system undergoes a transition to an ordered state for arbitrarily small concentrations of the magnetic ions and the conduction electrons. In the limitJ/? _F?1 we obtain the Ruderman-Kittel result for the Curie temperature, and forJ/? _F?1 a generalization of Thompson’s strong coupling result. (J is the exchange coupling constant between conduction electrons and magnetic ions and? _F is the kinetic Fermi energy at absolute zero.) The intermediate range is evaluated numerically.     

A numerical estimate of the upper limit for the Higgs boson mass

TheSU (2) Higgs model with a scalar doublet field is studied by Monte Carlo calculations on 12⁴ and 16⁴ lattices. The gauge coupling is chosen to be similar in magnitude to the physical value in the standard model. The numerical results at large scalar self-coupling imply an upper limitm _H /m _W ?9 for the ratio of the Higgs boson mass to the W-mass.     

Improvement of magnetic properties and read/write characteristics in SmCo5 perpendicular thin films

An SmCo₅ alloy is a promising candidate for ultra-high density magnetic recording media because of its strong uniaxial magnetocrystalline anisotropy, whose constant, K_u, is more than 1.1×10⁸ erg/cm³. Recently, we successfully obtained high perpendicular magnetic anisotropy for a sputter-deposited SmCo₅ thin film by introducing a Cu/Ti dual underlayer. However, it is necessary to improve magnetic properties and read/write (R/W) characteristics for applying SmCo₅ thin films to perpendicular magnetic recording media. In this study, we focused on reduction of magnetic domain size and change of a magnetization reversal process of SmCo₅ perpendicular magnetic thin films by introducing carbon (C) atoms into the constituent Cu underlayer. The magnetic domain size became small and the ratio of coercivity (H_c) against magnetic anisotropy (H_k) which is an index of the magnetization reversal process was increased by adding C atoms. We also evaluated the R/W characteristics of SmCo₅ double-layered media including C atoms. The medium noise was decreased and signal-to-noise ratio increased by introducing the C. The addition of C into the Cu underlayer is effective for changing the magnetization reversal process, reducing medium noise and increasing SNR.     

On the nonrelativistic limit of the Dirac theory

The relation between a “nonrelativistic” Hamiltonian of the formH ^∞=(A+B)²+C and a corresponding family of “Dirac-Hamiltonians”H(c) in the limitc → ∞ is investigated. It is shown that the resolvent (z ?H(c))^?1 and the relativistic perturbation of isolated eigenvalues ofH ^∞ are analytic in 1/c for sufficiently large |c|.     

Strange anisotropy in amorphous metals

The spin orientation in ferromagnetic amorphous Fe₈₃P₅C₁₂ in an external magnetic field, H_ext, applied perpendicular to the ribbon sample has been determined by magnetoresistance measurements and by the angular dependence of the hyperfine interaction. In small fields, H_ext ? 4 kOe, the spins tend to orient themselves perpendicular to H_ext due to the influence of the demagnetizing field and in large fields, H_ext ≈ 50 kOe, a complete alignment was not obtained.     

Specific features of magnetoresistance during the antiferromagnet—paramagnet transition in Tm1 − x Yb x B12

The transverse magnetoresistance Δρ/ρ(H, T) of Tm_{1 ? x} Yb _x B₁₂ single crystals is studied in the ytterbium concentration range corresponding to the antiferromagnet-paramagnet transition in a magnetic field up to 80 kOe at low temperatures. A magnetic H-T phase diagram is constructed for the antiferromagnetic state of substitutional Tm_{1 ? x} Yb _x B₁₂ solid solutions with x ≤ 0.1. The contributions to the magnetoresistance in the antiferromagnetic and paramagnetic phases of the dodecaborides under study are separated. Along with negative quadratic magnetoresistance -Δρ/ρ ∝ H ₂, the magnetically ordered phase of these compounds is found to have component Δρ/ρ ∝ H that linearly changes in a magnetic field. The negative contribution to the magnetoresistance of Tm_{1 ? x} Yb _x B₁₂ is analyzed in terms of the Yosida model for a local magnetic susceptibility.     

Strong-coupling constant in coordinate space

The coordinate-space behavior of (vector) strong-coupling constant in the background field α_B(r) is compared with that in standard perturbation theory α_v(r). The numerically calculated two-loop coupling constant α_B(r) is shown to exceed α_v(r) by 1–5% at very small distances, r?0.02 fm, and to be in agreement with lattice measurements of the static potential. At large distances, α_B(r) approaches the freezing value at r?0.5 fm. An analytic form of α_B(r) is proposed that approximates α_B(r) with a precision ?2% in the region r?0.5 fm.     

Magnetic properties and microstructures of Fe-Cr-10 wt% Co-M (M= Si/Ti/Ni/Mo/Ge/Ta) permanent magnet alloys

Effect of alloying elements and heat treatments on magnetic properties and microstructures of Fe-Cr-10 wt% Co alloys was studied by magnetic measurements and transmission electron microscopy (TEM). Among the alloying elements tried, it was found that only Si and Ti can greatly improve the hard magnetic properties of the Fe-Cr-Co alloys. An Fe-10Co-26Cr alloy added with 0.5 wt% Si-0.5 wt% Ti showed the best combination of magnetic properties of H_c = 43 kA/m.B_r = 1.42 T, (BH)_m = 41 kJ/m³.The variation in magnetic properties was correlated to quantitative microstructural features, particle size and aspect ration. It was found that for alloys of the same composition, H_c is determined by shape anisotropy induced by magnetic aging.     

Superlattice CoCrPt/Ru/CoFe structure fabricated by pulsed laser deposition

The synthetic antiferromagnets (SAF) have been used in spin-valve sensor in data storage industry [1]. We report a new hard/Ru/soft sandwich structure (SHBL) fabricated by pulsed lased deposition to replace current single layer structure for information recording application. SHBL consists of two magnetic layers separated by thin nonmagnetic layers, typically with Ru layers of 0.7-1.2 nm, through which antiferromagnetic coupling is induced. Varying the relative thickness of the magnetic layers, the spacer layers, and the type of magnetic materials can alter magnetic properties of CoCrPt/Ru/CoFe superlattice. The coercivity H_c and grain size of magnetic layer is also dependent on the laser fluence. High laser fluence results in both small grain size and high H_c. The observed phenomena are related to high quenching and deposition rates during PLD at high fluence, resulting in more pronounced phase segregation.     

Theory of negative magnetoresistance in three-dimensional systems

Theory of negative magnetoresistance in two-dimensional systems due to delocalization of electrons by magnetic field by Hikami, Larkin, and Nagoaka is extended to the case of three-dimension. The increase in conductivity by magnetic field is independent of the direction of the current relative to that of the magnetic field, and is proportional to $\text{H}{}^2\text{τ}{}_{\mathrm{?}}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$, τ_? being energy relaxation time of electrons, when the magnetic field H is small. If τ_? is large enough at low temperature and H is not too small, it is independent of the parameter characterizing the system, and is of the form $\text{0.918?H mho cm}{}^{\mathrm{?1}}$ (H in kOe).     

Floquet Hamiltonians with pure point spectrum

We consider Floquet Hamiltonians of the type $K_F : = - i\partial _t + H_0 + \beta V(\omega t)$ , whereH ₀, a selfadjoint operator acting in a Hilbert space ?, has simple discrete spectrumE ₁<E₂<... obeying a gap condition of the type inf {n ^?α(E _n+1?E_n); n=1, 2,...}>0 for a given α>0,t?V(t) is 2π-periodic andr times strongly continuously differentiable as a bounded operator on ?, ω and β are real parameters and the periodic boundary condition is imposed in time. We show, roughly, that providedr is large enough, β small enough and ω non-resonant, then the spectrum ofK _f is pure point. The method we use relies on a successive application of the adiabatic treatment due to Howland and the KAM-type iteration settled by Bellissard and extended by Combescure. Both tools are revisited, adjusted and at some points slightly simplified.     

Ab initio SCF-Cl studies on the large amplitude bending motion of the water molecule: Rigid,semirigid, and nonrigid bender models for the Hamiltonian

Self-Consistent Field (SCF) and Configuration Interaction (CI) studies are performed on the bending mode of the water molecule using a double zeta plus polarization basis set. The ab initio points are fitted to a three-parameter double minimum potential consisting of a quadratic plus Lorentzian terms. The vibration-rotation energies are then evaluated using the large amplitude Hamiltonian developed by P. R. Bunker and co-workers at various levels of approximations. It is found that the calculated frequencies improve significantly as one proceeds from approximate H_b⁰⁰(ρ) to rigid bender H_b⁰(ρ) [P. R. Bunker and J. M. R. Stone, J. Mol. Spectrosc.41, 310–332 (1972)] to semirigid bender H_b⁰(r, ρ) [P. R. Bunker and P. M. Landsberg, J. Mol. Spectrosc.67, 374–385 (1977)] Hamiltonian. With H_b⁰(r, ρ), the ab initio calculated bending frequency ν₂ differs from the observed value (1595 cm^?1) by 30 cm^?1 and the barrier height is 12 229 cm^?1. It is also shown that ν₂ and its first four overtones are better calculated by 45–98 cm^?1 when the ab initio potential is used directly instead of the three-parameter analytic potential fitted to ab initio data. Finally, rotation bending energy levels are calculated for v₂ ≤ 3 and J ≤ 10 on the basis of a nonrigid bender Hamiltonian of A. R. Hoy and P. R. Bunker [J. Mol. Spectrosc.74, 1–8 (1979)], using the ab initio quadratic force field of P. Hennig, W. P. Kraemer, G. H. F. Diercksen, and G. Strey, [Theor. Chim. Acta47, 233–248 (1978)]. These results show that the accuracy of calculated force constants and frequencies is critically dependent not only on the size of the basis set but also on the number and spacing of the ab initio points used to derive the force field.     

X-ray emission from non-Keplerian magnetically dominated accretion disks

Summary Most accretion disk models do assume Keplerian rotation as the ?natural? one. This is not so obvious and is somewhat aprioristic, as shown by some attempts to follow different approaches. In the present work we suggest a new approach to the problem, pointing out that the disk structure is deeply related to the balance between the magnetic energy produced by dynamo action and the rotational energy associated with the accreting flow. This balance, together with the conservation laws, allows us to derive analytically the angular-velocity field, the radial velocity field and the magnetic-energy distribution inside the disk. As far as the disk X-ray emission is concerned, we adopt the point of view that the buoyancy and emergence at the disk surface of the azimuthal magnetic field generated by differential rotation gives rise to looplike structures in a hot magnetically confined corona, which is not merely overimposed, as in other models, but ?naturally? fits to the disk structure. The magnetic-energy dissipation into the coronal medium, which sustains the X-ray luminosity, is due to twisting of the magnetic loops. we give an exact analytical expression for the integrated luminosityL _x in terms of the mass of the compact object, the accretion rate, the disk size and rotational velocity, and show that in the limitr _i≪r _e (wherer _i andr _e are the inner and the outer radius of the accretion disk, respectively) it agrees fairly well with the characteristic observed values of both galactic and extragalactic strong compact sources, within the reasonable range commonly accepted for the relevant parameters. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.     

Spin-wave resonance in [Co x Ni1 − x ]N and [Co x P1 − x ]N gradient films

Gradient films of ferromagnetic 3d metals with prescribed magnetic potential profile along the film thickness are obtained. It is found that the spin-wave resonance spectrum in these films is characterized by anomalous dependences of resonance fields of spin-wave modes H _r on the mode number: H _r(n) ～ n, H _r(n) ～ n ^2/3.     

Magnetic ordering of Fe atoms in icosahedral Al70−x BxPd30−y Fey quasicrystals

Novel icosahedral quasicrystals, in which Fe atoms possess a magnetic moment, have been found in Al_70?x B_xPd_30?y Fe_y compounds with 5<x<10 and 10<y<20. The compounds have ferromagnetic properties, and their Curie temperature ranges between 280 and 340 K, the saturation magnetization σ _s(5 K)≈7.5 emu/g. It follows from Mössbauer spectra that only a fraction of Fe atoms (12 to 15%) are magnetically ordered at 4.2 K, and the mean saturation field 〈H _hf〉=96 kOe. The isomer shift values confirm that the atomic volume of magnetic Fe sites is larger than that of nonmagnetic Fe sites. The magnetic properties of these quasicrystals can be interpreted in terms of large magnetic clusters with a size of 185 to 290 Å. This size correspond to about 4×10⁴ “unit cells,” hence the magnetic state can be described in terms of bulk parameters. The localized magnetic moment of Fe atoms is tentatively ascribed to bonding between Fe and B, similarly to that in the amorphous Fe_～50B_～50 alloy.     

Field polarity dependent nucleation of superconductivity in quasi-one-dimensional magnetic templates

We investigate the nucleation of superconductivity in an Al/Al₂O₃/Py trilayer system by electrical transport measurements. Magnetic force microscopy images taken at room temperature show that the 0.7 μm thick Py-film form stripes of magnetic domains with alternating out-of-plane stray field. After applying a strong out of plane magnetic field H the superconductor/normal phase boundary becomes asymmetric with respect to H = 0. This lack of field polarity symmetry results from the unbalanced size distribution of domains with opposite polarity.     

A scaling theory of clusters in the lattice gas

It is proposed that the number ofp-particle,k-bond lattice gas cluster configurations is of the form exp{σpf(k/p)} in the limitp→t8. A simple modification permits application to finite clusters, with the consequence that asymptotically the cluster partition function is of the droplet form, i.e., Z _p =exp[kp{itμp^1?1/d }+O(ln p)]. The scaling function for two-dimensional lattices is determined numerically and is found to be qualitatively universal. The scaling theory is used to investigate the size dependence of the surface free energy. The surface tension of small clusters can significantly exceed its limiting value. For intermediate cluster sizes (～100 particles) there is a modest reduction in surface tension, in accord with Tolman's prediction and the results of recent Monte Carlo studies.