Magnetic anisotropy in multilayers

Magnetic anisotropy between in-plane and out of plane magnetic alignments is studied in a variety of multilayer systems using Mössbauer spectrosopy to observe the (Fe) magnetic orientation. The surface anisotropy in Fe/Au (1 1 1) multilayers is measured as K _s = 0.9 × 10^?3 Jm^?2. In Fe/Ni multilayers the dependence of magnetic orientation on external field applied normal to the layers enables volume and interface anisotropies K _v = (?5 ± 1) × 10⁴ Jm^?3 and K _s = (?0.6 ± 0.4)× 10^?3 Jm^?2 to be evaluated. In similar applied field experiments coherent rotation of the magnetic Fe and NiFe layers in Fe/Cu/NiFe/Cu multilayers was observed for intervening Cu layer thickness x = 5 Å but independent rotation for x = 50 Å. Out of plane magnetic components are observed for DyFe₂, YFe₂ thin films and DyFe₂/YFe₂ multilayers. In fields of up to 0.25 T applied inplane only the moments of the YFe₂ film showed significant rotation.     

Self-oscillations of electronic and nuclear polarization during optical excitation of an Si/CaF2 nanostructure in a transverse magnetic field

We examine theoretically low-frequency and high-frequency self-oscillations of electronic and nuclear polarization in an Si/CaF₂ nanostructure in a transverse magnetic field. We show that the low-frequency self-oscillations are stable in zero field, and the analogous high-frequency oscillations are stable beyond the region of the maximum on the Hanle curve. The frequency of the low-frequency oscillations is 0.001–0.500 of the reciprocal nuclear longitudinal relaxation time; the frequency of the high-frequency oscillations is 10⁸–10⁹ Hz, and their amplitude reaches 50% of the initial electronic spin polarization. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 363–369, May–June, 2006.     

Self-oscillations of an ordered magnetic structure

The first observation of self-oscillations of an ordered magnetic structure is reported. They were detected by the Mössbauer effect in the Tb_0.8Y_0.2Fe₂ ferrimagnetic compound and occur with a period of several days. The oscillations were initiated by a single electric-field pulse (～10⁸ kV cm^?1 s^?1). A phenomenological model of the phenomenon is proposed.     

The quantum era

A universe is conceived as a gravitational quantum fluctuation stablized by coherent black hole creation out of expansion energy. The initial temperature is determined (T = 4.57 × 10¹⁶ GeV) and the adiabatic era causally explained. The theory has far-reaching implications for quantum gravity.     

Collective rearrangement of spin excitation spectrum of antiferromagnet with magnetic impurities in external magnetic field

Antiferromagnetic resonance (AFMR) in Co_1?cMn_cF₂ (c ～ 10^-4) in the external magnetic field is investigated. It was established that coherent and incoherent rearrangements (CR and ICR) of spectrum in the region of AFMR and impurity lines crossing are possible in the same sample depending on field orientation. Topology of the phase diagram of spin excitation (SE) spectrum in variables c-H is obtained.     

Calculation of Double-Quantum-Coherence Two-dimensional Spectra: Distance Measurements and Orientational Correlations

The double-quantum-coherence (DQC) echo signal for two coupled nitroxides separated by distances ?10 Å, is calculated rigorously for the six-pulse sequence. Successive application of six pulses on the initial density matrix, with appropriate inter-pulse time evolution and coherence pathway selection leaves only the coherent pathways of interest. The amplitude of the echo signal following the last π pulse can be used to obtain a one-dimensional (1D) dipolar spectrum (Pake doublet), and the echo envelope can be used to construct the 2D DQC spectrum. The calculations are carried out using the product space spanned by the two electron-spin magnetic quantum numbers m ₁, m ₂ and the two nuclear-spin magnetic quantum numbers M ₁, M ₂, describing, e.g. two coupled nitroxides in bilabeled proteins. The density matrix is subjected to a cascade of unitary transformations taking into account dipolar and electron exchange interactions during each pulse and during the evolution in the absence of a pulse. The unitary transformations use the eigensystem of the effective spin Hamiltonians obtained by numerical matrix diagonalization. Simulations are carried out for a range of dipolar interactions, D, and microwave magnetic field strength B ₁ for both fixed and random orientations of the two ¹⁴N (and ¹⁵N) nitroxides. Relaxation effects were not included. Several examples of 1D and 2D Fourier transforms of the time-domain signals versus dipolar evolution and spin-echo envelope time variables are shown for illustration. Comparisons are made between 1D rigorous simulations and analytical approximations. The rigorous simulations presented here provide insights into DQC electron spin resonance spectroscopy, they serve as a standard to evaluate the results of approximate theories, and they can be employed to plan future DQC experiments.     

Coherent State Path Integral for the Harmonic Oscillator and a Spin Particle in a Constant Magnetic field

Definition and formulas for harmonic oscillator coherent states and spin coherent states are reviewed in detail. The path integral formalism and its relation with the partition function of a system are also reviewed. The harmonic oscillator coherent state path integral is evaluated exactly at the discrete level and then used to find its continuum limit using various regularizations. The computation of the path integral for a particle of spin s put in a constant magnetic field is carried out using harmonic oscillator coherent states and spin coherent states, with a careful analysis of infinitesimal terms (in 1/N where N is the number of time slices) appearing in the Lagrangian. A mapping of the spin system into a CP¹ model is shown explicitly. The theory of a spinless particle in the field of a magnetic monopole and its relation with the spin system are explained. The equivalence of these two models is established up to infinitesimal order by the introduction of an external field correction. This gives a new representation of a coherent state path integral in terms of a more familiar Feynman path integral.     

Correlated states and transparency of a barrier for low-energy particles at monotonic deformation of a potential well with dissipation and a stochastic force

The features of the formation of correlated coherent states of a particle in a parabolic potential well at its monotonic deformation (expansion or compression) in finite limits have been considered in the presence of dissipation and a stochastic force. It has been shown that, in both deformation regimes, a correlated coherent state is rapidly formed with a large correlation coefficient |r| → 1, which corresponds at a low energy of the particle to a very significant (by a factor of 10⁵⁰–10¹⁰⁰ or larger) increase in the transparency of the potential barrier at its interaction with atoms (nuclei) forming the “walls” of the potential well or other atoms located in the same well. The efficiency of the formation of correlated coherent states, as well as |r|, increases with an increase in the deformation interval and with a decrease in the deformation time. The presence of the stochastic force acting on the particle can significantly reduce the maximum |r| value and result in the fast relaxation of correlated coherent states with |r| → 0. The effect of dissipation in real systems is weaker than the action of the stochastic force. It has been shown that the formation of correlated coherent states at the fast expansion of the well can underlie the mechanism of nuclear reactions at a low energy, e.g., in microcracks developing in the bulk of metal hydrides loaded with hydrogen or deuterium, as well as in a low-pressure plasma in a variable magnetic field in which the motion of ions is similar to a harmonic oscillator with a variable frequency.     

Improvement of magnetic intergranular isolation and evaluation of read/write characteristics on SmCo5 perpendicular magnetic thin films

SmCo₅ alloy is a promising candidate for ultra-high-density perpendicular magnetic recording (PMR) media because of its high uniaxial magnetocrystalline anisotropy K_u of more than 1.1×10⁸ erg/cm³. Previously, we successfully achieved high K_u in a sputter-deposited SmCo₅ thin film by introducing a Cu/Ti dual underlayer. However, in order to apply the SmCo₅ films to practical PMR media, it is necessary to decrease medium noise. A granulated magnetic film comprising of small and magnetically decoupled grains is effective in reducing the medium noise. In this paper, we have proposed a new granular film that is fabricated by partial thermodiffusion of Cu between the Sm-Co continuous layer and the Cu underlayer, which is granulated using compositional segregation caused by the addition of Ta₂O₅. We have analyzed the magnetic properties, magnetic domain size, and magnetization reversal process of the proposed SmCo₅ film. The magnetic domain size decreased and the magnetization reversal process changed from the magnetic-wall-motion mode to a coherent rotation mode to some extent on isolation of magnetic grains. The read/write characteristics of granulated SmCo₅ double-layered media were also evaluated. The medium noise decreased and the signal-to-noise ratio increased for the granulated double-layered (PMR) medium.     

Nanoionics of advanced superionic conductors

New scientific direction — nanoionics of advanced superionic conductors (ASICs) was proposed. Nanosystems of solid state ionics were divided onto two classes differing by an opposite influence of crystal structure defects on the ionic conductivity σ_i (energy activationE): I) nanosystems on the base compounds with initial small σ_i (large values ofE); and II) nanosystems of ASICs (nano-ASICs) withE ≈0.1 eV. The fundamental challenge of nanoionics as the conservation of fast ion transport (FIT) in nano-ASICs on the level of bulk crystal was first recognized and for the providing of FIT in nano-ASICs the conception of structure-ordered (coherent) ASIC//indifferent electrode (IE) heteroboundaries was proposed. Nano-ASIC characteristic parameterP=d/λ _Q (d is the thickness of ASIC layer with the defect crystal structure at the heteroboundary, and λ _Q is the screening length of charge for mobile ions of the bulk of ASIC) was introduced. The criterion for a conservation of FIT in nano-ASIC isP≈1. It was shown that at the equilibrium conditions the contact potentialsV at the ASIC//IE coherent heterojunctions in nano-ASICs areV«k _BT/e. Interface engineering approach “from advanced materials to advanced devices” was proposed as fundamentals for the development of applied nanoionics. The possibility for creation on the base of ASIC//IE coherent heterojunctions of the efficient energy and power devices (sensors and supercapacitors with specific capacity ≈10^?4 F/cm² and maximal frequencies 10⁹–10⁰ Hz,) suited for micro(nano)electronics, microsystem technology and 5 Gbit DRAM was pointed out.     

Spin wave relaxation and magnetic properties in [M/Cu] super-lattices; M=Fe, Co and Ni

In this work, we study the elementary excitations and magnetic properties of the [M/Cu] super-lattices with: M=Fe, Co and Ni, represented by a Heisenberg ferromagnetic system with N atomic planes. The nearest neighbour (NN), next nearest neighbour (NNN) exchange, dipolar interactions and surface anisotropy effects are taken into account and the Hamiltonian is studied in the framework of the linear spin wave theory. In the presence of the exchange alone, the excitation spectrum E(k) and the magnetization 〈S^z〉/S analytical expressions are obtained using the Green's function formalism. The obtained relaxation time of the magnon populations is nearly the same in the Fe and Co-based super-lattices, while these magnetic excitations would last much longer in the Ni-based super lattice. A numerical study of the surface anisotropy and long-ranged dipolar interaction combined effects are also reported. The exchange integral values deduced from a comparison with experience for the three super-lattices are coherent.     

Electrically stimulated motion of dislocations in a constant magnetic field

This paper reports on the results of investigations into the dislocation mobility in n-Si single crystals (N _d =5×10²⁴ m^?3) upon simultaneous exposure to electric (j=3×10⁵ A/m²) and magnetic (B≤1 T) fields. It is found that the introduction of dislocations (≈10⁹ m^?2) into dislocation-free silicon doped with phosphorus leads to the appearance of the paramagnetic component of the magnetic susceptibility. The paramagnetic component increases with an increase in the dopant concentration. Similar transformations in silicon account for the formation of magnetically sensitive impurity stoppers that respond to external magnetic perturbations. An analysis of the behavior of dislocations in electric and magnetic fields has revealed a parabolic dependence of the dislocation path length on the magnetic induction B. The effective charges and mobilities of dislocations are numerically calculated from the results obtained. A model is proposed according to which the observed increase in the dislocation mobility is associated with the decrease in the retarding power of magnetically sensitive stoppers due to a local change in the magnetic characteristics of the material and the spin-dependent reactions stimulated by a magnetic field.     

Nonlinear resonant polarization rotation under conditions of coherent population trapping

Nonlinear resonant optical rotation was studied over a wide range of experimental parameters at the Rb D ₁ F = 2→F′ = 1 transition in the ⁸⁷Rb vapor under conditions of coherent population trapping. The angle of rotation was found to depend nonmonotonically on the laser intensity and applied magnetic field. The effect of optical pumping out to the level F = 1 is discussed. It is demonstrated experimentally that the Faraday rotation angle increases twofold upon the compensation for pumping.     

Ferromagnetism of (Pb,Sn, Mn) Te under high pressure

Results of measurements of the magnetic susceptibility in the p-type diluted magnetic semiconductor [(PbTe)_1-x(SnTe)_x]_1-y(MnTe)_y with x = 0.72, y = 0.03 are presented. The temperature of the ferromagnetic ordering T_c increases (for hole concentration 3×10²⁰ cm^-3) < p < 7 × 10²⁰ cm^-3 or decreases (p > 7 × 10²⁰ cm^-3) as hydrostatic pressure is applied. The pressure causes an increase in the concentration of carriers responsible for the interactions between Mn ions. The observed shift of T_c with pressure is interpreted on the basis of the RKKY interaction mechanism and points out the importance of the band structure behavior in consideration of magnetic properties of the semiconductor.     

Statistical mechanics of a bounded,ideal magnetofluid

A statistical model of a three-dimensional, incompressible, cylindrically bounded, current-bearing magnetofluid is presented for the purpose of gaining insight into the nonlinear relaxation process routinely observed in reversed-field-pinch experiments. An absolute equilibrium ensemble is utilized that incorporates energy, magnetic helicity, and magnetic flux constraints. Results are extracted only after an extensive mathematical treatment of the properties of poloidal and toroidal fields. The model predicts the presence of magnetic fluctuations about a cylindrically symmetric, Bessel-function-model, mean magnetic field, which satisfies ▽ × 〈B〉 = μ〈B〉. As Taylor's ∵-parameter approaches 1.56, the model predicts that the significant region of the fluctuation spectrum narrows down to a single (coherent) m = 1 mode. An analogy between the Debye length of an electrostatic plasma and μ^?1 suggests the physical validity of the model's prediction of the magnetic-field-fluctuation autocorrelation tensor 〈δB(r) δB(r′)〉, when |;r ? r′| ≥ μ^?1.     

A comparative study of the dissociation of protons into low-mass pπ+π− systems in coherent and incoherent proton-nucleus collisions at 18.6 GeV/c

A decay angular moments analysis has been performed on 8500 pπ⁺π^? events produced by the interaction of high-energy protons with nuclear targets. The analysis was carried out separately for the low-t, coherent events and high-t, incoherent events. The moments of the two samples show significant differences and possible reasons for such differences are discussed.     

The equivalent photon approximation for coherent processes at colliders

We consider coherent electromagnetic processes for colliders with short bunches, in particular the coherent bremsstrahlung (CBS). CBS is the radiation of one bunch particles in the collective field of the oncoming bunch. It can be a potential tool for optimizing collisions and for measuring beam parameters. A new simple and transparent method to calculate CBS is presented based on the equivalent photon approximation for this collective field. The results are applied to theφ-factoryDAΦNE. For this collider about 5·10¹⁴ d E _γ/E _γ photons per second are expected in the photon energyE _γ range from the visible light up to 25 eV.     

The equivalent photon approximation for coherent processes at colliders

We consider coherent electromagnetic processes for colliders with short bunches, in particular the coherent bremsstrahlung (CBS). CBS is the radiation of one bunch particles in the collective field of the oncoming bunch. It can be a potential tool for optimizing collisions and for measuring beam parameters. A new simple and transparent method to calculate CBS is presented based on the equivalent photon approximation for this collective field. The results are applied to the??-factoryDA??NE. For this collider about 5·10¹⁴ d E _??/E _?? photons per second are expected in the photon energyE _?? range from the visible light up to 25 eV.     

The kinetics of the sensitized flourescence of naphthalene single crystals doped with 2-methylnaphthalene and anthracene

The kinetics of the sensitized flourescence in naphthalene crystals doped with 2-methylnaphthalene (2-MN) and anthracene (A) was studied as a function of the temperature. These experiments yield singlet exciton diffusion constants of (2 ± 1) × 10^-4cm²s^-1 at 4.2 K and roughly 10^-5 cm² s^-1 at 300 K. The exciton trapping radii can be estimated to be of the order of 60 Å for A and 10 Å for 2-MN. At temperatures below 40 K a partially coherent exciton motion can be inferred, above 40 K the exciton motion appears to be purely incoherent.     

Exotic molecular states in a strong magnetic field: H<Stack><Subscript>3</Subscript><Superscript>(2+)</Superscript></Stack> ion

The existence of the molecular ion H ₃ ⁽²⁺⁾ in a magnetic field in a triangular and a linear configuration is discussed. A variational method (with an optimization of the form of the vector potential) is used. It is shown that, in the range of magnetic fields 10⁸<B<10¹¹ G, the system (pppe), with the protons forming an equilateral triangle perpendicular to the magnetic line, has a well-pronounced minimum in the total energy. Also, for B?10¹⁰ G, if the protons are situated along a magnetic line (linear configuration), a well-pronounced minimum in the total energy appears. Both configurations are unstable under the decays H(atom) + p + p and H ₂ ⁽⁺⁾ + p. A possible connection between the H ₃ ⁽²⁺⁾ molecular ion and a recently discovered absorption feature in a neutron-star atmosphere is discussed.